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• Redox potential Redox systems
• Reversible redox system Reversible redox systems

It is generally accepted that atmospheric molecular oxygen is of biogenic origin, and its appearance is directly related to the formation of a new type of photosynthesis, in which water is used as an electron donor. Under the conditions of primitive Earth, before the emergence of oxygen-producing photosynthetic eubacteria, the only source of free oxygen was the reaction of photolysis of water vapor in the atmosphere, which occurs under the action of short-wave ultraviolet radiation. However, the amount of "photolytic" oxygen was negligible. The resulting oxygen was used to oxidize the gases of the primitive atmosphere and reduced minerals that make up the earth's crust.

Of all the organisms that carry out photosynthesis with the release of O 2, photosynthetic eubacteria (cyanobacteria, prochlorophytes) are the most primitively organized, and we have the right to assume that the appearance of molecular oxygen is associated with these organisms or with some of their very close ancestors.

Before the emergence of photosynthetic eukaryotes, and primarily higher plants, the content of free oxygen in the Earth's atmosphere was negligible compared to its content in the modern Earth's atmosphere. However, according to calculations, to switch the organism from fermentation to respiration, an oxygen concentration of 0.2%, i.e., 0.01 of its level in the modern atmosphere, is sufficient. The appearance and accumulation of O 2 in the Earth's atmosphere was an event whose significance for the subsequent evolution of life on Earth can hardly be overestimated. First of all, this meant a significant restructuring of everything that was formed on Earth in the "pre-oxygen" era, and primarily concerned living organisms.

The formation of O 2 in increasing amounts made it possible for oxidative reactions to occur on a large scale. The nature of the atmosphere has changed: from a reducing atmosphere it has become an oxidizing one. The latter entailed significant changes in relation to the donor-acceptor problem. If in an oxygen-free atmosphere the solution to the problem of an electron acceptor was dominant, then in an oxygen atmosphere the problem of an electron donor becomes the main one, since with the appearance of O 2 in the Earth's atmosphere a source of an excellent electron acceptor was formed.

INTERACTION OF PROKARYOTES WITH MOLECULAR OXYGEN

Initially, molecular oxygen appeared inside the cell, and this immediately created the problem of the interaction of the cell with O 2 . Obviously, the first photosynthetic organisms that produced molecular oxygen did not have enzyme systems not only for the beneficial use of this acceptor, but also for its neutralization in the cell. They also did not exist in other anaerobic life forms that existed. Therefore, it can be assumed that the first type of interaction with O 2 was based on a sharply negative attitude of the cell towards it. An example of this is the numerous data on the high toxicity of molecular oxygen for modern obligate anaerobic organisms 62 .

62 In this connection, interesting data show that in the period preceding the appearance of large amounts of free oxygen in the atmosphere, the prokaryotic community was more diverse than in subsequent times. The diversity of the prokaryotic community decreased significantly 1.5 billion years ago (see Fig. 52).

As O 2 accumulates, it becomes a constant component of the external environment, and only local conditions can be created where it is absent or contained in trace amounts. This led to two possible options for the subsequent interaction of prokaryotes with molecular oxygen. Some of the existing anaerobic forms "left" to habitats where O 2 is practically absent, and thus retained the "look of an anoxic era." Others were forced to follow the path of adaptation to "oxygen" conditions. This means that they formed new metabolic reactions that primarily serve to neutralize the negative effect of molecular oxygen.

So, the next step in the interaction of prokaryotes with oxygen is the ability to exist in the presence of O 2, neutralizing its negative effect. A certain idea of ​​the formed defense systems against molecular oxygen in prokaryotes can be obtained by studying the representatives of this group, located at different steps of the evolutionary ladder.

Toxic effects of molecular oxygen and its derivatives

As an environmental factor, O 2 affects modern prokaryotic organisms in two ways: on the one hand, it can be absolutely necessary, on the other hand, toxic effects on cells are associated with molecular oxygen and its derivatives.

Molecular oxygen. There are a number of hypotheses explaining the sensitivity of prokaryotes to O 2 . According to one of them, molecular oxygen itself is a toxic compound, the aggressive action of which is associated with the ability to oxidize cellular metabolites necessary for functioning in a reduced state. The toxic effect of O 2 depends on the conditions under which organisms interact with it: the concentration of dissolved O 2 , the duration of exposure, and the composition of the environment.

The toxicity of molecular oxygen, for example, may be the result of its active acceptance of electrons from soluble carriers that function in fermentation processes, which will lead to the depletion of the intracellular pool of reduced electron donors necessary for biosynthesis. Indeed, it was found that the activity of soluble flavoproteins capable of functioning as NAD(P)-H 2 -oxidase increased 5-6 times when grown Clostridium acetobutylicum under aerobic conditions compared to anaerobic ones. The shift under the influence of O 2 of electron carriers in the direction of their predominant presence in the oxidized state led to growth suppression and a change in the yield of fermentation products: the synthesis of butyric acid was stopped and the accumulation of a more oxidized product - acetic acid.

Finally, for the manifestation of the toxic effect of O 2, it is quite sufficient for it to oxidize any one key metabolite or enzyme, leading to their inactivation. There are three enzyme systems of prokaryotes that are particularly sensitive to molecular oxygen: nitrogenase, hydrogenase, and ribulose diphosphate carboxylase.

The nitrogenase system, which catalyzes the fixation of molecular nitrogen, is known to consist of two metalloproteins: a protein containing iron and molybdenum, and a protein containing only iron. Each protein is required for the manifestation of catalytic activity. Molecular oxygen has a damaging effect on both nitrogenase proteins, but is more sensitive to the O 2 Fe protein.

The sensitivity of nitrogenase proteins to O2 is primarily determined by the sensitivity of their metal centers, which are involved in both substrate binding and electron transfer. Since the stepwise reduction of O2 by the one-electron mechanism can also occur in this case, superoxide ions, hydrogen peroxide, and singlet oxygen appear as products of such reduction, which contribute to the oxidative damage of nitrogenase.

Nitrogenase proteins are not the only component of the nitrogen-fixing system sensitive to O 2 . Ferredoxins and flavodoxins, which donate electrons to nitrogenase, can autoxidize and undergo irreversible oxidative damage.

The hydrogenases of many prokaryotes also show a high sensitivity to molecular oxygen, which in vitro largely depends on the method of isolation and the degree of purification. As a rule, crude enzyme preparations are more resistant to O 2 . Compared to the membrane-bound enzyme, the resistance to O 2 of membrane-separated hydrogenase is generally lower. The enzyme obtained from anaerobic cells is more sensitive to O 2 than that isolated from aerobic prokaryotes.

The catalytic activity of ribulose diphosphate carboxylase, the enzyme catalyzing CO2 fixation in the vast majority of autotrophic prokaryotes, depends on the partial pressures of CO2 and O2 . At a high concentration of O 2 and a low concentration of CO 2, the oxygenase reaction predominates:

O 2 and CO 2 molecules compete for the catalytic center of the enzyme. And although the enzyme itself does not show an increased sensitivity to molecular oxygen and, therefore, is not damaged at its high concentration, an increase in O 2 in the medium leads to a change in the functioning of ribulose diphosphate carboxylase. The flow of the enzymatic reaction along the oxygenase pathway leads to the depletion of the cellular pool of ribulose diphosphate molecules and, as a consequence, a decrease in the activity of the reductive pentose phosphate cycle in the cell.

In addition to existing in the basic form, in biological reactions and under the influence of various physicochemical factors, products of incomplete reduction of O 2 appear, which are more reactive and highly toxic to the cell. As you know, for the complete reduction of molecular oxygen, leading to the formation of a water molecule, 4 electrons are required:

O 2 + 4H + + 4 e–  2H 2 O

Most prokaryotes have enzymes that catalyze reactions of simultaneous transfer of 4 electrons to O 2 , in which no intermediate products of O 2 reduction are found. These are cytochrome oxidases and some copper-containing enzymes. It is possible that short-lived products of incomplete O 2 reduction arise in these reactions, but they remain bound to enzymes, do not enter the cytoplasm, and practically do no harm to the cell.

superoxide anion. If the reduction of molecular oxygen occurs in steps, then when 1 electron is transferred to O 2, an overperoxide (superoxide) anion is formed:

O2+ e-  O 2 -.

The latter contains an unpaired electron, therefore it is a negatively charged radical (radical anion). It can be protonated to form a neutral hydroperoxide radical:

O2-. + H +  HO 2.

Recently, the point of view has been recognized, according to which the main danger to organisms is represented by products formed during the one-electron reduction of the O 2 molecule , one of which is the superoxide anion.

There are many biochemical reactions leading to its occurrence. Superoxide anions are generated by interaction with O2 molecules of various components (reduced flavins, quinones, thiols, FeS proteins), as well as in reactions catalyzed by a number of flavoprotein enzymes. Finally, during photosynthesis, there is a flow of electrons. Most photosynthesis reactions are single electron transfer reactions. Therefore, superoxide anions often appear in the system. In addition to reactions of a biological nature, O 2 -. can occur outside the cell in aqueous solutions when exposed to ultrasound, as a result of photochemical, chemical and electrochemical processes.

The danger of any reactive compounds largely depends on their stability. In this regard, O 2 - ions. are very dangerous, since their "lifetime" in an aqueous medium is longer than that of the other O 2 -derivative radicals. Therefore, exogenously arisen O 2 -. can penetrate into the cell and (along with endogenous) participate in reactions leading to various damages: peroxidation of unsaturated fatty acids, oxidation of protein SH groups, DNA damage, etc. The toxicity of superoxide anions can increase due to secondary reactions leading to the formation of hydroxide radicals (OH .) and singlet oxygen (*O 2).

Many prokaryotes belonging to different physiological groups, including strictly anaerobic species, have specific protection in the form of the superoxide dismutase enzyme, which intercepts O 2 - ions. and catalyzing their dismutation. The resulting superoxide anions dismutate in a reaction occurring spontaneously (3) or catalyzed by superoxide dismutase (4):

O2-. + O 2 -. + 2H +  H 2 O 2 + * O 2; (3)

O2-. + O 2 -. + 2H +  H 2 O 2 + O 2. (four)

The differences between the two reactions are in their rates (the rate of the enzymatic reaction is about four orders of magnitude higher than that of the spontaneous one), and also in the fact that in a spontaneous dismutation reaction, one of the initially arising products is singlet oxygen, while in an enzymatic reaction, the resulting oxygen is in the ground triplet state.

hydroxide radical. Superoxidanion can interact with H 2 O 2 to form a hydroxide radical (OH .), superior to O 2 -. according to oxidative activity and toxicity:

O2-. + H 2 O 2 + H +  O 2 + H 2 O + OH. . (5)

source of OH. one-electron oxidation reactions of hydrogen peroxide catalyzed by iron-containing compounds that are always present in cells can serve:

H 2 O 2 + Fe 2+  Fe 3+ + OH - + OH. . (6)

In addition to the above reactions, hydroxide radicals are also formed during the radiolysis of water and are usually present in low concentrations in aqueous solutions. Oh. of all known oxidizing agents, it is the strongest, causing radiation damage to many types of biopolymers.

Hydrogen peroxide. The transfer of 2 electrons to O 2 leads to the formation of peroxide anion (7) or hydrogen peroxide (8):

O 2 + 2 e–  O 2 2–; (7)

O 2 + 2H + + 2 e-  H 2 O 2. (eight)

The transfer of 2 electrons to O 2 can be catalyzed by flavin oxidases contained in prokaryotic cells and some cytochromes. The source of H 2 O 2 can be the autoxidation reactions of some non-heme FeS proteins, as well as the dismutation reactions of superoxide radicals described above (reactions 3 and 4). Hydrogen peroxide is formed in all aerobes and facultative anaerobes growing under aerobic conditions, so its occurrence in prokaryotic cells is a natural process.

Hydrogen peroxide is the most stable of the O 2 reduction intermediates, but also the least reactive. In most aerobic prokaryotes, H 2 O 2 is rapidly decomposed by the heme-containing enzymes catalase and peroxidase. In their absence, H 2 O 2 can accumulate in lethal concentrations for the organism.

H 2 O 2 causes oxidation of SH-groups in proteins, peroxidation of unsaturated fatty acids. However, these reactions proceed at measurable rates if the concentration of H 2 O 2 in the cell is four orders of magnitude higher than that which is usually achieved in vivo. Therefore, it is possible that hydrogen peroxide is dangerous not because of direct interaction with cell components, but because, reacting with O 2 -. (reaction 5) or Fe 2+ ions (reaction 6) can lead to the formation of a hydroxide radical.

In the 20s. the theory explaining the toxicity of O 2 by the accumulation of hydrogen peroxide in the cell was very popular. However, O 2 forms more toxic to the cell were later found among the primary and secondary products of its one-electron reduction (O 2 , OH . , *O 2).

singlet oxygen. Normally, O 2 is in a stable state, called the triplet state, which is characterized by the lowest level of molecular energy. Under certain conditions, the O 2 molecule passes into one of two excited singlet states (*O 2), which differ in the degree of energy and the duration of "life". In most living cells in the dark, the main source of singlet oxygen is the spontaneous dismutation of superoxide anions (see reaction 3). Singlet oxygen can also arise from the interaction of two radicals:

O2-. +OH.  OH - + * O 2. (nine)

Probably, any biological system in which O 2 is formed can be an active source of singlet oxygen. However, the latter also occurs in dark enzymatic reactions in the absence of O 2 .

It has long been known that the toxicity of molecular oxygen to living organisms increases in the light. This is facilitated by substances in the cell that absorb visible light - photosensitizers 63 Many natural pigments can be photosensitizers. In the cells of photosynthetic organisms, chlorophylls and phycobiliproteins are active photosensitizers. The oxidation of biologically important molecules under the influence of visible light in the presence of molecular oxygen and a photosensitizer is called the photodynamic effect.

63 Photosensitizers are molecules capable of absorbing light and inducing chemical reactions that would not occur in their absence. The ability to absorb light is due to the presence of chromophore groups in the molecules, which usually contain cyclic nuclei. More than 400 substances are known to have the properties of photosensitizers. Among natural substances, photosensitizers are chlorophylls, phycobilins, porphyrins and intermediate products of their synthesis, a number of antibiotics, quinine, riboflavin, etc. Some photosensitizers act only in the presence of O 2, causing a photodynamic effect.

The absorption of visible light leads to the transition of the photosensitizer molecule to an excited singlet state (*D):

Molecules that have passed into the singlet state can return to the ground state (D) or go over to the long-lived triplet state (*D), in which they are photodynamically active. Several mechanisms have been established by which an excited molecule (*D) can cause the oxidation of a substrate molecule. One of them is associated with the formation of singlet oxygen. The photosensitizer molecule in the triplet state reacts with O 2 and transforms it into an excited singlet state:

T D + O 2  D + * O 2.

Singlet oxygen oxidizes the substrate molecule (B):

B + * O 2  BO 2.

The photodynamic effect is found in all living organisms. In prokaryotes, as a result of photodynamic action, many types of damage are induced: the loss of the ability to form colonies, damage to DNA, proteins, and the cell membrane. The cause of damage is the photooxidation of certain amino acids (methionine, histidine, tryptophan, etc.), nucleosides, lipids, polysaccharides and other cellular components.

Cells contain substances that quench singlet oxygen and reduce the possibility of structural and other damage caused by it. One of the "quenchers" of singlet oxygen is carotenoids, which protect photosynthetic organisms from the lethal effects photosensitized by chlorophyll. *O 2 interceptors are also various biologically active compounds: lipids, amino acids, nucleotides, tocopherols, etc.

Ozone and atomic oxygen. The products of molecular oxygen are also atomic oxygen (O) and ozone (O 3). It is known that molecular oxygen strongly absorbs light in the far UV region (160–240 nm). The absorbed photon causes the oxygen molecule to dissociate into two atoms:

O 2 + h  2O.

Then a reaction proceeds spontaneously, leading to the formation of an ozone molecule:

Ozone can arise from molecular oxygen in the air during strong electrical discharges, as well as during the electrolysis of water and in some reactions where it accompanies the formation of O 2 . In some oxidation reactions with ozone, singlet oxygen is produced. As oxidizing agents, ozone and atomic oxygen are stronger than O 2 . Ozone can react with almost all types of compounds to form radicals.

Defense mechanisms of the cell

To neutralize toxic forms of O 2, existing prokaryotes have developed various defense mechanisms that can be divided into several types. The defense systems of the first type are based on the activity of special enzymes, for which the decomposition of toxic forms of O 2 is the main and in some cases the only function. In the second type of defense systems, certain cellular metabolites are used to destroy toxic forms of O 2 . As a rule, in this case participation in the protection of the cell from the toxic effects of O2 derivatives is not the only function of these metabolites. Finally, a number of adaptations developed by prokaryotes at different levels: population, physiological, structural, can be attributed to the protective mechanisms of a special type. It is more likely that they were created for other purposes, but have proven to be useful for O 2 detoxification as well.

Enzymatic defense systems. The front line of defense against the toxic effects of O 2 derivatives are enzymes: superoxide dismutase, which captures O 2 molecules (reaction 4), catalase and peroxidase, which trap H 2 O 2:

They minimize the concentration in the cell O 2 -. and H 2 O 2 and do not allow them to interact with the formation of OH. (reaction 5).

Superoxide dismutase was found in O2-using chemotrophic prokaryotes (obligate and facultative aerobic forms), as well as in studied representatives from the groups of photosynthetic prokaryotes. Among anaerobes, the enzyme was found in the vast majority of aerotolerant forms. Some lactic acid bacteria are an exception; however, the cells of most of them contain high concentrations (up to 30 mM) of divalent manganese ions. It turned out that Mn 2+ , which has been shown to be able to be oxidized under the action of O 2 , at such concentrations can just as effectively remove the formed superoxide ions as superoxide dismutase does, the content of which in the cell is usually maintained at the micromolar level. Thus, in these lactic acid bacteria, the function of neutralizing O 2 is performed by Mn 2+ ions.

Neither superoxide dismutase nor high concentrations of Mn 2+ were found in the cells of some types of lactic acid bacteria. These species are characterized by a very high sensitivity to O 2 .

Among obligate anaerobes, superoxide dismutase has been found in many members of the genus Clostridium. The study of their resistance to O 2 reveals a clear relationship with the content of this enzyme in the cells. Species that have superoxide dismutase are characterized by moderate or even high resistance to O 2 compared to species that lack this enzyme. Superoxide dismutase has been found in various types of strictly anaerobic bacteria. The number of organisms with superoxide dismutase not identified so far is very small.

The discovery of superoxide dismutase in strict anaerobes (much more common than previously thought) raises the question of its physiological role in these organisms. The ability of the latter to grow only in an oxygen-free environment makes it unclear the functions of the enzyme under these conditions. It is possible that only when a strict anaerobe enters aerobic conditions that are unfavorable for it, the synthesis of the enzyme is induced by molecular oxygen, which provides the body with protection from O 2 under these conditions.

Superoxide dismutase is an enzyme containing metal ions as a prosthetic group in the active center. In prokaryotes, these are manganese and / or iron atoms. Most of the studied superoxide dismutases are built from two identical subunits, each of which contains one metal atom. Fe- and Mn-enzymes are similar in amino acid sequence. Attempts to reveal the relationship between the physiological and other characteristics of organisms and the metal form of the enzyme contained in them have not led to a definite conclusion. Both forms of superoxide dismutase were found in representatives of gram-positive and gram-negative prokaryotes, among photo- and chemotrophs, obligate anaerobes, aerobes, and facultative anaerobic forms. Moreover, both metal forms of superoxide dismutase can be present in the same organism and even be part of the molecule of the same enzyme. For some species, it has been shown that the synthesis of one or another type of enzyme depends on the presence of metal ions in the cultivation medium.

Superoxide dismutase of the studied chemotrophic prokaryotes is an enzyme not associated with membranes and localized in the cytoplasm. At E. coli, whose cells contain Fe-, Mn-, and Fe/Mn-forms of the enzyme, Fe-superoxide dismutase is localized in the periplasmic space, and the Mn-containing enzyme is located in the cytoplasm. In this regard, it is suggested that the metal forms of the enzyme play different roles in protecting the cell from O 2 –. : Fe-containing enzyme protects the cell from exogenous superoxide anions, and Mn-containing - from endogenous.

The problem of protection from molecular oxygen and its derivatives in cyanobacteria cells is especially acute. It was probably they who first felt the consequences of the toxic effects of oxygen to the greatest extent. Superoxide dismutase is found in all cyanobacteria. In cages Anacystis nidulans (Synechococcus) Fe-superoxide dismutase, which makes up to 90% of the total amount of the enzyme, is localized in the cytosol of the cell, and the Mn-containing form is in thylakoids. The function of the last form of the enzyme is probably reduced to the interception of O 2 - ions. arising in the process of photosynthetic electron transport.

catalase and peroxidase. Hydrogen peroxide is degraded by two classes of related enzymes that catalyze its two-electron reduction to H 2 O and use H 2 O 2 as an electron donor in the case of catalase (reaction 10) or various organic compounds in the case of peroxidase (reaction 11).

Catalase and peroxidase activities were found in all obligate and facultative aerobic prokaryotes. Among obligate anaerobes, these enzymes are much less common than superoxide dismutase. Many strict and aerotolerant anaerobes containing superoxide dismutase but not catalase have been found. Among them are those lactic acid bacteria in which the dismutation of the formed ions O 2 -. is provided by Mn 2+, which is in cells in high concentrations.

The absence of catalase in lactic acid bacteria is due to the fact that they cannot synthesize the heme - the prosthetic group of the enzyme, but are capable of synthesizing apoenzyme. When heme groups are added from the outside, lactic acid bacteria form heme-containing catalase. In a number of lactic acid bacteria, catalase was found that does not contain a heme group, which is why it is called pseudocatalase. The isolated enzyme consists of six identical polypeptide chains linked together by non-covalent forces. Each subunit contains 1 manganese atom.

Hydrogen peroxide, resulting from the interaction of cells with O 2 , is also eliminated by non-enzymatic pathways. It is known that Fe 2+ ions in an aqueous solution accelerate the reduction of H 2 O 2 to H 2 O. A cell always contains a certain amount of iron ions. The destruction of H 2 O 2 can also occur due to the reduced substances released into the culture medium.

For anaerobic prokaryotes capable of tolerating contact with O 2 and its derivatives on a relatively small scale, the presence of superoxide dismutase, which “removes” O 2 –, is necessary in the cells. . The presence of catalase is not necessary in this case, since the hydrogen peroxide that occurs in the dismutation reaction and other reactions decomposes spontaneously or with the participation of non-enzymatic catalysts, and organisms generally cope with it under these conditions. Thus, in the implementation of energy metabolism of anaerobic type to eliminate the toxic effects of O 2 . one enzyme barrier in the form of superoxide dismutase is sufficient.

A sharp increase in the scale of interaction between prokaryotes and O 2 during the functioning of aerobic metabolism makes non-enzymatic ways of removing H 2 O 2 ineffective. For the decomposition of hydrogen peroxide, which is formed in large quantities, special enzymes are needed that increase the rate of decomposition of H 2 O 2 by several orders of magnitude. This is provided by catalase and peroxidase. Thus, under conditions of active interaction of cells with O 2, which makes aerobic life possible, the system of enzymatic protection against its toxic effects is formed with the participation of superoxide dismutase, catalase, and peroxidase as necessary components (Fig. 87).

Mechanisms of protection with the help of cellular metabolites. Protection against one of the most toxic derivatives of O 2 - singlet oxygen - is carried out with the help of various biologically important molecules. All types of quenching *O 2 can be divided into physical and chemical. A quenching that does not lead to the destruction of the quencher is called physical quenching:

*O 2 + A  O 2 + A

Chemical quenching results in the oxidation of the quencher:

*O 2 + A  O 2 + A ok

Predominantly according to the chemical mechanism, *O 2 quenching is carried out by saturated fatty acids, lipids, amino acids, nucleotides and other compounds. The mechanisms of chemical quenching are varied, but in most cases the initial stage is the formation of a labile cyclic peroxide followed by its decomposition, which leads to the formation of free radicals. Chemical quenching of *O 2 can lead to significant destructive consequences in the cell. Molecules of various chemical compounds are capable of quenching mainly by physical mechanism. The most effective in this regard are carotenoids, which are widely distributed in the world of prokaryotes. They are found in the cells of many aerobic chemotrophs and are an essential component of the pigment apparatus of all phototrophs. In the cells of photosynthetic organisms, as noted above, chlorophylls are active photosensitizers. However, the possibility of photooxidative effects under the conditions of functioning of the photosynthetic apparatus is rather low, firstly, because of the extremely short time (10–11 s) of chlorophyll in the excited state and, secondly, because of the protection of cells from photooxidation by carotenoids.

The role of carotenoids in preventing the lethal effect caused by photooxidation was first shown in the study of a carotenoid-free mutant of the purple bacterium Rhodopseudomonas sphaeroides. The initial culture grew well phototrophically under anaerobic conditions, but could also grow in the light and in the dark under aerobic conditions. The mutant obtained from it, devoid of carotenoids, had a low growth rate in the light under anaerobic conditions and in the dark under aerobic conditions, but quickly died when exposed to light + air.

Photooxidative damage can also develop in non-photosynthetic prokaryotes, as their cells also contain colored molecules that absorb visible light, which can function as photosensitizers. The action of carotenoids is not limited to their participation in protection against the photodynamic effect. They quench the singlet state of oxygen, irrespective of the reactions in which it occurs: in the light or in the dark.

The mechanism of the protective action of carotenoids in photosynthetic organisms is as follows (Fig. 88). A chlorophyll molecule that has absorbed light quickly (10–12 s) transfers the energy of the singlet excited state to the reaction center. Of the 10 4 absorbed light quanta, approximately 4 lead to the transition of the chlorophyll molecule to an excited triplet state. There is a possibility of photodynamic damage. Carotenoids can participate in three protective reactions: 1) directly quench the triplet state of chlorophyll, transferring it to the ground state (Fig. 88, BUT); the resulting triplet carotenoid molecule gives off excess energy in the form of heat and returns to the ground state; 2) triplet chlorophyll is not quenched by carotenoids; it interacts with O 2 , which transforms the latter into an excited singlet state; singlet oxygen is quenched by carotenoids (Fig. 88, B); 3) singlet oxygen, which has not undergone quenching by carotenoids according to the physical mechanism, can interact with them in a chemical reaction leading to the oxidation of carotenoids. The participation of carotenoids in any of the three reactions described above will reduce the level of formation of *O 2 in the cell.

Adaptations of prokaryotes that help them protect themselves from the toxic effects of molecular oxygen. In the cells of those obligate anaerobic clostridia in which neither superoxide dismutase nor catalase was found, the available means of neutralizing O 2 is its displacement from the cultivation medium with the help of actively released gaseous products (CO 2 and H 2) accompanying fermentation, as well as absorption by the cell suspension oxygen from the environment, leading to the death of some cells, but allowing the remaining ones to multiply under conditions of low O 2 content.

Lactic acid bacteria have made a certain step forward in the direction of protection against molecular oxygen. These bacteria are the only group of prokaryotes that do not have heme-containing catalase and are able to grow in the presence of air. The search for the mechanisms of neutralization of O 2 and its derivatives led to the discovery in them, in addition to superoxide dismutase, of a high intracellular concentration of Mn 2+ ions, which decompose O 2 –. , pseudocatalase, as well as catalase- and peroxidase-like activity. In some representatives of lactic acid bacteria, a more pronounced degree of adaptability to O 2 is seen, leading to attempts at a certain beneficial use of it. For some lactic acid bacteria of the genus Lactobacillus acceleration of glycolytic decomposition of glucose under aerobic conditions is shown. This is due to the fact that under aerobic conditions, hydrogen from NAD-H 2 can be directly transferred to O 2 , freeing a part of pyruvic acid from its acceptor function, as occurs in ordinary lactic acid fermentation. Released from this "duty", pyruvic acid can now be oxidized to acetyl-CoA, the subsequent metabolization of which to acetate leads to the synthesis of an ATP molecule. As you can see, the participation of oxygen in this process is not directly related to the receipt of energy by the cell (when transferring hydrogen from NAD-H 2 to O 2, energy in the form of ATP is not formed), i.e., all energy is obtained due to substrate phosphorylation, but O 2, taking on the acceptor function, releases part of the pyruvate, which can be used by the cell along the energy pathway, which ultimately leads to an increase in the energy yield of fermentation. Thus, direct oxidation of some of the reduced electron carriers during fermentation can have not only negative, but also positive consequences.

As noted above, the process of nitrogen fixation is very sensitive to O2. Despite this, the ability to fix N 2 is widespread among prokaryotes that differ in their attitude to molecular oxygen; it is inherent in chemotrophs and phototrophs, including cyanobacteria that carry out oxygen photosynthesis. Free-living forms and prokaryotes in symbiosis with eukaryotic organisms can fix N2.

The study of the means of protection of this process in prokaryotes showed that in most cases it is far from 100% effective. Among aerobic nitrogen fixers, only a few organisms can be distinguished that are capable of growing in an environment with N 2 in equilibrium with air. The majority can grow and fix N 2 only under conditions of low concentration of molecular oxygen, i.e. under microaerobic conditions. Protection of nitrogenase in the cells of facultative anaerobes is even less effective: they can actively fix nitrogen only under anaerobic conditions.

Aerobic nitrogen fixers include members of the genus Azotobacter, which have various protective devices. One of them is associated with a sharp increase in the respiratory activity of cells that perform nitrogen fixation under aerobic conditions. Respiration largely serves in this case for "binding" intracellular O 2 . At the same time, significant rearrangements were found both in the cellular structure of Azotobacter, expressed in the intensive development of the system of intracytoplasmic membranes, and in the organization of the respiratory chain itself, localized in these membranes. respiratory chain Azotobacter vinelandii quite complex, has branches in the electron transport pathways at the level of cytochrome b. Electron transport associated with phosphorylation occurs along the path:

cytochromes b  c 4  c 5  a 1 .

In the implementation of "respiratory" protection, the activity of electron transport increases along the branch: cytochromes b  d not related to energy storage. This leads to the fact that, despite an increase in the overall activity of respiration, the coupling of electron transport with energy storage decreases. Thus, there is a "burning" of a part of the carbon substrates, which are used to reduce O 2 , without storing energy by the cell.

In addition to forced "sacrificing" part of the carbon sources, high concentrations of O 2 cause reversible changes in the structure of nitrogenase in the cell, making the areas sensitive to molecular oxygen less accessible to it. Various suggestions have been made as to how the "conformational" protection is carried out. Perhaps, in this case, there is a change in the relative position of the two nitrogenase proteins. Participation in the protection of this type of cell membrane is not excluded. A certain stabilization of nitrogenase under conditions of a high concentration of O 2 occurs when divalent cations are added to the enzyme complex. Finally, special protective proteins have been found that form complexes with nitrogenase proteins and lead to an increase in their stability in the presence of O 2 . No other functions, except for the protective one, have yet been found in these proteins.

Most nitrogen-fixing prokaryotes are able to fix molecular nitrogen under microaerobic conditions. Their protective devices include: the formation of mucus, which prevents O 2 diffusion into the cell and thereby creates a microaerobic zone around it; the formation of cell clusters that impede the access of O 2 to cells located inside the cluster, which, thus, creates more favorable conditions for nitrogen fixation; the existence of nitrogen-fixing species in association with non-nitrogen-fixing aerobic heterotrophs that protect the nitrogenase of nitrogen-fixers from O 2 access.

Specific adaptations to protect nitrogenase from high concentrations of O 2 have been developed by symbiotic nitrogen fixers - nodule bacteria. Already the nodules themselves, places of active reproduction of bacteria and fixation of N 2 by them, should be considered as a structure, one of the purposes of which is to limit the access of molecular oxygen into the interior. The same function is performed by the leghemoglobin contained in the nodules (a protein similar to hemoglobin), which is able to actively bind O 2 and control its entry into bacteroids. In any case, when aerobic metabolism is carried out, respiration will also prevent the accumulation of molecular oxygen in the cell.

The most acute problem is the protection of nitrogen fixation from O2 in the group of cyanobacteria. In all cyanobacteria, nitrogenase is sensitive to O2, which is of extracellular and intracellular origin. In accordance with this, they can distinguish devices aimed at protecting against exogenous oxygen, and those designed to neutralize O 2. formed inside the cell during photosynthesis.

If all nitrogen-fixing cyanobacteria are considered from the point of view of the degree of protection of the nitrogen fixation process from O 2, then they can be divided into two groups. The first group includes cyanobacteria, in which the protection of nitrogen fixation from O 2 is the least effective; therefore, vegetative cells can fix N 2 only under anaerobic or microaerobic conditions. The second group consists of cyanobacteria, in which specialized cells, heterocysts, have been formed to carry out nitrogen fixation under aerobic conditions.

In non-heterocyst cyanobacteria, the protection of vegetative cell nitrogenase from O2, primarily endogenous, is carried out by separating the processes of photosynthesis and nitrogen fixation in time, continuous nitrogenase synthesis, and high activity of superoxide dismutase in combination with catalase and peroxidase activities. In the center of the filaments of some non-heterocyst forms, weakly pigmented vegetative cells are often distinguished, in which the ability to photosynthetic fixation of CO 2 is presumably suppressed and thus more favorable conditions for nitrogen fixation are created. (These are not heterocysts, but it is probably from them that heterocysts subsequently developed as centers of nitrogen fixation under aerobic conditions). The means of protection against exogenous O 2 is the synthesis of a large amount of mucus, often surrounding the cells of nitrogen-fixing cyanobacteria. Existence in the form of colonial forms can also ensure the creation of anaerobic conditions for cells located in the central part of the colony.

The most perfect protection against endogenous and exogenous molecular oxygen in heterocysts. Heterocysts are not capable of photosynthetic release of O 2 . And the high activity of the oxidative pentose phosphate pathway, which supplies electrons to the respiratory chain, where they are accepted by O 2, increased levels of superoxide dismutase compared to vegetative cells, the formation of molecular hydrogen by heterocysts, a thick multilayer shell that acts as a gas barrier - all this reliably protects the nitrogen-fixing system in heterocysts from inactivation by molecular oxygen.

Thus, one can only assume that the mechanisms of neutralization of molecular oxygen at different stages of the evolution of the interaction of cells with it were not the same. H 2 at some stage, enzymatic reactions arose that catalyze the inclusion of O 2 in the metabolism of prokaryotes.

MOLECULAR OXYGEN IN PROKARYOT METABOLISM

The fact that all prokaryotes existing on Earth, even strict anaerobes, absorb it in the presence of O 2 indicates that they carry out some reactions of interaction with molecular oxygen. In relation to O 2, all prokaryotes can be divided into several physiological groups (see Fig. 34). Such a subdivision speaks of the necessity or harm of molecular oxygen, but does not reveal the mechanisms of cell interaction with it. Indeed, we now know that O 2 may be necessary for a cell to obtain energy or to carry out just one reaction that does not have an energy value.

Rice. 89. Ways of using a portion of molecular oxygen absorbed by the cell. See the text for an explanation (according to Skulachev, 1969).

H 2 Based on the study of energy processes occurring in the mitochondria of animal cells, V. P. Skulachev proposed the following classification of reactions of cell interaction with molecular oxygen (Fig. 89). Portion absorbed by the cell O 2 can be divided into two unequal parts. The bulk of oxygen is consumed by the cell with the participation of cellular enzyme systems. Absorption by a cell of some part of O 2 is not connected with its enzymatic systems. The latter is illustrated by the well-known fact of the active absorption of oxygen by a suspension of cells killed by heating. In this case, oxygen uptake is a purely chemical process associated with the oxidation of certain cell chemicals, such as the SH groups of cellular proteins. It is impossible to exclude the possibility of processes of a similar nature occurring in a suspension of living cells. In turn, the enzymatic absorption of molecular oxygen - respiration 64 - is divided into oxidation, associated with the storage of energy, and free oxidation, i.e., not associated with the storage of energy for the cell. Oxidative enzymatic reactions involving O 2 , categorized as free oxidation, are reactions in which energy is released in the form of heat 65 . This category of processes includes reactions catalyzed by mono- and dioxygenases, in which oxygen is directly incorporated into the molecule of the oxidized substance, as well as reactions catalyzed by some oxidases.

64 The term "breathing" was first introduced to denote a certain process associated with the vital activity of higher organisms (plants and animals). Two main features characterized this process: gas exchange with the environment with the indispensable participation of O 2 ; necessary for the life of an organism. The fundamental similarity of the process of respiration at the cellular level in all higher organisms made the use of this term convenient, and the concept denoted by it quite clear. Difficulties arose when using the term "respiration" to designate functionally similar processes in prokaryotes due to their extraordinary diversity. In our understanding, the term "respiration" covers all processes of enzymatic absorption of molecular oxygen by the cell.

65 Free oxidation reactions are of great importance in the implementation of thermoregulation in animals when the body is cooled.

Enzymatic absorption of O 2 associated with energy storage is divided into processes not associated with phosphorylation and processes accompanied by phosphorylation. In the first case, oxidation associated with the storage of energy is not associated with the transformation of free energy into the form of macroergic phosphate bonds. It is known that there are two universal forms of energy in the cell: chemical and electrochemical ( H +). One of the ways to obtain energy in the form of a transmembrane electrochemical H + gradient is associated with the transfer of electrons to O 2 . Energy in this form can be used by the cell to perform various types of work (see Fig. 27). Chemical energy lies mainly in compounds containing high-energy phosphate bonds, and first of all in ATP molecules. But at the intermediate stages of catabolic processes, ultimately associated with the absorption of O 2, metabolites are formed containing energy-rich bonds, such as thioether (C ~ S - CoA). These compounds can directly provide energy for some biosynthetic processes.

Finally, during phosphorylating oxidation, the energy released during electron transport to molecular oxygen and initially occurring in the form of  H + is transformed into a chemical form in ATP molecules with the help of proton ATP synthase. In contrast to higher organisms, where a high degree of conjugation between electron transport and phosphorylation has been achieved, i.e., this pathway appears already in its current form, in modern prokaryotes we find different ways of electron transport and different degrees of conjugation of electron transport with phosphorylation. All of the above types of oxidative processes involving O 2 occurring in a highly organized cell are also found in prokaryotes.

The classification proposed by V. P. Skulachev is based on the consideration of all reactions of the interaction of a cell with molecular oxygen from the point of view of their "energetic significance". According to the chemical mechanisms underlying these reactions, they can all be divided into 2 types. The first type includes reactions catalyzed by oxygen transferases, or dioxygenases, in which an oxygen molecule is directly attached to a metabolite molecule:

A + O 2  AO 2.

One substrate molecule can accept both atoms of the oxygen molecule, as is the case in the above reaction. O 2 acceptors can be molecules of two different substrates:

X + Y + O 2  XO + YO.

All such reactions are free oxidation and are not associated with the production of energy by the cell.

In reactions of the second type, electrons go to oxygen, which acts as the final acceptor. In this case, 1, 2, or 4 electrons, depending on the nature of the carrier, are accepted by the oxygen molecule, which ultimately leads to its incomplete (O 2 , H 2 O 2) or complete (H 2 O) reduction. Reactions of this type are catalyzed by enzymes called oxidases and can be free oxidation and energy storage oxidation. Free oxidation reactions include reactions catalyzed by soluble oxidases localized in the cytosol of the cell. In addition to them, a number of membrane-associated oxidases of cytochrome and non-cytochrome nature have been described in prokaryotes, the transfer of electrons from which to O 2 is also not associated with energy storage.

Chemically intermediate reactions between the above are reactions in which the fate of each of the two atoms in the oxygen molecule is different:

A + O 2 + DH 2  AO + H 2 O + D.

In this case, 1 atom of the absorbed oxygen molecule is used to oxidize the substance by direct attachment to it. and the other is reduced to H 2 O in the presence of a suitable electron donor. Both reactions are catalyzed by a single enzyme belonging to the group of monooxygenases, or oxygenases (oxidases) with mixed functions. Monooxygenases in the cell are numerous and varied. They catalyze free oxidation reactions. Participation in the processes associated with the storage of energy by the cell is unlikely.

Thus, oxygenases are enzymes that catalyze the activation of O 2 and the subsequent incorporation of 1 or 2 of its atoms into the molecules of various substrates. If the substrate (O 2 acceptor) is hydrogen, the enzyme is called an oxidase. In this sense, oxidases can be considered as a specialized class of oxygenases.

Oxygenases play an important role in the processes of biosynthesis, degradation and transformation of cellular metabolites: aromatic amino acids, lipids, sugars, porphyrins, vitamins. The substrates that oxygenases act on are often highly reduced, water-insoluble compounds; their oxidation leads to the fact that the reaction products become more soluble in water and, therefore, biologically active, which is important for their subsequent metabolization. In strictly anaerobic prokaryotes, the oxygen included in the substrate molecule does not come from O 2 , but from other compounds, such as water.

Consequently, the totality of the interaction of molecular oxygen with the cell, from the point of view of the underlying chemical mechanisms, can be reduced to the participation of O 2 in two types of reactions, in the first of which it acts as the final electron acceptor, and in the second, it is directly introduced into a molecule of matter. Only the first type of reactions involving molecular oxygen can become a source of energy for the cell. Therefore, it is important for us to analyze the evolution of the cell's interaction with O 2 along the path of its formation of systems that include the use of molecular oxygen as the final electron acceptor.

FORMATION OF "OXIDASE MECHANISM" OF INTERACTION WITH MOLECULAR OXYGEN ASSOCIATED WITH ENERGY STORAGE

With the advent of O 2 in the atmosphere, it became possible to transfer electrons to it. In order for this transfer to be associated with energy production, it was necessary to form electron transport chains with carriers oriented in a certain way in the membrane, ensuring the movement of protons through the membrane at certain stages, and electrons to O 2, and an enzyme complex that converts the electrochemical energy arising during electron transport into a chemical stored in ATP molecules.

With formed electron transport chains localized in the membrane, containing all types of carriers and directly related to the receipt of energy by the cell, we already meet in the anaerobic eubacteria considered in Chapters 13 and 14 with the most simply organized energy of the chemotrophic (fermentation) and phototrophic (oxygen-free photosynthesis) type : some propionic acid bacteria, all photosynthetic purple and green bacteria. In the cell membranes of these organisms, ATP synthases associated with electron transport are localized and function.

P. Mitchell suggested that the electron and proton transport system and the proton-carrying ATPase arose independently of each other and, probably, non-simultaneously as different ways of generating  H +, which is necessary to provide energy for the process of selective transport of nutrients into the cell. The subsequent "meeting" of both systems in the cell marked the beginning of the conjugation of the processes of electron transport and phosphorylation as a result of the reversal of the work of ATPase. This made it possible to store the free energy of oxidation in ATP molecules. The similar composition and similar structure of energy-converting membranes, the great similarity of the coupling mechanisms in different groups of prokaryotes and eukaryotes indicate that the system of electron transport and phosphorylation coupling that arose at an early stage of evolution was used by all organisms without fundamental changes.

On the origin of reversible proton ATPase

The most ancient origin is probably proton ATPase. It is found in the cells of all organisms, including the primary fermenting anaerobes that synthesize ATP in reactions of substrate phosphorylation. Hypothetical primary cells received all their energy from substrate phosphorylation and had underdeveloped biosynthetic abilities. The intake of the necessary organic compounds from the external environment and the release of the end products of fermentation occurred according to the passive uniport mechanism (see Fig. 26). Primary cells probably did not have a cell wall, but were delimited from the surrounding sections only by an elementary membrane. Obviously, active transport processes that ensure the selective transfer of substances against their concentration gradients were necessary at very early stages of cellular evolution.

To accomplish this task, an ATP-dependent proton pump localized in the CPM was formed in cells. The energy of ATP hydrolysis, carried out by ATPase, was used to push protons out of the cell into the external environment. Hydrolysis of one ATP molecule leads to the transfer of 2 protons and thus the creation of a transmembrane electrochemical proton gradient. This was experimentally shown for lactic acid bacteria and clostridia, which do not have respiration, but ATPases are localized in the CPM, which break down ATP molecules formed during fermentation.

Thus, the use of ATP to create  H + on the membrane is an evolutionarily very ancient mechanism of the prokaryotic cell. Later, a mechanism for the synthesis of ATP at the expense of  H + arose. To do this, it was necessary to change the direction of the work of the proton ATPase complex.

Reversibly functioning proton ATPases are found in primordial anaerobes that obtain energy from fermentation. It was found that the release of lactic and acetic acids into the external environment by lactic acid bacteria and clostridia leads to the creation of a proton gradient at the CPM. In streptococci that carry out homofermentative lactic acid fermentation, lactic acid accumulates in the cell in the form of an anion, for which the CPM is practically impermeable. The release of lactate from the cell occurs in the process of electrically neutral symport with protons (Fig. 90). A similar picture is observed in Clostridium pasteurianum, in which acetate ions accumulating in the cell pass through the CPM in an undissociated form. Since the concentrations of lactic and acetic acids inside the cell at the beginning of fermentation are always higher than in the external environment, they are released using the appropriate carriers along the concentration gradient, i.e., in the process of facilitated diffusion that does not require energy costs. The transport of H + in symport with lactate or acetate leads to the generation of  H + on the CPM. With the accumulation of acids in the external environment, their concentration gradient gradually decreases, as a result of which the ability to form a proton gradient associated with the release of acids from the cell decreases. At high concentrations of lactic and acetic acids in the medium, the formation of  H + on the membrane depends only on ATP hydrolysis.

The electrochemical energy of the proton gradient, which occurs when acids are released from the cell during fermentation, can be used to transport soluble substances into it, as well as for the synthesis of ATP, which is carried out when the proton ATPase functions in the opposite direction, i.e. in the ATP synthase reaction. The energy output due to the release of fermentation products from the cell can be quite significant. With homofermentative lactic acid fermentation, according to estimates, it can reach 30% of the total amount of energy produced by the cell. Thus, in some eubacteria that obtain energy in the process of fermentation, ATP can be synthesized in reactions of substrate phosphorylation and additionally due to the use of  H +, which is formed when the end products of fermentation are released in symport with protons. Consequently, eubacteria with an obligately fermentative energy type already have proton ATPases that function in the direction of ATP hydrolysis and synthesis, i.e., they catalyze the reversible interconversion of two types of metabolic energy:

ATP   H +

Finally, some primary anaerobic fermenting eubacteria have been found to have ATP synthase activity coupled to short fragments of electron transfer via membrane-bound carriers (see below).

Soluble electron transfer systems on O 2 in primary anaerobes

As is known, electron transfer underlies all redox processes. In different types of fermentations discussed in Chap. 13, the transfer of electrons (hydrogen) from one organic molecule to another is usually carried out by soluble NAD-dependent dehydrogenases:

where dg are the corresponding dehydrogenases containing NAD as a coenzyme; A is an organic molecule serving as an electron acceptor. NAD-H 2 molecules are used in constructive metabolism, providing a reductant for biosynthetic processes, as well as in the energy metabolism system, participating in solving the "acceptor problem". In this case, the electron transfer does not lead to the receipt of energy by the cell, it is produced only in reactions of substrate phosphorylation.

In some eubacteria, a direct transfer of electrons from soluble NAD-dependent enzymes to O 2 has been described, leading to its reduction:

OVER-H 2 + O 2  OVER + + H 2 O 2

Oxidation of NAD-dependent dehydrogenases is also carried out through flavoproteins, which catalyze the transfer of 1, 2, or 4 electrons to O 2 , which leads to the formation of superoxide anion, hydrogen peroxide, or water, respectively. O 2 and H 2 O 2 can be further decomposed by the enzymes discussed earlier in this chapter.

In aerotolerant anaerobes, such as lactic acid bacteria and some Clostridia, flavoproteins act as the main link between the substrate and molecular oxygen. Such systems can be useful, for example, for creating anaerobic conditions as a result of the absorption of O 2 from the environment, but they are not related to obtaining energy by the cell. O 2 reduction, in which flavoproteins act as oxidases, i.e., enzymes that directly transfer electrons to molecular oxygen, is called "flavin respiration". Basically, during flavin respiration, a two-electron transfer to O 2 occurs. So, in lactic acid bacteria of the genus Streptococcus 90% of the absorbed O 2 is reduced to H 2 O 2 .

Finally, in some eubacteria, flavoprotein oxidases have been found that catalyze the direct oxidation of substrates, such as pyruvic and lactic acids, with molecular oxygen:

A common feature of the above electron transport pathways involving one or two O 2 mediators is the occurrence of reactions in the cytosol of the cell, i.e., without connection with cell membranes, and the absence of useful energy storage by the cell.

A fundamentally important step towards the creation of electron transport systems, leading to the receipt of energy by the cell, was the incorporation of electron carriers into the membrane.

Formation of membrane-bound electron transport pathways under anaerobic conditions

In the cells of primary anaerobes, short paths of electron transfer, carried out with the help of membrane-bound carriers, have been found. In some cases, this transfer is accompanied by the movement of protons across the membrane and leads to the formation of  H + and the synthesis of ATP. One of the most studied pathways of this type is the fumarate reductase system, which leads to the reduction of fumarate to succinate.

The reduction of fumarate to succinate can be used for anabolic purposes (the need for succinate for the synthesis of tetrapyrroles) or in catabolic processes. In the latter case, all components of the reaction can be soluble, and then the process serves only to accept electrons (Fig. 91, 4 ), or be in a state associated with the membrane (Fig. 91, B–D). According to available data, this does not always lead to the synthesis of ATP. The formation of a proton gradient on the membrane during the transfer of electrons to fumarate depends on the composition and location of electron carriers.

Electron donors for fumarate reduction can be NAD-H 2 , lactate, formate, or molecular hydrogen, from which electrons are transferred by substrate-specific dehydrogenases to membrane-bound carriers (Fig. 91, B). Among the carriers, FeS proteins, menaquinone, and cytochromes of the type b, however, this type of transfer is not associated with the cell receiving energy.

For the formation of a proton gradient, in some cases it is sufficient that the electron donor and their final acceptor are located on different sides of the membrane. The supply of electrons to the carrier localized on the outer side of the membrane leads to the release of protons into the medium, and the reduction of fumarate on the other side of the membrane is accompanied by their absorption from the cytoplasm, while the transfer of protons through the membrane does not occur. The discharge of the resulting proton gradient with the help of ATP synthase will lead to the synthesis of ATP (Fig. 91, B). If the electron donor and acceptor are localized on the same side of the membrane, then the creation of a proton gradient is provided by a combination of electron and hydrogen transfer along a chain containing several carriers (Fig. 91, G). The transfer of hydrogen across the membrane is carried out with the help of quinones. Further improvement of electron transport is associated with the inclusion of cytochromes in the membrane.

The possibility of ATP synthesis during the transfer of electrons from NAD-H2, formate, lactate, H2 to fumarate is confirmed by the corresponding values ​​of the redox potentials of donors and the final electron acceptor (see Table 11).

Functioning in the system of cellular catabolism, the reduction of fumarate to succinate was found in a number of eubacteria that obtain energy in the process of fermentation. One of the steps in the formation of propionic acid during propionic acid fermentation is the reduction of fumarate to succinate catalyzed by fumarate reductase (see Fig. 54). Fumarate reductase has also been found in some Clostridia and lactic streptococci.

The membrane-bound enzyme succinate dehydrogenase is well known to catalyze the oxidation of succinate to fumarate in the TCA cycle. Hydrogen accepted in this reaction by flavin adenine dinucleotide (FAD) directly enters the respiratory chain (see Fig. 92). Since fumarate reductase and succinate dehydrogenase catalyze the same reaction, but in different directions, it was originally thought that this was the same enzyme. It has now been shown that the reactions are carried out by different enzymatic proteins. Information about them is contained in different genes. Synthesis of succinate dehydrogenase is induced under aerobic conditions, while fumarate reductase is induced under anaerobic conditions.

Acetogenic clostridia were able to synthesize acetate from CO 2 and H 2:

2CO 2 + 4H 2  CH 3 -COOH + 2H 2 O.

They can grow chemolithoautotrophically on a medium containing H 2 as the sole source of energy. Therefore, in these organisms, the reduction of CO 2 to acetate, coupled under anaerobic conditions with the oxidation of H 2 , must be associated with the production of useful energy. It was found that flavodoxin, menaquinones, and cytochromes of the type b, i.e. carriers of the same type as during the functioning of the fumarate-reductase system.

Thus, in a number of primary anaerobes that receive energy in fermentation processes, short electron transport chains associated with the membrane have formed, the functioning of which leads to the formation of a proton gradient used for ATP synthesis. Due to the lack of a suitable terminal electron acceptor under anaerobic conditions, the energy yield in this type of process is low. However, the fundamental foundations for creating a new type of energy have been formed.

To switch to the use of light energy, it was necessary to create photoreceptor molecules and "connect" some of them to the existing electron transport chains. Such photoreceptors - Mg-porphyrins - were formed. Photosynthesis began, apparently, with the creation of a system of photoinduced cyclic electron transport and served first as an energy source, additional to the main one, which was the fermentation process. Primary photosynthetic organisms could obtain the reducing agent in the same ways as fermenters, or by spending part of the synthesized ATP for this in the process of reverse electron transfer. Later, for this purpose, the ability of direct photoreduction of NAD + was formed, which led to the creation of light-dependent non-cyclic electron transport. Further improvement of the photosynthetic apparatus led to the use of water as an electron donor, the by-product of which was the formation of molecular oxygen.

As a result of photosynthetic release of O 2, a chemical compound appeared that served as an active oxidizing agent. In response to the appearance of O2, most prokaryotes developed various defense mechanisms. In some lines, adaptation to O 2 ended there, resulting in the emergence of anaerobic forms with varying degrees of aerotolerance.

The next important step in the formation of the mechanism for using molecular oxygen as the final electron acceptor is the use of this process, which contains great energy potential, to obtain energy for the cell. Indeed, the amount of energy released during the transfer of a pair of electrons depends both on the nature of the donor and on the nature of the electron acceptor. For example, the redox potential of NAD-H 2 is – 320 mV, and that of molecular oxygen is +810 mV. For the formation of 1 ATP molecule, the transfer of a pair of electrons along an electrochemical gradient corresponding to a potential difference of approximately 200 mV is necessary.

To use O 2 as the final electron acceptor in the processes associated with the production of metabolic energy, it seemed the least difficult to convert photosynthetic electron transport into respiratory. For this purpose, it was necessary to add dehydrogenases to the low-potential end of the chain and cytochrome oxidase to the other, interacting directly with O 2 . All the necessary types of carriers and reversible proton ATPases had already been formed by this time.

The main task was to create an enzyme system for the four-electron reduction of O 2 (cytochrome oxidase), which would not release its toxic intermediates.

Phosphorylation coupled with the transfer of electrons from substrates in dark oxidative reactions is called oxidative phosphorylation. The development of the mechanism of oxidative phosphorylation made it possible to achieve the most complete extraction of free energy from oxidized substrates.

Thus, the appearance of molecular oxygen marked the beginning of the evolution of new types of life in the world of prokaryotes, which are based on obtaining energy through the processes of oxidative phosphorylation.

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) combined a number of phenomena associated with the boundary of the Archean and Proterozoic, under the name "Great Oxygen Event" (Great Oxygenation Event). The available data made it possible to represent this milestone as follows: the beginning of the activity of photosynthetic organisms, the accumulation of oxygen in connection with it, and the gradual transformation of the planet from a reducing to an oxidizing one. Subsequent work significantly corrected this model. Photosynthetic organisms that release oxygen originated at the dawn of Archean life, but free oxygen appeared at the turn of the Archean and Proterozoic due to changes in the nature of terrestrial volcanism. For 90% of its life, the planet had a practically oxygen-free hydrosphere and atmosphere, while in the Proterozoic, the oxygen content turns out to be significantly lower than previously thought, and extremely variable.

In the 50s of the XX century, data began to accumulate on the early Proterozoic oxygen jump (Oxygen catastrophe, or Great oxygenation event, “Great oxygen event”). There was an idea that the early atmosphere of the planet was reducing, and then 2.6–2.2 billion years ago, the atmosphere and the ocean gradually began to increase free oxygen. Oxygen was formed as a by-product of the activities of photosynthetics: for energy, they used the most readily available substance on the planet - water. This model was based on geochemical data. The main one was considered to be the high content of divalent (underoxidized) iron in the Archean rocks in the form of pyrite (FeS 2), magnetite (Fe 3 O 4), siderite (FeCO 3). In this case, pyrite grains could be well rolled, and, consequently, they were actively affected by surface waters and the atmosphere. Also indicative was the presence of graphite (non-oxidized carbon), lapis lazuli (Na 2 S - non-oxidized sulfur), as well as iron-manganese ores in the most ancient rocks. These latter are formed mainly under low oxygen conditions, since in the unoxidized state, iron and manganese migrate together, and with an increased oxygen content, iron loses its mobility, and their paths diverge. In the late 1960s, another important piece of evidence for a reducing atmosphere on ancient Earth was presented: sedimentary uraninite conglomerates. They could accumulate only in the absence of oxygen, so they are found only in the most ancient rocks. Minerals with a high degree of oxidation of elements began to predominate in the Proterozoic rocks, iron-manganese ores and uraninites disappeared. On the other hand, rare elements have appeared that are included in sedimentary minerals in the presence of oxygen.

Verification and refinement of this hypothesis took the next four decades. What caused the oxygen revolution? What are the dates for this event? Where did oxygen go before the great oxygen revolution, and did it exist at all? Why did the release of oxygen at the turn of the Archean and Proterozoic occur relatively quickly, while the accumulation of oxygen proceeded slowly? What is the role of living organisms in this process? All these questions had to be answered. On the pages Nature Timothy Lyons and colleagues from the Department of Geosciences at the University of California at Riverside have summarized what they have learned so far. The picture, as it turns out, is both more complex and more interesting than the original simple model shown schematically in Fig. 2.

In connection with discussions of this model, it is first necessary to ask question about dates oxygen event: yet when did it happen? Usually, when answering this question, reference is made to sulfur fractionation data. Due to different reactivity, sulfur isotopes accumulate in minerals in certain ratios - this is the essence of isotope fractionation. These ratios are used to judge the mechanisms of fractionation: mechanical according to the mass of isotopes (this is mass-dependent fractionation) or biological (this is mass-independent fractionation). The signal about the change from mass-independent fractionation to mass-dependent fractionation is easily read in Archean and Proterozoic rocks. It was believed that sulfate-reducing bacteria provided mass-independent fractionation: they preferred lighter isotopes for their needs. Therefore, the Archean time with a mass-independent signal was considered an anaerobic world of sulfate reducers. And when, in the ensuing oxygen abundance, their reducing world was supposed to shrink to tiny enclaves, the biological fractionation of sulfur basically stopped. And according to this signal, the onset of the Great Oxygen Revolution was dated. However, it was beautifully proved that the shift from mass-independent to mass-dependent fractionation of sulfur isotopes is not at all explained by the overthrow of sulfate reducers from their dominant positions (for this, see the news The most ancient bacteria of the Archaean were not sulfate reducers, "Elements", 09/28/2012). This transition was associated with changes in the Archean atmosphere (its transparency, density, types and volume of volcanic emissions). This does not mean that there were no sulfate reducers, it does not mean that there was no biological mass-independent sulfur fractionation. This means that the dating of sulfur fractionation events should not be associated with the oxygen revolution. Sulfate reducers - their course, and sulfur fractionation - their own, and where the oxygen supply is located is unknown. Moreover, the signal of mass-independent fractionation can be "smeared" in time due to the constant geological cycle of sulfur. Minerals that carry one or another fractionation signal could have formed in more ancient times, then become buried, then rise again to the surface. Thus, an ancient signal may also appear in younger samples. Therefore, today it is difficult, firstly, to associate the signal of mass-independent fractionation with a certain time, secondly, with a certain biological mechanism, and thirdly, with an oxygen event.

Another possible approach to dating the oxygen event is based on the search for traces of oxygen producers - cyanobacteria and other chlorophyll-containing organisms. In this way, you can kill two birds with one stone - and estimate the time of the onset of the oxygen era, and figure out who is behind it. Paleontologists find many Archean fossils that are interpreted as certain microorganisms. But their morphology is so simple that it is difficult to say with certainty that their metabolism was based on oxygen photosynthesis.

It was also believed that in reasoning about Archean life, one could rely on data on biomarkers - molecules that specifically indicate one or another type of metabolism and/or type of microorganisms. Such, for example, are steran molecules, inherent only in eukaryotes; oxygen is required for their synthesis. Steranes were found in rocks 2.7 billion years old. While scientists were discussing whether oxygen is really necessary for the synthesis of steranes, and if necessary, in what quantity, it turned out that the steranes that excited everyone are late pollution (read about this in the news The oldest traces of eukaryotes and cyanobacteria on Earth are recognized as late pollution, “Elements ", 10/29/2008). In addition, some recent work casts doubt on the reliability of biomarker data: many of them may be late contamination. But again, this does not mean that photosynthetics did not exist. They were, and even with a high probability.

To confirm their assumptions, Lyons and colleagues suggest paying attention to the distribution of organic matter in the sedimentary rocks of the Archean (Fig. 3).

Amazing! The amount of organic carbon produced in the Archean was the same as in the inhabited Neogene. Theoretically, iron bacteria, which oxidize Fe 2+ to Fe 3+, and sulfate reducers, which oxidize hydrogen sulfide, and some other exotic photo- and chemosynthetics, can also be represented as producers of this organic matter. But geochemical data do not allow us to consider these producers as a decisive force. Nevertheless, first of all, one has to turn to oxygen photosynthesis in order to explain the high production of organic matter in the Archaean. Consequently, photosynthetics were already in full swing in the Archaean. This conclusion is based more on logic than evidence. In addition, although it pushes the beginning of oxygen life deep into the Archaea, it does not help to date the events of the oxygen revolution.

Changes in the nature of organic synthesis were judged by sharp jumps in the δ 13 С isotopic curve (Fig. 4). In the Early Proterozoic, about 2.4 billion years ago, a high positive excursion appeared on the curve (that is, there was an increase in the share of buried biological carbon production), and about 2.2–2.1 - a negative excursion. As it turns out, the Early Proterozoic δ 13 C peak is asynchronous, which means that it cannot simply be interpreted as a widespread increase in organic production. Rather, it is necessary to consider the increase in buried organic matter as a result of an imbalance between the processes of accumulation (burial) and decomposition of organic matter. It is clear that if these two processes proceed at the same rate, then nothing accumulates and is not subjected to burial, which means that we probably will not receive any signal. The shift on the isotopic curve is interpreted as a violation of this balance towards accumulation.

In any case, oxygen is formed, but is quickly consumed for the oxidation of some products. In the Archaean, as the authors of the article point out, these products were probably volcanic gases - hydrogen sulfide, sulfur dioxide, methane and hydrogen. Changes in the nature of volcanism reduced the flow of these gases, oxygen eventually began to accumulate. All of this together suggests that the Great Oxygen Event should be viewed as the result of changes in volcanic processes and geochemical relationships, rather than shifts in biological activity and metabolism.

From these positions it is convenient to interpret the onset of the Huron glaciation, probably the first glaciation that turned the planet into a snowball. During changes in volcanic activity, firstly, less methane and other greenhouse gases began to enter the atmosphere, and secondly, methane was quickly oxidized by the oxygen that appeared. For the then planet with its dim sun (the luminosity of the Sun in the Archaean was 70-80% of the modern one), the decrease in the amount of greenhouse gases turned out to be critical: a long cold set in, the planet froze.

Surprising as it may seem, but after the oxygen event at the turn of the Archean and Proterozoic (it is already clear that it should not be called great, since there was no actual event), there was no gradual increase in oxygen, as one would expect with the onset of the era of photosynthetics. The amount of oxygen either decreased or increased again, planetary glaciations either came or ended... So, about 2.08–2.06 billion years ago, the amount of oxygen dropped sharply. Accordingly, the amount of buried bioorganics also fell. The reasons for these jumps are still unknown. The presence of unoxidized chromium and manganese in Proterozoic paleosols is also alarming: in the presence of oxygen, these metals should have been oxidized extremely quickly.

The hypothesis of the existence of a stratified ocean with oxygenated surface waters and deep waters saturated with hydrogen sulfide (the Black Sea model) also turned out to be untenable. Most likely, on the contrary, hydrogen sulfide layers were located in shallow waters (Fig. 5). And this was precisely the result of active life and high organic production of the shallow waters of the photic zone. Although, of course, the oxygen stratification of the ocean took place one way or another.

As a result of summing up all these data and reasoning, it turns out that the oxygen content in the atmosphere and ocean during the Proterozoic was not constant. It slightly increased compared to the Archaean, although it remained relatively low - lower than previously thought. It should be noted that no special changes in biota are associated with oxygen fluctuations.

Thus, the history of oxygen on the planet appears to be somewhat different than previously thought (Fig. 6). Oxygen photosynthesis and, accordingly, photosynthetics using it have existed since the earliest Archean times. Free oxygen, a by-product of their metabolism, could accumulate locally (blue arrows in the diagram), but the scale of early photosynthesis on the planet is still difficult to estimate. All this oxygen was spent on the oxidation of organics and other elements, in particular, volcanic gases. Changes in the nature of volcanism on the planet began in the Late Archean. They were associated with the formation and stabilization of continental plates. As a result of these geological processes, the balance of oxygen supply and removal was sharply disturbed: free oxygen began to enter the atmosphere. These interconnected processes took a significant amount of time, and did not happen at the end of the Archaean with the wave of a magic “photosynthetic” wand. During the Proterozoic, oxygen levels changed, sometimes by an order of magnitude, but remained low on average. The deep layers of the ocean remained anoxic. At the end of the Proterozoic, the ocean was saturated with oxygen to the very depths.

The second oxygen jump that occurred at the end of the Proterozoic remains a mystery. It is associated with the emergence of multicellular life. Paradoxically, with a large number of deposits of this age and, accordingly, an impressive amount of data on this critical interval, it is now difficult to formulate any complete model of this oxygen shift. It is important that not long before it a very large amount of deposits of organics enriched in light isotopes appeared, and then a great glaciation followed and the planet turned into a snowball. After the glaciation, organic matter with a low 13 C isotopic signal was buried. In other words, the sequence of global events resembles that of the Early Proterozoic sequence. It is clear that in this case, too, the balance between the production and sink of oxygen could be disturbed.

The review clearly shows that our knowledge of the most ancient times of our planet is not complete, or even terribly poor. It remains only to hope for future researchers, and that this unyielding material will nevertheless reveal its secrets to them.

Oxygen catastrophe (oxygen revolution) - a global change in composition atmosphereEarth, which occurred at the end of the Archean - the beginning Proterozoic, about 2.4 billion years ago (period siderium). The result of the oxygen catastrophe was the appearance in the composition of the atmosphere of free oxygen and a change in the general character of the atmosphere from reducing to oxidizing. The assumption of an oxygen catastrophe was made on the basis of a study of a sharp change in the nature of sedimentation.

Prior to the significant increase in atmospheric oxygen, almost all existing forms of life were anaerobes, that is, the metabolism in living forms depended on forms of cellular respiration that did not require oxygen. The access of oxygen in large quantities is detrimental to most anaerobic bacteria, so at this time most of the living organisms on Earth disappeared. The remaining life forms were either immune to oxidation and the damaging effects of oxygen, or spent their life cycle in an oxygen-deprived environment.

Accumulation of O 2 in the Earth's atmosphere:
1. (3.85-2.45 billion years ago) - O 2 was not produced
2. (2.45-1.85 billion years ago) O 2 was produced but absorbed by the ocean and seabed rocks
3. (1.85-0.85 billion years ago) O 2 leaves the ocean, but is consumed by the oxidation of rocks on land and the formation of the ozone layer
4. (0.85-0.54 billion years ago)
5. (0.54 billion years ago - present) O 2 reservoirs are filled and accumulation in the atmosphere begins

Primary composition of the Proterozoic atmosphere

The exact composition of the Earth's primary atmosphere is currently unknown, but it is generally accepted that it was formed as a result of degassing of the mantle and was of a restorative nature. Its basis was carbon dioxide, hydrogen sulfide, ammonia, methane. This is evidenced by:

• non-oxidized deposits visibly formed on the surface (for example, river pebbles from oxygen-resistant pyrite);
• no known significant sources of oxygen and other oxidizing agents;
• study of potential sources of the primary atmosphere (volcanic gases, composition of other celestial bodies).

Causes of the oxygen catastrophe

The only significant source of molecular oxygen is the biosphere, more precisely, photosynthetic organisms. Appearing at the very beginning of the existence of the biosphere, photosynthetic archaebacteria they produced oxygen, which was almost immediately spent on the oxidation of rocks, dissolved compounds and atmospheric gases. A high concentration was created only locally, within bacterial mats(the so-called "oxygen pockets"). After the surface rocks and gases of the atmosphere turned out to be oxidized, oxygen began to accumulate in the atmosphere in a free form.
In addition, one of the likely factors that influenced the change in microbial communities was a change in the chemical composition of the ocean. So, one of hypotheses, the functioning of ancient bacterial mats could be suppressed by a decrease in the concentration nickel, which plays an important role in methanogenesis. The decrease in the concentration of this and other substances could be caused by the extinction of volcanic activity.

Consequences of an oxygen catastrophe

Biosphere

Since the vast majority of organisms of that time were anaerobic, unable to exist at significant oxygen concentrations, there was a global change of communities: anaerobic communities have changed aerobic, previously limited only by "oxygen pockets"; anaerobic communities, on the contrary, were pushed aside in “ anaerobic pockets" (figuratively speaking, "the atmosphere turned inside out"). Subsequently, the presence of molecular oxygen in the atmosphere led to the formation ozone shield, which significantly expanded the boundaries of the biosphere and led to the spread of a more energetically favorable (compared to anaerobic) oxygen respiration.

Lithosphere

As a result of the oxygen catastrophe, almost all metamorphic and sedimentary rocks, which make up most of the earth's crust, are oxidized.

Siderius (from other Greekσίδηρος - iron) - geological period, part Paleoproterozoic. Covers the time period from 2.5 to 2.3 billion years ago. The dating is purely chronological, not based on stratigraphy.

At the beginning of this period, there is a peak of the appearance iron-containing x breeds. They were formed under conditions anaerobic algae produced spent oxygen, which, when mixed with iron, formed magnetite(Fe 3 O 4, iron oxide). This process cleared the iron from oceans. Eventually, when the oceans stopped absorbing oxygen, the process led to the formation of oxygenated atmosphere which we have today.

Atmosphere

As a result of changes in the chemical composition of the atmosphere after the oxygen catastrophe, its chemical activity changed, the ozone layer formed, and the the greenhouse effect. As a result, the planet entered an era Huronian glaciation.

Huron glaciation

From Wikipedia, the free encyclopedia

The Huron glaciation is the oldest and longest glaciation of the Earth. Started and ended in an era Paleoproterozoic, lasted about 300 million years.

Causes of glaciation

1. The root cause of the Huron glaciation was oxygen catastrophe, at which a large amount of oxygen generated photosynthetic organisms. Methane, which was previously present in the atmosphere in large quantities and made the main contribution to the greenhouse effect, combined with oxygen and turned into carbon dioxide and water. Changes in the composition of the atmosphere, in turn, led to a decrease in the number methanogens, which caused an additional decrease in methane levels.

2. The colossal scale and duration of the Huron glaciation can also be associated with the so-called weak young sun paradox.

3. Theory "Snowball Earth" (English Snowball Earth) - hypothesis , assuming that Earth was completely covered ice in part cryogenic and Ediacaran periods Neoproterozoic era, and possibly in other geological eras. Explains the cooling by the dissolution of carbon dioxide CO 2 in the oceans and its transformation into limestones Ca CO 3

4. Concentration of continents in the form of the Rodinia supercontinent and the emergence of an ice sheet similar to Antarctica.

And a change in the general character of the atmosphere from reducing to oxidizing. The assumption of an oxygen catastrophe was made on the basis of a study of a sharp change in the nature of sedimentation.

Primary composition of the atmosphere

The exact composition of the Earth's primary atmosphere is currently unknown, but by default, scientists believe that it was formed as a result of degassing of the mantle and was of a restorative nature. Its basis was carbon dioxide, hydrogen sulfide, ammonia, methane. This is evidenced by:

• non-oxidized sediments visibly formed on the surface (for example, river pebbles from oxygen-labile pyrite);
• no known significant sources of oxygen and other oxidizing agents;
• study of potential sources of the primary atmosphere (volcanic gases, composition of other celestial bodies).

Causes of the oxygen catastrophe

The only significant source of molecular oxygen is the biosphere, more precisely, photosynthetic organisms. Photosynthesis, apparently, appeared at the dawn of the existence of the biosphere (3.7-3.8 billion years ago), but archaebacteria and most groups of bacteria did not produce oxygen during photosynthesis. Oxygen photosynthesis originated in cyanobacteria 2.7-2.8 billion years ago. The released oxygen was almost immediately spent on the oxidation of rocks, dissolved compounds and gases of the atmosphere. A high concentration was created only locally, within the bacterial mats (the so-called "oxygen pockets"). After the surface rocks and gases of the atmosphere turned out to be oxidized, oxygen began to accumulate in the atmosphere in a free form.

One of the probable factors that influenced the change of microbial communities was the change in the chemical composition of the ocean, caused by the extinction of volcanic activity.

Consequences of an oxygen catastrophe

Biosphere

Since the vast majority of organisms of that time were anaerobic, unable to exist at significant oxygen concentrations, a global change of communities occurred: anaerobic communities were replaced by aerobic ones, previously limited only to "oxygen pockets"; anaerobic communities, on the contrary, were pushed into "anaerobic pockets" (figuratively speaking, "the biosphere turned inside out"). Subsequently, the presence of molecular oxygen in the atmosphere led to the formation of an ozone screen, which significantly expanded the boundaries of the biosphere, and led to the spread of more energetically favorable (compared to anaerobic) oxygen respiration.

Atmosphere

As a result of changes in the chemical composition of the atmosphere after the oxygen catastrophe, its chemical activity changed, the ozone layer formed, and the greenhouse effect sharply decreased. As a consequence, the planet entered the era of the Huronian glaciation.

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Notes

Links

• - Nature 458, 750-753 (04/09/2009)(English)
• - CNews, 03.08.2010
• Naimark, Elena. elementy.ru (2.03.14). .

evolutionary processes Factors of evolution Population genetics Origin of life Historical concepts Modern theories Evolution of taxa

An excerpt characterizing the Oxygen catastrophe

The battle of Borodino, followed by the occupation of Moscow and the flight of the French, without new battles, is one of the most instructive phenomena of history.
All historians agree that the external activity of states and peoples, in their clashes with each other, is expressed by wars; that directly, as a result of greater or lesser military successes, the political strength of states and peoples increases or decreases.
No matter how strange the historical descriptions of how some king or emperor, having quarreled with another emperor or king, gathered an army, fought with the army of the enemy, won a victory, killed three, five, ten thousand people and, as a result, conquered the state and the whole people in several million; no matter how incomprehensible why the defeat of one army, one hundredth of all the forces of the people, forced the people to submit, - all the facts of history (as far as we know it) confirm the validity of the fact that greater or lesser successes of the army of one people against the army of another people are causes or, according to at least essential signs of an increase or decrease in the strength of the peoples. The army won, and immediately the rights of the victorious people increased to the detriment of the defeated. The army has suffered a defeat, and immediately, according to the degree of defeat, the people are deprived of their rights, and with the complete defeat of their army, they completely submit.
So it has been (according to history) from ancient times to the present. All the wars of Napoleon serve as confirmation of this rule. According to the degree of defeat of the Austrian troops - Austria is deprived of its rights, and the rights and forces of France increase. The victory of the French at Jena and Auerstet destroys the independent existence of Prussia.
But suddenly, in 1812, the French won a victory near Moscow, Moscow was taken, and after that, without new battles, not Russia ceased to exist, but an army of 600,000 ceased to exist, then Napoleonic France. It is impossible to force facts on the rules of history, to say that the battlefield in Borodino was left to the Russians, that after Moscow there were battles that destroyed Napoleon's army - it is impossible.
After the Borodino victory of the French, there was not a single not only general, but any significant battle, and the French army ceased to exist. What does it mean? If this were an example from the history of China, we could say that this phenomenon is not historical (a loophole of historians when something does not fit their standard); if it were a case of a short-term clash in which small numbers of troops would participate, we could take this phenomenon as an exception; but this event took place before the eyes of our fathers, for whom the question of life and death of the fatherland was decided, and this war was the greatest of all known wars ...
The period of the campaign of 1812 from the Battle of Borodino to the expulsion of the French proved that a won battle is not only not the cause of conquest, but not even a permanent sign of conquest; proved that the power that decides the fate of peoples lies not in the conquerors, even in armies and battles, but in something else.
French historians, describing the situation of the French army before leaving Moscow, argue that everything in the Great Army was in order, except for cavalry, artillery and carts, but there was no fodder for horses and cattle. Nothing could help this disaster, because the surrounding peasants burned their hay and did not give it to the French.
The battle won did not bring the usual results, because the peasants Karp and Vlas, who, after the French had come to Moscow with carts to rob the city, did not personally show heroic feelings at all, and all the countless number of such peasants did not bring hay to Moscow for the good money that they offered, but burned it.

Let's imagine two people who went out to a duel with swords according to all the rules of fencing art: fencing went on for quite a long time; suddenly one of the opponents, feeling wounded - realizing that this was not a joke, but about his life, threw down his sword and, taking the first club that came across, began to toss with it. But let us imagine that the enemy, having so wisely used the best and simplest means to achieve the goal, at the same time inspired by the traditions of chivalry, would want to hide the essence of the matter and would insist that he, according to all the rules of art, won with swords. One can imagine what confusion and obscurity would result from such a description of the duel that took place.
The fencer who demanded the fight according to the rules of the art was the French; his opponent, who dropped his sword and raised his club, were Russians; people who try to explain everything according to the rules of fencing are historians who wrote about this event.
Since the fire of Smolensk, a war has begun that does not fit under any previous legends of wars. The burning of cities and villages, the retreat after battles, the blow of Borodin and the retreat again, the abandonment and fire of Moscow, the catching of marauders, the capture of transports, the guerrilla war - all these were deviations from the rules.

Accumulation of O 2 in the Earth's atmosphere:
1 . (3.85-2.45 billion years ago) - O 2 was not produced
2 . (2.45-1.85 billion years ago) O 2 was produced but absorbed by the ocean and seafloor rocks
3 . (1.85-0.85 billion years ago) O 2 leaves the ocean, but is consumed by the oxidation of rocks on land and the formation of the ozone layer
4 . (0.85-0.54 billion years ago) all rocks on land are oxidized, the accumulation of O 2 in the atmosphere begins
5 . (0.54 billion years ago - present) modern period, the content of O 2 in the atmosphere has stabilized

Oxygen catastrophe(oxygen revolution) - a global change in the composition of the Earth's atmosphere that occurred at the very beginning of the Proterozoic, about 2.4 billion years ago (siderian period). The result of the Oxygen Catastrophe was the appearance of free oxygen in the composition of the atmosphere and a change in the general character of the atmosphere from reducing to oxidizing. The assumption of an oxygen catastrophe was made on the basis of a study of a sharp change in the nature of sedimentation.

Primary composition of the atmosphere

The exact composition of the Earth's primary atmosphere is currently unknown, but it is generally accepted that it was formed as a result of degassing of the mantle and was of a restorative nature. Its basis was carbon dioxide, hydrogen sulfide, ammonia, methane. This is evidenced by:

• non-oxidized sediments visibly formed on the surface (for example, river pebbles from oxygen-labile pyrite);
• no known significant sources of oxygen and other oxidizing agents;
• study of potential sources of the primary atmosphere (volcanic gases, composition of other celestial bodies).

Causes of the oxygen catastrophe

The only significant source of molecular oxygen is the biosphere, more precisely, photosynthetic organisms. Appearing at the very beginning of the existence of the biosphere, photosynthetic archaebacteria produced oxygen, which was almost immediately spent on the oxidation of rocks, dissolved compounds and atmospheric gases. A high concentration was created only locally, within the bacterial mats (the so-called "oxygen pockets"). After the surface rocks and gases of the atmosphere turned out to be oxidized, oxygen began to accumulate in the atmosphere in a free form.

One of the probable factors that influenced the change of microbial communities was the change in the chemical composition of the ocean, caused by the extinction of volcanic activity.

Consequences of an oxygen catastrophe

Biosphere

Since the vast majority of organisms of that time were anaerobic, unable to exist at significant oxygen concentrations, a global change of communities occurred: anaerobic communities were replaced by aerobic ones, previously limited only to "oxygen pockets"; anaerobic communities, on the contrary, were pushed into "anaerobic pockets" (figuratively speaking, "the biosphere turned inside out"). Subsequently, the presence of molecular oxygen in the atmosphere led to the formation of an ozone screen, which significantly expanded the boundaries of the biosphere and led to the spread of more energetically favorable (compared to anaerobic) oxygen respiration.

Lithosphere

As a result of an oxygen catastrophe, almost all metamorphic and sedimentary rocks that make up most of the earth's crust are oxidized.

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