The dangerous liaisons of oxygen and life

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oxygene - oxygene vie - nebuleuse carene oxygene espace

Oxygen is life! In any case today many biological processes could not take place without molecular oxygen, O2, and many organisms die when they are deprived of it. But other organisms do very well without oxygen, it is even a violent poison for some of them. And in the past? In fact, for nearly two billion years, life developed on Earth without oxygen, simply because there was little or no oxygen in the Earth’s atmosphere or in the water of the oceans (water that is absolutely essential for life!). In fact, the oxygen molecule is a by-product of the metabolism of certain bacteria, the cyanobacteria. Life created oxygen! Then it had to manage to continue its story in the presence of this powerful oxidant. It is this turbulent confrontation between life and oxygen that is discussed throughout this article.

The oxygen molecule, O2, also known as dioxygen, is very closely associated with our idea of life. Along with the water molecule, it is probably the most important molecule that characterizes life in the eyes of the general public. No life without oxygen! So much so that the possible decrease in its rate in the air that would be induced by the massive deforestation of the Amazon is presented as the harbinger of a planetary catastrophe [1]. However, there is nothing absolute about the life-oxygen link. There was a time when, on our planet, life developed and persisted in the absence of oxygen, and even today many forms of life still do very well without it. In fact, it is often a terrible poison for them. There is a good chance that if it had been abundant in the early history of our planet, life would never have appeared on it.

1. What’s oxygen?

oxygene - molecule oxygene
Figure 1. The oxygen molecule, O2, in its singlet (highly reactive) and triplet (much more stable) form. Ozone, O3, is the other important molecule made up exclusively of oxygen atoms. O4 and O8 are forms detected in the laboratory. [Source: © Authors’ diagram]
The element oxygen has the atomic number 8 (Figure 1) [2]: eight electrons gravitate around its nucleus. At any given time, six of these electrons are capable of participating in the formation of chemical bonds. Two bonds are commonly formed between two oxygen atoms, each bringing an extra electron. Each oxygen atom is therefore surrounded by eight electrons, thus respecting the octet rule that an atom must be surrounded by eight binding electrons. This is how, for example, the oxygen molecule, O2, is formed. The simplest way to represent it is to place a double bond between the two atoms. However, this is an inaccurate representation and in the most stable form of the molecule each oxygen atom retains one “single” electron. This is called triplet oxygen. The drawing with the double bond (Figure 1) corresponds to another form of the molecule called singlet oxygen, much less stable and more reactive than the triplet form.

Other molecules consisting only of oxygen atoms exist. The best known is ozone, O3, which is present in the Earth’s upper atmosphere, where it is produced from O2. Ozone effectively protects us from some of the solar ultraviolet radiation that it is able to absorb. Other infinitely rarer forms of oxygen can be obtained in the laboratory: O4, and even O8 in solid oxygen.

2. What’s the oxygen for?

sucr glucose schema
Figure 2. Sugars are our main source of energy. This energy comes from the oxidation of sugars by O2. At the same time O2 is reduced to water. All oxidation-reduction reactions are electron exchanges, as the two examples of iron and sodium clearly show. [Source: © Scheme of the authors]
It is almost a truism: oxygen is an oxidant. This is its role in biology. For example, it oxidizes the sugars we ingest (Figure 2). Oxidations produce energy which is then used to promote other processes, chemical, such as the synthesis of biological molecules, or physical, for example, stretching a muscle, walking, running, thinking…

The mechanisms of oxidation reactions are complex and are always associated with reduction mechanisms. We talk about redox processes. [3] In the reaction in Figure 2, glucose is oxidized (to CO2 and H2O), while oxygen is reduced (to H2O). All the reactions involved can be interpreted in terms of electron transfer. A molecule is reduced when it accepts an extra electron, it is oxidized when it loses an electron. Oxygen, O2, which is reduced to water, is therefore an electron acceptor. At the end of a chain of redox reactions, for example the respiratory chain, it is the oxygen that captures the electrons involved. It is said to be the final electron acceptor.

Humans, like all animals and fungi, use organic molecules (e.g. sugars) as electron sources and as acceptor O2: they breathe! Plants also breathe and need oxygen to live (even if they are able to produce it through photosynthesis; see Shedding light on Photosynthesis). All this makes us unconditionally associate life and oxygen. Yet other electron acceptors are possible.

3. Life without oxygen today

sulfates - sulfites - sulfites
Figure 3. The reduction of sulphates and sulphites to sulphides. Sulphides can be converted to hydrogen sulphide (H2S). [Source: © Authors’ diagram]
Oxygen is a very good electron acceptor. The organisms that we mentioned above that use oxygen are called ‘aerobic’ (“with air”). Almost all multi-celled life is aerobic as well as many single-celled life forms. But there are other types of organisms that use other terminal electron acceptors. They are said to function anaerobically (“airless”). This is the case, for example, with bacteria of the family Desulfobacteraceae. They use sulphate anions as electron acceptors, which they reduce to sulphides (Figure 3). However, they are also able to use elemental sulphur. Sulphur lies below oxygen in the Mendeleev Periodic Table. These are elements that have similar properties (two “chalcogens”). It is therefore hardly surprising that organisms use it. In a way: for lack of oxygen, why not its cousin sulphur! And the reduction of sulphur to sulphide, which can give hydrogen sulphide, H2S, is strictly analogous to the reduction of oxygen to water.

archee methanobrevibacter
Figure 4. The archaea Methanobrevibacter smithii reduces carbon dioxide to methane. The illustration shows the composition of archaea membranes, which is different from that of bacteria. [Source: Reproduced from Gootlieb et al, ref [5], License CC BY-NC 4.0]
Some single-celled organisms are capable of producing methane, CH4. They are called methanogenic archaea. [4] They do not reduce oxygen or sulfur, but for example CO2. Much of the methane present in the Earth’s atmosphere is produced by this type of archaea. They live in the wet sediments of estuaries, marshes or heaths (it is possible that the bright will-o’-the-wisps that populate heaths are in fact flames resulting from the combustion -among others- of the methane emitted by archaea) [5] as well as in extreme environments such as the boiling waters of geysers and the surroundings of “black smokers” on the seabed (See Microbes in extreme environments). Others live in the intestines of ruminants and… humans, making us methane emitters. 10% of the microorganisms present in our intestines belong to the species Methanobrevibacter smithii, a methanogenic archae (Figure 4) [6]. Although we lead an undeniably aerobic life, we also harbor a whole anaerobic life in our intestines. Taking it a step further, and considering that a human being is the sum of a multi-celled organism and a multitude of single-celled organisms, or ‘holobionte’ (see The adaptation of life to environmental constraints & Symbiosis and parasitism), one could even say that our metabolism is (very) predominantly aerobic and (very) minority (although not anecdotally) anaerobic.

For many organisms that live in anaerobic conditions, oxygen is not only useless, it is also toxic. It’s a poison that destroys them quickly. For them, oxygen is synonymous with death.

4. Life without oxygen in the beginning

Four billion years ago, when life was beginning to develop, the Earth’s atmosphere was devoid of molecular oxygen. Of course, it’s difficult to be certain about the composition of the early atmosphere. At first it was thought to be largely made up of hydrogen, methane, ammonia. Then it would have been very reductive. Especially if a few oxygen molecules had formed, they would have quickly disappeared, reacting with hydrogen to give water (O2 would have been reduced). But it now seems clear that the atmosphere was mainly composed of carbon dioxide, CO2, and water vapour, a more acceptable medium for molecular oxygen to remain in the atmosphere. But the water of the terrestrial ocean contained a lot of ferrous iron (Fe2+), which is soluble in water. Any trace of oxygen would have reacted by oxidizing the ferrous iron to ferric iron (Fe3+), its most oxidized form.

It is therefore quite certain that the first life on Earth functioned anaerobically, without oxygen (see Once upon a time when life appeared: chemistry in the earth’s ocean 4 billion years ago). If we consider that life on Earth appeared about 4 billion years ago, which seems reasonable, then for more than a third of that time, oxygen was not the final electron acceptor (the oxidant) of any living form! But then what was the oxidant? We have seen that today it could be sulfates or sulfur. Nitrogen derivatives, nitrates and nitrites, can also be suitable. They’re all potential oxidizers for the origin of life.

But an electron acceptor still effective today was then omnipresent, CO2, which is therefore a serious candidate for the role of primitive oxidant. We then think back to the methanogenic archaea, such as Methanobrevibacter smithii, which pulls in our intestines. If this was the case, then methane was formed on Earth 4 billion years ago. It is hardly possible to imagine finding any trace of this original methane today. All of the molecules formed have since been recycled to form other organic products and/or give back CO2. And even if there were very small amount, how could it be distinguished from the current biological methane? Perhaps by looking elsewhere? Methane is found on Mars. The hypothesis has been put forward that it was formed at the beginning of the Red Planet’s history, when it was partly covered by ocean and life began to develop there. Analogues of methanogenic archaea on the very young Mars? We’re a long way from little green men, but we would have more plausible Martians there! Life having then disappeared, the Martian methane could have survived until today, and would thus be a fossil of archaea.

5. The history of oxygen and its influence on living things

oxygene atmosphere
Figure 5. Evolution of oxygen partial pressure in the atmosphere as a function of time since the origin of the Earth. Logarithmic representation; BP = Before present [Source: Diagram adapted from J. Hirshfeld, Licence CC BY-NC 4.0]
During this first third of the history of life, there were only prokaryotic single-celled organisms (without a cell nucleus). As there are now two main families, bacteria and archaea, it is reasonable to think that they were already present. Among the bacteria, one group would play a dramatic role: cyanobacteria. [7] They are the main actors in the greatest upheaval our Earth has ever known: the arrival of oxygen. They appeared at least 2.7 billion years ago. Above all, they are capable of photosynthesis, i.e. of making organic molecules comprising several carbon atoms (sugars) from CO2. To do this, they use the light energy provided by the sun. O2 is a by-product of photosynthesis (see Shedding lLight on Photosynthesis). So much so that little by little this oxygen oxidized everything that could reduce it (including ferrous iron) until, 2.4 billion years ago, there was too much of it and it began to accumulate (see The Biosphere, a major geological player).

5.1. The great oxidation: a few percent of oxygen

This considerable event, which lasted from 2.4 to 2 billion years, is called the Great Oxidation Event (GOE). [8] This “oxygen crisis” was catastrophic for a considerable number of anaerobically metabolized species. Probably most of them disappeared. This is perhaps the largest mass extinction on Earth. A mass extinction of “tiny microbes”, less telegenic than the extinction of the dinosaurs, but with huge consequences. Just as some avian dinosaurs (birds) survived the Cretaceous extinction, while all the other dinosaurs disappeared, some ancient bacteria and archaea, by taking refuge in non-oxygenated biotopes, or by adapting, have successfully passed the GEO crisis (so that prokaryotes still exist).

Among the adaptive processes, one has allowed for considerable good evolution. Before the GOE, no organism had a cell nucleus, by the end of the GOE some had. Eukaryotes were born! It is thought that they were archaea that transformed. To resist oxidative stress, they would have developed internal membranes, one of which would have become the nuclear membrane. Within this extra membrane, the DNA of the new species would have been better protected from oxygen attack. Then, or at the same time, bacteria would have come to phagocytize these proto-eucaryotes, perhaps finding protection there. These ex-archae with a nucleus and bacterial hosts (such as mitochondria), are the eukaryotes (see Symbiosis and evolution: at the origin of the eukaryotic cell). Thanks to them (and prokaryotes with aerobic metabolism), it is possible to associate life and oxygen.

5.2. And even more oxygen!

Meganeura libellule
Figure 6. Replica of Meganeura, a dragonfly of size XXL. [Source : Image de Yinan Che, licence CC0]
After the GOE, there was therefore oxygen on Earth, but in much smaller quantities than today. 21% of the Earth’s atmosphere is currently made up of oxygen, which is about one billion billion tons of oxygen (see The Earth’s atmosphere and gaseous envelope). After the GOE and for a good billion years this percentage remained stable but low, at about 3-4%. However, a little less than a billion years ago, it started to increase again, perhaps reaching more than 30%. It is then that life as we perceive it today, with animals, plants, mushrooms, a multitude of eukaryotic and multicellular species, was established.

It is more than likely that the two events, the significant increase in the amount of available oxygen and the multiplication of large multicellular species, are linked. This increase is the consequence of relative variations between photosynthesis and trapping of organic matter in sediments that produce oxygen and other phenomena that consume it, such as the oxidation of iron of magmatic origin, mineral sulphur, ancient sedimentary rocks, etc.

Aerobic breathing, which consumes O2, then provided a massive supply of energy that made this explosion of life possible. Here at least, oxygen is life, even in its excessiveness. It is perhaps thanks to its abundance in the Permian (23% as opposed to 21% today), 300 million years ago, that a kind of giant dragonfly 30 centimeters long with a wingspan of 70 centimeters, Meganeura, (Figure 6) was able to develop. According to the authors who present this hypothesis, there would not be enough oxygen on earth today to allow Meganeura to live, and the largest dragonfly today has a wingspan of just 20 centimeters (which is already not bad!).

5.3. Too much oxygen?

But getting to “too much” oxygen can lead to serious problems, starting with ice ages. As long as there wasn’t too much, methane would remain in the atmosphere, and methane is a powerful greenhouse gas. It maintained a climate that could be described as “globally tropical” on our planet. But the increase in oxygen levels led to its oxidation into CO2. CO2 (we talk a lot about it, and rightly so, with the current global warming) is also a greenhouse gas, but much less than methane (more than 20 times less per unit of mass). Replacing methane with carbon dioxide therefore means cooling the Earth, so that gigantic glaciations took place at a time aptly named the Cryogenian (from the ancient Greek krùos, cold), 700 to 600 million years ago.

It is even conceivable that the entire surface of the planet has been frozen, creating a “snowball Earth“. It was after this period, during the Ediacaran era, that multicellular life really took hold (see The first complex ecosystems). The fauna of the Ediacaran was extremely rich and diverse, yet it completely disappeared 545 million years ago, replaced by a new fauna, the Cambrian fauna, which is the origin of all the animals we know today. It is likely that the increased presence of oxygen in the surface water of the oceans allowed the Ediacaran fauna to explode. Its sudden disappearance may have been caused by a destabilization of the ocean water layers, leading to the upwelling of a powerful toxic gas, hydrogen sulphide H2S.

triangle de feu
Figure 7. The triangle of fire. To grow, a fire needs oxygene from air, heat and fuel. [Source: Gustavb / CC BY-SA 3.0 License]
Oxygen has another major defect: it burns organic matter, and fire every year destroys considerable areas of forest and heathland, killing almost everything, plants and animals. The 21% oxygen in our atmosphere is sufficient to ignite wood, especially if it is dry or not too wet. But with even more oxygen, fires would be much more catastrophic, and much more dangerous for life as a whole (Figure 7).

Sixty-five million years ago, when a gigantic meteorite hit the Yucatan Peninsula in Mexico, causing the disappearance of all non-avian dinosaurs (see Massive extinctions in geological time), oxygen was a few percent more abundant than it is today. These few percent must have had significant consequences. The first dinosaurs (and other animals) to disappear were those of the Yucatan. Then the shock wave reached North and South America, the front of the wave spreading in a huge circle, and crushing everything in its path. A huge Tsunami completed the picture and drowned everything that was not high enough above sea level as far as Europe and Africa.

So far, the excess oxygen had nothing to do with it. But then… glowing ash rained down on the entire planet. They started millions of fires that soon joined together to form one on the scale of the entire Earth. And that’s when the few percent extra oxygen played a nasty trick on the dinosaurs. Many of them lived in wetlands. With 21% oxygen as it is today, some forests might not have burned. In any case, there would have been some mangroves, some swampy heaths that wouldn’t have burned. And a number, perhaps small compared to their initial population, but not negligible, of dinosaurs would have passed this stage. Perhaps enough for some to successfully face the climatic cooling that followed, caused by the impenetrable layer of smoke that prevented the sun’s rays from reaching the ground. Dinosaurs have been known to live in areas with harsh winters. So why not? But, definitely, too much oxygen is dangerous.

6. A precarious balance

As we’ve seen, indiscriminately associating life and oxygen is a mistake. There has been and still is (even inside human beings) life without oxygen. For a third of its history, life on Earth has fared very well without it. It even feared it, to the point of a serious crisis when cyanobacteria produced it en masse in the oceans and in the atmosphere. Fortunately, the great oxidation was a slow enough event (several hundred million years) for bacteria and archaea to adapt and survive this kind of first world chemical warfare. Evolution allowed the selection of archaea that had a nucleus and associated with bacteria (strength in numbers!). Eukaryotes were born. They will take a long time to become animals and plants. This time oxygen will help them do so. But it’s never far away from the point where that same oxygen could burn all the organisms on Earth.

Terrestrial oxygen is taken up in a vast geochemical cycle, linked to the cycle of other elements, mainly carbon, via the respiration of living beings, their decomposition, their burial, the formation of carbonates (layers of limestone) or that of hydrocarbon and coal deposits. It is also linked to the sulphur cycle, by forming sulphates. The stability of the oxygen level is therefore the result of numerous chemical equilibriums, but also of physical processes. If all the organic matter produced in the oceans was oxidized, they would not release any. In fact, it is because a very small part of this organic mass is buried before being oxidized that the balance remains positive. The equilibrium is very precarious and is played out to almost nothing: to within 0.0001%! [9]

cyanobactérie Prochlorococcus sp
Figure 8. The cyanobacterium Prochlorococcus sp. is the smallest and most abundant component of phytoplankton and is responsible for most of the oxygen production in the oceans. [Source: Luke Thompson from Chisholm Lab & Nikki Watson from Whitehead, MIT / CC0]
Coming back to deforestation, it is a bad idea for the biodiversity it contains and for its very specific human populations to raze the Amazon rainforest a little more each day, but no, neither the Amazon nor all the world’s forests are the ‘lungs’ of the Earth [1]. Trees produce oxygen, it is true, but they also consume it, so the whole is balanced (see The biosphere, a major geological player). Only a young tree really absorbs more CO2 (to grow) than it releases. Most of the oxygen in our atmosphere comes from the sea, precisely from the micro-organisms that make up phytoplankton. And what do we find en masse in phytoplankton (next to eukaryotic algae): cyanobacteria! All the prokaryotes of the phytoplankton are cyanobacteria, the descendants of those responsible for the great oxidation. Today they are in danger because of the warming of the oceans. Of course, if the oxygen balance of the oceans is positive, it is only because a small part of their organic matter settles to the bottom of the oceans and is therefore not oxidized. It is therefore not so much a matter of O2 production, but a consequence of its very partial “non-consumption”. Nevertheless, if we want to save our oxygen, it is the cyanobacteria that we must first save (Figure 8).

7. Messages to remember

  • It is essentially geochemical and geological phenomena that determine the amount of oxygen, O2, present in the atmosphere.
  • For more than a billion years, life developed on Earth in the absence of oxygen.
  • A little over two billion years ago, the appearance of oxygen, the great oxidation, was a considerable event, which turned life on Earth upside down. It was then that eukaryotic cells (with a nucleus) appeared.
  • It was when the oxygen level increased again to reach 20 to 30% that pluricellular life, as we know it today, appeared and diversified (animals, fungi, plants).
  • Oxygen is essential to life today, but it is a powerful oxidant. Too much oxygen could be detrimental to its maintenance, or in any case, modify it very seriously.

Notes and References

Cover image. Image of oxygen in the Great Carina Nebula. [Source: Original photo by Dylan O’Donnell, deography.com; derivative work by Tobias Frei / CC0]

[1] Zimmer K., Why the Amazon doesn’t really produce 20% of the world’s oxygen ? National Geographic, 28 Août 2019

[2] Oxygène (Futura Sciences)

[3] Réaction d’oxydo-réduction : un transfert d’électron (La chimie.net)

[4] Les archées : rencontre du troisième type (MNHN).

[5] Boisson T., Feux follets : que sont en réalité ces étranges lumières ? Trust my Science, 2018,

[6] Gootlieb K., Wacher V., Sliman J. & Pimentel M., AP&T, 2016, 43, 197, DOI : 10.1111/apt.13469

[7] Claire König. Les cyanobactéries : apparition, adaptation et reproduction. (Futura Sciences)

[8] The event that transformed Earth (BBC).

[9] Field C.B., Behrenfeld M.J., Randerson J.T. & Falkowski P., Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components. Science, 1998, 281:237-240. DOI: 10.1126/science.281.5374.237

 


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: LEQRAA Naoual, VALLÉE Yannick (July 22, 2020), The dangerous liaisons of oxygen and life, Encyclopedia of the Environment, Accessed December 9, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/the-dangerous-liaisons-of-oxygen-and-life/.

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氧气与生命之间的危险联系

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oxygene - oxygene vie - nebuleuse carene oxygene espace

  氧气即生命!在如今任何环境下,许多生物过程如果没有氧气(O2)的参与就不可能发生,绝大数生物一旦离开了氧气就会死亡。但有些生物在没有氧气的情况下依然能够生存,甚至对某些生物来说,氧气是一种剧毒物质。那在过去呢? 实际上在过去近20亿年的时间里,地球上的生命是在没有氧气的环境下发展起来的,因为在地球的大气中或海洋中的水中很少或没有氧气(水是生命的必需品!),而氧分子是某些细菌如蓝藻细菌代谢的副产物。是生命创造了氧气!然后它必须设法在这种强烈的氧气环境中继续生存。本文讨论的正是生命与氧气之间的这种激烈的对抗关系。

  在我们的认知里,氧气(氧分子)与生命密切相关。与水分子一样,在公众的眼中,氧气可能是表征生命的最重要的分子。没有氧气就没有生命。二者关系非常密切,亚马逊雨林遭到大面积砍伐而导致空气中氧气含量下降,甚至有可能演变成地球灾难。然而,生命与氧气之间并没有绝对的联系。曾经,地球上生命是在没有氧气的情况下生存和发展的,即使在今天,许多生物仍然在没有氧气的环境中生存,氧气对它们来说经常是一种致命的毒药。如果氧气在早期充斥着地球,那么地球很有可能永远不会出现生命。

1. 氧气是什么?

环境百科全书-氧气与生命之间的危险联系-氧分子以单线态和三线态形式存在
图1 氧分子以单线态(高活性)和三线态(更稳定)形式存在。臭氧(O3)是另一种完全由氧原子组成的重要分子,O4 和 O8 可在实验室中检测到的。【来源:作者的简图】Oxygene singulet 单线态氧,oxygene triplet 三线态氧,Ozone 臭氧,Complexe entre deux molecules d’O2 两个氧气分子之间的络合物,Complexex entre quater molecules d’O2 四个氧气分子的络合物。

  氧元素的原子序数为8(图1)[2]8个电子围绕其原子核旋转,其中的6个电子在任何给定的时间都能参与化学键的形成。两个氧原子之间通常形成两个键,每个键都带一个额外的电子。因此每个氧原子被8个电子包围着,因此根据八隅规则,一个原子必须被8个结合电子包围着。氧气分子 O2 的形成过程就是这样。最简单的表示方法是在两个原子之间放置一个双键。但这是一种不准确的表述,在分子最稳定的形式中,每个氧原子保留一个“单”电子,这叫做三线态氧。带有双键的图(图 1)对应的是分子的另一种形式,称之为单线态氧,比三线态氧相比,其稳定性低得多并且更具反应性。

  还有一些分子组成只有氧原子参与,例如臭氧(O3),它存在于地球上层大气中,由氧分子结合形成,它能有效吸收的一些太阳紫外线辐射保护人类免受其伤害。在实验室中还可以制备其他极其稀有的氧:O4 及固体氧中的O8

2. 氧气的作用

环境百科全书-氧气与生命之间的危险联系-糖是主要的能量来源,被氧化产生能量
图2 糖是主要的能量来源,被氧化产生能量。同时氧气被还原为水。所有的氧化还原反应都是电子交换,铁和钠的两组反应可以论证。【来源:作者的方案】 Glucose(sucre)葡萄糖(糖),Reaction globale daxydation du glucose  葡萄糖全氧化反应, Oxydation 氧化,Reduction 还原,Energie 能量,Deux exemples simples de reactions redox 氧化还原反应的两个简单例子。

  这几乎是不言而喻的,氧气是一种氧化剂。这就是它在生物学中的作用。例如它可以氧化我们摄入的糖(图 2),氧化产生能量随后为其他生命过程供能,如生物分子的合成等化学过程以及拉伸肌肉、行走、跑步、思考等物理过程。

  氧化反应的机理是复杂的,往往与还原反应有关,我们说的是氧化还原过程[3]。在图2所表示的反应中,葡萄糖被氧化生成CO2和H2O,氧气被还原生成H2O。我们可以用电子传递来解释所有的反应。当分子接受额外的电子时将被还失去电子时被氧化。氧是电子受体,被还原生成水。在氧化还原反应链的末端(如呼吸链),氧捕获了所涉及的电子,成为最后的电子受体。

  和所有动物和真菌一样,人类使用有机分子(例如糖)作为电子源和氧气受体:它们呼吸!植物也需要氧气和呼吸才能生存(即使它们能够通过光合作用产生氧气;请参阅:阐明光合作用)。这一切使我们下意识地将生命和氧气联系在一起,当然其他的电子受体存在也是可能的

 3. 如今不需氧的生命

环境百科全书-氧气与生命之间的危险联系-硫酸盐和亚硫酸盐被还原成硫化物
图3 硫酸盐和亚硫酸盐被还原成硫化物。硫化物可转化为硫化氢(H2S) 【来源:作者的简图】
Desulfobacteraceae 脱硫杆菌科 Reduction des sulfates 硫酸盐还原,Sulfate 硫酸盐,matiere organique 有机物,Anaerobie 厌氧,Bacteries 细菌,Sulfure d’hydrogene 硫化氢, Oxygene 氧,
Soufre 硫,Reduction des sulfites  亚硫酸盐还原,Sulfite 亚硫酸盐,Bacteries 细菌,Anion sulfure 阴离子硫化物,Soufre elementaire 硫单质,Sulfite 亚硫酸盐,Sulfate 硫酸盐。

  氧是一种很好的电子受体。上文提到的使用氧气的生物被归为“需氧型”(“需要空气”)。几乎所有多细胞生命以及单细胞生物都是需要氧气的。但也有其他类型的有机体是厌氧的(“无需空气”),它们使用其他最终电子受体。例如脱硫杆菌科的细菌使用硫酸根阴离子作为电子受体,将其还原为硫化物(图 3)。然而,它们也可以利用硫单质,在门捷列夫元素周期表中,硫元素位于氧元素的下面,它们是具有相似属性的元素(两个“硫族元素”),因此有机体使用硫元素也就不足为奇了。从某种意义上讲,既然缺氧,为何不使用氧的“表亲”硫元素呢?将硫还原成硫化物,生成硫化氢,这与氧气被还原为水的过程是十分相似的。

环境百科全书-氧气与生命之间的危险联系-古细菌史密斯甲烷短杆菌将二氧化碳还原为甲烷
图4 古细菌史密斯甲烷短杆菌将二氧化碳还原为甲烷。图中显示的是古菌细胞膜的组成,这与细菌细胞膜是不同的。【来源:转载自 Gootlieb 等人,引用[5],许可 CC BY-NC 4.0】Methanobrevibacter smithii 史密斯甲烷短杆菌,Archaeologist(bilayer)二醚脂质(双层膜), Caldarchaeol(monolayer)跨膜脂质(单层),Cell membtabe 细胞膜,Archaea lipid 古菌脂类,Branched isoprene chains 支链的异戊二烯链,Ether links 醚键,Bacteria lipid 细菌脂类,Unbranched fatty acids 无支链的脂肪酸。

  一些单细胞生物能够产生甲烷(CH4),它们被称之为产甲烷古菌,这些细菌不会还原氧或硫,但会还原二氧化碳[4]。地球大气中的大部分甲烷都是由这类古菌产生的。产甲烷古菌生活在河口、沼泽或荒地湿沉积物中(荒野上明亮的光可能是古细菌排放的甲烷燃烧产生的火焰)以及在极端环境中[5],如沸腾的温泉和黑色海底烟柱附近(请参阅:极端环境中的微生物)。另一些则生活在反刍动物和人类的肠道里,如史密斯甲烷短杆菌——一种产甲烷古菌——在人类肠道微生物中占比达10%[6](图4),这让人类也成为了甲烷的排放者。尽管人类一直在有氧环境下生活,但肠道内也存在无氧的生命过程。

  更进一步,考虑到人类是多细胞有机体和多个单细胞有机体的共同组成或“共生体生物”(请参阅:生命对环境约束的适应&共生和寄生),新陈代谢主要是有氧过程,(很)少数是厌氧过程。

  对于许多生活在无氧环境中的生物来说,氧气不仅没有用甚至是有害的。对于它们来说,氧气是一种能迅速摧毁它们的毒药,是死亡的代名词。

4. 最初无氧的生命

  40 亿年前生命开始出现时,地球大气中并没有氧分子。早期大气的组成很难确定,人们最初认为它主要由氢气、甲烷和氨气组成,但这种组成的大气很容易被还原,特别是当氧气出现,与氢反应生成水(氧气被还原),大气会迅速消失。现在可以确定大气主要由二氧化碳和水蒸气组成,水蒸气是更容易将氧分子保留在大气中的一种介质。但陆地海洋的水中含有大量的亚铁离子(Fe2+),能够溶于水,任何痕量的氧气都会在将二价铁氧化成三氧化二铁(Fe3+),这是其最易被氧化的形式。

  可以肯定的是,地球上的第一个生命是在无氧的环境中进行厌氧运作的(请参阅:生命出现的时代:40亿年前地球海洋化学)。如果地球上的生命出现在大约40亿年前(这种认知是合理的),在地球超过三分之一的历程中,氧不是任何生命的最终电子受体(氧化剂),那么什么是氧化剂呢?今天我们已经知道的硫酸盐、硫磺、氮衍生物、硝酸盐和亚硝酸盐,它们都可能是生命起源的潜在氧化剂。

  但有一种至今仍然活跃的电子受体当时也是无处不在的,那就是二氧化碳。对于最初的氧化剂的角色而言,二氧化碳是一个重要的候选者。然后我们再回想下产甲烷古菌,如在我们肠道中运作的史密斯甲烷短杆菌,如果真是这样,那么40 亿年前地球上就已经产生了甲烷,但今天几乎不可能找到这种原始甲烷的痕迹,因为形成的分子都被回收利用,形成其他有机物和/或生成二氧化碳。即使存在少量的原始甲烷,它又怎么能和现在的生物甲烷区别开呢?也许是找别的地方?目前人们已经在火星上发现了甲烷。有人认为甲烷是在火星形成之初出现的,当时火星部分地表被海洋覆盖,生命开始在那里发展。在非常年轻的火星上,是否存在产甲烷古菌的类似物呢?我们离发现小绿人(小说里的一种外星人)还有很长的路要走,但是很有可能存在过火星人!生命消失后,火星上的甲烷可能一直保留到今天,将成为古菌的化石。

 5. 氧气的历史及其对生物的影响

环境百科全书-氧气与生命之间的危险联系-自地球起源以来大气中氧分压随时间变化的演变
图5 自地球起源以来大气中氧分压随时间变化的演变,对数表示;BP=目前之前【来源:图表改编自 J. Hirshfeld,许可 CC BY-NC 4.0】
Hadeen 冥古代,Arcneen 太古代,Proterozoique 元古代,Phanerozoique 超生代,Photosynthese 光合作用,Grande oxydation 大氧化时代,Oxygene 氧气,Niveau doxygene incertaion 氧水平不确定,Bacteries Cyanobacterie 蓝藻细菌, Archae 古细菌,Algues 海藻类,Animaux 动物,
Plantes 植物,Temps(milliards dannees BP)年代表(十亿年)。

  在生命出现的前三分之一时间中,仅存在原核单细胞生物(无细胞核)。由于现在存在细菌和古菌两个主要类别,我们有理由认为当时它们已经存在。在这些细菌中,蓝藻发挥着重要作用[7]。蓝藻在氧气产生这一地球史上最大的巨变中发挥了重要的作用。蓝藻至少在27亿年前已经存在了,最重要的是它们能进行光合作用,将二氧化碳作为原料制造出由碳原子组成的有机分子(糖),为了实现这一过程,它们利用太阳提供的光能。氧气是光合作用的副产物(请参阅:阐明光合作用),大量氧气的出现几乎逐渐氧化了所有可以还原它的物质(包括亚铁)。直到24亿年前,氧气含量升高并开始积累(请参阅:生物圈——主要的地质参与者)。

5.1. 大氧化事件:氧气增多

  在24亿到20亿年前期间,地球出现了大氧化事件(GOE)[8]。这种“氧气危机”对大部分厌氧代谢生物来说是毁灭性的,这也许是地球历史上最大规模的物种灭绝,绝大数物种都消失了。微生物的大规模灭绝,虽然没有恐龙灭绝被人熟知,但其影响巨大。正如一些鸟类恐龙在白垩纪大灭绝中幸存下来,而其他恐龙都消失了一样,一些古老的细菌通过在非氧化生物群落中避难或通过适应,成功地度过了这些大氧化危机(因此原核生物仍然存在)。

  在适应性过程中,生物进化得相当完善。在 GEO 之前,任何生物都不具有细胞核,到 GEO 结束时,一些生物进化出细胞核,真核生物就此诞生。有人认为真核生物是古菌是为了抵抗氧化应激而演化出的,它们会形成内膜,其中一层会变成核膜。在这层膜中,新物种的 DNA 会得到更好的保护从而免受氧气的攻击。然后,或者与此同时,细菌会来吞噬这些原始真核生物,也许在那里找到保护。这些具有细胞核和细菌宿主(如线粒体)的古生物是真核生物(请参阅:共生与进化:真核细胞的起源)。正是因为它们(以及有氧代谢的原核生物)的出现,才有可能将生命和氧气联系起来。

5.2. 富氧时代到来

环境百科全书-氧气与生命之间的危险联系-巨脉蜻蜓
图6 巨脉蜻蜓(XXL 号蜻蜓)的复制品【来源:Yinan Che 图片,CC0 许可证】

  在GOE之后,地球有了氧气,但含量比今天少得多。目前地球大气氧的含量约为21%,大概有10亿吨氧(请参阅:地球大气层和气体包裹层)。在GOE之后的10亿年里,氧含量占比一直较低且保持稳定,大约为3-4%。但在不到10亿年前,氧气含量又开始增长,可能达到30%以上。当时地球上的生命,包括动物、植物、蘑菇,以及大量的真核生物和多细胞物种开始涌现。

  氧气有效含量的显著增加和大型多细胞物种的繁殖,这两个事件很可能是存在联系的。这种增加是光合作用以及沉积物中产生氧气的有机物的捕获和其他耗氧现象之间的相对变化的结果,例如岩浆中铁离子的氧化、矿物硫、古老的沉积岩等的氧化。

  消耗氧气的有氧呼吸提供了大量的能量,使生命爆发成为了可能。至少在地球,氧气就是生命,即使氧气过量。也许是由于3亿年前二叠纪氧气含量较高(现在空气中氧气含量只有21%,曾经有23%),出现了一种30厘米长,翼展70厘米的巨型蜻蜓(图 6))。提出该假设的作者认为今天的地球上没有足够的氧气来支撑巨型蜻蜓生存。现在最大的蜻蜓的翼展只有20厘米(这已经很不错了)。

5.3. 氧气含量过多?

  从冰河时期开始,过多的氧气会导致严重的问题。甲烷是一种强大的温室气体,如果含量不是很高的情况下就会留在大气中。在地球上甲烷可以维持一种叫做“全球热带”的气候,氧气含量的增加将导致它被氧化成二氧化碳。二氧化碳(现在人们总在谈论二氧化碳,这是对的,因为现在人们面临着全球变暖的形势)也是一种温室气体,但温室效应强度比甲烷小得多(每单位质量比甲烷低20多倍)。因此,如果甲烷被大量氧化成二氧化碳意味着要地球气候将变冷,因此7亿到6亿年前的大型冰川开始出,这个时期被称为成冰纪(来自于古希腊语:寒冷)

  可以想像,在这个期间整个地球表面都被冻结了,形成了一个“雪球”。这个时期之后即埃迪卡拉纪,多细胞生命才真正占据了主导地位(请参阅:第一个复杂的生态系统)。埃迪卡拉纪的动物区系极其丰富多样,但在5.45亿年前完全消失了,取而代之的是一种新的动物群——寒武纪生物群,是我们今天已知生物的来源。海洋表层海水中氧气含量增加,可能是导致埃迪卡拉纪动物群的爆发的主要原因,由于海水层的不稳定,促使有毒气体硫化氢上涌,最终导致了当时生物群的突然消失。

环境百科全书-氧气与生命之间的危险联系-火焰三角
图7.火焰三角:火焰燃烧需要空气中的氧气以及热量和燃料【来源:Gustavb / CC BY-SA 3.0许可】
Oxygen氧气,Heat加热,Fuel燃料。

  氧气还有另一个主要缺陷:它能燃烧有机物,每年的火灾都会摧毁相当大面积的森林和荒地,几乎会摧毁所有的植物和动物。空气中21%的含量的氧气足以点燃木材,尤其是在干燥或不太潮湿的情况下。如果氧气含量更高,那么火灾破坏性更强,对整个生态系统来说是十分危险的(图7)。

  六千五百万年前,当一颗巨大的陨石撞击墨西哥尤卡坦半岛,导致所有非禽类恐龙的消失(请参阅:地史时期的大规模物种灭绝事件),那时空气中氧气的含量比今天高出了几个百分点。但百分之几的含量差能产生了十分重要的影响。尤卡坦半岛的恐龙(和其他动物)最早消失,撞击冲击波传到了北美和南美,波峰的前部呈圆型散开并压碎了沿途的所有物体。引发的巨大的海啸淹没了所有海拔低的地方,甚至波及到了非洲和欧洲。

  到目前为止,空气中高浓度的氧气与海啸等并没有关系,但后来,发光的火焰灰烬如雨点般撒落在星球上。地球上发生数百万次的大火,这些火很快汇聚在了一起,然后遍布了整个地球。当时比现在高出的百分之几的氧气给恐龙带来了巨大打击。许多恐龙都生活在湿地中,如果按照如今的氧气含量,其中的森林有可能不会燃烧,至少一些红树林和沼泽地不会被烧毁。与恐龙最初的数量相比,能存活下的恐龙的数量可能很小但也不容忽视,它们在这一阶段存活了下来。太阳光无法穿越火灾引起的烟雾,无法到达地面,导致随后气温降低,也许存在一些恐龙能够适应随后的寒冷气候。可以肯定的是,过多的氧气是危险的。

6. 不稳定的平衡

  正如我们所看到的,不加区别地将生命和氧气联系起来是错误的。不需要氧气的生物过去一直存在,且现在仍然存在(甚至在人类体内)。在过去地球三分之一的历史中,生命是不需要氧气的。当蓝细菌(一种制氧细菌)大量繁殖时,海洋和大气层中氧气增加,当时这可能会成为一场严重的地球危机。幸运的是大氧化事件持续时间很长(几亿年),细菌和古菌有充足的时间适应这种环境,进化使具有细胞核并与细菌相关的古菌得以选择(数量上的优势!)。真核生物就此诞生,但它们要花很长时间才能演化成动植物,同时氧气会促进这一过程,但氧气能够燃烧生物的麻烦依然存在。

  陆地上的氧在一个巨大的地球化学循环中被吸收,通过生物的呼吸、分解、埋藏、碳酸盐(石灰岩层)或碳氢化合物和煤藏的形成,与其他元素(主要是碳)的循环相联系。它还通过形成硫酸盐与硫循环有关。因此氧水平的稳定既是许多化学平衡的结果,也是物理过程的结果。如果海洋中产生的所有有机物质都被氧化,海洋就不会释放出任何物质。实际上,正是因为有机质的一小部分在被氧化之前就被埋藏了,所以平衡才保持为正。这种平衡是非常不稳定的,保持平衡的大概在0.0001%以内[9]

环境百科全书-氧气与生命之间的危险联系-蓝藻原绿球藻是浮游植物中最小和最丰富的组成部分
图8 蓝藻原绿球藻是浮游植物中最小和最丰富的组成部分,负责海洋中大部分氧气的生成【来源:Chisholm实验室的Luke Thompson和MIT / CC0 Whitehead的Nikki Watson】

  回到砍伐森林的问题上来,对于亚马逊雨林的生物多样性保护,尤其是对于人类的生存来说,砍伐面积与日俱增不是一个好主意。但亚马逊和世界上其他所有的森林并不是地球的“肺”[1]。树木可以产生氧气同时也在消耗氧气,因此整个过程是平衡的(请参阅:生物圈——主要的地质参与者),只有幼树吸收二氧化碳比释放的多。地球大气中的大部分氧气来自海洋,确切的说来自构成浮游植物的微生物。我们在浮游植物中(紧邻真核藻类)发现了蓝藻,浮游植物中的所有原核生物都是蓝藻,而蓝藻是造成大氧化事件细菌的后代。如今由于海洋变暖,蓝藻生存环境正处于危险之中。海洋的氧平衡之所以是正的,是因为海洋中一小部分有机物沉淀到海底,因而没有被氧化。因此与其说是海洋产生了氧气,不如说是氧气部分“非消耗”的结果。但如果想保留下更多氧气,首先必须保护蓝藻(图8)。

7. 需记住的信息

  • 从本质上说,是地球化学和地质现象决定了大气中氧的含量。
  • 10多亿年来,地球上的生命是在缺氧的环境下发展起来的。
  • 大约20多亿年前,氧气的出现,也就是大氧化,是一个重大事件彻底颠覆了地球上的生命。这时,真核细胞(有细胞核)出现了。
  • 当氧气含量再次增加到20%到30%时,我们今天所知道的多细胞生命出现并多样化(动物、真菌、植物)。
  • 氧是当今生命所必需的,但它是一种强大的氧化剂。过多的氧气可能不利于生命的维持,或许亦会对生命带来巨大变化。

 


参考资料及说明

封面照片:大船底座星云中的氧气图像[图片来源:迪伦·奥唐纳的原始照片;托比亚斯·弗雷/CC0的衍生作]

[1] Zimmer, Why the Amazon doesn’t really produce 20% of the world’s oxygen ? National Geographic, 28 Août 2019

Zimmer K.,为什么亚马逊河流域不能生真正产生世界上20%的氧气?国家地理,2019年8月28日

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To cite this article: LEQRAA Naoual, VALLÉE Yannick (March 13, 2024), 氧气与生命之间的危险联系, Encyclopedia of the Environment, Accessed December 9, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/vivant-zh/the-dangerous-liaisons-of-oxygen-and-life/.

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