Organisms that need little oxygen, but are able to survive for a long time at its concentrations below the pasteur point, are called facultative anaerobes. Organisms that need little oxygen, but are capable of
However, there are organisms that do not need oxygen at all for their vital functions. They were first discovered by the French biologist Louis Pasteur in 1861. They turned out to be bacteria from the genus Clostridium of the family of bacilli, carrying out butyric acid fermentation. This caused a real sensation in science, since it was previously believed that life without oxygen was impossible. Such organisms, Pasteur called anaerobic.
Oxygen for anaerobes is a deadly poisonous gas. Pasteur found that they are able to live only in those environments where the oxygen content does not exceed 1% of its current content in the atmosphere, i.e. less than 0.21% of the air volume ( pasteur point). Such conditions take place in the deep layers of the earth's crust, bottom sediments of water bodies, in the earth's crust, in the internal cavities of organisms, etc.
Anaerobic organisms lack mitochondria, and the processes of obtaining energy occur in the cytoplasm. From a biochemical point of view, it is more correct to call these processes not anaerobic respiration, but fermentation.
One form of fermentation is alcoholic fermentation,or the breakdown of glucose to ethyl alcohol and carbon dioxide:
C 6 H 12 0 6 → 2 C 2 H 5 OH + 2 CO 2
Another type of fermentation is lactic acid fermentation, or the breakdown of a glucose molecule into two lactic acid molecules:
C 6 H 12 0 6 → 2 C 3 H 6 O 3
In both cases, when one glucose molecule is cleaved, only 2 ATP molecules are formed, instead of 38 during aerobic respiration.
Subsequently anaerobic microorganisms in relation to oxygen, anaerobes were divided into two groups. Microorganisms that can exist at concentrations only at oxygen concentrations below the Pasteur point are named obligateanaerobes.
A very important indicator characterizing the oxygen conditions of the environment is redox potential(rH 2), which is the ratio between the content of O 2 and H 2. It is determined electrometrically using a potentiometer. The unit for rH 2 is volt. The rH 2 values \u200b\u200brange from 0 to 40 volts. Pasteur's point corresponds to rH 2 \u003d 14 volts.
Aerobic microorganisms can survive at rH 2 from 14 to 40. Obligate anaerobes survive rH 2 from 0 to 14. Facultative anaerobes exist at rH 2 from 7.4 to 20.
More than 2000 species of obligate anaerobes are currently known. In the absolute majority, these are prokaryotic microorganisms from different taxonomic groups. One order of prokaryotes - chladimia - is represented exclusively by obligate anaerobes.
Most recently, anaerobic bacteria have been discovered that live in oil fields at depths of up to 700 m, in the complete absence of oxygen. They receive energy from the decomposition of organic compounds in oil. Another discovery of this kind is the discovery of bacteria living at depths of up to 3 km in the thickness of limestone rocks. They are named lithotrophic bacteria.
There are many obligate anaerobes among chemoautotrophic organisms. Among photoautotrophic prokaryotes (cyanobacteria and a number of bacteria), anaerobic forms are unknown. There are no anaerobes among nitrogen fixing bacteria.
All free-living protists are strictly aerobic forms.
All multicellular eukaryotic organisms are aerobic. However, many species of fungi and animals are, to one degree or another, capable of facultative anaerobiosis, since they are able to survive for a certain period of time in conditions of a lack of oxygen, receiving energy for their life through fermentation.
Among fungi, yeast is an example. Under anaerobic conditions, such as dough, the yeast goes over to fermentation, which is used in cooking.
Even individual tissues of multicellular animals differ in their ability to anaerobiosis. Muscle tissue is highly capable of anaerobiosis. With intensive tower work, animals often lack oxygen. Then the muscle protein glycogen is broken down to lactic acid, which has toxic properties. It builds up in muscles, causing muscle fatigue. From here, the organisms, after intense work, breathe intensively in order to more quickly oxidize lactic acid to carbon dioxide and water.
Plants in the daytime are not very sensitive to oxygen deficiency in the external environment, since they themselves produce it during photosynthesis. In the dark, on the contrary, they become very sensitive to a decrease in its concentration. Therefore, in water bodies where the oxygen concentration is very low, plants, like other autotrophic organisms, are absent or are in a depressed state.
On the other hand, the roots of terrestrial plants, in soil saturated with water, stop growing. However, it is not entirely clear whether this is caused by an oxygen deficiency or an excess of methane and hydrogen sulfide released by anaerobic bacteria.
Anaerobic organisms appeared on Earth much earlier than aerobic ones and dominated it for a long time. However, with the advent of aerobic organisms, anaerobic organisms could not compete with them due to the very low efficiency of their energy processes. Therefore, they survived only in those places where there is no oxygen, in which aerobic organisms are unable to survive. Although not all types of anaerobes have been discovered yet, their total number is undoubtedly much lower than that of aerobic organisms.
In general, terrestrial animals very rarely experience oxygen deficiency for breathing. Therefore, terrestrial organisms living at low altitudes (up to 3 km) do not have special physiological adaptations for existence with a lack of oxygen.
With altitude, the partial pressure of oxygen in the atmosphere decreases, so organisms develop " oxygen starvation". In high-mountain mammals, an increase in the content of erythrocytes in the blood was noted, and the erythrocytes themselves were larger. This increases the oxygen capacity of the blood.
The decrease in oxygen with altitude is a serious limiting factor limiting the spread of living organisms in the mountains. The highest of the major cities is the capital of Tibet, Lhasa, which is located at an altitude of about 4500 m.
A small bird, a lentil from the order of passerines, makes nests in the Himalayas at an altitude of up to 6000 m. Climbers at altitudes above 7000 m cannot stay for more than a few hours, therefore, when climbing high heights, they must use oxygen devices.
However, the influence of oxygen content on the distribution of invertebrates and plants in the mountains is often very difficult to distinguish against the background of other factors, in particular, the height of the snow cover and temperature.
Summer deaths are explained by the increased temperature and an increase in the concentration of organic substances in the water, since oxygen is consumed for their oxidation. The most severe deaths are observed in shallow standing lakes and ponds.
A decrease in the concentration of oxygen in water as a result of its consumption of magnesium for the oxidation of organic substances in the water column is called biochemical oxygen consumption (BOD).
Many species of aquatic invertebrates and fish, especially those living in the bottom layers of water and bottom sediments, where the lack of oxygen is felt most strongly, have a number of adaptive mechanisms that allow them to exist for some time in an almost complete absence of oxygen. Their organisms have acquired a higher resistance to the increased content of lactic acid. The lower the temperature, the longer these organisms can survive without oxygen.
In the waters of the seas and oceans, the oxygen content is on average lower than in continental water bodies, averaging 4 - 5 mg / l. As a rule, the oxygen concentration at depths is somewhat lower than at the surface.
In general, in sea water bodies, the water in which is constantly mixed as a result of horizontal and vertical currents, the oxygen content changes in much narrower limits than in continental ones. Therefore, in the seas and oceans, oxygen deficiency in water is very rare.
Light factor.The main source of light in the Earth's biosphere is the Sun. Visible in the sky, the Moon and a number of nearby planets of the Solar System shine with the reflected light of the Sun. Electromagnetic radiation from stars and artificial light sources in the Biosphere are not of great importance. However, it is assumed that birds during their seasonal migrations can navigate by the constellations of the starry sky.
The following quantitative characteristics of light are important for an ecologist:
– wavelength;
– luminous intensity(the amount of radiation energy supplied per unit time per unit area);
– photoperiod(the ratio between the light and dark phase of the day).
The human eye perceives electromagnetic waves (visible light) in a very narrow range - from 3900 Å (blue light) to 7600 Å (red). Radiation with a lower wavelength of UV, X-ray and gamma radiation, and a higher one - infrared radiation, radio waves, etc., the human eye does not perceive. However, some insects can see ultraviolet light, and many nocturnal animals can see infrared (thermal) radiation emitted from objects that are hotter than the ambient temperature.
Green plants for photosynthesis use waves in the range " photosynthetically active radiation"(PAR) from 3800 to 7100 Å.
Prokaryotes have photosynthetic pigments that use radiation energy outside the PAR range, namely wavelengths of 8000, 8500 and 8700 - 8900 Å. In general, the PAR accounts for about 44% of the sun's radiant energy falling on the Earth's surface.
The maximum efficiency of using PAR for photosynthesis is no more than 3 - 4.5%. It was observed in the culture of seaweed under twilight lighting. In tropical forests this value is 1 - 3%, in temperate forests - 0.6 - 1.2%, in agricultural crops - no more than 0.6%.
The intensity of light is important for the rate of photosynthesis, or the amount of organic matter produced per unit of time. Have different types of photosynthetic organisms, the maximum rate of photosynthesis is achieved at different values \u200b\u200bof the intensity of the light flux. For example, sun-loving cereals reach the maximum photosynthesis at more high level illumination intensity, those of aquatic diatoms. On this basis, plants are divided into light- and shade-loving.
However, in all species in very light, the intensity of photosynthesis decreases sharply.
The light of the full moon in a cloudless sky is quite enough for photosynthesis in higher plants... The spread of plants into the depths of the reservoir is determined by the depth of light penetration. The latter, in turn, depends on the content of dissolved and suspended substances in it. In the clear waters of the World Ocean, light penetrates to a depth of 200 m. In clean freshwater lakes, lakes, light can penetrate to a depth of 60-70 m (Baikal). In Lake Naroch, this indicator is currently 6 - 8 m. In polluted water bodies, light penetrates to a depth of several meters to several centimeters.
The depth of penetration of light into water can be determined by probably the simplest of scientific instruments - disc Secchi.It is a white metal disc that is lowered into the water by a rope. On the first ever scientific oceanographic vessel, the British ship Challenger, a white porcelain plate was used for this purpose. Now the depth of light penetration and illumination at different depths can be measured with very high accuracy using luxmeter.
A significant part (up to half) of the organic matter created by plants during photosynthesis is immediately spent on their respiration. Therefore, a plant can exist only in such light conditions under which the amount of organic matter created during photosynthesis will exceed or at least be equal to its amount used for respiration.
There are a number of autotrophic protist species from the subtype of plant flagellates capable of bioluminescence. An example of this is the usual nightlife in the Black Sea. Noctiluca mirabilis.At night, its clusters generate enough light for their photosynthesis process.
Heterotrophic organisms with visual organs use visible light for orientation in space. Certain nocturnal organisms can also perceive infrared radiation, and insects - ultraviolet radiation.
Some species, especially cave, underground and deep-sea ones, do without light at all. However, most heterotrophic organisms need a certain amount of light, for example, for the production of vitamins and other substances in the skin.
The annual nature of the change in the ratio between the light (C) and dark (T) phases of the day ( photoperiod) is subject to strict laws, which is due to the rotation of the Earth around the Sun.
At the equator, the photoperiod throughout the year is strictly constant and amounts to 12C: 12T. With the advancement to higher latitudes in the directions towards both poles, the photoperiod changes regularly.
In the Northern Hemisphere to latitude about 67 o N day length is minimal on December 22 (winter solstice), then it constantly increases. March 22 (vernal equinox) on the entire planet, day is equal to night.
The length of the day reaches its maximum on June 22 (summer solstice). The higher the latitude, the longer the daylight hours. For example, in Minsk (54 o N) on June 22, the photoperiod is approximately 17C: 7T, and in St. Petersburg (60 o N) - 22C: 2T ( white Nights).
The higher the latitude, the faster the increase in daylight hours and the decrease in nighttime.
In polar latitudes (over 67 ° N), approximately between May 22 and August 22, the Sun does not set beyond the horizon at all; comes polar day, i.e. photoperiod 24C: 0T.
After June 22, the duration of daylight hours everywhere, except for the equator, decreases and reaches a minimum on December 22. In Minsk on this day, the photoperiod is approximately 7C: 17T.
At polar latitudes, approximately between November 22 and January 22, the Sun does not rise above the horizon at all; comes polar night, i.e. photoperiod 0С: 24Т.
The duration of daylight hours per day at a certain point of the Earth is strictly constant, unlike other important environmental factors - temperature, precipitation, etc. Therefore, for many organisms, especially birds, the photoperiod is a signal factor for many of the most important stages of their life cycle, for example, the beginning of reproduction, departure of birds for wintering, etc.
The presence of water and moisture.All organisms need water as it is the main component of their cell cytoplasm. Therefore, living organisms are 60 - 99% water.
Water is used for photosynthesis.
Water is one of the main habitats. A number of types of living organisms are composed exclusively or almost exclusively of aquatic species (echinoderms, fish). Many other species are associated with water at certain stages of their life cycle (amphibians, semi-aquatic, insects).
Many organisms have adapted to the existence of water scarcity. Plants that live in arid zones store water in their tissues ( succulents). Their most famous example is cacti.
Many animals, in the event of a lack of water, use metabolic water, obtained by oxidizing the stored fat in their bodies. These include many insects that have fat reserves in fatty body,as well as some desert mammals such as camels and mouse rodents.
In this case, the amount of water received exceeds the amount of broken down fat, since almost all the oxygen in metabolic water is obtained from atmospheric oxygen.
The age of the Earth is about 4.6–4.7 billion years. The composition of the ancient atmosphere is considered to be close to the composition of gases emitted from modern volcanoes. Chemical analysis of gas bubbles in the oldest rocks of the Earth showed complete absence of free oxygen in them, about 60% CO 2 about 35% H 2 S, SO 2, NH 3, HCl and HF, some nitrogen and inert gases. At present, there is already quite a lot of indisputable evidence that the early Earth's atmosphere was anoxic. Life that arose on Earth gradually changed these conditions and transformed the chemistry of the planet's upper shells.
The history of the Earth is divided into three large segments: archaea – the first approximately two billion years of its existence, proterozoic – next 2 billion years and phanerozoic, which began about 570 million years ago. Dophanerozoic time is called cryptozoic, that is, an era of hidden life, since the ancient rocks do not contain skeletal imprints of macro-fossils.
Until recently, it was believed that the emergence of life on Earth was preceded by a very long (billions of years) chemical evolution, including the spontaneous synthesis and polymerization of organic molecules, their integration into complex systems preceding cells, the gradual formation of metabolism, etc. The possibility and ease of flow abiogenic synthesis of organic monomers under conditions simulating the atmosphere of the ancient Earth, was convincingly proven back in the 50s in many laboratories of the world, starting with the well-known experiments of S. Miller and G. Urey. However, the path from simple organic molecules to the simplest living cells with the ability to reproduce and the apparatus of heredity was considered very long. In addition, the ancient breeds seemed lifeless. With the development of sophisticated methods for studying organic molecules contained in Archean and Proterozoic rocks, as well as the remains of microscopic cellular structures, this opinion has changed. One of the most amazing paleontological discoveries of recent decades is the registration of traces of life even in the most ancient rocks of the earth's crust. Consequently, the evolution from organic compounds to living cells proceeded in a very short time, at the very beginning of the Earth's history. Photosynthetic organisms also appeared very early. Rocks with an antiquity of 3.8 billion years already indicate the presence of cyanobacteria (blue-green algae) on Earth, and, consequently, the existence of photosynthesis and biogenic release of molecular oxygen. On the border of the Archean and Proterozoic, cyanobacteria were already represented by a rich set of forms similar to modern ones. Along with the fossil remains of blue-green cells, traces of their large-scale geological activity were found in the Archean layers - rocks composed of stromatolites. These characteristic banded and columnar fossils arise from the functioning of cyanobacterial communities, where photosynthetic blue-green and a number of other types of bacteria, destructors and chemosynthetics are closely spatially united. Thus, each colony represents a separate ecosystem, in which the processes of synthesis and decay of organic matter are coupled. Modern stromatolites arise only in extremely extreme conditions - in salty or hot waters, where there is no more highly organized life.
Thus, we can assume that already in the middle archaea life on Earth was represented by various types of prokaryotes, beginning to influence its geological history. In a reducing environment, oxygen released by cyanobacteria was first consumed for the oxidation of various compounds and did not accumulate in a free form in the atmosphere. In this case, ammonia was oxidized to molecular nitrogen, methane and carbon monoxide - to CO 2, sulfur and hydrogen sulfide - to SO 2 and SO 3. The composition of the atmosphere gradually changed.
The development of life proceeded against the background of the geological development of the planet. In the Archean, due to chemical and physical weathering and erosion of the land, the formation of the first sedimentary rocks in the ocean began, their granitization took place, and the cores of future continental platforms were formed. According to some assumptions, at the beginning of the Proterozoic they formed a single continent, called Megagea, and were surrounded by a single ocean.
The tectonic activity of the Earth, as the age of igneous rocks shows, is not constant over time. Short periods of increased activity alternate with longer periods of rest. This cycle takes up to 150–500 million years. In the history of the planet, geologists count 19 tectonic-magmatic epochs, four of which are Phanerozoic and 15 - Cryptose. As a result, there was an increase in the heterogeneity of the earth's crust. Increased volcanism, mountain building processes, or, conversely, the subsidence of platforms changed the areas of shallow water and the conditions for the development of life. On Earth, the climatic zoning was either weakening or strengthening. Traces of ancient glaciers have been known since the Archean era.
It is believed that early life had a local distribution at first and could exist only at shallow depths in the ocean, from about 10 to 50 m. The upper layers, up to 10 m, were penetrated by destructive ultraviolet rays, and below 50 m there was not enough light for photosynthesis. Salts of the ancient ocean were different increased content magnesium versus calcium in accordance with the composition of the rocks of the primary earth's crust. In this regard, one of the main sedimentary rocks of the Archean is magnesium-containing dolomites. Sulfate precipitation did not occur in the ocean, since there were no oxidized sulfur anions. In ancient rocks, there are many easily oxidized, but not completely oxidized substances - graphite, lapis lazuli, pyrite. In the Archean, as a result of the activity of anaerobic iron bacteria, significant strata of magnetite and hematite were formed - ores containing undeoxidized bivalent iron. At the same time, it was found that the oxygen present in the composition of these rocks is of photosynthetic origin.
The gradually increasing scale of photosynthetic activity of cyanobacteria led to the appearance and accumulation of free oxygen in the environment. The transition of a reducing atmosphere to an oxidizing one was outlined at the beginning proterozoic, oh as evidenced by changes in the chemical composition of terrestrial rocks.
In the history of atmospheric oxygen, several of its threshold values \u200b\u200bare important. On Earth, devoid of photosynthesis, oxygen is formed in the atmosphere due to the photodissociation of water molecules. Its content, according to G. Yuri's calculations, cannot exceed 0.001 of the modern (Yuri point) and automatically keeps at this level. With this oxygen content, only anaerobic life can exist. The emergence of molecular oxygen through photosynthesis made it possible for living cells to breathe, which is a much more efficient way of releasing energy than anaerobic fermentation. From these positions, the value of 0.01 oxygen content from the current level is important - the so-called pasteur point. There are a number of microorganisms that can switch their energy metabolism from respiration to fermentation and back when oxygen fluctuates below or above the Pasteur point. At the same time, life was able to spread almost to the surface of water bodies, since ultraviolet rays, due to a weak ozone screen, could now penetrate to depths of no more than a meter.
The third threshold content of О 2 (Berkner-Marshall point) corresponds to 10% of the modern one. It determines such a formation of the ozone screen in which the streams of hard ultraviolet rays from the sun no longer reach the earth's surface and do not interfere with the development of life. By modern research, the transition of the Pasteur point could have occurred already 2.5 billion years ago, and the 10% oxygen content (Berkner – Marshall point) was reached already in the period 1.8–2.0 billion years from the present.
Thus, for more than two billion years, the biosphere was formed exclusively by the activity of prokaryotes. They completely changed the geochemical situation on Earth: they formed an oxygen atmosphere, cleared it of toxic volcanic gases, tied up and transferred huge amounts of CO 2 into carbonate rocks, changed the salt composition of the ocean and formed huge deposits of iron ores, phosphorites and other minerals.
The formation of an oxidizing atmosphere led to the rapid development of eukaryotic life, the energy of which is based on the process of respiration. It is obvious that eukaryotic life is closely related to the aerobic environment prepared for it by prokaryotes. The first aerobic organisms could have arisen quite early in the composition of cyanobacterial communities, which, according to paleontologists, were a kind of "oxygen oases" in an anaerobic environment.
In general, the oxygen released by early photosynthetic organisms was toxic and deadly for anaerobic life forms. After its accumulation in water and atmosphere, anaerobic prokaryotic communities were pushed into the depths of the ground, to the bottom of water bodies, i.e., to local habitats with a lack of O 2.
In the second half of the Proterozoic, different groups of unicellular algae and protozoa appeared in the seas. Eukaryotic phytoplankton has increased the scale of photosynthesis. In turn, cyanobacteria also left huge deposits of stromatolites at this time, which indicates their high photosynthetic activity. At the end of the Proterozoic, so many biological products were already created in the seas that ancient oil and gas deposits arose on its basis.
The last stage of the Proterozoic, which takes about 100 million years (vend), demonstrates an explosion of multicellular diversity. It is possible that multicellularity appeared earlier, since there is still no clarity about a number of controversial paleontological finds, but only in the Vendian does a huge variety of aquatic animals and plants of a sufficiently high organization appear. Large localities of the Vendian biota were found in different regions of the world: Australia, South Africa, Canada, Siberia, on the coast of the White Sea. Among the animals, coelenterates and worms predominated, there were forms resembling arthropods, but in general most of them differed in their peculiar appearance and were not found in the later layers. Among the benthic algae, there were many ribbon-like thallus forms. A distinctive feature of the entire Vendian biota is skeletalness. The animals already reached large sizes, some up to a meter, but had jelly-like gelatinous bodies that left imprints on soft grounds. Good and massive preservation of prints indirectly indicates the absence of corpse eaters and large predators in the Vendian biocenoses.
Organic matter of biogenic origin has become a permanent and indispensable component of sedimentary rocks from the second half of the Proterozoic.
A new stage in the development of the organic world is the mass appearance of various external and internal skeletons in multicellular organisms. Since that time, it dates back to the Phanerozoic - "the era of explicit life", since the preservation of skeletal remains in the earth's layers allows us to reconstruct the course of biological evolution in more detail. AT phanerozoic the impact of living organisms on the geochemistry of the ocean, atmosphere and sedimentary rocks is sharply increasing. The very possibility of the appearance of skeletons was prepared by the development of life. Due to photosynthesis, the World Ocean lost CO 2 and was enriched in O 2, which changed the mobility of a number of ions. In the bodies of organisms, mineral components began to be deposited as a skeletal basis.
By extracting a number of substances from the aquatic environment and accumulating them in their bodies, organisms are no longer indirect, but the direct creators of many sedimentary rocks, burying themselves at the bottom of water bodies. The accumulation of carbonates has become predominantly biogenic and calcareous, since CaCO 3 is more intensively used for the formation of skeletons than MgCO 3. The ability to extract calcium from water is acquired by many types. At the beginning of the Phanerozoic, large deposits of phosphorites also appeared, created by fossils with a phosphate skeleton. Chemical precipitation of SiO 2 also becomes biogenic.
Three eras are distinguished within the Phanerozoic: paleozoic ,mesozoic and cenozoic , which, in turn, are subdivided into periods. The first period of the Paleozoic - cambrian - characterized by such an explosion of biological diversity that it was called the Cambrian Revolution. The Cambrian rocks are rich in numerous organisms. During this period, almost all types of currently existing animals arose and a number of others that have not survived to our time. Archaeocyates and sponges, brachiopods, famous trilobites, various groups of molluscs, shell crustaceans, echinoderms and many others appeared. Among the protozoa, radiolarians and foraminifers arose. Plants are represented by a variety of algae. The role of cyanobacteria has diminished since stromatolites have become smaller and fewer in number.
During ordovician and silurian the diversity of organisms in the ocean grew and their geochemical functions became more and more diverse. The ancestors of vertebrates appeared. The reef-forming role has passed from stromatolites to coral polyps. The main event of the Paleozoic was the conquest of land by plants and animals.
It is possible that the surface of the continents was inhabited by prokaryotes as early as the Precambrian, if we take into account the hardiness of some forms of modern bacteria to hard radiation. However, complex life forms were able to master the land only with the formation of a full-fledged ozone screen. This process, apparently, began in the Silurian time, but the Devonian became the main period of its development. The first terrestrial plants - the composite group of psilophytes - are already characterized by a number of primitive anatomical and morphological adaptations to living in the air: conducting elements, integumentary tissues, stomata, etc. appear. In other features of their structure, psilophytes are still very similar to algae. Terrestrial vegetation evolved so rapidly that, by the end of the Devonian, forests of lyre, horsetail, and ferns arose in wet and submerged habitats. Moss appeared on land even earlier. This spore vegetation could exist only in humid semi-flooded biotopes and, buried in anaerobic conditions, left deposits of a new type of fossil - coal.
In the seas devonian, along with the jawless, different forms of fish prevailed. One of the groups - cross-finned, acquired a number of adaptations for living in small reservoirs littered with dying plants, gave rise to the first primitive amphibians. The first terrestrial arthropods are known since the Silurian. In the Devonian, small soil arthropods already existed, apparently consuming rotting organic matter. However, the destructive process on land was still insufficiently effective, and the biological cycle was not closed. The mass burial of plant organic matter and its exit from the biological circulation system resulted in the accelerated accumulation of O 2 in the air. The atmospheric oxygen content at the beginning of the Phanerozoic was about one third of the present. In the Devonian, and especially in the next period - carbonone, it has reached the modern and even surpassed it. Carboniferous forests are the pinnacle of spore vegetation development. They consisted of treelike lymphoids - lepidodendrons and sigillaria, giant horsetails - calamites, powerful and diverse ferns. High plant production was stimulated and sufficient high content CO 2 in the atmosphere, which was about 10 times higher than today. Carboniferous coal contains a large amount of carbon removed from the air CO 2 reserves during that period.
Already in the Carboniferous, plants and animals appeared capable of conquering dry land areas: the first gymnosperms - cordaites and the first reptiles. The first flying insects mastered the air. Cartilaginous and bony fishes, cephalopods, corals, ostracods and brachiopods flourished in the seas. The end of the Paleozoic, the Permian period, was characterized by a sharp change in climatic conditions. Intense volcanism and mountain building processes (the end of the Hercynian tectonic epoch) led to the regression of the sea and the high standing of the continents: the southern supercontinent Gondwana and northern Laurasia. Geographic zoning has sharply increased. In Gondwana, traces of extensive glaciation have been found. In Laurasia, in the arid climate zone, large areas of evaporation sediments occur - gypsum, rock and potassium salt (Solikamsk deposits), anhydrite, dolomite. In tropical regions, however, the accumulation of coal continues (Kuzbass, Pechora, China). Spore vegetation is in sharp decline. The mass of oxygen in the atmosphere decreases to values \u200b\u200bcharacteristic of the beginning of the Paleozoic.
On the border of the Paleozoic and Mesozoic eras, at the end perm and beginning triassic there was, against the background of a change of flora, a deep renewal of the marine and terrestrial fauna. Gymnosperms dominate among plants - cicadaceae, ginkgo and conifers. Many groups of amphibians and early reptiles die out, trilobites disappear in the seas.
In the Mesozoic, the disintegration of Gondwana into separate continents and their divergence from each other began. Mid Mesozoic (Yura) characterized again by the expansion of shallow waters, an even warm climate and a weakening of geographical zoning. Jurassic forests were much more diverse in composition than carbonaceous ones, less moisture-loving and grew not only in swamps and along the edges of water bodies, but also inside continents. They also left deposits of coal along the valleys and floodplains. Among vertebrates on land, reptiles dominate, which have also mastered the air and secondary aquatic environment. Various groups of dinosaurs, pterosaurs, ichthyosaurs and many other forms arise.
In the Mesozoic, the deposition of carbonate rocks is sharply reduced, one of the reasons for it is considered a further decrease in CO 2 in the atmosphere and ocean in connection with the consumption of photosynthesis. The very nature of carbonate deposits is also changing - they are mainly represented by biogenic chalk and marls with an increased calcium content. At the beginning of the Mesozoic, a new group of unicellular algae appears - diatoms with silicon shells, and due to them, thin silicon oozes and new rocks - diatomites - begin to form. Their strata reach 1600 m in places in the World Ocean at a rate of accumulation of 7–30 cm per 1000 years. The intensity of photosynthesis and the scale of the burial of organic matter are very high, the oxygen consumption for the oxidation of rocks in the intertectonic period is insignificant, therefore, by the middle of the Mesozoic, there is a sharp increase in the mass of oxygen in the air, which exceeds the current one.
The development of vegetation led to the emergence of a new progressive group - angiosperms. This happened in chalk period, by the end of which they, rapidly spreading across all continents, significantly pressed the flora of gymnosperms. In parallel with flowering plants, various groups of pollinating insects and consumers of angiosperm tissues are rapidly evolving. Flowering plants are distinguished by their accelerated growth and development rates, a variety of synthesized compounds. Being independent of water in fertilization processes, they are characterized, nevertheless, by a higher moisture consumption for transpiration processes, and a more intensive involvement of ash elements in the circulation, especially nitrogen. With the advent of vegetation of angiosperms, the water cycle on the planet by 80–90% was determined by their activity. Under their influence, similar to modern soils began to form with surface aerobic decomposition of plant residues. Coal accumulation processes slowed down significantly.
Throughout the Cretaceous period, reptiles dominated, many of which reached gigantic proportions. There were also toothed birds, placental mammals arose, descending from primitive Triassic ancestors. By the end of the period, birds close to modern ones spread. Bony fishes, ammonites and belemnites, foraminifera flourished in the seas.
The end of the Cretaceous period was characterized by the beginning of a new tectonic epoch and global cooling. The change of floras also led to the change of faunas, which was intensified as a result of the influence of global tectonic and climatic processes. On the border of the Mesozoic and Cenozoic eras, one of the most grandiose extinctions occurred. Dinosaurs and most other reptiles disappeared from the face of the Earth. Ammonites and belemnites, rudists, a number of planktonic unicellular organisms and many other groups became extinct in the seas. The intensive adaptive radiation of the most progressive groups of vertebrates - mammals and birds - began. Insects have begun to play an important role in terrestrial ecosystems.
The onset of the Cenozoic era was characterized by an increase in aerobic conditions in the biosphere not due to an increase in the mass of oxygen, but due to a change in soil regimes. The completeness of biological cycles has increased. Paleogene humid forests still left significant accumulations of black and brown coal. At the same time, the flourishing of active vegetation of angiosperms lowered the content of CO 2 in the atmosphere to the current level, as a result of which the overall efficiency of photosynthesis also decreased. AT neogene the growing aerobiosis of soils and water bodies stopped the formation of coal and oil. In the modern era, only peat formation occurs in marshy soils.
During the Cenozoic, abrupt climate changes took place. As a result of the evolution of angiosperms during periods of dryness in the middle of the era, herbaceous plant formations and new types of landscapes arose - open steppes and prairies. At the end, climatic zoning increased and the ice age began with the spread of ice over a significant part of the Northern and Southern Hemispheres. The last wave of glaciers retreated only about 12 thousand years ago.
Development of the organic world
Physical processes developing in the atmosphere determine the weather conditions on the earth's surface and are one of the main reasons for the planet's climate. These include the processes of changes in the composition and circulation of the atmosphere, which affect the absorption of solar radiation and the formation of long-wave radiation fluxes.
The spherical shape of the Earth promotes various absorption of solar radiation by the earth's surface. Most of it is absorbed in low latitudes, where the air temperature at the earth's surface is much higher than in middle and high latitudes. Although this temperature difference existed throughout the entire geological history of the planet, it was quantitatively expressed differently at different time intervals. The main planetary factors that affect the distribution of temperature are the variability of the composition of the atmosphere, the different areas of land and sea, and the location of the continents on the earth's sphere. The temperature difference between equatorial and polar latitudes was the main reason for the movement of air masses in the atmosphere and water masses in the seas and oceans. With a significant difference between temperatures in polar and equatorial latitudes, as, for example, in the modern era or during the largest glaciations, large horizontal and vertical movements of water and air masses occurred.
Among the components of the atmosphere, the so-called thermodynamically active impurities have the greatest influence on the temperature regime of the Earth. These are carbon dioxide, aerosol and water vapor. Although the oxygen concentration does not have an effective direct effect on the temperature regime, its influence on the development of the organic world and the creation of the ozone screen is very great.
Atmosphere composition has a direct impact on the development of biological processes. Oxygen, ozone and carbon dioxide are of the greatest importance for the development of biological processes. Oxygen and carbon dioxide are used by organisms for the effective course of metabolic processes and are spent on mineralization and oxidation of organic substances.
The development of the atmosphere is closely related to geological and geochemical processes and the activity of living organisms. At the same time, the atmosphere itself has an impact on the earth's surface, being a powerful factor of weathering and denudation.
Without touching upon the features of the origin of the atmosphere and the evolution of its composition in the Archean and Proterozoic, we will consider only the main trends in the change in the composition of the atmosphere during the Phanerozoic history of the Earth. According to L. Berkner and L. Marshall, in the Vendian era, the so-called Pasteur point was reached, when the oxygen content in the atmosphere increased to 0.01% compared to the present. This point is a very important borderline for the development of organisms. At the Pasteur point, a number of microorganisms go over to oxidative reactions during respiration, and in the process of metabolism, about 30-50 times more energy is released than during anaerobic (enzymatic) fermentation. L. Berkner and L. Marshall believe that when the Pasteur point was reached, the efficiency of the ozone screen increased. The appearance of the ozone screen limited the arrival of the shortest-wavelength part of ultraviolet radiation on the earth's surface. A small dose of ultraviolet radiation reaching the earth's surface was no longer able to penetrate, as before, through the entire water column. This opened up the opportunity for organisms to populate the vastness of the seas and oceans. Relatively quickly, already at the beginning of the Cambrian, many groups of invertebrates appear and quickly settle.
Subsequently, according to Berkner and Marshall, approximately on the border between the Silurian and Devonian, that is, about 400 million years ago, the oxygen content further enhanced the effectiveness of the ozone screen, which absorbed part of the ultraviolet solar radiation that was destructive for living organisms and the surface of the seas and sushi has become safe for organisms. It was at this time that the emergence of primitive plant forms on land, the appearance of phyto- and zooplankton, and the relatively rapid occupation of the land by vegetation and terrestrial organisms belong.
Current oxygen content in the atmosphere was reached around the middle of the Devonian, as a result of rapid photosynthesis taking place in primitive woodlands. According to a number of researchers, the current level of oxygen in the atmosphere was even surpassed at the beginning of the Carboniferous, when there was a rapid flowering of vegetation. Mass death of vegetation at the end of the Early Carboniferous caused a decrease in oxygen concentration. Not all researchers agree with the point of view of Berkner and Marshall. According to M. Rutten, the oxygen content in the atmosphere has reached pasteur points not in the Vendian, but much earlier, in the middle of the Archean, and the 10% oxygen content in the Late Riphean.
At present, the main reservoirs of oxygen are sedimentary rocks, which contain about 6-1022 g of oxygen, 1.4-1024 g of oxygen in the ocean, 1.2 1021 g in the atmosphere, and about 1019 g of oxygen in the biosphere. All of these oxygen reservoirs are interconnected through the atmosphere. Moreover, the greatest supply of oxygen to the atmosphere occurs through the biosphere. The annual production of oxygen in the biosphere is approximately (1 ... 1.5) -1017 g. Such a high productivity makes it possible to assume that all oxygen in the atmosphere could be created in several tens of millennia. In fact, during this period of time, oxygen in the atmosphere is completely renewed. The oxygen produced by the biosphere is spent on respiration, on the oxidation of organic substances and volcanic gases, and on weathering. In particular, during weathering, 3-10 g of oxygen is consumed annually. This means that in the complete absence of oxygen sources, all atmospheric oxygen would be consumed for weathering and oxidation of minerals in just 4-106 years.
Based on modern reviews, the main source of oxygen in the atmosphere was the process of photosynthesis. MI Budyko and AB Ronov calculated that 7.3-1021 g of organic carbon accumulated in the Phanerozoic deposits and 19-1021 g of oxygen were formed. These data refer only to the continents, but the actual oxygen supply should be large, since in this case photosynthesis in the oceans was not taken into account. According to the authors' calculations, it turned out that over 90% of the total oxygen input in the Phanerozoic was spent on the oxidation of mineral compounds and weathering. The rate of this flow rate during the Phanerozoic history was not the same; it fully depended on the mass of atmospheric oxygen. Knowing the total amount of organic carbon at certain intervals, oxygen consumption for oxidation, and the value of the calculated proportionality coefficient allowed Budyko and Ronov to calculate the change in the amount of oxygen in the atmosphere during the Phanerozoic (7.3).
According to this calculation, the mass of atmospheric oxygen at the beginning of the Phanerozoic was "one-third of its modern content. During the Phanerozoic, the amount of oxygen increased unevenly. The first sharp increase in the oxygen content in the atmosphere occurred in the Devonian and Carboniferous, when its total amount reached the present level. At the end of the Paleozoic, the mass of oxygen decreased, reaching at the beginning of the Triassic or at the end of the Permian the values \u200b\u200bcharacteristic of the early Paleozoic In the middle of the Mesozoic there was a new increase in the amount of oxygen, which was later replaced by a slow decrease.
The calculated oxygen content in the atmosphere is in good agreement with a number of geological facts. Thus, an increase in oxygen in the Devonian and Carboniferous coincided with the highest flowering of vegetation, its occupation of the land and a significant increase in phytoplankton productivity in the seas and oceans. The development of arid conditions in the Permian and Triassic led to a reduction in plant mass, and in connection with this, oxygen consumption for oxidation processes increased sharply. In turn, this contributed to a decrease in the total amount of atmospheric oxygen.
The most important role in the formation of the climate belongs to another thermodynamic active impurity - carbon dioxide, which creates greenhouse effect... In some periods of the geological history of the Earth, the concentration of carbon dioxide in the atmosphere significantly exceeded the modern one, but over time, there was a gradual removal of carbon dioxide from the atmosphere.
Carbon dioxide was removed from the atmosphere and hydrosphere as a result of the formation of carbonates, both chemogenic and organogenic. AB Ronov, based on the distribution and thickness of carbonate and carbonaceous rocks within the modern continents, calculated the consumption of carbon dioxide in different periods of the Phanerozoic. These data show that the volumes of carbonates do not change monotonically, but undergo significant fluctuations.
MI Budyko and AB Ronov calculated the change in the mass of carbon dioxide in the atmosphere during the Phanerozoic. Due to the fact that the initial data do not include materials on the oceans and their accuracy is naturally limited by the incompleteness of the geological record, the results obtained are not absolute, but relative and characterize the intensity of planetary processes at different intervals of geological time.
Using data on the amount of carbon dioxide concentrated in sedimentary rocks during the Cenozoic era, the authors found that 0.05-1021 g of CO2 accumulated over the last million years. Based on the modern content of carbon dioxide in the atmosphere (0.03%), the proportionality coefficient was determined, due to which the concentration of carbon dioxide in the atmosphere of the Earth was obtained in different periods of the Phanerozoic (7.4).
Carbon dioxide content in the atmosphere throughout the Phanerozoic it varied unevenly from 0.4 to 0.03%. The first significant decrease in the total amount of carbon dioxide in the atmosphere occurred at the end of the Early Paleozoic. At the end of the Mesozoic era, a gradual decrease in the concentration of carbon dioxide began. The last significant decrease in CO2 concentration in the atmosphere occurred in the Pliocene. In the modern era, the content of carbon dioxide in the atmosphere has reached its lowest value in the entire history of the Earth.
Fluctuations in volcanic activity affect the amount of carbon dioxide in the atmosphere. The concentration of carbon dioxide in the atmosphere changed consistently along with the change in the level of volcanic activity, the maximum activity was accompanied by an increase in CO2.
The content of water vapor in the modern atmosphere is 0.23% and its role in creating the greenhouse effect is very great. The saturating concentration of water vapor in the atmosphere increases with increasing temperature. The more water vapor in the atmosphere, the stronger the greenhouse effect, the higher the temperature. At the present time, there is no possibility, at least indirectly, to obtain data on the amount of water vapor in one or another epoch of the Phanerozoic. To some extent, the known periods of humidization of the climate can testify to the relatively high content of water vapor.
But back to the question of aerobic and anaerobic, oxygenated and oxygen-free lifestyle. As we already know, microorganisms use many more ways to obtain energy than higher organisms. Let's first look at photosynthesis and respiration. Both of these processes are literally aerobic: they are possible only in contact with the atmosphere. But photosynthesis can be both oxygen-free and oxygen-free, while respiration is necessarily an oxygen process.
However, microorganisms also know other methods for extracting energy from the environment. The most famous of these methods is fermentation. The variety of fermentation options is amazing, but the bottom line is always the same: an electron is gradually transferred by the body from a donor to an electron acceptor. The energy released during this transfer is used by the body in vital processes *.
* (In this case, the substance from which the electron is taken away is oxidized, and the substance that receives the electron is reduced. Therefore, these reactions are called "exergonic redox reactions". Since they do not use the radiant energy of sunlight, they are also called "dark". When breathing, the final acceptor of an electron is free oxygen, which is reduced by accepting an electron and forms water with hydrogen. But microorganisms can use many other compounds, both organic and inorganic, as the final electron acceptor.)
According to Stanje et al. , the following main types of microbial metabolism can be distinguished:
1) using radiant energy (sunlight) - organic photosynthesis;
2) using redox reactions that give energy (the so-called "dark" reactions): a) respiration (the final electron acceptor is free oxygen); b) anaerobic respiration (the final electron acceptor is another inorganic substance); c) fermentation (the final electron acceptor is organic matter).
This variety of metabolic processes characteristic of microorganisms leads to the "smearing" of the transition zone between oxygen and anoxic life. Instead of two clearly demarcated groups of organisms - those incapable of living without oxygen (the so-called obligate aerobes) and those incapable of living in the presence of oxygen (obligate anaerobes) - among microorganisms we find many forms that are capable of living in both oxygen and anoxic environment. They are called facultative anaerobes. It would be better to say of such forms of early life that they possessed the facultative breathing capacity.
Yeast and the vast majority of bacteria belong to facultative anaerobes. When the oxygen content falls below a certain level, they switch to fermentation, above this level they exist due to respiration. The transition point is called the Pasteur point.
This transition in facultative anaerobes is clearly expressed. Pasteur's point corresponds to the oxygen content of about 0.01 of its content in the modern atmosphere (according to the personal communication of L. Berkner and L. Marshall, 1966) *.
* (In the modern atmosphere, the partial pressure of free oxygen can be taken equal to 159 mm Hg. Art. Then the Pasteur point corresponds to a pressure of 1.59 mm Hg. Art.)
The Pasteur point should not be confused with the level of free oxygen below which strictly aerobic organisms cannot breathe. This level is indicated by P 50. Its values \u200b\u200bfor different vertebrates are shown in table. eleven.
At a pressure of O 2 corresponding to Pasteur's point, organisms capable of facultative respiration gain significant advantages over those that are unable to live in the presence of oxygen. After all, the breathing process gives much more energy than fermentation. This is explained by the fact that the end product of fermentation is an organic substance that has accepted an electron, while free oxygen serves as an electron acceptor during breathing. Organic matter, which is the end product of fermentation, is oxidized to the end during respiration, giving an additional large amount of energy. As can be seen from the equations presented here, the energy output of respiration is more than ten times the output of energy during fermentation.