**9. Conclusions**

The Early Archean oxygen concentration was up to 0.02−0.08% (Urey's level) (Rutten, 1971) [2]. From the Late Archean (3.0–2.7 Ga) to Middle Proterozoic (2.2–2.0 Ga) oxygen concentration reached 0.21% (Pasteur's level) [2, 32, 33]. In the Neoproterozoic (Tonian– Cryogenian– Ediacaran, 1000−550 Ma), oxygen concentration was estimated to be up to 5 %, reaching 8% in the end of Ediacaran [34]. In the Early Paleozoic (Cambrian, 541–485 Ma), according to different estimates, oxygen concentration was 12–13% [29, 37, 38]. In the Middle Paleozoic (Ordovician– Silurian, 485–420 Ma), oxygen concentration has been 13–15% [39]. In the Late Paleozoic (Carboniferous–Early Permian, 350–280 Ma), oxygen concentration reached about 30−35% [37, 38]. In the Early Mesozoic (Triassic) (250–200 Ma), oxygen concentration has reduced down to 15–17% [39, 40]. In the Miocene epoch of Neogene (23–5 Ma), oxygen content again increased

The oxygen growth is explained by photosynthesis expansion. In parallel, another product of global photosynthesis, "the living matter," transforming into "buried organic matter," was accumulated in sedimentary rocks. How long could it last? As free oxygen accumulated in the atmosphere, photosynthesizing organisms have acquired photorespiration which was in reciprocal relations with CO2 uptake [42]. As known, carbon dioxide uptake increases biomass production, whereas photorespiration reduces it. As a result, photorespiration decreased the expansion of photosynthesis and brought down the accumulation of buried organic matter. Despite the absolute growth of buried organic matter in sediments, its relative input became lesser with each new orogenic cycle. The contribution of the above processes to metabolism depends on the СО2/О2 ratio in the environment. The latter ratio steadily grew down in the course of orogenic cycles. Thus photosynthesis performed a regulatory role in carbon cycle.

For individual photosynthesizing organisms of C3-type, which were the first representatives of photosynthesizing life on the Earth, a term "compensation point" is applicable. It is a metabolic state of the organism at a particular concentration ratio of СО2 and О2 when CO2 uptake becomes equal to CO2 release. Below this point, the rate of photorespiration (together with respiration) exceeds the rate of photoassimilation, and the physiological existence of organisms becomes impossible. As СО2 and О2 are mutually related, the compensation point

It was shown that the plants placed in a closed chamber due to reverse links (reciprocal relations) between main photosynthetic processes, CO2 assimilation and photorespiration, make the CO2/O2 ratio in chamber atmosphere stable [43, 44]. Considering these results, Tolbert et al. [43] assumed that the same feedback mechanism acts in nature and is responsible for the achievement of stationary СО2 and О<sup>2</sup> concentrations in the atmosphere. They introduced the term "ecological compensation point." The above processes are the driving forces in achieving the ecological compensation point. The numerous oxidation processes of the reductive branch, due to O2 consumption and CO2 evolution, play a regulatory role via common reaction intermediates, defining the real position (concentrations) of the ecological compensation point. In this position, the full conversion of the reduced carbon into the oxidized forms and back occurs. The total interaction of CO2 assimilation and photorespiration provides the excess of reductive carbon over oxidative in period from photosynthesis origin up to the moment of achieving the ecological compensation point. The excess of the reduced carbon was accumu‐

may be determined via consideration of either СО2 or О2 concentrations [43].

to 25% level [41].

68 Applied Photosynthesis - New Progress

New redox carbon cycle model is suggested. It claims a dominant role of photosynthesis in the mechanism of cycle functioning. According to the model, carbon transfer between geospheres and biosphere is accompanied by the changes in the redox state of carbon. Photosynthesis provides the transfer of carbon from the oxidative to the reduced state. The reverse transfer takes place in numerous processes of oxidation, including the respiration of living organisms and the processes of direct and indirect oxidation of buried organic matter. The final powerful oxidation of organic matter occurs in sulfate reduction proceeding in the subduction zones (plates' collisions zone).

Photosynthesis connects the Earth crust and biosphere processes. This link is realized by means of CO2 pulses appearing in lithospheric plates' collisions when they move. The source of CO2 is the oxidation of the buried organic matter in sulfate reduction in subduction zone. The last reaction is the coupling point of natural carbon and sulfur cycles.

Lithospheric plates' movement has two phases. In the short-term orogenic phase, the CO2 coming from subduction zone fills the "atmosphere–hydrosphere" system, causing climate warming. In the long-term geosynclynal phase, weathering and photosynthesis become dominant processes, depleting the oxidative forms of carbon followed by glaciations. By this way, photosynthesis transmits the impact of Earth crust processes on biosphere. The irregular periodicity exerts an impact on climate, biodiversity, distribution of organic matter in sedi‐ mentary deposits, etc.

Thus due to the unique combination of CO2 assimilation and photorespiration, which are in reciprocal relations, photosynthesis plays a key regulatory role in the carbon cycle. During photosynthesis, the expansion of O2 concentration in the environment permanently grew while that of CO2 dropped. When they reached values limiting physiological boundaries of life, the changes ceased and the CO2/O2 ratio became stable. This state does correspond to ecological compensation point. In terms of global photosynthesis, it means that all biomass, produced by photosynthesis, is oxidized in numerous processes of respiration, microbial and chemical oxidation in sediments. It happened when photosynthesis had spread over the whole land. The last splash of oxygen in the atmosphere occurred in the Miocene.

Prior to this moment, the excess of photosynthesis production was accumulated in sediments as a buried organic carbon. Thus, the process of organic carbon accumulation in sediments went on from the origin of photosynthesis up to the ecological compensation point.

On achieving the ecological compensation point, further accumulation of buried organic carbon has ceased. The amount of organic matter and its derivatives in sediments became stationary. It means that the amount of oils generated by source rocks also stabilized.

It should be underlined that the isotope techniques now are a powerful instrument of exami‐ nation of the processes in the past. It became possible since in many cases carbon isotopic discrepancies remain unchangeable and preserve its memory about the processes. Among them, carbon isotope technique is of special interest. In accordance with actualism principle, carbon isotopic difference between carbonate and coeval organic matter (*ε* parameter) is the analog of 13C isotope discrimination in modern plants, while *ε* parameter is a function of CO2/ O2 ratio in the environment. Moreover, the above isotopic differences are the most widely used data in geology studies. The established changes of the CO2/O2 ratio, found by means of carbon isotope data, reflect many events of evolution of the atmosphere, climate, and other coupled phenomena in the biosphere. Combining this technique with adequate carbon cycle model, researchers obtain a very delicate tool to study various phenomena in the past, including evolution itself. Very interesting and important results can be obtained in combination of isotopic techniques of different elements.

## **Author details**

#### A.A. Ivlev

Russian State Agrarian University – Moscow Agricultural Academy of K.A. Timiryazev, Russian Federation

#### **References**

way, photosynthesis transmits the impact of Earth crust processes on biosphere. The irregular periodicity exerts an impact on climate, biodiversity, distribution of organic matter in sedi‐

Thus due to the unique combination of CO2 assimilation and photorespiration, which are in reciprocal relations, photosynthesis plays a key regulatory role in the carbon cycle. During photosynthesis, the expansion of O2 concentration in the environment permanently grew while that of CO2 dropped. When they reached values limiting physiological boundaries of life, the changes ceased and the CO2/O2 ratio became stable. This state does correspond to ecological compensation point. In terms of global photosynthesis, it means that all biomass, produced by photosynthesis, is oxidized in numerous processes of respiration, microbial and chemical oxidation in sediments. It happened when photosynthesis had spread over the whole land.

Prior to this moment, the excess of photosynthesis production was accumulated in sediments as a buried organic carbon. Thus, the process of organic carbon accumulation in sediments

On achieving the ecological compensation point, further accumulation of buried organic carbon has ceased. The amount of organic matter and its derivatives in sediments became

It should be underlined that the isotope techniques now are a powerful instrument of exami‐ nation of the processes in the past. It became possible since in many cases carbon isotopic discrepancies remain unchangeable and preserve its memory about the processes. Among them, carbon isotope technique is of special interest. In accordance with actualism principle, carbon isotopic difference between carbonate and coeval organic matter (*ε* parameter) is the analog of 13C isotope discrimination in modern plants, while *ε* parameter is a function of CO2/ O2 ratio in the environment. Moreover, the above isotopic differences are the most widely used data in geology studies. The established changes of the CO2/O2 ratio, found by means of carbon isotope data, reflect many events of evolution of the atmosphere, climate, and other coupled phenomena in the biosphere. Combining this technique with adequate carbon cycle model, researchers obtain a very delicate tool to study various phenomena in the past, including evolution itself. Very interesting and important results can be obtained in combination of

Russian State Agrarian University – Moscow Agricultural Academy of K.A. Timiryazev,

went on from the origin of photosynthesis up to the ecological compensation point.

stationary. It means that the amount of oils generated by source rocks also stabilized.

The last splash of oxygen in the atmosphere occurred in the Miocene.

isotopic techniques of different elements.

**Author details**

Russian Federation

A.A. Ivlev

mentary deposits, etc.

70 Applied Photosynthesis - New Progress


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