**8. Ecological compensation point**

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66 Applied Photosynthesis - New Progress

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**the Miocene epoch of Cenozoic era**

2001) [31] (b) with stratigraphic subdivisions.

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**Figure 6.** Comparison of distribution of explored extractable oil fields in the largest world reservoirs (Vishemirsky, Kontorovich, 1997) [30] (a) and revealed oil fields (% of the total number of oil fields) of the former USSR (Korchagin,

**7. 13C enrichment of oils reflects O2 growth in the atmosphere and display four orogenic cycles from the Cryogenian period of Neoproterozoic era to**

Having examined the extensive collection of oils (504 oil samples) differing in age and origin, Andrusevich et al. [35, 36] established that in the course of geological time, the oils and their components have been consistently enriched in 13C (Figure 7). Model's logic allows concluding that the most likely reason for this enrichment is the intensified photorespiration of photo‐

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Global natural redox carbon cycle is a developing and self-organizing system. This feature is provided due to photosynthesis which is an essential part of the cycle. Its origin occurred in anoxic atmosphere. The photosynthesis evolution was followed by atmospheric oxygen growth. The literature data, given below, illustrate oxygen growth in the atmosphere over geological time. Atmospheric O2 concentration prior to photosynthesis was determined by water dissociation under ultraviolet action and was equal to one thousandth part of the present oxygen level (Urey level). With the emergence of photosynthesis, the average oxygen concen‐ tration in the atmosphere began to increase from cycle to cycle.

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 to 25% level [41].

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

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‐ lated in deposits in the form of buried organic matter. The corresponding amount of oxygen was accumulated in the atmosphere.

The glacial–interglacial oscillations of CO2 have emerged as a consequence of proximity of the system to the ecological compensation point [22,45−48]. This possibly happened in the end of Carboniferous when the great expansion of photosynthesis took place and covered the land [49]. The ecological compensation point was most likely achieved in the Neogene (Miocene). At this time, after some decline, there was a burst of atmospheric oxygen to the maximum level associated with global cooling. Right after cooling in the Miocene, there was considerable warming in the Pliocene period [50−52]. The warming was followed by mass extinction of organisms, and by the formation of sediments rich in organic matter. These sequences of climatic changes are characteristic of transitions from one orogenic cycle to another.

After achieving the ecological compensation point, global carbon cycle became very sensitive to separate plates' collisions, what is in agreement with short-term glacial–interglacial oscillations of CO2. It allows assuming that long-term orogenic cycles and short-term oscilla‐ tions have the same physical nature.

The most important conclusion from the existence of the ecological compensation point is the following. After the achievement of ecological compensation point, the system of global carbon cycle gets its stationary level. No additional accumulation of buried organic carbon occurs. It means no additional oil generation takes place, and hence the amount of oil in the Earth crust becomes stable and limited. Hence it should be concluded if the rate of oil production is greater than the rate of generation, oil resources should be exhausted with time.
