Evolution of Biogeochemical Cycling

Structural Peculiarities of Biogeochemical Cycles' Development

Although ecological systems can be considered as the next level of matter self-organization after the biological one, it is quite probable that biogeochemical cycling preceded the origin of separate organisms. First, the cyclic chemical reactions, including both synthetic and decomposing processes, were formed in the ocean. Then their main links were shaped as separate self-reproduced organisms. The formation of the biological cycling was not the result of unification and cooperation of organisms or populations that existed before; all other forms of biological units arose in the course of structurization of biological cycles (as a result of special processes of discretization or corpusculariza-tion). Such a view on forming ecosystems is analogous to the model of M. Eigen (his famous hypercycle) of first organisms' origin.

Another important point is the fact that evolution of the organic world never totally destroyed the cycles that previously existed, but only supplemented and transformed them. All new forms are forced to adapt to existing conditions and are not 'interested' in their destruction. Besides, the biosphere is characterized by extremely various conditions; in some cases 'old-fashioned' cycles still were the most effective. Thus, biogeochemical evolution is not a process of the change of old cycles by new ones; it is a process of cycles 'layering', development of complex dynamical networks. Such a multilevel, diverse character of cycling provides unique vitality to the biosphere, which successfully develops in spite of various natural cataclysms, which took place during the existence of life.

The stability of biosphere is explained substantially by a complex dialectic interaction between its living and nonliving components. Biota is under strong influence of abiotic environment and should adapt to it, but at the same time actively transforms it, making life more convenient. Such important environment parameters, as chemical composition of air, water, and lithosphere, climate, character of solar radiation on the surface, etc., are under essential control of the biosphere.

Although such external factors as volcanism, ocean regressions, transgressions, etc., cannot be influenced by biota, they also contribute to the life development. Abrupt or continuous changes in external conditions play the role of oscillations in the process of 'sifting' of living forms. The most impressive advances of the organic world took place after essential changes in the environment; only by radical disturbance of a system it can pass from a local optimum to other, more optimal steady state.

Forming of Biogeochemical Cycling

The last micropaleontological research has shown that the life and, correspondingly, biogeochemical cycling took place from very early stages of the Earth's history. They originated during its first billion years; the age of first primitive prokaryotic organisms is about 4 billion years. Probably, first cycles were based mostly on che-motrophic organisms, which used for biomass synthesis chemical energy of inorganic or simple organic substances (by way of their degradation with the use of oxygen or sulfur). Ecosystems of the black geysers can give an estimate of the first biological cycles. First chemotrophs lived in the age of forming the Earth crust and essentially influenced its composition. In the initial biosphere, as well as in the present one, the processes of decomposition of silicates, sedimentation of silicon, iron, phosphorus, manganese, cycling of sulfur, etc., took place.

The first cycles operated in condition of high temperature and dense hydrogen and helium atmosphere. The last elements permanently dissipated in the interplanetary space, and about 2.7 billion years ago the initial atmosphere disappeared. It was the first ecological catastrophe, which caused essential degradation and further reformation of biogeochemical cycling.

Dynamics of Oxygen and Carbonic Gas Concentration in the Atmosphere

During next 400 000 years, forming and intensive development of phototrophs took place. Correspondingly, the atmosphere became more and more oxygenic. The accumulation of oxygen was promoted by methanogenic bacteria, which consolidated free hydrogen. The main pathway of hydrogen (originated from water dissolution) led to its dissipation in the outer space through methane, which migrated into upper atmosphere layers and was destructed under the influence of ultraviolet radiation. The development of oxygenic atmosphere caused the next ecological catastrophe (sometimes called 'oxygen revolution'), because free oxygen was toxic for organisms of those ages. Some of them evolved to aerobic forms, others remained in anaerobic conditions, but the majority died out.

From 2.3 to 0.3 billions years ago the oxygen concentration in the atmosphere permanently increased. As a result, the character of photosynthesis was changed. According to Gaffron, the first phototrophs used energy of ultraviolet radiation. Since free atmospheric oxygen produced the ozone layer protecting the surface from ultraviolet radiation, phototrophs evolved to use the visible light, forming the chlorophyll photosynthesis and recent type of biogeochemical cycling.

The formation of the ozone screen about 700 million years ago (with oxygen concentration corresponding to Pasteur point) gave a possibility of starting the terrestrial life and terrestrial biogeochemical cycling.

Decreasing of carbonic gas concentration in the atmosphere accompanied the increasing of oxygen one. The control of carbonic gas is one of the most evident functions of the biosphere; without this control the gas concentration can reach 98%, as it takes place, for example, in the abiotic atmosphere of Venus. Probably, the antagonism between carbonic gas and oxygen in the atmosphere (determined by biotic factors) produces long-term oscillation of their concentrations. The stage of relative abundance of carbonic gas in carbon (generated by high geological activity and led to further conservation of essential amount of carbon in the form of coal in the Earth crust) was changed by the stage of high concentration of oxygen in Mesozoic era (stimulated development of huge forms of animals).

The abundance of carbon gas is quite important for development of vegetation. It is the main biomass-gener-ating substance, a peculiar ecological 'currency', for which, similarly to the real one, little inflation should take place. Volcanic activity permanently adds the gas to the cycling and creates such inflation. One of the pessimistic ecological forecasts is connected with gradual decrease of geological activity. When the carbonic gas entry to the biosphere finishes, 'stagnation' of the biogeo-chemical cycling is inevitable.

At the same time, ecological problems of the recent moment are connected mostly with increase of carbonic gas concentration in the atmosphere, accompanied by the notorious greenhouse effect. The problem is a result of large-scale activity of human society, which is deeply involved in the current biogeochemical cycling.

Mankind as a Biogeochemical Factor

The origin of the sentient life about 500 000 years ago and the civilization about 10 000 years ago determined a principally new stage of the global cycles' development. The fact that people are very effective consumers with highlevel abilities on information processing is not so significant in this context. Much more important factors are essential for changing of matter pathways as a result of human impact. The invention of fire led not only to efficiency of consuming biomass use (because of its use in cooking) and areal broadening (because of heating), but for people it created the possibility to play the role of reducers, promptly transforming useless biomass to mineral substances. The slash-burn clearing produced a new kind of intensive biological cycling.

Agricultural and industrial activity of man is accompanied by more and more large-scale involvement of new substances into the cycling. The humanity has intensified water cycling (by creation of water reservoirs, artesian wells, etc.), and added to the turnover a big amount of different elements, including toxic ones. This impact is not always negative for the biosphere (if it is possible to talk about the use of interference into the nature). Enrichment of elements in biogeochemical cycling can hasten it. For example, in the course of the so-called eutrophication, a natural process of lakes transformation into bogs and then to meadows can run much quicker. But such process of cycling acceleration is considered, from human point of view, as a negative impact.

Involvement of a huge amount of carbon as a result of use of coal and oil deposits has led to an increase of carbon gas in the atmosphere and, correspondingly, to the greenhouse effect. At the same time, plant nutrition improves; it can partially compensate forest destruction. Besides, the greenhouse effect can compensate the already-mentioned global tendencies to the climate getting colder.

Human impact on forming biological cycles became determinative during the last centuries. It has a geological and planetary character. The new stage ofbiosphere development was called as the noosphere by V. I. Vernadsky in 1944, who used the term, proposed by E. Le Roy with a 1927, with a somewhat different meaning.

Humanity should coordinate its activity with global biogeochemical cycling. According to D. H. Meadows, three main principles of sustainable development are closely connected with integrity of global cycling: the rate of use of renewable resources should correspond to the rate of their regeneration; rate of use of nonrenewable resources should correspond to the rate of their change by renewable resources; the rate of production of pollutants should correspond to the rate of their decomposition in the environment.

Both in agriculture and industry, humanity was forced to embed elements of biogeochemical cycles as a part of general technologies. The recent ecological situation and tendencies of its change demand another approach: to embed technologies in the global biogeochemical cycling. Conscious control of the cycling, creation of real noo-sphere (sphere of intellect), is the only one way for humanity to survive.

General Tendencies of Biogeochemical Cycles' Developments

The question about direction, driving forces, and 'purport' of biosphere evolution is quite complex and vexed. An answer can be based on different philosophical concepts. Let us consider the energetic approach, which was actively developed by A. Lotka, V. I. Vernadsky, and H. Odum. From this point of view the main direction of biogeochemical cycles' development is permanent increase of the energy flow through biosphere.

Energetic efficiency is a key parameter of species in the process of competitive selection. Ability of better assimilation of solar energy or energy collected by other organisms is a prior evolutional advantage. More and more effective populations are involved into global cycling, increasing its intensity. As a result, the biosphere power (consumed energy per unit time) permanently grows. V. I. Vernadsky formulated three main tendencies of biosphere evolution: biogenic migration tends to maximum manifestation; biological evolution causes intensification of biogenic migration; covering the Earth by life is maximally possible for current abilities of the biosphere.

The intensification of cycling is mainly a result of competition of producers, which are forced to maximize production for keeping place in ecosystem. Another extremely important factor is activity of consumers. They withdraw producers' biomass and additionally intensify cycling. Probably, the global role of consumers in the biosphere consists exactly in the spin-up of ecological cycles.

Successions

The process of increasing biosphere power includes not only improvement of existing cycles, but very often it is replacement of less effective cycles by more effective ones (and, not so often, forming new type of cycling). Since the type of cycling geographically corresponds to biogeocoenoses, one can talk about perfection of cycling in the course of biogeocoenoses change or, in other words, during successions. The latter can take place after either change of external conditions or disturbance of ecosystem steady state (climax) as a result of external impact (or, rarely, of ecosystem elements evolution).

It is important to take into consideration that change in biogeocoenoses is a result of 'struggle' (or competition) between biological cycles of different types. Such a struggle geographically takes place on territories between different types of biogeocoenoses (e.g., between forest and grassland). The corresponding ecosystem including two or more types of cycles is called ecotone or amphicoenose. By the definition of A. L. Belgard, amphicoenose is a system of several antagonistic biological cycles. Intermediate conditions do not allow one cycle to displace another; it is a case of 'glitching' succession. On the other hand, amphicoenoses ecosystems, including two struggling cycles, are natural intermediate stages of any succession. The amphicoeno-tic character of ecosystems dynamics is one of the main sources of their biodiversity.

A model of amphicoenose can be designed as a combination of models [1] or [2] of coenome. The scheme of amphicoenose structure for the two-dimensional case is represented in Figure 4. The two coenomes are unified by the soil block. The model correctly describes the main dynamic properties of amphicoenoses: competitive exclusion of plant associations, crossing of reducer populations between associations, etc. The result of computer simulation of amphicoenose dynamics (which was considered as a struggle of biological cycles of different types) is shown in Figure 5. The use of the cyclic models determines much more complicated dynamics than using the standard Volterra-type models of successions.

e1(q*1 - q + {q\ - q)2 + 6)x1 e2(q*2- q + (q*2 - q)2 + 6)x2

Figure 4 Matter flows in an amphicoenose.

Figure 4 Matter flows in an amphicoenose.

V

aI V /

---A >—^

f y \/ i i

h \

N^ / \ $

\Vq

J\\

' 1 /

Figure 5 Results of simulation of the amphicoenose dynamics; x-i, x2 - contents of the element in the producer blocks, q - the same for the soil block.

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