From abiogenic cycles to biogeochemical ones
Before life originated on Earth, the circulation of chemical elements occurred exclusively without the participation of organisms. Such early cycles are called abiogenic geochemical cycles.
In that epoch, the primary source of elements was numerous volcanic gases abundantly emitted from the planet's interior. Upon entering the primordial gas envelope, these gases underwent oxidation due to various photochemical reactions. The final stage of such an abiogenic cycle was the removal of oxidized chemical compounds from the atmosphere: they precipitated onto the planet's surface along with atmospheric precipitation and became fixed in the lithosphere.
As soon as the first living organisms appeared on the planet, the pattern of element movement changed dramatically. Ancient abiogenic cycles evolved and transformed into biogeochemical cycles. Circulation now became inextricably linked with the vital activity of organisms, which shaped the modern balanced appearance of the biosphere.
Modern atmospheric composition
The transformation of biogeochemical cycles directly affected Earth's gas envelope. Today's atmospheric composition is the result of the long-term operation of biogeochemical cycles. It is dominated by two main gases that support respiration and nutrition processes in the vast majority of living organisms:
- Nitrogen ($N_2$): accounts for the overwhelming majority of the volume — 78.8%.
- Oxygen ($O_2$): accounts for 20.95%.
In addition to them, atmospheric air constantly contains the inert gas argon ($Ar$), with a share of 0.93%, and carbon dioxide ($CO_2$), which is extremely important for plant photosynthesis and makes up only 0.036%.
Also, trace amounts of impurities are regularly recorded in the atmosphere, including hydrogen ($H_2$), hydrogen sulfide ($H_2S$), carbon monoxide ($CO$), methane ($CH_4$), ammonia ($NH_3$), as well as various nitrogen and sulfur oxides. These compounds are also integral participants in biogeochemical transformations.
Structure of cycles: reservoir and exchange pools
Any material cycle in the biosphere has a clear dual structure. The entire mass of circulating chemical elements is generally divided into two large functional categories called pools.
- Reservoir pool. It concentrates the bulk of the biosphere's chemical substances. The main feature of this pool is the extremely slow movement of elements. The reservoir pool represents a purely abiotic part of the cycle, meaning it is generally not directly connected to living organisms. It is worth noting that it is precisely by the location of reservoir pools that all cycles in the biosphere are usually divided into three large groups.
- Mobile (exchange) pool. This pool includes a significantly smaller mass of chemical substances compared to the reservoir pool, but it is characterized by rapid movement and high functional activity. It is within the exchange pool that the direct exchange of chemical elements between living organisms and their surrounding environment occurs, which maintains the continuity of biological processes.
Briefly: answers to topical questions
Which groups of microorganisms ensure the biogeochemical nitrogen cycle?
The biogeochemical nitrogen cycle is ensured by several physiological groups of microorganisms.
- Nitrogen-fixing bacteria — assimilate free nitrogen and synthesize organic compounds (nodule bacteria, Azotobacter, Clostridium, Beijerinckia).
- Ammonifying bacteria — cause the putrefaction of organics and the decomposition of urea with the formation of ammonia (aerobes: B. subtilis, B. mesentericus, Serratia marcescens, Proteus; anaerobes: C. sporogenes, C. putrificum; ureabacteria: Urobacillus pasteuri, Sarcina urea).
- Nitrifying bacteria — oxidize ammonia to nitrous acid (Nitrosomonas) and convert it into nitric acid (Nitrobacter).
- Denitrifying bacteria — reduce nitrates to molecular nitrogen (Pseudomonas, Bacillus, Micrococcus).
What is the essence of V.I. Vernadsky's teaching on the biogenic migration of atoms?
The essence of the teaching is that chemical elements continuously circulate in the system between the external environment and living organisms. The stability of these cycles is a crucial condition for the existence of the biosphere. Before the appearance of life, abiogenic geochemical cycles formed without the participation of organisms, and with the emergence of living organisms, they transformed into biogeochemical cycles.
Which chemical elements belong to the main biogenic macronutrients (organogens)?
The main biogenic macronutrients (organogens) include four chemical elements that account for 98% of the composition of living organisms.
- Carbon (C) — accounts for 45–55% of the chemical composition of bacteria, acting as a structural component of organic substances.
- Oxygen (O) — accounts for 25–30%, participating in the synthesis of organic substances and the realization of their functions.
- Nitrogen (N) — accounts for 8–15%, necessary for the synthesis of organic substances.
- Hydrogen (H) — accounts for 6–8%, being a structural component of water and organic compounds (proteins, nucleic acids, carbohydrates, lipids).
For quick memorization of the cycle structure, associate the pools with a bank: the "Reservoir" is a gold vault (large mass, motionless, abiotic), while the "Exchange" is a cash withdrawal desk (little money, but rapid turnover, direct exchange with client organisms occurs).
Class and exam questions
What is the main difference between abiogenic and biogeochemical cycles?
Abiogenic geochemical cycles proceed without the slightest participation of living organisms, as was the case before the appearance of life (e.g., the oxidation of volcanic gases and their precipitation). Biogeochemical cycles, conversely, are inextricably linked with the vital activity of organisms and involve the direct exchange of substances between the biota and the environment.
What gases mainly make up the modern atmosphere?
The absolute basis of the gas envelope is molecular nitrogen (78.8%) and oxygen (20.95%). Argon, carbon dioxide, and impurities in the form of methane, hydrogen sulfide, ammonia, and oxides are present in significantly smaller proportions.
What is the reservoir pool of a cycle?
This is the abiotic part of the cycle, which contains the bulk of chemical substances. In this pool, elements move very slowly and practically do not interact directly with living organisms.
What else to study on this topic
- Classification of cycles into three groups by the location of reservoir pools
- Mechanisms of photochemical oxidation of volcanic gases in the primordial atmosphere
- The role of atmospheric impurities (methane, hydrogen sulfide, ammonia) in biogeochemical cycles
- Rate of metabolism in the mobile pool
- Sedimentation processes during the transition of elements from the atmosphere to the lithosphere
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