GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

21. MICROORGANISMS AND THE ENVIRONMENT

21.1. THE ROLE OF MICROORGANISMS IN BIOGEOCHEMICAL CYCLES

Based on their roles and Functions in the natural balance, Living organisms are divided into three groups. Green plants synthesize organic matter using solar energy and carbon dioxide, and are therefore called producers. Animals act as consumers, utilizing a significant portion of primary biomass to build their own bodies. PLANT AND ANIMAL remains are ultimately subject to decomposition, a process that converts Organic compounds into mineral, inorganic substances. This process is known as mineralization. It is carried out primarily by Fungi and Bacteria, which serve as decomposers in nature's balance.

Carbon cycle. In the carbon cycle, microorganisms ensure the mineralization of carbon previously converted by green plants into organic compounds, thereby maintaining a rather delicate equilibrium. Atmospheric air contains slightly more than 0,03 % carbon. The photosynthetic productivity of green plants is so immense that atmospheric CO₂ reserves would be depleted in approximately 20 years. Green plants would have to cease CO₂ fixation if lower animals and microorganisms did not ensure its return to the atmosphere through the continuous Mineralization of organic material. In the overall balance of matter in nature, soil bacteria and fungi play a role no less significant than that of photosynthesizing green plants. The interdependence of all living beings on Earth is most clearly reflected in the carbon cycle.

Another feature of the mineralization process is that a small fraction of the mineralized carbon (1,0-1,5 %) enters the atmosphere not as CO₂, but in the form of methane (СН₄). This gas is produced from organic matter in environments inaccessible to oxygen (such as tundra soils, rice paddies, and the rumen of ruminant animals). Methanogenic bacteria participate in methane production.

At first glance, the oceans appear to be a vast reservoir of carbon dioxide. However, it should be noted that The rate of exchange between atmospheric CO₂ and oceanic CO₂—where more than 90 % of this compound exists in the form of HCO₂—is relatively low: only about a tenth of the atmospheric Carbon dioxide is exchanged in this manner annually. Moreover, only a thin surface layer of Water participates in gas exchange between the sea and the atmosphere. Over recent decades, the carbon dioxide content in the air has been steadily increasing. This can be attributed to two main factors: the combustion of oil and gas, and the decline in photosynthetic CO₂ fixation resulting from the deforestation of large woodland areas and soil degradation. It should be emphasized that the World Ocean acts as a powerful buffer system, striving to maintain atmospheric CO₂ at a relatively stable level.

Photosynthetic CO₂ fixation by green plants primarily yields sugars and related compounds. The bulk of fixed carbon in both woody and herbaceous plants is stored in the form of polymeric CARBOHYDRATES. Since Polysaccharides predominate among the assimilation products of green plants, sugars play a major role in the Nutrition of All living organisms that require organic food. Glucose and other sugars in polymeric form serve as the quantitatively dominant substrate for mineralization processes in nature, whereas in monomeric form, they provide optimal nutrients for heterotrophic microorganisms.

Nitrogen cycle. Ammonium occupies a central place in The Nitrogen Cycle. It is a breakdown product of Proteins AND AMINO acids that enter the soil along with plant and animal residues. In well-aerated soils, ammonium undergoes nitrification: bacteria of the genera Nitrosomonas and Nitrobacter oxidize it to nitrite and nitrate. Plants can utilize both ammonium and nitrate as nitrogen sources. In the absence of oxygen, nitrate is converted into molecular gas nitrogen (denitrification). Bacteria involved in denitrification use nitrate as a terminal electron acceptor in the anaerobic Respiratory Chain (anaerobic "nitrate" Respiration). Denitrification is accompanied by a loss of nitrogen from soils. At the same time, certain bacteria are capable of fixing molecular nitrogen. Nitrogen-fixing bacteria occur both as free-living organisms and in symbioses with higher plants.

Phosphorus cycle. In the biosphere, phosphorus is represented almost exclusively in the form of phosphates. In living organisms, phosphoric acid exists as esters. Following Cell death, these esters rapidly decompose, releasing phosphoric acid ions. The form of phosphorus accessible to plants in soils is the free orthophosphate ion (Н₂РО₄)^-; its concentration is often low, and plant growth is typically limited not by a total shortage of phosphates, but by The formation of poorly soluble compounds (such as apatites and heavy metal complexes). In many areas, fertilizer-derived phosphates enter flowing water bodies. Because the concentrations of iron, calcium, and aluminum ions in these waters are low, the phosphate remains in a soluble form, leading to water eutrophication, which is particularly favorable for the proliferation of nitrogen-fixing cyanobacteria. In soils, however, phosphates rapidly become unavailable to plants due to the formation of insoluble salts.

Sulfur cycle. In living Cells, sulfur is represented by sulfhydryl groups within Sulfur-Containing Amino Acids (Methionine, Cysteine, homocysteine). Sulfur accounts for 1 % of the dry matter of organisms. During the anaerobic decomposition of organic matter, sulfhydryl groups are cleaved by desulfurases; The production of hydrogen sulfide during mineralization under anaerobic conditions is also referred to as desulfurization. However, the largest amount of sulfur is produced via dissimilatory sulfate reduction, carried out by sulfate-reducing bacteria. These bacteria utilize sulfate as a terminal electron acceptor in the anaerobic respiratory chain ("sulfate" respiration).

Hydrogen sulfide formed in the absence of oxygen in aquatic sediments can be oxidized by anaerobic phototrophic bacteria (Chromatiaceae) to sulfur and sulfate. When hydrogen sulfide penetrates oxygenated zones, it is oxidized either abiotically or by aerobic sulfur bacteria into sulfate. Plants and some microorganisms obtain the sulfur required for the synthesis of sulfur-containing amino acids via assimilatory sulfate reduction, whereas animals obtain reduced sulfur compounds through their diet.

Phosphorus and nitrogen as factors limiting biomass production. Phosphorus and nitrogen are the primary elements limiting Plant Growth and, consequently, biomass production both on land and in the oceans. Based on the data in the table, one can calculate how much biomass can be synthesized from the elements contained in 1 m³ of seawater. From 28 g of carbon, 60-100 g of biomass can be formed, compared to only 6 g from 0.3 g of nitrogen, and just 5 g from 0.03 g of phosphorus. Consequently, biomass production is primarily limited by phosphates. In seawater, even nitrogen-fixing organisms such as cyanobacteria lack a selective advantage over others.

Class="center">Distribution of bioelements in seawater