Microbes are distributed everywhere i.e. soil, water and air even they are present deep in side the earth as deep sea vents. The microbes play an important role in recycling of biological elements such as oxygen, carbon, nitrogen, sulphur and phosphorus.

Microbes in biogeochemical cycles: Oxygen comprises 70% of cell's weight and 23% oxygen in atmosphere which is available to all the microbes. On the other hand , oxygen occurs in mineral deposit as salts of carbonates, silicate, aluminate and other oxides. About 80% of oxygen is unavailable to biological organisms. Cyanobacteria, heterotrophs and chemolithotrophs use it. Water is s product of aerobic processes hence again remains available for photosynthesis.

About 20% earth's carbon is an organic compound, and less than 1% is in fossil fuel such as petroleum, coal, etc. Carbon is present in the form of CO2 in the atmosphere. CO2 is produced during the burning of fossil fuel, biological operations and microbial decomposition.Photosynthetic and chemosynthetic microorganisms convert CO2 into organic carbon.

Methane (CH4) is generated anaerobically from CO2 and H2 by methanogenic archaeobacteria. Soil bacteria and fungi mainly use organic matter present in the soil. Without the cycling action of these microbes life on this planet would suffer due to non- availability of nutrients essential for life. Microbes through their enzymatic machinery are able to release soluble products, making them available to microbes and plants. The dead remains of plants and animals are decomposed by microbes.

About 78% of N2 is present in atmosphere while 9-15 percent of a cell's dry weight contains essential cellular element N which contains amino acids, nucleic acid and in some coenzymes. The inorganic forms of nitrogen are interconverted by the metabolic activities of microorganisms, which maintain the natural nitrogen balance. Nitrogen fixing bacteria reduce nitrogen gas N2 to ammonia required for plant growth. Some organic nitrogen (amino acid, nucleic acid) is recycled by the process called ammonification. The ammonia produced is either incorporated into biomass or becomes the substrate for nitrification. The aerobic oxidation of ammonia to nitrate (NO3-) is carried out by nitrifying bacteria represented by Nitrosomonas and Nitrosococcus.

Examination of aquatic environments where eutrophication is actively underway of several fascinating variety of microorganism. Among these would be protozoa, algae and less obvious visually but no less important functionally are non-motile organisms and smaller forms, including fungi, and the bacteria, certain viruses also show their presence if special procedures are adopted for the detection. Viruses that infect all the biological grouping have a bearing on water pollution problems. Plant and algal viruses may prevent the unrestricted development of the host. It has been suggested that excessive growth of blue green algae might be controlled by seeding with apecific viruses (LPP and AS viruses). Similarly, bacterial viruses (bacteriophage) may help in controlling the size of bacterial population through the lysis of susceptible groups. Of special interest, the possible role of bacteriophage is the destruction of bacteria pathogenic to man, and bacterial indicators of faecal pollution. On the other hand, bacteria is especially fit to exploit a wide range of environmental opportunities. Bacteria play an important role in waste decomposition. Some actinomycetes do not seem responsible for the earthy odour .

Biological sewage treatment and self-purification have much in common. Both result in the mineralization of organic pollutants and in the utilization of dissolved oxygen. Complex microbial associations play an important role in self- purification, and such communities dominate the ecology of treatment plants. Some organic molecules present in industrial effluents are decomposed readily a variety of microorganisms, while some compounds appear to resist biological attack completely.

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