Oxidation of Sulphur and Sulphur Compounds by Bacteria

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Oxidation of Sulphur and Sulphur Compounds by Bacteria

	Oxidation of Sulphur and Sulphur Compounds by Bacteria - Bacteria which oxidize various forms of sulphur, special1y hydrogen sulphide, belong to two different nutritional types-photosynthetic and chemolithotrophic. Photosynthetic bacteria which oxidize hydrogen sulphide to elemental sulphur belong to the families Chlorobiaceae and Chromatiaceae. Hydrogen sulphide acts as an electron donor for CO₂ reduction in photosynthesis.
Sulphur is oxidized to sulphate by chemolithotrophic bacteria of the family Thiobacteriaceae. The most important organism is Thiobacillus thioxidans which oxidizes sulphur and thiosulphates to sulphuric acid under aerobic conditions. The formation of sulphate is benefical to agriculture in three ways. 1) Sulphate is the most suitable source of sulphur for the plants.
2) Sulphate is the anion of a strong mineral acid (H₂SO₄) and prevents excessive alkalinity due to ammonia-formation by microorganisms. 3) Accumulation of H₂SO₄ solubilizes inorganic salts that contain plant nutrients such as phosphates and metals. In the other hand Thiobacillus denitrificans oxidizes sulphur with the reduction of nitrate. This is an important factor in the loss of fertility in certain anaerobic soils.
Reduction of sulpha:Sulphate in the soil is assimilated by plants and microorganisms and is incorporated into proteins. This is known as assimilatory sulphate reduction. There is also a dissimilatory sulphate reduction similar to dissimilatory nitrate reduction. Organisms utilize sulphate as a terminal electron acceptor in anaerobic respiration with the production of hydrogensulphide. Organic compounds are dehydrogenated and, hydrogen is used in the reduction of sulphate to hydrogen sulphide through sulphite.
	The most important sulphate reducing bacteria are those belonging to the genus Desulfovibrio, and the best known species is Desulfovibrio desulfuricans. Certain obligate anaerobic organisms belonging to the genus Desulfotomaculum are also sulphate reducers. Phosphorus exists in an organic form in the protoplasm. On death or living organisms this organic phosphorus is changed to inorganic phosphoric acid. This is soon converted into insoluble salts of calcium, iron, magnesium and aluminium. Phosphorus thus alternates between organic and inorganic and soluble and insoluble forms. Insoluble phosphorus is solubilized by various acids produced by microorganisms.

Microbial activities involved in the cycling of carbon, nitrogen, sulphur and phosphorus arc absolutely essential for maintenance of soil fertility. However, there are also unfavourable relationships between plants and soil organisms, causing various plant diseases. This is partly taken care off by microbial antagonism. Since almost all types of microorganisms are present in the soil, there is a competition among them selves for survival. Microorganisms produce a wide variety of antibiotics which are inhibitory to other microorganisms. Plant pathogens, which cause number of plant diseases, are thus inhibited or killed by different antibiotics.

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Sulphur Cycle Ammonification in Soil Nitrification in Soil Nitrate Reduction in Soil Denitrification in Soil Nitrogen Fixation Symbiotic Nitrogen Fixation Rhizobium Specificity of Bacteria and Legumes Non-Symbiotic Nitrogen Fixation Non-symbiotic Nitrogen Fixing Microorganisms Mechanism of Nitrogen Fixation Oxidation of Sulphur and Sulphur Compounds by Bacteria

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