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Index >> Applications of Microbial Interactions >> Oxidation of Sulphur

Oxidation of Sulphur

	Oxidation of Sulphur In the presence of oxygen, reduced sulphur compounds are capable of supporting chemolithotrophs like Beggiatoa, Thioplaca, Thiothrix and filamentous bacteria capable of oxidation of H₂S. H₂S + 1/2O₂ → S° +H₂ Sulphur globules are deposited within the cells. In the absence of H₂S, these globules are slowly oxidised to sulphates. These bacteria are found inthe interface of anaerobic environment or the sediment where the partially oxygenated water is in contact with the sediment.
Some species of Thiobacillus also oxidise the H2S sine they have low tolerance of acidity and deposit elemental sulphur rather than generating H2SO4 by further oxidation. These organisms are called facultative chemolithotrophs. Acidophilic organisms like T. thiooxidans that are obligate chemolithotrophs further oxidise elemental sulphur to sulphuric acid. These organisms are acid-tolerant and can withstand up to 2-3 pH and obtain their energy exclusively from oxidation of inorganic sulphur thus aiding in denitrification step of nitrogen cycle.
Thiobacillus denitrificans utilises nitrates as terminal electron acceptor during oxidation of inorganic sulphur. 3S° + 4NO₃^2- 3SO₄^2- +2N₂ H2S is also subjected to phototrophic oxidation in anaerobic environment. CO2 + H₂S → CH₂O + S° Microbial oxidation of reduced sulphur is needed for continued availability of this element in non-toxic form. Sulphur oxidation produces high amount of mineral acids which leads to solubilisation of phosphorus and other mineral nutrients. Typical temperature range for sulphur oxidation is 34-37°C.
Mesophiles (T ferrooxidans) are active at 40°C. Sulfolobus species are active at 80°C and S.acidodurans has an optimum pH of 0.5. It grows in hot acidic sulphate soils. Beggiatoa grow in rhizosphere of rice and protect it from H₂S in flooded sediment by its oxidation to elemental sulphur. An important practical implication of sulphur cycle is the anaerobic corrosion of steel and iron structure, destroying pipe set in sulphate containing soils and sediments. The surface of iron reacts with water forming a layer of iron hydroxide and hydrogen.
	Fe²⁺ + H₂O → Fe(OH)₂ + H₂ Desulfovibrio desulphuricans removes the hydrogen and forms H₂S. H₂ + SO₄^2- → H₂S + 2OH^- + 2H₂O The released H₂S attacks iron, forming iron sulphide. H₂S + Fe²⁺ → FeS + H₂ Thus sulphur cycle is maintained in the respective environments chiefly by the microorganisms involved in the oxidation and reduction of sulphur.

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