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sulphur cycle The cycling of sulphur between the biotic (living) and abiotic (nonliving) components of the environment (see biogeochemical cycle). Most of the sulphur in the abiotic environment is found in rocks, although a small amount is present in the atmosphere as sulphur dioxide (SO2), produced by combustion of fossil fuels. Sulphate (SO42–), derived from the weathering and oxidation of rocks, is taken up by plants and incorporated into sulphur-containing proteins. In this form sulphur is passed along food chains to animals. Decomposition of dead organic matter and faeces by anaerobic sulphate-reducing bacteria returns sulphur to the abiotic environment in the form of hydrogen sulphide (H2S). Hydrogen sulphide can be converted back to sulphate or to elemental sulphur by the action of different groups of photosynthetic and sulphide-oxidizing bacteria (see sulphur bacteria). Elemental sulphur becomes incorporated into rocks.