Fundamentals of Microbiology

4.3.6 Sulphur cycle

The cyclic movements of sulphur between the living organisms and the environment are referred as sulphur cycle. Sulphur is an essential element for all living organisms. It is present in free as well as combined states. Plant, animal and microbial proteins, aminoacids – cystine and methionine contain sulphur. In soil, it occurs both in organic (sulphur amino acids, vitamins, etc.) as well as in the inorganic form (sulphur and sulphates) and is readily metabolized. Four distinct transformations are recognized; these are:

(i) decomposition of larger organic sulphur compounds to smaller units and their version into inorganic compounds (mineralization),

(ii) microbial associated immobilization,

(iii) oxidation of organic ions and compounds such as sulphides, thiosulphates and sulphur,

(iv) reduction of sulphates to sulphides.