SUMMARY: A fractionation of sulphur isotopes was found in all metabolic processes investigated except those in which elemental sulphur was the starting substrate for growth of and sp. and for reduction by . Except for polythionates formed during sulphide oxidation by or sp., the products of metabolism were enriched in S relative to the starting substrates. The magnitudes of the enrichment differed for different processes and for the same overall process carried out by different organisms. The δS values (%) ranged from -46·0 for sulphide from sulphate reduction by to +19·0 for polythionate formed during growth on sulphide by . Fractionation during sulphate reduction was inversely proportional to rate of reduction when lactate and ethanol were electron donors and directly proportional with molecular hydrogen as the electron donor. Temperature and sulphate concentration, within the normal physiological ranges of these parameters, influenced fractionation only in so far as they influenced rate of reduction. However, anomalous fractionation effects were obtained at low temperatures and when a resting suspension reducing sulphite was subjected to changes in temperature. The data are discussed with reference to the mechanism(s) of fractionation.


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