A steady-state model for sulphur isotope fractionation in bacterial reduction processes

A model is developed to explain the isotope fractionation effects produced in laboratory experiments involving the reduction of sulphate to hydrogen sulphide by the bacterium Desulfovibrio desulfuricans. The approach differs from previous ones in its use of zero-order kinetics to describe the uptake of sulphate by the bacterium. Expressions are developed which relate the overall isotope effects produced by the bacterium to the ratios of backward to forward flows between its internal sulphur reservoirs. Other applications of this type of model are discussed, including the differences of isotopic composition between sulphate and hydrogen sulphide in the Black Sea and an unusual isotope effect observed in the course of bacterial nitrate reduction.