Sulfur isotope fractionation and kinetic studies of sulfite reduction in growing cells of Salmonella heidelberg.

A pulsed feeding technique was used during studies of sulfite reduction by Salmonella heidelberg in order to realize large percentages of SO(3) (=) conversion while simultaneously maintaining a reasonably stable cell population. As a consequence, much data for conventional kinetic and sulfur isotope fractionation computations were obtained in any one experiment. Under the conditions of supplying 150 mug glucose per ml of medium every 6 hr, anaerobiosis, and varying the SO(3) (=) concentration, the following observations were made: 1. Below 0.01% w/v Na(2)SO(3), the reduction strictly followed first order kinetics with respect to SO(3) (=) concentration. At higher concentrations, the rate of SO(3) (=) reduction fell below that predicted by first order kinetics suggesting that a saturation effect was occurring. 2. At lower concentrations, the ratio of the isotopic rate constants k(1)/k(2) was 1.02 whereas at higher SO(3) (=) levels, k(1)/k(2) values of 1.04 were found. These latter effects are much higher than those obtained in the equivalent chemical reduction. On the basis of these observations, a model is considered which features two isotopically dependent steps and an intermediate reservoir which forms at higher SO(3) (=) concentrations. Results of an experiment under aerobic conditions and an experiment wherein the reduction rate was thermally altered, are also presented.