S-Isotope Fractionation Behavior of Desulfobacter Latus Under Sulfide Stress Conditions

We investigate whether the fractionation of sulfur isotopes during microbial sulfate reduction is affected by cell external sulfide concentrations (Brunner and Bernasconi~2005). We report time series data of δ34S ratios of dissolved sulfate measured in two sets of batch culture experiments with the marine sulfate reducer Desulfobacter latus. The initial sulfide conditions for the 1st set were 0.3~mM reaching up to ~~21~mM at the end whereas the 2nd set grew from 22.2~mM to ~~40~mM sulfide. We obtain enrichment factors (ɛ) of 12.9±0.1~‰ and 13.5±0.3~‰ for the first and second experiment respectively, assuming a Rayleigh-type fractionation process. However, the residuals of the regression analysis for the 2nd experiment reveal a curved trend which suggests that the data are not linearly correlated whereas the 1st set of data has a linear trend. To evaluate our experimental results, we develop a single box model that allows forward and backward fluxes between the microbial cell and respective cell external sulfate and sulfide pools (Brunner and Bernasconi~2005). By introducing a backward flux for sulfide, we are now able to minimize the residuals between modelled curve and experimental data significantly, which results in a better fit of the measured values in the 2nd experiment. Assuming that the backward flux increases with a fixed ratio based on the total sulfide concentration outside the cell, we obtain enrichment factors of ~~12.7~‰ for the 1st and ~~16.3~‰ for the 2nd experiment. Our data suggest that Rayleigh-derived enrichment factors might be underestimated in cases where sulfide re-flux is present but not identified. Brunner, B. and S. M. Bernasconi (2005). A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria. Geochim. Cosmochim. Acta 69(20): 4759-4771