Sulfur isotope fractionation in marine sediments and bacterial cultures.

In this study, the sulfur fractionation during bacterial transformation of SO4(2-), SO3(2-), S2O3(2-) and S0 has been quantified to provide a better understanding 0f the stable isotope data available from modern and ancient sediments. For the first time, a detailed study of the sulfur fractionation during sulfate reduction by natural populations of sulfate-reducing bacteria is reported. The isotope fractionation by the natural populations was inversely related with the sulfate reduction rate (rate per volume sediment) and showed similar trends as previously measured by pure culture, where the fractionation was related to the specific rate of sulfate reduetion (rate per cell). The isotope fractionation in sediments likely depends on the specific rate of sulfate reduction and not the absolute rate. While absolute rates of sulfate reduetion could vary over a larger range, a large variation of the specific rate of sulfate reduction might not occur in sediments as the bacteria population size should adjust to substrate availability. Similar fraetionation values as measured in this study can therefore be expected during sulfate reduetion in any kind of sediment.