Sulfur isotope fractionation during bacterial sulfate reduction in organic-rich sediments

Isotope fractionation during sulfate reduction by natural populations of sulfate-reducing bacteria was investigated in the cyanobacterial microbial mats of Solar Lake, Sinai and the sediments of Logten Lagoon sulfuretum, Denmark. Fractionation was measured at different sediment depths, sulfate concentrations, and incubation temperatures. Rates of sulfate reduction varied between 0.1 and 37 mu mol cm(-3)d(-1), with the highest rates among the highest ever reported from natural sediments. The depletion of S-34 during dissimilatory sulfate reduction ranged from 16 parts per thousand to 42 parts per thousand, with the largest S-34-depletions associated with the lowest rates of sulfate reduction and the lowest S-34-depletions with the highest rates. However, at high sulfate reduction rates (>10 mu mol cm(-3)d(-1)) the lowest fractionation was 20 parts per thousand independent of the rates. Overall, there was a similarity between the fractionation obtained by the natural populations of sulfate reducers and previous measurements from pure cultures. This was somewhat surprising given the extremely high rates of sulfate reduction in the experiments. Our results are explained if we conclude that the fractionation was mainly controlled by the specific rate of sulfate reduction (mass cell(-1) time(-1)) and not by the absolute rate (mass volume(-1) time(-1)). Sedimentary sulfides (mainly FeS2) were on average 40 parts per thousand depleted in S-34 compared to seawater sulfate. This amount of depletion was more than could be explained by the isotopic fractionations that we measured during bacterial sulfate reduction. Therefore, additional processes contributing to the fractionation of sulfur isotopes in the sediments are indicated. From both Solar Lake and Logten Lagoon we were able to enrich cultures of elemental sulfur-disproportionating bacteria. We suggest that isotope fractionation accompanying elemental sulfur disproportionation contributes to the S-34 depletion of sedimentary sulfides at our study sites. Copyright (C) 1997 Elsevier Science Ltd.