Sulfur isotope fractionation during bacterial reduction and disproportionation of thiosulfate and sulfite

In bacterial cultures we measured sulfur isotope fractionation during transformations of thiosulfate (S2O32-) and sulfite (SO32-), pathways which may be of considerable importance in the cycling of sulfur in marine sediments and euxinic waters. We documented isotope fractionations during the reduction and disproportionation of S2O32- and SO32- by bacterial enrichments and pure bacterial cultures from marine and freshwater environments. We also measured the isotope fractionation associated with the anoxygenic phototrophic oxidation of H2S to S2O32- by cyanobacteria. Except for SO32- reduction, isotope fractionations for these processes have not been previously reported. During the dissimilatory reduction of SO32-, H2S was depleted in S-34 by 6 parts per thousand, and during the reduction of S2O32- to H2S, depletions were between 7 parts per thousand and 11 parts per thousand. The largest observed isotope fractionation was associated with the bacterial disproportionation of SO32- which caused a S-34 depletion in H2S of 20-37 parts per thousand and a S-34 enrichment in sulfate of 7-12 parts per thousand. During the bacterial disproportionation of S2O32-, isotope fractionations between the outer sulfane sulfur and H2S and between the inner sulfonate sulfur and SO42- were <4 parts per thousand. We observed isotope exchange between the two sulfur atoms of S2O32- leading to a depletion of S-34 in H2S by up to 12 parts per thousand with a comparable enrichment of S-34 in SO42-. No isotope fractionation was associated with the anoxygenic phototrophic oxidation of H2S to S2O32-. The depletion of S-34 into H2S during the bacterial reduction and disproportionation of S2O32- and SO32- may, in addition to sulfate reduction and the bacterial disproportionation of elemental sulfur, contribute to the generation of S-34-depleted sedimentary sulfides. Copyright (C) 1998 Elsevier Science Ltd.