Abstract:
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 34S by 6%%, and during the reduction of S2O32- to H2S, depletions were between 7%% and 11%%. The largest observed isotope fractionation was associated with the bacterial disproportionation of SO32- which caused a 34S depletion in H2S of 20-37%% and a 34S enrichment in sulfate of 7-12%%. 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%%. We observed isotope exchange between the two sulfur atoms of S2O32- leading to a depletion of34S in H2S by up to 12%% with a comparable enrichment of 34S in SO42-. No isotope fractionation was associated with the anoxygenic phototrophic oxidation of H2S to S2O32-. The depletion of 34S 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 34S-depleted sedimentary sulfides.