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

Abstract

In bacterial cultures we measured sulfur isotope fractionation during transformations of thiosulfate (S 2O 3 2−) and sulfite (SO 3 2−), 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 S 2O 3 2− and SO 3 2− 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 H 2S to S 2O 3 2− by cyanobacteria. Except for SO 3 2− reduction, isotope fractionations for these processes have not been previously reported. During the dissimilatory reduction of SO 3 2−, H 2S was depleted in 34S by 6‰, and during the reduction of S 2O 3 2− to H 2S, depletions were between 7‰ and 11‰. The largest observed isotope fractionation was associated with the bacterial disproportionation of SO 3 2− which caused a 34S depletion in H 2S of 20–37‰ and a 34S enrichment in sulfate of 7–12‰. During the bacterial disproportionation of S 2O 3 2−, isotope fractionations between the outer sulfane sulfur and H 2S and between the inner sulfonate sulfur and SO 4 2− were <4‰. We observed isotope exchange between the two sulfur atoms of S 2O 3 2− leading to a depletion of 34S in H 2S by up to 12‰ with a comparable enrichment of 34S in SO 4 2−. No isotope fractionation was associated with the anoxygenic phototrophic oxidation of H 2S to S 2O 3 2−. The depletion of 34S into H 2S during the bacterial reduction and disproportionation of S 2O 3 2− and SO 3 2− may, in addition to sulfate reduction and the bacterial disproportionation of elemental sulfur, contribute to the generation of 34S-depleted sedimentary sulfides.