The oxidation of sulfide, generated by bacterial sulfate reduction, is a key process in the biogeochemistry of marine sediments, yet the pathways and oxidants are poorly known. By the use of 35S-tracer studies of the S cycle in marine and freshwater sediments, a novel shunt function of thiosulfate (S2O32-) was identified. The S 2O32- constituted 68 to 78 percent of the immediate HS-oxidation products and was concurrently (i) reduced back to HS -, (ii) oxidized to SO42-, and (iii) disproportionated to HS-+SO42-. The small thiosulfate pool is thus involved in a dynamic HS-. S 2O32- cycle in anoxic sediments. The disproportionation of thiosulfate may help account for the large difference in isotopic composition (34S/32S) of sulfate and sulfides in sediments and sedimentary rocks.
