Microbial transformations of methylated sulfur compounds in anoxic salt marsh sediments

Abstract

Anoxic salt marsh sediments were amended with several methylated sulfur compounds. Sediment microbes transformed the added compounds into other volatile methylated sulfur compounds and eventually mineralized the compounds to CH₄ and presumably to CO₂ and H₂S. The principal methyl-sulfur product of dimethylsulfoniopropionate (DMSP) was found to be dimethylsulfide (DMS), with only small amounts of methane thiol (MSH) produced. By contrast, methionine and S-methyl cysteine were degraded mostly to MSH and to lesser amounts of DMS. Dimethylsulfoxide (DMSO) was biologically converted to DMS. Dimethyldisulfide (DMDS) was rapidly reduced to MSH by the sediment microflora, and some DMS was also produced. DMS, whether added directly or when derived from other precursors, was metabolized with the production of MSH. Methane thiol was also metabolized, and evidence suggests that MSH may be biologically methylated to form DMS. Experiments with selective microbial inhibitors were used to ascertain which microbial groups were responsible for the observed transformations. Based on these experiments, it appears that both sulfate-reducing and methane-producing bacteria may be involved in transforming and mineralizing methylated sulfur compounds. A simple scheme of how methylated sulfur compounds may be transformed in the environment is presented.