Role of methanogens and other bacteria in degradation of dimethyl sulfide and methanethiol in anoxic freshwater sediments

The roles of several trophic groups of organisms (methanogens and sulfate- and nitrate-reducing bacteria) in the microbial degradation of methanethiol (MT) and dimethyl sulfide (DMS) were studied in freshwater sediments. The incubation of DMS- and MT-amended slurries revealed that methanogens are th e dominant DMS and MT utilizers in sulfate-poor freshwater systems. In sedi ment slurries, which were depleted of sulfate, 75 mu mol of DMS was stoichi ometrically converted into 112 mu mol of methane. The addition of methanol or MT to DMS-degrading slurries at concentrations similar to that of DMS re duced DMS degradation rates. This indicates that the methanogens in freshwa ter sediments, which degrade DMS, are also consumers of methanol and MT. To verify whether a competition between sulfate-reducing and methanogenic bac teria for DMS or MT takes place in sulfate-rich freshwater systems, the eff ects of sulfate and inhibitors, like bromoethanesulfonic acid, molybdate, a nd tungstate, on the degradation of MT and DMS were studied. The results fo r these sulfate-rich and sulfate-amended slurry incubations clearly demonst rated that besides methanogens, sulfate-reducing bacteria take part in MT a nd DMS degradation in freshwater sediments, provided that sulfate is availa ble. The possible involvement of an interspecies hydrogen transfer in these processes is discussed. In general, our study provides evidence for methan ogenesis as a major sink for MT and DMS in freshwater sediments.