Microbial populations involved in cycling of dimethyl sulfide and methanethiol in freshwater sediments

Although several microorganisms that produce and degrade methanethiol (MT) and dimethyl sulfide (DMS) have been isolated from various habitats, little is known about the numbers of these microorganisms in situ. This study rep orts on the identification and quantification of microorganisms involved in the cycling of MT and DMS in freshwater sediments. Sediment incubation stu dies revealed that the formation of MT and DMS is well balanced with their degradation. MT formation depends on the concentrations of both sulfide and methyl group-donating compounds. A most-probable number (MPN) dilution ser ies with syringate as the growth substrate showed that methylation of sulfi de with methyl groups derived from syringate is a commonly occurring proces s in situ. MT appeared to be primarily degraded by obligately methylotrophi c methanogens, which were found in the highest positive dilutions on DMS an d mixed substrates (methanol, trimethylamine [TMA], and DMS). Amplified rib osomal DNA restriction analysis (ARDRA) and 16S rRNA gene sequence analysis of the total DNA isolated from the sediments and of the DNA isolated from the highest positive dilutions of the MPN series (mixed substrates) reveale d that the methanogens that are responsible for the degradation of MT, DMS, methanol, and TMA in situ are all phylogenetically closely related to Meth ano-methylovarans hollandica. This was confirmed by sequence analysis of th e product obtained from a nested PCR developed for the selective amplificat ion of the 16S rRNA gene from M. hollandica. The data from sediment incubat ion experiments, MPN series, and molecular-genetics detection correlated we ll and provide convincing evidence for the suggested mechanisms for MT and DMS cycling and the common presence of the DMS-degrading methanogen M. holl andica in freshwater sediments.