Isolation and characterization of Methanomethylovorans hollandica gen. nov., sp nov., isolated from freshwater sediment, a methylotrophic methanogen able to grow on dimethyl sulfide and methanethiol

A newly isolated methanogen, strain DMS1(T), is the first obligately anaero bic archaeon which was directly enriched and isolated from a freshwater sed iment in defined minimal medium containing dimethyl sulfide (DMS) as the so le carbon and energy source. The use of a chemostat with a continuous DMS-c ontaining gas stream as a method of enrichment, followed by cultivation in deep agar tubes, resulted in a pure culture. Since the only substrates util ized by strain DMS1(T) are methanol, methylamines, methanethiol (MT), and D MS, this organism is considered an obligately methylotrophic methanogen lik e most other DMS-degrading methanogens. Strain DMS1(T) differs from all oth er DMS-degrading methanogens, since it was isolated from a freshwater pond and requires NaCl concentrations (0 to 0.04 M) typical of the NaCl concentr ations required by freshwater microorganisms for growth. DMS was degraded e ffectively only in a chemostat culture in the presence of low hydrogen sulf ide and MT concentrations. Addition of MT or sulfide to the chemostat signi ficantly decreased degradation of DMS. Transient accumulation of DMS in MT- amended cultures indicated that transfer of the first methyl group during D MS degradation is a reversible process. On the basis of its low level of ho mology with the most closely related methanogen, Methanococcoides burtonii (94.5%), its position on the phylogenetic tree, its morphology (which is di fferent from that of members of the genera Methanolobus, Methanococcoides, and Methanohalophilus), and its salt tolerance and optimum (which are chara cteristic of freshwater bacteria), we propose that strain DMS1(T) is a repr esentative of a novel genus. This isolate was named Methanomethylovorans ho llandica. Analysis of DMS-amended sediment slurries with a fluorescence mic roscope revealed the presence of methanogens which were morphologically ide ntical to M. hollandica, as described in this study. Considering its physio logical properties, M. hollandica DMS1(T) is probably responsible for degra dation of MT and DMS in freshwater sediments in situ. Due to the reversibil ity of the DMS conversion, methanogens like strain DMS1(T) can also be invo lved in the formation of DMS through methylation of MT. This phenomenon, wh ich previously has been shown to occur in sediment slurries of freshwater o rigin, might affect the steady-state concentrations and, consequently, the total flux of DMS and MT in these systems.