Formation of Methane and Carbon Dioxide from Dimethylselenide in Anoxic Sediments and by a Methanogenic Bacterium

Anaerobic San Francisco Bay salt marsh sediments rapidly metabolized ¹⁴C]dimethylselenide (DMSe) to ¹⁴CH₄ and ¹⁴CO₂. Addition of selective inhibitors (2-bromoethanesulfonic acid or molybdate) to these sediments indicated that both methanogenic and sulfate-respiring bacteria could degrade DMSe to gaseous products. However, sediments taken from the selenium-contaminated Kesterson Wildlife Refuge produced only ¹⁴CO₂ from ¹⁴C]DMSe, implying that methanogens were not important in the Kesterson samples. A pure culture of a dimethylsulfide (DMS)-grown methylotrophic methanogen converted ¹⁴C]DMSe to ¹⁴CH₄ and ¹⁴CO₂. However, the organism could not grow on DMSe. Addition of DMS to either sediments or the pure culture retarded the metabolism of DMSe. This effect appeared to be caused by competitive inhibition, thereby indicating a common enzyme system for DMS and DMSe metabolism. DMSe appears to be degraded as part of the DMS pool present in anoxic environments. These results suggest that methylotrophic methanogens may demethylate methylated forms of other metals and metalloids found in nature.