An S-methyltransferase that produces the climate-active gas dimethylsulfide is widespread across diverse marine bacteria.

Hydrogen sulfide (H 2 S), methanethiol (MeSH) and dimethylsulfide (DMS) are abundant sulfur gases with roles in biogeochemical cycling, chemotaxis and/or climate regulation. Catabolism of the marine osmolyte dimethylsulfoniopropionate (DMSP) is a major source of DMS and MeSH, but both also result from S-methylation of H 2 S via MddA, an H 2 S and MeSH S-methyltransferase whose gene is abundant in soil but scarce in marine environments. Here we identify the S-adenosine methionine (SAM)-dependent MeSH and H 2 S S-methyltransferase 'MddH', which is widespread in diverse marine bacteria and some freshwater and soil bacteria. mddH is predicted in up to ~5% and ~15% of seawater and coastal sediment bacteria, respectively, which is considerably higher than mddA. Furthermore, marine mddH transcript levels are similar to those for the most abundant DMSP lyase gene dddP. This study implies that the importance of H 2 S and MeSH S-methylation pathways in marine environments is significantly underestimated.