Aerobic methylation of hydrogen sulfide to dimethylsulfide in diverse microorganisms and environments

Abstract

Dimethylsulfide (DMS) is the major biosulfur source emitted to the atmosphere with key roles in global sulfur cycling and potentially climate regulation. The main precursor of DMS is thought to be dimethylsulfoniopropionate. However, hydrogen sulfide (H₂S), a widely distributed and abundant volatile in natural environments, can be methylated to DMS. The microorganisms and the enzymes that convert H₂S to DMS, and their importance in global sulfur cycling were unknown. Here we demonstrate that the bacterial MddA enzyme, previously known as a methanethiol S-methyltransferase, could methylate inorganic H₂S to DMS. We determine key residues involved in MddA catalysis and propose the mechanism for H₂S S-methylation. These results enabled subsequent identification of functional MddA enzymes in abundant haloarchaea and a diverse range of algae, thus expanding the significance of MddA mediated H₂S methylation to other domains of life. Furthermore, we provide evidence for H₂S S-methylation being a detoxification strategy in microorganisms. The mddA gene was abundant in diverse environments including marine sediments, lake sediments, hydrothermal vents and soils. Thus, the significance of MddA-driven methylation of inorganic H₂S to global DMS production and sulfur cycling has likely been considerably underestimated.