Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process.

Dimethylsulfoniopropionate (DMSP) is one of the Earth's most abundant organosulfur molecules, a signalling molecule ¹ , a key nutrient for marine microorganisms ^2,3 and the major precursor for gaseous dimethyl sulfide (DMS). DMS, another infochemical in signalling pathways ⁴ , is important in global sulfur cycling ² and affects the Earth's albedo, and potentially climate, via sulfate aerosol and cloud condensation nuclei production ^5,6 . It was thought that only eukaryotes produce significant amounts of DMSP ^7-9 , but here we demonstrate that many marine heterotrophic bacteria also produce DMSP, probably using the same methionine (Met) transamination pathway as macroalgae and phytoplankton ¹⁰ . We identify the first DMSP synthesis gene in any organism, dsyB, which encodes the key methyltransferase enzyme of this pathway and is a reliable reporter for bacterial DMSP synthesis in marine Alphaproteobacteria. DMSP production and dsyB transcription are upregulated by increased salinity, nitrogen limitation and lower temperatures in our model DMSP-producing bacterium Labrenzia aggregata LZB033. With significant numbers of dsyB homologues in marine metagenomes, we propose that bacteria probably make a significant contribution to oceanic DMSP production. Furthermore, because DMSP production is not solely associated with obligate phototrophs, the process need not be confined to the photic zones of marine environments and, as such, may have been underestimated.