Dimethylsulfoniopropionate (DMSP) and its catabolite dimethyl sulfide (DMS) are key marine nutrients 1,2 , with roles in global sulfur cycling 2 , atmospheric chemistry 3 , signalling 4,5 and, potentially, climate regulation 6,7. DMSP production was previously thought to be an oxic and photic process, mainly confined to the surface oceans. 2 However, here we show that DMSP concentrations and DMSP/DMS synthesis rates were higher in surface marine sediment from e.g., saltmarsh ponds, estuaries and the deep ocean than in the overlying seawater. A quarter of bacterial strains isolated from saltmarsh sediment produced DMSP (up to 73 mM), and previously unknown DMSPproducers were identified. Most DMSP-producing isolates contained dsyB 8 , but some alphaproteobacteria, gammaproteobacteria and actinobacteria utilised a methionine methylation pathway independent of DsyB, previously only associated with higher plants. These bacteria contained a methionine methyltransferase 'mmtN' gene-a marker for bacterial DMSP synthesis via this pathway. DMSP-producing bacteria and their dsyB and/or mmtN transcripts were present in all tested seawater samples and Tara Oceans bacterioplankton datasets, but were far more abundant in marine surface sediment. Approximately 10 8 bacteria per gram of surface marine sediment are predicted to produce DMSP, and their contribution to this process should be included in future models of global DMSP production. We propose that coastal and marine sediments, which cover a large part of the Earth's surface, are environments with high DMSP and DMS productivity, and that bacteria are important producers within them. Approximately eight billion tonnes of DMSP is produced by phytoplankton in the Earth's surface oceans annually 9. However, surface sediment from saltmarsh ponds, an estuary and the deep ocean (with high pressures and no light) contained DMSP levels (5-128 nmol DMSP g-1) that were up to ~three orders of magnitude higher than the overlying seawater (0.01-0.70 nmol DMSP ml-1) (Fig. 1a-b, Supplementary Tables 1a and 2), a phenomenon also observed in 10,11. DMSP concentration decreased with depth, being much lower in anoxic sediment, but even in deeper sediments the concentration was approximately an order of magnitude higher than in the overlying seawater (Supplementary Table 1a). This study focused on DMSP synthesis in coastal surface sediments, where DMSP concentrations were highest. The

Ruegeria pomeroyi DSS-3 is a model Roseobacter marine bacterium, particularly regarding its catabolism of dimethylsulfoniopropionate (DMSP), an abundant anti-stress molecule made by marine phytoplankton. We found a novel gene, dddW, which encodes a DMSP lyase that cleaves DMSP into acrylate plus the environmentally important volatile dimethyl sulfide (DMS). Mutations in dddW reduced, but did not abolish DMS production. Transcription of dddW was greatly enhanced by pre-growth of cells with DMSP, via a LysR-type regulator. Close DddW homologs occur in only one other Roseobacter species, and there are no close homologs and only a few related sequences in metagenomes of marine bacteria. In addition to DddW, R. pomeroyi DSS-3 had been shown to have two other, different, DMSP lyases, DddP and DddQ, plus an enzyme that demethylates DMSP, emphasizing the importance of this substrate for this model bacterium.

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