Dimethylsulfoniopropionate in anoxic intertidal sediments: A precursor of methanogenesis via dimethyl sulfide, methanethiol, and methiolpropionate

The most important volatile sulfur compound in the marine environment is dimethyl sulfide (DMS), which mainly originates from the algal osmolyte dimethylsulfoniopropionate (DMSP). Conversion of DMSP in anoxic marine sediments involves a cleavage to acrylate and DMS, which can further be converted by methanogenic Archaea and possibly by sulfate-reducing bacteria, or a demethylation to 3-methiolpropionate (MMPA); MMPA can be demethylated to 3-mercaptopropionate or demethiolated to methanethiol and presumably acrylate. Little is known about the types of anaerobic microorganisms that are involved in the demethylation pathway. We found that the marine Desulfobacterium strain PM4 converts DMSP to MMPA and that three marine Methanosarcina strains demethylate MMPA to S-mercaptopropionate and methane. Methanogenic conversion of MMPA in anoxic sediment slurries was only found when antibiotics that inhibit Bacteria but not Archaea were added. Under non-inhibited conditions MMPA was rapidly converted to methanethiol and subsequently to methane. The numbers of MMPA-metabolizing methanogens were significantly lower than those of DMS-, trimethylamine-, or methanol-metabolizing methanogens. It is concluded that in anoxic intertidal sediments DMSP is not only a source of DMS with its well-known counteractive effect on global warming, but also of the potent greenhouse gas methane.