Biogeochemical fates of methanethiol in seawater

The volatile gas methanethiol (CH3SH; MeSH; methyl mercaptan), the simplest organic sulfur compound, is present in the surface ocean at picomolar to low nanomolar concentrations. MeSH in oxic seawater is thought to derive mainly from the degradation of the phytoplankton metabolite dimethylsulfoniopropionate (CH3S+CH2CH2COO-; DMSP), although other precursors (e.g. methionine) are potentially significant. Earlier work suggested that MeSH was very reactive in seawater, yet very little was known about its turnover and biogeochemical fates. My project focused on characterizing biotic and abiotic fates of MeSH with an emphasis on characterization of dissolved non-volatile sulfur products (DNVS) formed from MeSH in coastal and open ocean seawater. To carry out these studies I used a combination of gas chromatography and the highly sensitive radiotracer 35S-MeSH.; MeSH was found to have high loss rate constants in whole (unfiltered) seawater, ranging from 0.48 to 1.1 h-1 in coastal waters, but a somewhat lower range in the open ocean (0.08 to 0.72 h-1). Biological fates of MeSH consisted of bacterial incorporation into protein methionine and biological oxidation to sulfate (SO42). Volatile MeSH was rapidly converted into DNVS in seawater by biological and abiological processes. The abiotically-produced DNVS consisted of the intact methanethiol molecule associated with metal cations and/or high molecular weight (>1000 Da) metal-dissolved organic matter (DOM) complexes. The formation of these metal-thiol complexes is the largest single factor controlling the abiotic loss rate of volatile MeSH in seawater. Experimental evidence suggested that much of the MeSH-derived DNVS formed as a result of reversible reactions. The rest existed in the form of partially oxidized sulfur consistent with methylthiol-containing oxidation products. Such abiotic reactions limited the bioavailability of MeSH because only the free, volatile MeSH was available for biological uptake, whereas MeSH-derived DNVS was no longer bioavailable.