Production of methanethiol from dimethylsulfoniopropionate in marine surface waters

Degradation of nM levels of dissolved dimethylsulfoniopropionate [DMSP(d)] in surface water samples from the Gulf of Mexico and Gulf of Maine was accompanied by the accumulation of both dimethylsulfide (DMS) and methanethiol (MeSH). The mean net yields for DMS and MeSH, in terms of sulfur from DMSP, were 32% (range 12-66%) and 22% (range 3-64%), respectively. In six out of seventeen experiments, maximum net accumulations of MeSH were equivalent to, or greater than, those obtained for DMS. No relationship between net DMS and MeSH accumulations was found when all seventeen experiments were considered, Inhibition of DMSP(d) degradation with 50 mu M glycine betaine substantially lowered production of both MeSH and DMS, indicating that degradation of DMSP was required to produce these sulfur gases. The most likely route for MeSH formation is from demethiolation of 3-methiolpropionate (MMPA), a product of DMSP demethylation. Experimental additions of MMPA confirmed that MeSH could be produced from this compound. The MeSH produced from DMSP was rapidly lost in all water samples tested, much more rapidly than DMS. Direct determinations of MeSH loss rate constants showed these to fall in the range of 0.14-1.4 h(-1) in different water samples. Filtration of water through 0.2-mu m membrane filters resulted in a 1.3-4.5-fold decrease in the whole water loss rate constants, suggesting biological or particle sinks for MeSH. Addition of Suwannee River humic acid accelerated the loss of MeSH from filtered water, suggesting a possible interaction between MeSH and DOM. The results of this study indicate that a substantial fraction of the DMSP(d) degraded in aerobic seawater is converted to MeSH. The diversion of DMSP-sulfur to MeSH represents an important biogeochemical control on the production of climatically active DMS. In addition, the production of highly reactive MeSH suggests that the degradation of DMSP may have a more important impact on the chemistry of marine surface waters than previously recognized.