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 Main e was accompanied by the accumulation of both dimethylsulfide (DMS) an d 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%), re spectively. In six out of seventeen experiments, maximum net accumulat ions of MeSH were equivalent to, or greater than, those obtained for D MS. No relationship between net DMS and MeSH accumulations was found w hen all seventeen experiments were considered, Inhibition of DMSP(d) d egradation with 50 mu M glycine betaine substantially lowered producti on of both MeSH and DMS, indicating that degradation of DMSP was requi red to produce these sulfur gases. The most likely route for MeSH form ation is from demethiolation of 3-methiolpropionate (MMPA), a product of DMSP demethylation. Experimental additions of MMPA confirmed that M eSH could be produced from this compound. The MeSH produced from DMSP was rapidly lost in all water samples tested, much more rapidly than D MS. 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. Filtra tion 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 biolo gical or particle sinks for MeSH. Addition of Suwannee River humic aci d accelerated the loss of MeSH from filtered water, suggesting a possi ble interaction between MeSH and DOM. The results of this study indica te that a substantial fraction of the DMSP(d) degraded in aerobic seaw ater is converted to MeSH. The diversion of DMSP-sulfur to MeSH repres ents an important biogeochemical control on the production of climatic ally active DMS. In addition, the production of highly reactive MeSH s uggests that the degradation of DMSP may have a more important impact on the chemistry of marine surface waters than previously recognized.