Dynamics of dimethyl sulfide and dimethylsulfoniopropionate in oceanic water samples

Abstract

The concentrations of dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP) were measured in incubated surface water samples collected from the Eastern Pacific Ocean and the Southern Sargasso Sea. During 12–48 h incubations, time courses of DMS concentrations in the water samples displayed a variety of patterns, including little net change, immediate decreases, and accumulation followed by removal. DMSP pools including particulate (DMSP part) and dissolved (DMSP diss) fractions declined significantly during 24 h dark incubations, and changes in DMSP pools could have affected DMS concentrations. Additions of DMSP diss in the 10–30 nM range to water samples produced an immediate stimulation of DMS production. The degradation of DMSP diss was partially inhibited by antibiotics, but was unaffected by chloroform (500 μM). DMS metabolism, on the other hand, was completely inhibited by chloroform. Depth profiles of DMS consumption rates for two stations in the Eastern Pacific revealed that rates were higher in the surface mixed layer than at 100 m depth. The rates of DMSP diss and DMS degradation in experiments were directly related to the concentrations of these compounds in the water samples. However, in Sargasso Sea water, apparent saturation of these respective consumption activities was observed above 20 nM for DMSP diss and 11 nM for DMS. During incubations, the DMSP lost from water samples was much greater than the DMS accumulated in these same samples. Concurrent removal of DMS by biological or photochemical mechanisms cannot account for the lower DMS accumulations; therefore, an alternative pathway for DMSP metabolism involving demethylation is suggested. This alternative pathway may be a major fate of DMSP in the ocean, thereby limiting the production of DMS. The results of this study indicate a dynamic biological cycling of DMS in ocean waters, with significant rates of simultaneous production and consumption. These biological processes are likely to play a key role in controlling the concentrations of DMS in seawater.