Marine microbial production of dimethylsulfide from dissolved dimethylsulfoniopropionate
Ledyard, Kathleen Mei
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Dimethylsulfide (DMS) plays a central role in the transfer of sulfur from the ocean to the atmosphere, and ultimately to land. The most abundant volatile organosulfur compound in seawater, DMS is believed to account for the bulk of the sea-to-air biogenic sulfur flux. DMS has also been implicated as the major precursor of submicron-sized sulfate aerosol over the ocean. This aerosol acts as an effective site for cloud droplet condensation, suggesting a possibly important role for DMS in marine cloud formation. In the ocean, the precursor of DMS is presumed to be the zwitterionic sulfonium compound dimethylsulfoniopropionate (DMSP), a common osmoticum in certain classes of marine algae. While some algae can cleave DMSP intracellularly to form DMS, correlation of DMS concentrations with indicators of algal productivity on a local scale is poor. This thesis focuses on an alternative pathway of DMS formation: microbial cleavage of dissolved (extracellular) DMSP. In laboratory studies, bacteria able to cleave DMSP to form DMS were isolated from seawater by a DMSP enrichment technique, and the kinetics of DMSP uptake and DMS production were examined closely in pure cultures of a bacterial isolate from the Sargasso Sea. The isolate could grow with both DMSP and acrylic acid, one of the products of DMSP cleavage, as the sole source of carbon and energy, and the enzyme catalyzing DMSP cleavage appeared to be induced by both of these compounds. Kinetic parameters were estimated for DMSP uptake and cleavage by whole cells. Comparison of the 16S rRNA sequence of this isolate with that of known eubacteria showed that it was most closely related to Erythrobacter longus, an aerobic, bacteriochlorophyll-containing member of the α proteobacteria. DMS production from dissolved DMSP, along with microbial DMSP and DMS removal, was investigated in seawater incubation experiments with the goal of establishing turnover times for DMSP and DMS. These were determined to be on the order of days in both coastal (Vineyard Sound) and oligotrophic (Sargasso Sea) seawater. Loss of DMSP from the dissolved phase always occurred more rapidly than production of DMS; on average, microbial removal processes turned DMSP over on timescales of less than a day. This suggests that processes which do not result in DMS production, such as demethylation, may be important sinks for DMSP. Kinetic parameters for DMSP uptake and DMS production varied, possibly as a function of season. However, more data are needed to assess the seasonal dependence of this process. These results imply that microbial production of DMS from dissolved DMSP is likely to be a quantitatively significant mode of DMS formation in both coastal and oligotrophic marine environments. Timescales of DMS turnover with respect to this process are comparable to published estimates of DMS turnover times due to biological consumption, indicating that coupling between these two microbially-mediated processes may constitute an important control on oceanic DMS levels.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1993
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