Significance of slowly-degrading dimethylsulfoniopropionate (DMSP) pools in the biogeochemical cycling of DMSP-sulfur and carbon in marine waters

Dimethylsulfoniopropionate (DMSP) is an organosulfur compound produced by many species of phytoplankton and some higher plants. DMSP and its degradation products, including dimethylsulfide (DMS), are found in a wide range of aquatic environments. Upon release to the extracellular environment, dissolved DMSP (DMSPd) is readily utilized by microorganisms as a source of carbon, sulfur and energy. The residence time of DMSPd in seawater is usually shorter than one day. However, evidence suggests the existence of two pools of DMSPd with much slower turnover times, a refractory component and a cellular pool retained in bacterial cells. The existence of these pools and some methodological issues related to measurement of DMSPd can result in the overestimation of the DMSPd turnover rates determined using conventional procedures. My dissertation research was aimed at reassessing the microbial degradation rates of DMSPd, by evaluating the impacts of the slowly-degrading DMSPd pools and methodological caveats that are largely unaccounted for in the usual rate calculations. The existence of a refractory component within what is typically measured as DMSPd was shown in oligotrophic open ocean waters, where it can represent up to 27% of the DMSPd pool. This refractory "DMSPd" was also detected in several different types of aquatic environments, confirming its widespread occurrence and importance on the global scale. A series of operational tests suggested that this refractory component is not authentic DMSP, but something else that produces DMS upon alkaline treatment (the standard technique used to measure DMSP). Investigation of another slowly-degrading pool, the microbially-retained DMSPd, showed that natural communities can take up nanomolar levels of DMSP and accumulate it at significant intracellular concentrations to use as an osmolyte. The osmoprotective function of DMSP retention by bacteria was confirmed by the increasing amount of DMSP retained as the external salinity was increased. Providing DMSPd to salt-stressed bacteria, even at concentration as low as 2.5 nM, alleviated the inhibition of bacterial production compared to DMSP-free treatments, explaining why a fraction of what we measure as DMSPd (operationally-defined as that passing through GF/F filters) is retained on 0.2 microm filters in estuarine and oceanic water samples. Still, even when accounting for the refractory and the microbially-retained pools, the DMSPd consumption rates measured using two alternative methods showed major discrepancies, especially in nearshore Gulf of Mexico waters. Potential causes for this mismatch could be the release of DMSPd from the particulate pool during rate determination (bottle effect) and the existence of other labile non-DMSP compounds that are included in the alkaline hydrolysis determination of DMSP. Finally, other uncertainties related to the estimation of bacterial production and respiration rates, as well as the use of multiple conversion factors, were identified as additional factors limiting the accurate quantification of the contribution of DMSPd turnover to bacterial C and S demands, DMS production and ultimately its importance in global biogeochemistry.