Annual DMSP contribution to S and C fluxes through phytoplankton and bacterioplankton in a NW Mediterranean coastal site

Abstract

The contribution of dimethylsulfoniopropionate (DMSP) to the fluxes of carbon and sulfur through phytoplankton and bacterioplankton was investigated throughout an annual cycle in the Blanes Bay Microbial Observatory (coastal NW Mediterranean). DMSP accounted for 0.3-7% of biovolume-estimated phytoplankton carbon and 4-93% of calculated phytoplankton sulfur, with higher contributions in 'summer' (highly irradiated, oligotrophic waters, May-September) and lower in 'winter' (October-April). DMSP biosynthesis rates accounted for 0.8-7% of carbon fixation and 11-88% of sulfur assimilation through primary production, with slightly higher shares in summer. Upon release from the algal cells, DMSP supplied 0.5-6% of the total carbon demand of heterotrophic bacteria, and 3->100% of the sulfur demand over the year. Uncertainties associated with these calculations are due to a scarce knowledge of C:S ratios in marine bacteria. Bacterial DMSP-sulfur assimilation (measured with 35S-DMSP) was positively correlated with bacterial heterotrophic production rates (measured with 3H-leucine). In summer waters, characterized by higher ratios of particulate DMSP to chlorophyll a (DMSPp:Chl a), DMSP-sulfur assimilation by bacteria was higher and contributed a larger share of the bacterial sulfur demand. We propose the DMSP:Chl a ratio as a good indicator of the relative role of DMSP in the carbon and sulfur fluxes through the first levels of the planktonic food-web