DMSP-Controlled Diversity and Function of Antarctic Marine Microbial Communities

Antarctic phytoplankton produce large amounts of the compatible solute dimethylsulfoniopropionate (DMSP). This organic molecule provides a source of carbon, sulfur and energy to marine bacteria. Just as dissolved DMSP may influence the structure and composition of microbial assemblages, it is probable that the composition of the microorganisms present at a given time or location will affect the magnitude (and direction) of DMSP cycling. The extent to which microbes influence DMSP cycling depends on their genomic potential, the commonality of demethylation genes, and the depth of their diversity. We show that DMSP can structure the composition and diversity of the Antarctic bacterial community as a function of DMSP concentration, growth rate and seasonality. DMSP was supplied experimentally at controlled, constant rates to microbial communities collected off Palmer Station, Antarctica during the austral summer of 2016-17 and fall 2018. Bacterial diversity and composition changed in experimental treatments relative to batch and ambient controls in summer and late fall, over weekly time-scales. The summer and fall Antarctic bacterial assemblages responded very quickly to the addition of DMSP, though with different levels of activity, biomass and diversity. The productivity of the heterotrophic bacterial community increased over time, except in late fall, and quickly reduced the amount of dissolved DMSP. During both seasons, the DMSP genes detected were most similar to those that have been revealed from other polar locations, but not identical to the DMSP genes from any cultivated marine microbes. The expression of DMSP degradation genes by marine bacteria in response to additions of DMSP and seasonality is being quantified. Joining these two approaches, we discuss whether DMSP cycling can thus act as a model to quantify bacterial-protistan networks and their associated interactions.