Osmoprotective role of dimethylsulfoniopropionate (DMSP) for estuarine bacterioplankton

Abstract

Dimethylsulfoniopropionate (DMSP) is synthesized and used by marine phytoplankton as an osmolyte. Previous studies have shown that some of the dissolved DMSP (DMSPd) in seawater is taken up by bacterioplankton and not degraded. We tested the hypothesis that retention of DMSP provides some benefits to marine bacteria. In experiments with coastal seawater filtrates containing mainly bacteria, acute osmotic stresses of +5 and +10 ppt NaCl significantly inhibited bacterial production (BP) over 6 h, while the availability of 20 nM DMSPd relieved most of the BP inhibition. Partial relief of salt-induced inhibition of BP was observed with DMSPd concentrations as low as 2.5 nM, and DMSP was more effective at relieving osmotic stress than other compounds tested. Osmotic stresses resulted in a faster and greater overall uptake of DMSPd and accumulation of untransformed DMSP in bacterial cells (DMSPcell). Retained DMSP reached osmotically-significant intracellular concentrations of 54 mM in salt-stressed bacterial populations. Retention of DMSP was accompanied by a lower production of methanethiol (MeSH), suggesting a down-regulation of the demethylation/demethiolation pathway under osmotic stress. These results show that estuarine bacterioplankton can use DMSP as an osmoprotectant, retaining up to 54% of the available dissolved DMSP un-transformed in their cells. This benefit provided by DMSP may help explain why some DMSP is retained in bacteria in the ocean, even under unchanging salinity. This retention slows down the cycling of DMSP, with potential implications for the trophic transfer of DMSP and its contributions to sulfur and carbon fluxes in the ocean.