Sunlight effects on the DMSP-sulfur and leucine assimilation activities of polar heterotrophic bacterioplankton

The influence of solar ultraviolet radiation and photosynthetically active radiation (PAR) on summertime marine bacterial uptake and assimilation of sulfur from radiolabeled dimethlysulfoniopropionate (35S-DMSP) was studied at four Arctic and two Antarctic stations. Incubations with 3H-leucine were also conducted for comparative purposes as a measurement of bacterial activity. Arctic waters were characterized by large numbers of colonial Phaeocystis pouchetii and higher DMSP concentrations than in the two diatom-dominated Antarctic samples. Exposure to full sunlight radiation (280–700 nm), and to a lesser extent to PAR + UVA (320–700 nm), generally decreased the bacterial assimilation of 3H-leucine with respect to darkness, and caused variable effects on 35S-DMSP assimilation. By using a single-cell approach involving microautoradiography we found high percentages of sulfur assimilating cells within the bacterial groups Gammaproteobacteria, Bacteroidetes, SAR11 and Roseobacter despite the varying DMSP concentrations between Arctic and Antarctic samples. The dominant SAR11 clade contributed 50–70% of the cells assimilating both substrates in the Arctic stations, whereas either Gammaproteobacteria or SAR11 were the largest contributors to 3H-leucine uptake in samples from the two Antarctic stations. Only one station was analyzed for single-cell 35S-DMSP assimilation in Antarctica, and Gammaproteobacteria were major contributors to its uptake, providing the first evidence for Antarctic bacteria actively taking up 35S-DMSP. PAR + UVA repeatedly increased the number of SAR11 cells assimilating 3H-leucine. This pattern also occurred with other 35S-DMSP assimilating groups, though not so consistently. Our results support a widespread capability of polar bacteria to assimilate DMSP-sulfur during the season of maximum DMSP concentrations, and show for the first time that all major polar taxa can be highly active at this assimilation under the appropriate circumstances. Our findings further confirm the role of sunlight as a modulator of heterotrophic carbon and sulfur fluxes in the surface ocean.