Effect of visible light on dimethylsulfoniopropionate assimilation and conversion to dimethylsulfide in the North Pacific Subtropical Gyre

The aim of the present study was to assess the effect of photosynthetically active radiation (PAR, 400 to 700 nm) on the utilization of dissolved dimethylsulfoniopropionate (DMSPd) by microbial communities in the oligotrophic North Pacific Subtropical Gyre, using 35 S-labeled substrate. Rates of DMSPd-sulfur (DMSPd-S) assimilation into macromolecules of microorganisms in surface mixed layer seawater were 39 to 78% higher in samples that were incubated in the light than in dark controls. There was no photoinhibition in the light range tested (up to 1200 µmol pho- tons m �2 s �1 ). Leucine assimilation, an index of bacterial protein production, was also photostimu- lated and was significantly correlated (r = 0.93) to DMSPd-S assimilation, suggesting that cells can respond to an increase in S requirements for protein synthesis by assimilating DMSPd-S. Light- driven changes in DMSPd-S assimilation were inversely and significantly correlated (r = �0.57) to changes in the corresponding dimethylsulfide (DMS) yield (i.e. DMS produced per DMSP con- sumed), in support of the hypothesis that DMS is produced only after cellular sulfur requirements are met. While light-driven changes in DMSPd-S assimilation appear to affect DMS yield, more than half of the DMSPd product pool remained unidentified; thus, in addition to assimilation, there may be other metabolic processes or products that can regulate DMS production.