Methionine And Dimethylsulfoniopropionate As Sources Of Sulfur To The Microbial Community Of The North Pacific Subtropical Gyre

Methionine (Met) and dimethylsulfoniopropionate (DMSP) are 2 important substrates that can serve as sources of sulfur and carbon to microbial communities in the sea. We studied the vertical and diel distributions and the assimilation rates of dissolved Met (dMet) and dissolved DMSP (dDMSP) into proteins of different microbial groups at Stn ALOHA, in the oligotrophic North Pacific Subtropical Gyre (NPSG). Concentrations of dMet never exceeded 50 pM and were at their daily minimum during the night-time (< 0.17 pM). dMet assimilation into proteins accounted for < 30% of the dMet lost from the dissolved pool, suggesting that other metabolic pathways were also important. Concentrations of dDMSP ranged from 0.35 to 1.0 nM in surface waters and did not present a distinguishable diel pattern. Cell-sorted Prochlorococcus, high nucleic acid (HNA), and low nucleic acid (LNA) non-pigmented bacteria showed a clear diel pattern for dMet and dDMSP assimilation, with higher rates during the night-time. Among the different groups, HNA bacteria had the highest per-cell assimilation rate for dMet and dDMSP, but when accounting for cell numbers in each group, the HNA and LNA bacterial group assimilation rates were comparable for both dDMSP and dMet. Integrated water column (0 to 125 m) dDMSP assimilation rates by the entire microbial assemblage were 1.7-to 5.3-fold faster than those for dMet, suggesting that dDMSP constitutes a more important source of sulfur than dMet to the microbial community of the NPSG during the time of our study.