Microbial dimethylsulfoniopropionate (DMSP) dynamics along a natural iron gradient in the northeast subarctic Pacific

Abstract

We characterized the effect of an inshore‐offshore gradient in Fe in the northeast subarctic Pacific on the bacterioplankton and phytoplankton assemblages and on the microbial cycling of particulate and dissolved dimethylsulfoniopropionate (DMSP p and DMSP d ) and dimethylsulfide (DMS). Averaged concentrations of total dissolved Fe (TDFe) decreased linearly with increasing water density along the transect, from 3.4 nmol L −1 at the two inshore stations to 1.0 nmol L −1 at the offshore stations, as a result of the vertical and lateral mixing between the Fe‐rich coastal water and the Fe‐poor Alaska Current. The Fe‐rich inshore stations were dominated by diatoms and characterized by low DMSP p : chlorophyll a (Chl a ) ratios (ca. 26 nmol [g −1 ) and bacterial DMS yield (< 4%). In contrast, the Fe‐poor offshore stations were dominated by prymnesiophytes and exhibited high DMSP p : Chl a ratios (ca. 84 nmol [g −1 ) and bacterial DMS yield (8%). Chl a , DMSPp, and the abundance of total bacteria and three bacterial clades ( Gammaproteobacteria , Roseobacter , and Betaproteobacteria ) were positively correlated with the TDFe gradient. At the Fe‐poor offshore stations, the positive correlation found between TDFe and the DMSPp : Chl a ratios suggests that Fe supplied by mixing stimulated DMSP production in the prymnesiophyte‐dominated assemblage, a response similar to that generally observed during the first days of most of the large‐scale ocean iron fertilizations (OIFs). These results suggest that the stimulation of DMSP production takes place whatever the Fe supply mode: atmospheric dust deposition, as simulated by OIFs, or mixing, as reported in this study.