Bacterial community structure associated with a dimethylsulfoniopropionate-producing North Atlantic algal bloom

The bacteria associated with oceanic algal blooms are acknowledged to play important roles in carbon, nitrogen, and sulfur cycling, yet little informa tion is available on their identities or phylogenetic affiliations. Three c ulture-independent methods were used to characterize bacteria from a dimeth ylsulfoniopropionate (DMSP)-producing algal bloom in the North Atlantic, Gr oup-specific 165 rRNA-targeted oligonucleotides, 16S ribosomal DNA (rDNA) c lone libraries, and terminal restriction fragment length polymorphism analy sis all indicated that the marine Roseobacter lineage was numerically impor tant in the heterotrophic bacterial community, averaging >20% of the 16S rD NA sampled. Two other groups of heterotrophic bacteria, the SAR86 and SAR11 clades, were also shown by the three 16S rRNA-based methods to be abundant in the bloom community. In surface waters, the Roseobacter, SAR86, and SAR 11 lineages together accounted for over 50% of the bacterial rDNA and showe d little spatial variability in abundance despite variations in the dominan t algal species. Depth profiles indicated that Roseobacter phylotype abunda nce decreased with depth and was positively correlated with chlorophyll a, DMSP, and total organic sulfur (dimethyl sulfide plus DMSP plus dimethyl su lfoxide) concentrations. Based on these data and previous physiological stu dies of cultured Roseobacter strains, we hypothesize that this lineage play s a role in cycling organic sulfur compounds produced within the bloom. Thr ee other abundant bacterial phylotypes (representing a cyanobacterium and t wo members of the or Proteobacteria) were primarily associated with chlorop hyll-rich surface waters of the bloom Ill to 50 m), while two others (repre senting Cytophagales and delta Proteobacteria) were primarily found in deep er waters (200 to 500 m).