Bacterioplankton community composition in nearshore waters of the NW Black Sea during consecutive diatom and coccolithophorid blooms

In this study we analyzed the dynamics of bacterioplankton community composition during coastal Black Sea phytoplankton blooms using a combination of whole-cell fluorescence in situ hybridization with rRNA-targeted oligonucleotide probes and catalyzed reporter deposition (CARD-FISH). Bacterial and algal assemblages were sampled in eutrophic shelf surface waters in Constanta Bay between May and August 2004. While diatoms dominated the spring phytoplankton bloom, the late summer bloom (16-August) involved mainly coccolithophorids and dinoflagellates. The coccolithophorid Emiliania huxleyi was the numerically abundant phytoplankton species in August (∼1.1 × 10⁶ cells/L). The composition of bacterioplanktonic communities was dominated by members of alpha-Proteobacteria (mainly the Roseobacter clade) and gamma-Proteobacteria, which together accounted for up to 31% of total prokaryotic abundance during the summer phytoplankton bloom. Our results suggest that members of gamma-Proteobacteria and the Roseobacter clade are associated with Emiliania huxleyi blooms in the Black Sea.