Abundance and activity of major groups of prokaryotic plankton in the coastal North Sea during spring and summer

The dynamics of the abundance and activity of selected heterotrophic prokaryotic groups were determined in the coastal North Sea during a coastal spring bloom dominated by the prymnesiophyte Phaeocystis globosa and in the subsequent spring and summer season using catalyzed reporter deposition-fluorescence in situ hybridization combined with microautoradiography (MICRO-CARD-FISH). Both Crenarchea and Euryarchaea were detected throughout the study period, albeit never at levels that exceeded 2% of the total prokaryotic abundance, corresponding to a maximum abundance of 2.8 x 10(8) Archaea l(-1). Euryarchaea were generally more abundant than Crenarchaea. On average, 21% of the Crenarchaea and 35% of the Euryarchaea were taking up leucine throughout the study period. Members of the Bacteroidetes were abundant during the P. globosa bloom period. At the senescent stage of the bloom, Bacteroidetes comprised up to 63% of the prokaryotic community. The abundances of members of the Roseobacter clade as well as the SAR86 cluster were low during the phytoplankton spring bloom period (mean 2% each), but increased during August. On average, 51% of the Roseobacter, 38% of the Bacteroidetes and 39% of the SAR86 cluster were taking up leucine. The percentages of active Bacteria increased during the decline of the P. globosa spring bloom. The Bacteroidetes cluster showed the strongest increase, indicating that members of this cluster are likely to play a major role in the degradation of organic matter produced in the P. globosa spring bloom.