Comparison of Cellular and Biomass Specific Activities of Dominant Bacterioplankton Groups in Stratified Waters of the Celtic Sea

A flow-sorting technique was developed to determine unperturbed metabolic activities of phylogenetically characterized bacterioplankton groups with incorporation rates of (35)Smethionine tracer. According to fluorescence in situ hybridization with rRNA targeted oligonucleotide probes, a clade of alpha-proteobacteria, related to Roseobacter spp., and a Cytophaga-Flavobacterium cluster dominated the different groups. Cytometric characterization revealed both these groups to have high DNA (HNA) content, while the alpha-proteobacteria exhibited high light scatter (hs) and the Cytophaga-Flavobacterium cluster exhibited low light scatter (ls). A third abundant group with low DNA (LNA) content contained cells from a SAR86 cluster of gamma-proteobacteria. Cellular specific activities of the HNA-hs group were 4- and 1.7-fold higher than the activities in the HNA-ls and LNA groups, respectively. However, the higher cellular protein synthesis by the HNA-hs could simply be explained by their maintenance of a larger cellular protein biomass. Similar biomass specific activities of the different groups strongly support the main assumption that underlies the determination of bacterial production: different bacteria in a complex community incorporate amino acids at a rate proportional to their protein synthesis. The fact that the highest growth-specific rates were determined for the smallest cells of the LNA group can explain the dominance of this group in nutrient-limited waters. The metabolic activities of the three groups accounted for almost the total bacterioplankton activity, indicating their key biogeochemical role in the planktonic ecosystem of the Celtic Sea.