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 a ctivities of phylogenetically characterized bacterioplankton groups with in corporation rates of [S-35] methionine tracer. According to fluorescence in situ hybridization with rRNA targeted oligonucleotide probes, a clade of a lpha -proteobacteria, related to Roseobacter spp., and a Cytophaga-Flavobac terium 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-Flavoba cterium cluster exhibited low light scatter (is). A third abundant group wi th low DNA (LNA) content contained cells from a SAR86 cluster of gamma -pro teobacteria. Cellular specific activities of the HNA-hs group were 4- and 1 .7-fold higher than the activities in the HNA-1s and LNA groups, respective ly. However, the higher cellular protein synthesis by the HNA-hs could simp ly be explained by their maintenance of a larger cellular protein biomass. Similar biomass specific activities of the different groups strongly suppor t the main assumption that underlies the determination of bacterial product ion: different bacteria in a complex community incorporate amino acids at a rate proportional to their protein synthesis. The fact that the highest gr owth-specific rates were determined for the smallest cells of the LNA group can explain the dominance of this group in nutrient-limited waters. The me tabolic activities of the three groups accounted for almost the total bacte rioplankton activity, indicating their key biogeochemical role in the plank tonic ecosystem of the Celtic Sea.