Similar community structure of biosynthetically active prokaryotes across a range of ecosystem trophic states

Variability in both the abundance and phylogenetic diversity of biosynthetically active prokaryotes has implications for global carbon cycling. In the present study, our primary goal was to determine the extent of variability in phylogenetic diversity of biosynthetically active prokaryotes from 3 regions in the California Current System off the Oregon coast, ranging from eutrophic shelf to oligotrophic basin. Assimilation of super(3)H-leucine, as determined by microautoradiography, was combined with fluorescence in situ hybridization (MICROFISH) to identify biosynthetically active prokaryotes. Oligonucleotide probes targeted 2 domains (Bacteria and Archaea), and 4 groups within the Bacteria (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Cytophaga-like cells). We found that the Alphaproteobacteria and Cytophaga-like cells comprised the largest proportion of bacterial cells assimilating leucine. Alphaproteobacteria was the only group in which the abundance of active cells was significantly correlated to in situ phytoplankton stocks. Archaea were present in low numbers in most samples. However, in deep (>250 m) samples from the oligotrophic basin station, 43% of cells identified as Archaea were biosynthetically active. In general, we observed a similar change in the proportional abundance of cells assimilating leucine for all identified phylogenetic groups. Thus, at this phylogenetic level, our data set is evidence for tandem increase or decrease in biosynthetic activity by the whole prokaryotic community, rather than for shifts in activity by specific phylogenetic groups.