Marine microbial communities are complex and dynamic and their ecology impacts biogeochemical cycles in pelagic ecosystems. Yet little is known about the relative activity of different microbial populations within genetically diverse communities. Here we use rRNA as a proxy for activity to quantify the relative specific activity (rRNA/rDNA) of eubacterial populations and to identify locations or clades for which there is an uncoupling between specific activity and abundance. After analyzing 1.6 million sequences from 16S rDNA and rRNA (cDNA) libraries from two euphotic depths from a representative site in the Pacific Ocean, we show that although there is an overall positive relationship between the abundance (rDNA) and activity (rRNA) among populations of the bacterial community, for some populations these measures are uncoupled. Different ecological strategies are exemplified by the two numerically dominant clades at this site: the cyanobacterium Prochlorococcus is abundant but disproportionately more active while the heterotrophic SAR11 is abundant but less active. Other rare populations, such as Alteromonas, have high specific activity in spite of their low abundance suggesting intense population regulation. More detailed analyses using a complementary qPCR-based approach of measuring relative specific activity for Prochlorococcus populations in the Pacific and Atlantic Oceans also show that specific activity, but not abundance, reflects the key drivers of light and nutrients in this system, but also suggests substantial top-down regulation (e.g. grazing, viruses or organismal interactions) or transport (e.g. mixing or immigration/emigration) of these populations. Thus, here we show that abundance and specific activity can be uncoupled in open ocean systems and that describing both is critical to characterizing microbial communities and predicting marine ecosystem functioning and response to change.
Hunt DE, Lin Y, Church MJ, Karl DM, Izzo LK, Tringe SG and Johnson ZI. The relationship between abundance and specific activity of bacterioplankton in open ocean surface waters. Applied and Environmental Microbiology DOI: 10.1128/AEM.02155-12