Effect of top-down control on cell-specific activity and diversity of active marine bacterioplankton

Abstract

Mortality processes such as grazing and viral lysis can alter both phylogenetic diversity and cell-specific activity of bacterioplankton. We conducted experiments to examine the effect of reduction in grazer or viral abundance on metabolically active bacterioplankton at a eutrophic shelf station and an oligotrophic basin station. Leucine assimilation was used as a proxy to characterize metabolically active bacterial cells. The phylogenetic affiliation of marine bacterioplankton assimilating leucine was identified with fluorescence in situ hybridization and microautoradiography. Flow cytometric sorting of leucine-labeled cells quantified cell-specific activity of high nucleic acid (HNA) and low nucleic acid cells. The activity and diversity of the bacterial community at the oligotrophic station was more responsive to a reduction in mortality compared to the community at the eutrophic station. HNA cells at the oligotrophic station showed a 4-fold increase in cell-specific leucine incorporation when the abundance of flagellates was reduced and an 8-fold increase when the abundance of flagellates, viruses, and bacterial cells was reduced. The abundance of active Alphaproteobacteria increased at the oligotrophic station when either grazer or viral abundance was reduced. Activity responses were less striking at the eutrophic station, and the abundance of active Gammaproteobacteria showed a greater increase with reduced flagellates compared to the treatment with reduced flagellates, viruses, and bacterioplankton. Our results indicate both the presence and type of mortality process play a key role in structuring a bacterial community, and this effect varies in ecosystems of differing trophic state. © Inter-Research 2010.