Growth rates of bacterioplankton in the coastal NW Mediterranean

Abstract

Growth rate is a crucial trait to understand the contribution of prokaryotes to food web dynamics and biogeochemical cycling. Bulk bacterial growth rates are generally low, (0.05 to 0.10 day-1), although it has been demonstrated that particular groups can grow much faster. However, there is a general lack of information on what controls the growth rates of different prokaryotes. In the last years, we have carried out manipulation experiments at several seasons under different light regimes, in which grazing, resource competition and viruses were reduced, using coastal seawater from the Blanes Bay Microbial Observatory, NW Mediterranean. Growth rates of different bacterioplankton groups, including the Bacteroidetes, Rhodobacteraceae, SAR11, Gammaproteobacteria and its subgroups Alteromonadaceae and the NOR5/OM60 clade, were calculated from changes in cell numbers using Catalyzed Reporter Deposition Fluorescence In situ Hybridization. Maximal growth rates were achieved when both predation pressure and nutrient limitation were minimized, but in general the response to predation removal was stronger than to nutrient availability. In all the experiments conducted, the group Alteromonadaceae presented the highest rates, although all groups largely responded to the different treatments. Comparison of light and dark treatments pointed out in some experiments to a certain increase in the growth rates of groups containing photoheterotrophs, such as the Rhodobacteraceae and the NOR5 clade. Additionally, we measured the cellular activity at the single-cell level with Bioorthogonal noncanonical aminoacid tagging. Overall, these results are valuable to set thresholds on growth rates and to estimate prokaryotic contribution to the structure and function of microbial communities