Genome analysis of Marinobacter phage AS1 suggests its close interactions with host Marinobacter sp.

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Functional responses of prokaryotes and viruses to grazer effects and nutrient additions in freshwater microcosms

International audienceFor aquatic systems, there is little data on the interactions between viruses, prokaryotes, grazers and the availability of resources. We conducted a microcosm experiment using a size fractionation approach to manipulate grazers, with a purpose to examine the effects of inorganic and organic nutrients on viral and prokaryotic standing stocks and activities, and on prokaryotic community composition as assessed by fluorescent in situ hybridization (FISH) method. Experiments were performed during periods of severe phosphate (P)-limiting conditions in the oligotrophic Sep Reservoir (Massif Central, France). In the absence of nutrient addition, the presence of grazers in microcosms stimulated prokaryotic growth and viral proliferation, likely through nutrient and substrate enrichment. Addition of nutrients had a stronger effect on viral infection of prokaryotes than grazing. Addition of P led to the most pronounced increase in prokaryotic abundance, production and growth efficiency, thus providing direct evidence of P limitation of prokaryotes. Enhanced prokaryotic activity in P treatments also stimulated viral abundance and viral-induced lyses of prokaryotes. Changes in prokaryotic community composition due to nutrient additions were evident in the grazer-free samples. Prokaryotic populations hybridizing for the probes bacteria, beta-Proteobacteria and alpha-Proteobacteria responded to nutrient enrichment with significant increases in their relative abundances, whereas cells hybridizing for Archaea and Cytophaga-Flavobacterium (now known as Bacteroidetes) probes failed to show any functional response. Cells hybridizing for the latter cluster increased towards the end of incubation period in the control samples (that is, without nutrient additions) with grazers present, suggesting the development of grazing resistant forms. From our nutrient enrichment microcosm experiments, we conclude that the presence of grazers is a stimulating factor for prokaryotic growth and viral proliferation in the plankton, probably through nutrient regeneration process

Growth and mortality rates of prokaryotes in the hypolimnion of a deep freshwater lake (Lake Biwa, Japan)

The presence of pico-sized cyanobacteria (genus Synechococcus) in hypolimnetic waters has been reported, and investigators have suggested that Synechococcus contribute more to ecological processes in the hypolimnion than previously hypothesized; however, the ecological role of Synechococcus in food webs and/or matter cycling in the hypolimnion remains unknown. To address this issue, we assessed protistan grazing and the virus-mediated mortality of Synechococcus in the oxygenated hypolimnion of a large freshwater lake (Lake Biwa, Japan) during the stratification period. In addition, we compared the carbon flux through mortality of Synechococcus to that of heterotrophic bacteria to evaluate the role of Synechococcus in ecological processes within a hypolimnetic ecosystem. Our results suggest that the biomass of Synechococcus and heterotrophic bacteria in the hypolimnion was removed primarily by protistan grazing. The abundance of Synechococcus was highest in August, when the average Synechococcus:bacteria carbon biomass and daily grazing loss ratios were 10.8% and 11.0%, respectively. Thus, the Synechococcus biomass is likely an important seasonal component of the carbon flux in the hypolimnetic microbial loop. Our results provide the first data on carbon flux through the mortality of both Synechococcus and bacteria in a hypolimnetic ecosystem