Viral control of bacterial growth efficiency in marine pelagic environments

We tested the hypothesis that viruses can control bacterial growth efficiency (BGE) in marine pelagic environments. In the Bay of Villefranche, Northwestern Mediterranean, three experiments were conducted on different months to determine bacterial and viral variables in seawater cultures. In December, phosphorus (P) addition enhanced bacterial growth 6-9-fold with concomitant increase in viral production (4-15-fold), but little enhancement of bacterial respiration (BR). In other months, P enrichment increased BR 2-6-fold and viral production 2-5-fold, but did not increase in bacterial abundance (Aug, Feb) or growth (Feb). BGE depended on the fraction of bacterial production destroyed by viruses (shunting efficiency, v; i.e., when v was low, nutrient enrichment enhanced BGE, whereas when v was high, nutrient enrichment mainly led to low BGE). Viral production and bacterial production and respiration in the Western North Pacific and other data from the literature showed that BGE was negatively correlated with shunting efficiency. Predictions from a carbon flow model were consistent with the above results showing that decreased BGE over a broad range of values (from 0.7 to 0.001) could be largely explained by viral-induced conversion of bacterial biomass to dissolved organic carbon. Viruses exert the major influence on patterns in carbon fluxes mediated by bacteria in marine pelagic environments.