Effect of seawater-freshwater cross-transplantations on viral dynamics and bacterial diversity and production

Dilution experiments were carried out to investigate the community composition and the metabolic response of seawater and freshwater bacteria to cross-transplantation, and the effects of non-indigenous bacterial hosts on viral dynamics. Changes in viral and bacterial abundance and production, as well as bacterial respiration, carbon demand and diversity were regularly monitored over a 6 d period. Bacterial production in the transplanted seawater (SB-t) and freshwater (FB-t) bac- teria treatments was stimulated up to 256 and 221%, respectively, compared to controls. The stimu- lation of bacterial production and carbon demand was accompanied by a decrease in bacterial rich- ness. Net viral production was stimulated by 81% in SB-t and repressed by 75% in FB-t. Transplantation increased the virus-induced mortality of marine bacteria, but decreased it for fresh- water bacteria. These results suggest that (1) marine bacteria can readily oxidize freshwater dis- solved organic matter, and (2) freshwater viruses might be able to infect marine hosts, thus highlight- ing their potential role in fueling bacterial growth under resource stress or nutrient-depleted conditions.