The relative importance of viral lysis and nanoflagellate grazing for prokaryote mortality in temperate lakes

1. We explored the factors that potentially affect viral abundance, viral life strategies (lytic versus lysogenic cycle) and the relative importance of viral lysis and heterotrophic nanoflagellate grazing for prokaryote mortality in the euphotic zone of 19 freshwater lakes in a single geographical region (the French Massif Central). 2. Flow cytometry indicated that viral abundance ranged from 0.5 to 26.3x10(7) viruses mL(-1), exceeding prokaryote abundance by c. 16-fold on average. Multiple regressions revealed that prokaryotic abundance was the best predictor of viral abundance (r(2)=0.74), suggesting that the prokaryotes were the major hosts for viral proliferation in these lakes. 3. Viral life strategies varied with month of sampling (June, July, August 2010). Lytic infection was the prominent mode of viral infection in June and August, whereas lysogenic infection was more evident in July in the majority of sampled lakes. This temporal shift in viral strategy was controlled by the availability of phosphate and chlorophyll concentration via host cell growth, with high concentrations favouring lytic infection and low concentrations favouring lysogenic infection. Such antagonistic interactions between viral life strategies support the hypothesis that lysogeny may represent a survival strategy for viruses in harsh nutrient/host conditions. 4. The dominance of viruses over flagellates, or vice versa, as prokaryotic mortality factors differed between the lakes. By taking into account both forms of mortality, our study indicates that viruses and flagellates are likely to act additively in their effects on prokaryotes by regulating the proportion of the most active members of the prokaryotic community. Such a combined effect of viruses and flagellates may strongly influence the planktonic food web and whole-ecosystem functioning.