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Summary

Viruses have evolved different life strategies for coping with environmental challenges and this is a key explanation for their omnipresence in aquatic systems. However, factors that determine the balance between lytic versus lysogenic decision within natural virioplankton are poorly documented, primarily in freshwaters. This study was designed to investigate the experimental short-term (24 h incubation) effects of added organic and inorganic nutrients on the two viral lifestyles in nutrient-depleted freshwater microbial (i.e. < 0.8 μm fraction) microcosms, using mitomycin C as prophage inductor agent. In the absence of mitomycin, viral lytic production increased as a functional response to the strong stimulation of bacterial growth rates (0.7–0.8 day−1) by the added nutrients, primarily the organic nutrients which appeared scarcer than inorganic nutrients and was related to the sampling period and the geomorphological peculiarities of Lake Pavin. In the presence of mitomycin, temperate phage production (frequency of lysogenically infected bacterial cells, FLC = 17–19% of total cells) significantly exceeded lytic production (frequency of lytically infected bacterial cells, FIC = 9–11%) in natural samples (i.e. without nutrient additions) as a result of enhanced prophage induction, which relatively increased with the decreasing contact probability between viruses and their potential hosts. In contrast, addition of nutrients drastically reduced FLC (< 4%) and increased FIC (> 22%). Both variables were antagonistically correlated as was the correlation between FLC and bacterial growth rates, supporting the idea that lysogeny may represent a maintenance strategy for viruses in harsh nutrient/host conditions which appeared as major instigators of the trade-off between the two viral lifestyles. Overall, at the community level, we reject the hypothesis that nutrients but mitomicyn C stimulate temperate phage induction, and retained the hypotheses that nutrients rather (i) stimulate lytic viruses via enhanced host growth and (ii) when limiting, promote lysogenic conversion in natural waters.