Such a stimulation of viral production by the presence of small eukaryotes (grazers) was observed in all experiments for the two lakes. These results corroborate the findings of Jacquet et al. [27] who

observed a clear and positive relationship between flagellate concentration and VIBM (virus-induced bacterial mortality) in Lake Bourget (r = 0.99, P < 0.05) at three different periods of the year (winter, spring and Tariquidar order summer), suggesting a synergistic cooperation between grazer and virus activity. Our new results extend the occurrence of this process at other periods of the year and in the oligotrophic Lake Annecy. Similar beneficial effects of protozoan grazing on viruses have been reported in various lacustrine systems with different trophic statuses [21, 23, 26]. This means that the trophic status cannot be ‘used’ as an environmental factor to change the balance between positive and negative effects of flagellates on viruses [29], and it is likely that there are probably different processes involved in enhancing viral activities in response to grazing activity [21]. To the best of our knowledge, Šimek et al. [19] were first to suggest that protozoan grazing may influence and increase viral lysis. From that time, other studies

reported such a synergistic effect in contrast to freshwater systems [21, 26, 27]. Nevertheless, an antagonistic interaction between these two compartments was also noted elsewhere buy AZD8931 [30, 31]. Mechanisms by which HNF affect viral activity are still unclear and many hypotheses have been proposed to explain such a cooperative interaction (reviewed by Miki and Jacquet [29]). In brief, grazing activity could stimulate bacterial PTK6 growth rates, by releasing organic and Barasertib manufacturer inorganic nutrients. Higher bacterial growth rates might be associated with enhanced receptor formation on cell surface which may result in a greater chance of phage attachment and in fine higher infection frequencies.

Thus, grazer stimulation of viral proliferation could occur through cascading effects from grazer-mediated resource enrichment [23]. We observed, in this study, a strong stimulation of bacterial production in treatments with grazers which may corroborate this assumption in both lakes. A link between infection and host production has been reported previously (summarized in Weinbauer [11]) and, recently, experimental studies showed that viruses may preferentially lyse active cells [18, 32]. Our results showed that autotrophic activity contributed to this stimulation, mainly in the early summer experiment (for both lakes), while heterotrophic flagellates were always involved in this positive feedback. A shift in the bacterial community structure could also contribute to the synergistic interaction observed in this study. According to Weinbauer et al.