Frequency of virus‐resistant hosts determines experimental community dynamics

Parasites, such as bacterial viruses (phages), can have large effects on host populations both at the ecological and evolutionary levels. In the case of cyanobacteria, phages can reduce primary production and infected hosts release intracellular nutrients influencing planktonic food web structure, community dynamics, and biogeochemical cycles. Cyanophages may be of great importance in aquatic food webs during large cyanobacterial blooms unless the host population becomes resistant to phage infection. The consequences on plankton community dynamics of the evolution of phage resistance in bloom forming cyanobacterial populations are still poorly studied. Here, we examined the effect of different frequencies of a phage‐resistant genotype within a filamentous nitrogen‐fixing Nodularia spumigena population on an experimental plankton community. Three Nodularia populations with different initial frequencies (0%, 5%, and 50%) of phage‐resistant genotypes were inoculated in separate treatments with the phage 2AV2, the green alga Chlorella vulgaris, and the rotifer Brachionus plicatilis, which formed the experimental plankton community subjected to either nitrogen‐limited or nitrogen‐rich conditions. We found that the frequency of the phage‐resistant Nodularia genotype determined experimental community dynamics. Cyanobacterial populations with a high frequency (50%) of the phage‐resistant genotype dominated the cultures despite the presence of phages, retaining most of the intracellular nitrogen in the plankton community. In contrast, populations with low frequencies (0% and 5%) of the phage‐resistant genotype were lysed and reduced to extinction by the phage, transferring the intracellular nitrogen held by Nodularia to Chlorella and rotifers, and allowing Chlorella to dominate the communities and rotifers to survive. This study shows that even though phages represent minuscule biomass, they can have key effects on community composition and eco‐evolutionary feedbacks in plankton communities. Consumer–resource interactions represent one of the key building blocks in virtually any ecological community. In plankton food webs, the most extensively studied consumer resource interaction has been the one between the phytoplankton and their zooplankton grazers. However, the important role of viral parasites has been increasingly acknowledged during recent decades (Suttle 1994, 2007, Weinbauer et al. 2003, Brussaard 2004, Wilhelm and Matteson 2008, Wigington et al. 2016). Viruses can alter community composition and element cycling and be the major cause of the mortality