The predation paradox: Synergistic and antagonistic interactions between grazing by crustacean predator and infection by cyanophages promotes bloom formation in filamentous cyanobacteria

In this study, we assessed the impact of synergistic/antagonistic interactions between grazing by crustacean predator (Daphnia magna) and infection by cyanophage (Vb-AphaS-CL131) on the population dynamics of the harmful bloom-forming filamentous cyanobacterium Aphanizomenon flos-aquae. We observed synergistic effect of cyanophage infection on D. magna survival and grazing trough the lysis-induced shift in cyanobacteria population structure toward the shorter filaments. However, lysis-mediated- and grazing-enhanced removal of short A. flos-aquae filaments resulted in the dominance of grazing- and infection-resistant A. flos-aquae population. In addition, the presence of D. magna generated a trait-mediated response in the A. flos-aquae population by promoting the aggregation of filaments into colony-like structures. Experiments using temperate A. flos-aquae colony isolates from natural environment demonstrated that colony-embedded A. flos-aquae filaments are insensitive to the addition of both D. magna and cyanophages. Therefore, we propose that interactions between crustacean grazers and cyanophages may promote the emergence of defensive A. flos-aquae genotypes and population traits that eventually lead to bloom formation in aquatic environments.