Daphnia magna tolerance to toxic cyanobacteria in the presence of an opportunistic infection within an evolutionary perspective

In aquatic environments, interactions between cyanobacteria and their grazers are crucial for ecosystem functioning. Cyanobacteria are photosynthetic prokaryotes, which are able to produce large blooms and associated toxins, some of which are able to suppress grazer fitness. Cyanobacterial blooms are intensified by global warming and eutrophication. In our experiments, the tolerance of Daphnia magna (Straus, 1820), an efficient grazer of toxic cyanobacteria, was studied. We used different D. magna clones sampled from different sediment depths, which corresponded to different time periods of eutrophication. Our results showed that different clones had a different tolerance towards the toxic cyanobacterial species, Microcystis aeruginosa , confirming the presence of genetic variation in D. magna tolerance to cyanobacteria. However, there was not a significant adaptive effect of sediment depth. As expected, in general under controlled, infection-free conditions M. aeruginosa reduced D. magna survival. However, a coincidental, non-intended opportunistic fungal infection in a first experiment allowed us to compare the response of D. magna to M. aeruginosa in infected individuals and non-infected individuals (from a second experiment). In the presence of this opportunistic infection, there was no negative effect of M. aeruginosa in the D. magna clones, suggesting that exposure to the infection provided protection for Daphnia individuals towards Microcystis . Biotic interactions can thus be important in the interpretation of cyanobacterial effects in zooplankton grazers and in finding appropriate solutions to reduce the occurrence of cyanobacterial blooms.