Daphnia magna Exudates Impact Physiological and Metabolic Changes in Microcystis aeruginosa

International audienceWhile the intracellular function of many toxic and bioactive cyanobacterial metabolites is not yet known,microcystins have been suggested to have a protective role in the cyanobacterialmetabolism, giving advantage to toxic over nontoxic strains under stress conditions. The zooplankton grazerDaphnia reduce cyanobacterial dominance until a certain density, which may be supported by Daphnia exudates, affecting the cyanobacterial physiological state andmetabolites’ production. Therefore,we hypothesized that D.magna spentmediumwill impact the production of cyanobacterial bioactivemetabolites and affect cyanobacterial photosynthetic activity in the nontoxic, but not the toxic strain. Microcystin (MC-LR and des-MC-LR) producingM. aeruginosa PCC7806 and its non-microcystin producing mutant were exposed to spent media of different D. magna densities and culture durations. D. magna spent medium of the highest density (200/L) cultivated for the shortest time (24 h) provoked the strongest effect. D. magna spent medium negatively impacted the photosynthetic activity of M. aeruginosa PCC7806, as well as the dynamics of intracellular and extracellular cyanobacterial metabolites, while its mutant was unaffected. In the presence of Daphnia medium, microcystin does not appear to have a protective role for the strain. On the contrary, extracellular cyanopeptolin A increased inM. aeruginosa PCC7806 although the potential anti-grazing role of this compound would require further studies