Exudates Impact Physiological and Metabolic Changes in .

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