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The ecological role of nodularin in cyanobacterium Nodularia spumigena is still largely unknown, as are the conditions that promote toxin production. We report a modulating effect of mesozooplankton grazing on cellular nodularin content in N. spumigena expressed as a decrease in cell-bound toxin concentration in the presence of copepods compared with the cyanobacterium in similar assemblages without copepods. In our experiments, N. spumigena was incubated in an ambient plankton (&lt;90 μm) assemblage (Expt I) or in 0.2-μm filtered seawater (Expt II), with and without the copepod Eurytemora affinis. Following ∼28-h incubation, we measured the changes in N. spumigena abundance and nodularin concentration, frequency of Nodularia DNA occurrence in copepods as a proxy for grazing pressure on the cyanobacterium and individual RNA content in E. affinis as a proxy for copepod growth response. In all copepod-free treatments, intracellular nodularin concentrations were up to four times higher than in the treatments containing copepods. In Experiment I, the copepods also had a positive effect on the cyanobacterium growth, presumably due to a selective removal of more edible algal species and thus decreased competition for nutrients. Nodularia DNA was detected with high frequencies, 18–80%, increasing in treatments with no alternative food or high copepod densities. Simultaneously, no noxious effects of N. spumigena on the copepods were detected as indicated by higher RNA content in copepods exposed to N. spumigena with or without ambient plankton organisms compared with starved controls. These findings stress the need to understand the importance of intra-specific interactions for nodularin production in relation to population dynamics of N. spumigena.

Toxin concentration in Nodularia spumigena is modulated by mesozooplankton grazers