Nitrogen availability increases the toxin quota of a harmful cyanobacterium, Microcystis aeruginosa.

An important objective in understanding harmful phytoplankton blooms is determining how environmental factors influence the toxicity of bloom-forming species. We examined how nutrients and grazers (dreissenid mussels) affect the production of microcystin (a liver toxin) by the cyanobacterium Microcystis aeruginosa, via a combination of field and laboratory experiments, and field observations in Lake Erie. The field experiment revealed no effect of mussel density on microcystin quota (particulate microcystin per unit Microcystis biomass). In contrast, in both field and laboratory experiments, nitrogen-limited conditions led to substantially reduced microcystin quota relative to phosphorus-limited or nutrient-saturated conditions. In the field experiment, microcystin per unit of mcyB gene was strongly reduced under nitrogen-limited conditions, indicating a phenotypic response. Results from a seasonal survey in the western basin of Lake Erie revealed a similar negative influence of nitrogen limitation (as indexed by nitrate concentration) on microcystin quota. Our results are consistent with stoichiometric considerations in that the cell quota of a nitrogen-rich secondary metabolite, microcystin, was reduced disproportionately under nitrogen limitation.