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

Abstract 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 experimentrevealed no effect of mussel density on microcystinquota (particulate microcystinper unit Microcystisbiomass). In contrast, in both field and laboratory experiments, nitrogen-limited conditions led to substantially reduced microcystinquota relative to phosphorus-limited or nutrient-saturated conditions. In the field experiment, microcystinper 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 Erierevealed a similar negative influence of nitrogen limitation (as indexed by nitrate concentration) on microcystinquota. 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.