Cylindrospermopsin is regulated by resource stoichiometry in the cyanobacterium, Aphanizomenon

While all the drivers of HAB formation are not well understood, eutrophic conditions increase the likelihood of HABs. Cyanotoxins are classified as nitrogen (N)-rich or carbon (C)-rich, with the Redfield ratio (C:N 6.6) separating the two classes. The stoichiometric hypothesis of toxin production states that conditions that cause N to be in excess [e.g., high N:phosphorus (N:P)] will cause increased N-rich cyanotoxin production. This hypothesis is supported in Microcystis with microcystin production; however, other cyanotoxins and cyanobacteria remain poorly understood. Here, we explored how a gradient of N:P affects the biomass, elemental stoichiometry, light-harvesting pigments, and cylindrospermopsin (CYN) concentration and cell quotas in an N-fixing cyanobacterium, Aphanizomenon. We found that cultures grown in low N:P conditions produced the same biomass, implying a high N-fixation efficiency that did not cause an N-fixation growth tradeoff. Even with no changes in biomass CYN concentrations increased over the N:P 1 to 100 gradient. The proportion of dissolved CYN increased with resource N:P that ranged between 0 and 75% of the CYN concentration. Our results enforce the need to control both N and P to lower the N:P in receiving waters that may decrease the potential for cyanobacteria to produce intracellular and extracellular toxins.