Environmental modulation of microcystin and β-N-methylamino-l-alanine as a function of nitrogen availability

• Combined nitrogen modulates MC and BMAA content in freshwater toxigenic cyanobacteria.
• An inverse relationship exists between cellular MC and BMAA content.
• Conditions for MC and BMAA presence in the field are in accordance with observations in laboratory cultures.
• Nutrient and light gradients in dense cyanobacterial blooms may result in both toxins being present at a single site.

The most significant modulators of the cyanotoxins microcystin and β-N-methylamino-l-alanine in laboratory cyanobacterial cultures are the concentration of growth-medium combined nitrogen and nitrogen uptake rate. The lack of field studies that support these observations led us to investigate the cellular content of these cyanotoxins in cyanobacterial bloom material isolated from a freshwater impoundment and to compare these to the combined nitrogen availability. We established that these toxins typically occur in an inverse relationship in nature and that their presence is mainly dependent on the environmental combined nitrogen concentration, with cellular microcystin present at exogenous combined nitrogen concentrations of 29 μM and higher and cellular BMAA correlating negatively with exogenous nitrogen at concentrations below 40 μM. Furthermore, opposing nutrient and light gradients that form in dense cyanobacterial blooms may result in both microcystin and BMAA being present at a single sampling site.