Medium N:P ratios and specific growth rate comodulate microcystin and protein content in Microcystis aeruginosa PCC7806 and M. aeruginosa UV027.

Hepatotoxin production in cyanobacteria has been shown to correlate to external stimuli such as light and nutrient concentrations and ratios, although conflicting results have been reported. Specific growth rates and protein and microcystin content of M. aeruginosa PCC7806 and M. aeruginosa UV027 were determined under nonlimiting batch culture conditions for a range of medium nitrogen and phosphorous atomic ratios. Both strains exhibited a similar optimal medium N:P ratio for increased cellular microcystin levels. Additionally, total cellular protein content and intracellular microcystin content were significantly correlated to each other (r2 = 0.81, p < 0.001). Microcystin and protein content increased considerably as the maximum specific growth rate for the experimental conditions was reached. The significant correlation of cellular protein and microcystin content and their relative increase with increasing specific growth rate, within defined ranges of medium N:P ratios, suggest a close association between microcystin production and N:P ratio-dependent assimilation of nitrogen, and resulting total cellular protein levels, which may be further modulated by specific growth rate.