Production of Cyanobacterial Hepatotoxin Microcystin in a Nutrient-limited Culture of Microcystis viridis

The production of microcystin by a nutrient-starved Microcystis viridis culture with a nutrient gradient was examined. The specific production rate of microcystin increased with increasing nitrate and phosphate concentrations. The specific production rates of nitrate-starved cultures after nitrate supply were higher than those of phosphate-starved cultures after phosphate supply showing that nitrogen is a more important nutrient for microcystin production than phosphorus. The time course of microcystin production by nitrate-starved M.viridis after nitrate supply was evaluated at 2h intervals. The microcystin-RR content increased to more than 2-fold within 24 hrs. An increase in dry weight was not observed within 24 hrs, showing microcystin was produced prior to cell growth for nitrogen-starved M. viridis. The specific production rate of microcystin-RR seemed to increase with increasing nitrate cell quota, which suggested that the nitrogen cell quota might regulate the rate of production of microcystin in lakes and reservoirs.