Effect of nitrate, ammonium and phosphate on the growth and microcystin production of Korean Microcystis species

Aim: This study aimed to investigate the effects of nitrogen (NO₃-N, NH₃-N) and phosphorus (PO₄-P) on the growth and microcystin production of two bloom-forming Microcystis species (toxic M. aeruginosa MAHC160824 and non-toxic M. viridis MVHC160824). Methodology: The two Microcystis species were isolated from the lower reaches of the Nakdong River, South Korea. In the culture experiments, the e average nutrient concentrations (NH₃-N, NO₃-N and PO₄-P) at which Microcystis appeared (> 15°C) was used as control medium. Different concentrations of NH₃-N, NO₃-N and PO₄-P were then employed in nutrient testing (control, vs. 4 times and 16 times higher than the control). Microcystin levels were measured using a UPLC^™ (LC MS/MS) system. Results: Both toxic and non-toxic Microcystis strains exhibited a maximum cell density at 30°C and a maximum growth rate at 25-30°C. In the nutrient addition assays, the maximum growth of two Microcystis species were found at nutrient concentrations 4 to 16 times higher than the control (NH₃-N: 0.468 mg l^-1, PO₄-P: 0.100 mg l^-1, NO₃-N: 32.5 mg l^-1). The highest microcystin production levels were found under optimal growth conditions. The microcystin levels of toxic M. aeruginosa MAHC160824 were below the detection limit despite a higher number of cells (> 300,000 cells ml^-1) at the same nutrients concentrations as those found in raw water from the Nakdong River. Interpretation: Higher production of microcystin occurs when there is an increase in NH₃-N and PO₄-P within a restricted range in toxic species M. aeruginosa MAHC160824, else the production is low.