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 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