Effect of different concentrations and ratios of ammonium, nitrate, and phosphate on growth of the blue-green alga (cyanobacterium) Microcystis aeruginosa isolated from the Nakdong River, Korea

Microcystis aeruginosa causes harmful algal blooms in the Nakdong River of Korea. We studied the effect of different concentrations and ratios of ammonium (NH4+), nitrate (NO3-), and phosphate (PO43-) on growth of this species in BG-11 medium: each nutrient alone, NO3- : NH4+ ratio, the N : P ratio with fixed total N (TN), and the N : P ratio with fixed total P (TP). The single nutrient experiments indicated that M. aeruginosa had the highest growth rate at NH4+ and NO3- concentrations of 500 mu M, and at a PO43- concentration of 5 mu M. The NO3- : NH4+ ratio experiments showed that M. aeruginosa had the highest growth rate at a ratio of 1 : 1 when TN was 100 mu M and 250 mu M, and the lowest growth rate at a ratio of 1 : 1 when the TN was 500 mu M. The N : P ratio with fixed TN experiments indicated that M. aeruginosa had the highest growth rates at 50 : 1, 20 : 1, and 100 : 1 ratios when the TN was 100, 250, and 500 mu M, respectively. In contrast, the N : P ratio with fixed TP experiments showed that M. aeruginosa had the highest growth rates at 200 : 1 ratio at all tested TP concentrations. In conclusion, our results imply that the NO3- : NH4+ ratio and the PO43- concentration affect the early stage of growth of M. aeruginosa. In particular, our results suggest that the maximum growth of M. aeruginosa is not simply affected by the NO3- : NH4+ ratio and the N : P ratio, but is determined by the TN concentration if a certain minimum PO43- concentration is present.