The effects of nitrate and phosphate concentrations on the production of three microcystins, namely, types LR, RR and YR by Microcystis viridis NIES 102, were determined in batch culture experiments at in situ level concentrations. The yield of microcystin‐RR during the exponential growth phase increased favorably in nitrate concentrations ranging from 0.2 to 0.8 mg l−1 of NO3-N. The effect plateaued at 1.0 mg l−1. This tendency was similar to the effect of nitrate on the specific growth rate. There was no significant correlation between the amounts of intracellular microcystins at exponential growth phase and nitrate concentrations. On the other hand, the specific growth rate was constant regardless of the phosphate concentrations. However, the yield of the three types of microcystins in the culture broth and the amounts of intracellular microcystins decreased with increasing the phosphate concentration. Furthermore, it was found that the correlation between total yield of microcystins (LR, RR and YR) and the ratio of microcystins during the exponential growth phase showed almost constant values, as follows: microcystin LR (0.24), RR (0.66) and YR (0.10).

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