The effects and possible mechanisms of action of L- phenylalanine on the growth of Microcystis aeruginosa cells were explored by cell counting and flow cytometry assays. L- phenylalanine promoted the growth of Microcystis aeruginosa at concentrations between 0.078 and 0. 312 μg/mL, but inhibited growth at concentrations between 0. 625 and 20μg/mL in 24 h exposure. The dose-effect and time-course relationships between exposure to L-phenylalanine and growth inhibition of Microcystis aeruginosa were observed. The IC50 value of L-phenylalanine for growth inhibition of Microcystis aeruginosa was 6. 2 μg/mL （95% confidence interval was 0. 005 to 16. 76 μg/mL）. The membrane integrity of the cells showed significant variations after 24 h exposure to L-phenylalanine. Meanwhile, no effects on esterase activity of the cells were observed until after 48 h exposure to L-phenylalanine. In conclusion, L-phenylalanine has hormesis effects and algae control effects on Microcystis aeruginosa. The latter is closely related to alterations or disorders in the cell membrane and with variation of esterase activity in the cells.
