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Harmful algal blooms (HABs) have been emerging as a worldwide issue due to their detrimental effect to water quality and the threat to ecological health. However, there were scarcely reports on whether existing agrochemicals such as herbicides have potential effects on Microcystis aeruginosa ( M. aeruginosa ) which may form large surface blooms. In this study, we aimed to demonstrate the effects of glufosinate, a widely use herbicide in the world, on the growth of M. aeruginosa . The change of the content of intracellular (intra-) and extracellular (extra-) levels of microcystin-LR (MC-LR) were also determined by HPLC. The mRNA level of mcy A, mcy D, mcy H related to the production of MC-LR was detected after 48 h exposure to glufosinate. Our results revealed that low concentrations of glufosinate exposure (0.5 mg/L, 1 mg/L) would promote the growth of M. aeruginosa. It was worth noting intra-MC-LR contents of algal cells were also significantly increased at low concentrations of glufosinate exposure (1 mg/L). While at high concentrations (5 mg/L, 10 mg/L), glufosinate could induce the MDA level and inhibit the growth of M. aeruginosa that would limit the production of MC-LR to the water body. At last, the transcription levels of algae toxin synthesis related genes mcy A and mcy D were increased significantly at environmentally relevant concentrations of glufosinate, which may affect the MC production. This study not only illuminated effects of glufosinate on growth of and MC-LR synthesis of M. aeruginosa at environmentally relevant concentrations, but also reminding us the potential risk caused by herbicide use should arouse concerns. [ABSTRACT FROM AUTHOR]