Effects of glyphosate on microcystin-LR production and release from Microcystis aeruginosa at different temperatures

Cyanobacterial blooms and their associated toxins are growing issues for many aquatic ecosystems. Microcystin-LR (MC-LR) is a toxic and common cyanobacterial toxin, whereas glyphosate is a commonly used herbicide that is massively applied in agriculture. In this study, the effects of glyphosate on the growth of Microcystis aeruginosa and MC-LR synthesis and release from M. aeruginosa at different temperatures are investigated. In addition, the MC-LR pollution in the Huangpu River in Shanghai urban area is studied. Results indicated that the MC-LR concentration in the Huangpu River is related to water temperature. The laboratory experiments revealed that the growth of M. aeruginosa was slightly promoted at 15 °C and glyphosate concentrations of 1 and 5 mg/L and inhibited in the presence of glyphosate and high temperatures (20 °C, 25 °C, 30 °C, and 35 °C). The intracellular MC-LR contents were remarkably increased by glyphosate at 15 °C, 20 °C, 25 °C, and 30 °C and remarkably decreased at 35 °C. Meanwhile, the extracellular MC-LR contents were remarkably increased at all temperatures and all concentrations except when treated with 1 mg/L glyphosate at 35 °C. The highest extracellular MC-LR content, which was 143.9% higher compared with that of the control, was observed at 30 °C and treatment with 10 mg/L glyphosate. These results were consistent with those of MC-LR investigation in Huangpu River. Furthermore, in accordance with the intracellular MC-LR contents, the ability of a single cell to synthesize MC-LR was enhanced at 15 °C, 20 °C, 25 °C, and 30 °C and decreased at 35 °C. These results provide an understanding on the toxic effects of glyphosate on cyanobacteria and the effects of temperature on MC release. Moreover, these results will be helpful in protecting aquatic ecosystems and human health.