Effects of the herbicides quizalofop-p-ethyl and quizalofop-ethyl on the physiology, oxidative damage, synthesis, and release of microcystin-LR in Microcystis aeruginosa

Abstract

Quizalofop-Ethyl (QE) is a widely used phenoxy herbicide. It contains two enantiomers, and only its R-enantiomer quizalofop-p-ethyl (QpE) has herbicidal activity. Cyanobacterial blooms caused by Microcystis aeruginosa pose a threat to human health and aquatic environments, but the potential effects of chiral herbicides on M. aeruginosa are rarely studied. In this study, we carried out experiments using 0, 0.5, 1, 2, and 5 mg/L of QE and QpE to explore the effects on the growth, metabolism, synthesis, and release of microcystin-LR (MC-LR) in M. aeruginosa, which is the dominant species of algal blooms and indicator organism of water contamination. The biomass and chlorophyll a and protein content were consistent, showing that QpE at high concentrations (2 and 5 mg/L) showed greater acute toxicity than QE. On the expression of photosynthesis-related genes psaB and psbD1, QpE at 2 and 5 mg/L showed a stronger inhibition effect than QE. The oxidative stress caused by QE and QpE indicated that 144 h of exposure increased the concentration of MDA and enhanced the activities of superoxide dismutase and catalase. The synthesis of intracellular MC-LR in M. aeruginosa was stimulated by 144 h of QpE and QE exposure, and QpE showed a stronger stimulation effect than QE at high concentrations. The content of extracellular MC-LR increased after 144 h of QpE exposure and remained stable under QE exposure compared with that in the control group. Moreover, the expression of MC-LR-related gene mcyH was upregulated by QpE (144 h exposure) but downregulated by QE. These results are helpful to understand the toxicological effects of chiral herbicides on nontarget aquatic organisms, thus providing a theoretical basis for the rational use and the ecological risk assessment of chiral pesticides.