Enantioselective Physiological Effects of the Herbicide Diclofop on Cyanobacterium <i>Microcystis aeruginosa</i>

Water blooms caused by cyanobacteria are currently major global environmental issues. The outbreaks induced by nutrient elements have attracted much attention; however, the effects of environmental pollutants on the cyanobacteria are themselves poorly understood, especially those due to chiral chemicals. To explore the enantioselective eco-effects of the chiral herbicide diclofop-methyl (DM) and its major metabolite diclofop acid (DA), the physiological characteristics of <i>Microcystis aeruginosa</i> were investigated. The results showed that using both biomass and protein content as growth parameters is necessary to access the impact of the herbicides, that stimulation biomass production by <i>R</i>-DA and <i>S</i>-DA was apparent (nonessential), and that the concentration of 5 mg/L is worth noting. Ultrastructure changes in gas vacuoles, thylakoids, glycogen, cyanophycin granules, poly beta-hydroxybutyrate, polyhedral body, and lipids indicated different toxicity modes among the four chemicals. The different effects between <i>R</i>-DA and <i>S</i>-DA demonstrated that <i>R</i>-DA probably acts as a proton ionophore shuttling protons across the plasmalemma, whereas <i>S</i>-DA did not demonstrate such action. The toxicity order in the present study is <i>S</i>-DA < <i>R</i>-DA < DM < DA. Stimulation of the growth of <i>M. aeruginosa</i> during the first 3 days by herbicidally inactive <i>S</i>-DA was greater than that due to <i>R</i>-DA, which is adverse to water quality in water bodies. Therefore, using the herbicidally active <i>R</i>-enantiomer is recommended. These results are helpful in understanding the enantioselective effects of chiral pesticides on cyanobacteria, which is important for environmental assessment and protection. It is also helpful for guiding the application of chiral pesticides in agricultural settings