| Abstract |
Unicellular algae, Raphidocelis subcapitata and Desmodesmus subspicatus, belonging to class Chlorophyceae, order Sphaeropleale, are used for ecological risk assessment of pesticides such as herbicides. Pyrazosulfuron-ethyl (PE), a sulfonylurea herbicide, shows a lower impact on cell growth in Desmodesmus than in Raphidocelis. We investigated the cause of the difference in PE sensitivity between these two algae from the aspect of target-site resistance (TSR) and nontarget-site resistance (NTSR). The cells of two algae showed similar accumulation levels of PE with no detection of a presumed PE metabolite with cytochrome P450 (CYP), indicating no differences in its uptake or metabolism. The 72-hour ErC50s for CYP inhibitors were >2.4 and <0.023 mg/L in R. subcapitata and D. subspicatus, respectively, for ketoconazole, whereas they were 60 and 160 mg/L in R. subcapitata and D. subspicatus, respectively, for 1-aminobenzotriazole (ABT). Co-exposure to ABT with PE showed no effect on the PE-dependent inhibition of cell growth in Raphidocelis, and rather mitigation of the inhibition in Desmodesmus, indicating no detoxification metabolism by CYP in either species. These data indicated no difference in terms of NTSR. As for acetolactate synthase, the target enzyme of sulfonylurea herbicides, PE-dose dependency of inhibition of its activity showed no great difference, and the amino acid residues important for sulfonylurea herbicides binding were conserved between the species. Therefore, no difference was shown in terms of TSR. In this study, the cause of the difference in PE sensitivity was not clarified, but different effects of CYP inhibitors were demonstrated in the two algal species.
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