Fipronil metabolism and dissipation in a simplified aquatic ecosystem

Several phenylpyrazole derivatives are selective inhibitors of chloride channel activities in insects. In this chemical family, fipronil is a powerful insecticide now widely used for several purposes. The dissipation of this molecule in a simplified aquatic ecosystem has been studied for 3 months, using C-14-labeled fipronil. The main features of the complex process leading to fipronil transformation in this system were the following. The fipronil aqueous solution was submitted to two chemical transformations: the photodependent desulfuration of the side chain bound to the 4-position of the heterocyclic ring and the chemical hydrolysis of the nitrile function bound to the 3-position. Fipronil, rapidly transferred from the water solution to the organic matter, was protected from the previously mentioned chemical transformations but evolved to give two main metabolites, which were either reduced or oxidized in the side chain on the 4-position. These derivatives were powerful insecticides as shown by LC50 measurements on Aedes aegypti larvae (LC50 for CF3-S-R and CF3-SO2-R = 8.8 nM). During the course of this experiment, nitrile hydrolysis took place slowly, originating either from the chemical hydrolysis in the aqueous solution or from enzymatic hydrolysis inside the microbial biomass. The fipronil-amide (3-NH2-CO-R') derivative, although much more polar than fipronil itself, was mostly bound to the organic matter. Other more polar derivatives were also detected but in very small amounts. No (CO)-C-14 emission was observed during the experiment.