Fipronil Metabolism and Dissipation in a Simplified Aquatic Ecosystem

Abstract

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 (14)C-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 LC(50) measurements on Aedes aegypti larvae (LC(50) for CF(3)-S-R and CF(3)-SO(2)-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-NH(2)-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 (14)CO emission was observed during the experiment.

Environmental fate and toxicology of fipronil

“…For instance, fipronil adsorption was found to increase as soil organic matter content increased from 0.1 to 6.5%. 5) A similar observation was noted using eight Australian soil types, with K oc values ranging from 542-1176 (average, 825Ϯ214), depending on soil type, and indicating low-tomoderate sorptive capability (Table 1). 17) Similar K oc values were obtained from studies of fipronil with British and French loams (427-1248).…”

“…5) It has a low-to-moderate affinity for organic carbon (e.g., humus in soils) as well as biota (log octanolϪwater partition coefficient [K ow ]ϭ3.9-4.1), and falls between highly insoluble pesticides such as DDT (log K ow ϭ6.2) and soluble ones such as atrazine (K ow ϭ2.6). 12) Fipronil is more dense than water, not particularly volatile, is degraded via photolysis, susceptible to hydrolysis under alkaline conditions, and possesses varying microbial degradation rates depending on soil conditions (anaerobic versus aerobic; Table 1).…”

“…5) It interferes with the function of g-aminobutyric acid (GABA)-gated channels; fipronil disrupts normal neuronal influx (e.g., passage of chloride ions) by targeting the GABA-gated chloride channel and, at sufficient doses, causes excessive neural excitation, severe paralysis, and death. 5,[13][14][15] Fipronil possesses a greater binding affinity for the GABA-regulated chloride channels of insects than mammalian GABA receptors. 16) …”

“…5) Under anaerobic conditions, fipronil degrades slowly in water. The t 1/2 on average is 123 days, much longer than any aerobic soil system where both the fipronil and fipronildesulfinyl t 1/2 is only 5 days.…”

Behaviour of Fipronil in Paddy Field Ecosystem

“…On the other hand, the carbamates are cholinesterase inhibitors. These are classical insecticides and due to common mode of action, many insects have developed resistance against them [4]. Fipronil acts in a different way by inhibition of principal nerve transmitter of insect i.e.…”