Comparison of Diazinon Toxicity to Temperate and Tropical Freshwater Daphnia Species

In recent decades, most of the developed countries are shifting toward a chemical-free agriculture, also known as ‘‘organic’’ agriculture/farming, or in other words, decreasing the use of pesticides . This has resulted in substantial improvements in agronomic practices guided by stringent legislations and in the quality of natural water bodies, that is, less risk to the aquatic lives . Most of the developing countries in the tropical regions are increasing their use of pesticides and fertilizers as they become wealthier . As a result, the concentration of pesticides in tropical environments has constantly increased, causing a decline in species diversity. Although pesticides have certain beneficial effects for agricultural crops, their use can cause a wide range of toxic effects on different nontarget organisms .

Diazinon (O,O-diethyl-O-[2-isopropyl-6-methyl-4-pyrimidinyl] phosphorothioate) is a nonsystemic organophosphate insecticide. It was commonly used for insert control in agricultural crops . The toxicity of diazinon results from inhibiting the enzyme acetylcholinesterase (AChE), leading to the accumulation of neurotransmitters and altered signal transmission in chemical synapses . In the domestic and international markets, there are more than 500 registered products that contain diazinon as the active substance . Due to its widespread use, diazinon is frequently found in freshwater ecosystems . In a recent study, Montuori et al. reported that Diazinon is prevalent in aquatic systems all over Europe, with the highest concentration recorded to be 785 ng·L−1 in the Ebro River, Spain. Moreover, high concentrations of Diazinon up to 1.5 μg·L−1 have been found in urban waterways in California [8]. In recent decades, Diazinon has been widely used in tropical regions . However, pesticide laws and regulations have not been implemented adequately in these regions despite its heavy applications. The toxic effects of diazinon are available in the literature for the temperate cladoceran D. magna. However, the toxic effects of Diazinon on tropical zooplanktons have not been well documented and studied in the literature.

Diazinon generates high acute toxicity to a wide variety of aquatic organisms, leading to a wide range of sublethal biochemical effects, damage to specific target organs and tissues, and adverse ecological impacts. The toxicity of diazinon has extensively studied in fish and crustaceans , and it has been reported to be moderately toxic to early life stages of zebrafish . The 96 h LC50 values ranging from 0.32 to 1.53 mg·L−1 for larval and from 2.2 to 10.3 mg·L−1 for adult of several fish have already been recorded . Yen et al. reported that diazinon lowers larval swimming activity and inhibits AChE activity in zebrafish, as well as an increase in the Hsp70 content. In cladocerans, diazinon at the concentration from 0.18 to 0.30 μg·L−1 caused adverse effects on the survival of D. magna . These Diazinon concentrations have been reported to cause a decrease in mean total young per female, mean brood size, and rate of population increase (RPI) and development of D. magna. Only the concentration above 0.25 μg·L−1 caused a delay in the time to first reproduction . Toxicokinetic and toxicodynamic models of diazinon and its by-product 2-isopropyl-6-methyl-4-pyrimidinol in crustacean species were investigated by Kretschmann et al. [15]. Results suggested that the oxidative dearylation of diazinon to pyrimidinol is a crucial cellular detoxification step which is catalyzed by the enzyme P450 . However, most ecotoxicological studies use the temperate cladoceran D. magna as an ecotoxicological model; the toxicity of insecticide on tropical zooplankton species has not been examined to the same extent .

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