An assessment of the toxicity of some pesticides and their metabolites affecting a natural aquatic environment using the Microtox (TM) system

The conservation and preservation of aquatic ecosystems is of utmost importance due to the high diversity and density of species and their complex food network. The evaluation of the potential adverse environmental impact caused by pesticides entering water bodies is an important parameter in aquatic toxicity. The toxicity of the insecticide, Fenitrothion and two of its metabolites, 3-methyl-4 nitrophenol and 3-methyl-4-nitroanisole, and of the herbicides Thiobencarb and Molinate, commonly used in rice fields in Valencia near the protected area of lake Albufera, has been tested by using the Microtox™ system. The 15 min EC50 values obtained with the marine luminescent bacterium Vibrio fischeri showed that the Thiobencarb was the most toxic of the three tested pesticides with an EC50 value of 0.03 mg/l. The EC50 values of the first two steps of the Fenitrothion hydrolysis indicated that while the first metabolite, 3-methyl-4-nitrophenol, was as toxic as its parent compound, a decreased toxicity was observed for the second metabolite, 3-methyl-4-nitroanisole. In order to analyze the toxic effects of pesticides in complex mixtures the EC50 values of Fenitrothion, Molinate and Thiobencarb as pure substances and their mixtures were compared. The impact of the pesticides in the natural ecosystem was also studied and a protective effect of lake water was observed.