Influence of temperature on the toxicity of the herbicides sulfentrazone (Boral 500SC®) and clomazone (Gamit®) in tadpoles of Eupemphix nattereri (Leiuperidae) and Rhinella schneideri (Bufonidae)

Temperature is an important environmental factor influencing the metabolism, development and behavior of cold-blooded animals. However, indirect effects of temperature on natural systems, such as the potential of interaction with environmental contaminants have not been well-characterized for many taxa. Changes in temperature patterns can potentially alter the nature and distribution of many contaminants in the environment, and therefore the current challenge for ecotoxicologists is to predict the risks established by these interactions at biological level. Pesticides are the main group of toxicants present in natural systems and a large part of these compounds are used in sugarcane crops for the control of invasive plants in Brazil. In the northwest of the country, many species of anurans are distributed in areas common to agriculture. Most of them have reproductive activity restricted to the rainy season, which coincides with the period of greatest environmental contamination. This study aimed to investigate the influence of temperature on the toxicity of the herbicides sulfentrazone, clomazone and diuron in anuran larvae, using specific physiological, biochemical and molecular biomarkers. Our first study investigated the effects of the herbicides sulfentrazone (Boral SC500C®) and clomazone (Gamit®) in tadpoles of Eupemphix nattereri and Rhinella schneideri under different temperatures (28, 32 and 36 ºC), evaluating the antioxidant responses and lipid peroxidation. In our second study, we evaluated the influence of temperature (28 and 34 °C) on the action of diuron and its metabolite 3,4-dichloroaniline (3,4-DCA) on thyroid function and metamorphosis in tadpoles of Lithobates catesbeianus. Our results showed that temperature is an important factor influencing toxicity of the studied herbicides for the different tadpole’s species. Moreover, studied biomarkers were also changed by isolated effects of temperature. Exposure of E. nattereri and R. schneideri to the herbicides clomazone and sulfentrazone altered the antioxidant enzymes activities in tadpoles, in addition to increases in malondialdehyde (MDA) levels, indicating the occurrence of oxidative stress; however, these responses were directly related to the temperatures of exposure for both species. Most of the enzymes had their activities more pronounced at higher temperatures and in combination with the presence of contaminants. These data are probably due to the increased toxicity of contaminants at higher temperatures as a result of increased metabolic rates of cold-blooded tadpoles. Differential activation of the antioxidant system in E. nattereri and R. schneideri suggests that tadpoles of different species may use diverse strategies and adaptive mechanisms to cope with the effects generated by exposure to agrochemicals in habitats where the temperature is a relevant influencing factor. The results of our second study showed that the herbicide diuron and its metabolite 3,4-DCA are potent thyroid deregulators in L. catesbeianus tadpoles, however these endocrine effects are more pronounced at higher temperatures. Besides the induction on expression of TH-related genes, we observed changes in plasma levels of triiodothyronine (T3) and an acceleration in the metamorphosis of tadpoles exposed to the combined effects of high temperature and the presence of contaminants, diuron or 3,4-DCA. Overall, our results suggest that realistic environmental temperatures should be increasingly taken into account in risk studies evaluating the exposure of amphibians to environmental contaminants, such as pesticides. The lack of approach of these variables in laboratory tests can lead to underestimated interpretations of the toxic effects of these and other compounds for different species of amphibians. (AU)