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  • Not all metals are susceptible

    2021-09-22

    Not all metals are susceptible of being accumulated by insects. Tissue concentrations of essential metals like Cu and Zn are regulated in many insects and hence cannot be used to biomonitor metal pollution (Hare, 1992). A similar behaviour could explain the observed stability of Zn and Cu relative to other metals present in caddisfly larvae. For the other metals, trace levels observed in sites 1 and 2 could be considered basal levels only influenced by municipal wastewater from small villages and farms (<900 population). Alternatively, the observed steady increase of metal concentrations towards reaches 3–7 reflected the effects of agriculture activities, sewage treatment plant effluents, collectors, discharges and industrial activities with salinization being probably one of the most important sources (Casas et al., 2003). Indeed, the previous argument was supported by the rather high correlation observed between metal levels and physicochemical parameters including salinization (measured as conductivity) and levels of Zn, Cu, Cr, Ni, Pb, Co, Pb, and organic pollution monitored as nutrient loading (water levels of phosphate, nitrite, nitrate and ammonium) and concentrations of Pb. Most organisms have baseline levels of antioxidant systems, involved in a variety of detoxication reactions, to assure the maintenance of a balance between production and removal of endogenous ROS and other pro-oxidants. Hydropsyche larvae were found to possess key antioxidant and phase II enzyme activities. SOD (7–12Umg−1 protein) and CAT (150–450μmolmin−1mg−1 protein) activities were within the range reported for other insect species (0.7–200Umg−1 protein for SOD, 19–960μmolμmin−1mg−1 protein for CAT). GST activity in Hydropsyche was in the range of 90–250nmolmin−1mg−1 protein), and values between 1.4–14,000nmolmin−1mg−1 protein have been reported for GST in other insect species (Ahmad, 1992, Orr and Sohal, 1994, Jonaisse and Storey, 1998, Choi et al., 1999, Egaas, 2000, Ottea et al., 2000, Wang et al., 2001, Jovanovic-Galovic et al., 2004). More specifically, observed enzymatic activities can be considered moderate to high when compared with those measured in most species (Ahmad, 1992, Ahmad and Pardini, 1990). Gutathione peroxidase activity towards H2O2 was not detected in Hydropsyche larvae, thus measured peroxidase activity towards cumene was mainly due to Altretamine synthesis of organic hydroperoxides, probably by Se independent GST peroxidases (GSTPX), which in insects play an important role decomposing lipid peroxides but do not have activity towards H2O2 (Ahmad, 1992, Weinhold et al., 1990). The above-mentioned results are consistent with the absence of the seleno enzyme glutathione peroxidase (EC 1.11.1.9) in a wide range of insects, which detoxifies H2O2 and organic hydroperoxides (Smith and Shrift, 1979, Ahmad, 1992). Indeed, GSTPX activity towards cumene hydroperoxide observed in Hydropsyche larvae (30–56nmolmin−1mg−1 protein) could be considered moderate when compared to that reported in other insects (1–253nmolmin−1mg−1 protein; Smith and Shrift, 1979, Weinhold et al., 1990, Ahmad, 1992, Jonaisse and Storey, 1998, Jovanovic-Galovic et al., 2004). Enzymatic activities are regarded as fast prognostic indices of individual reaction to environmental stress, and should allow prediction of the consequences of pollution at the population and community level (Depledge and Fossi, 1994). The relationship between contaminant toxicity in animals, free radical processes and defensive responses of free radical scavenging system are thought to be crucial (Livingstone, 2001). In the present study, altered activity patterns of two (CAT, GST) out of four enzymatic systems assayed and increased levels of lipid peroxidation of Hydropsyche larvae collected along the Llobregat river system denoted increasing oxidative stress towards Altretamine synthesis downstream reaches where metal body burdens were the highest. Of particular interest are transition metals like iron (Fe), and chromium (Cr) that showed up to three-fold increase from high to downstream reaches and that through the Fenton reaction are able to facilitate the conversion of superoxide anion and hydrogen peroxide to the highly reactive hydroxyl radical, a species frequently proposed to initiate lipid peroxidation. Other metals including cadmium (Cd), nickel (Ni) and lead (Pb), which may produce oxidative stress by indirectly depleting glutathione levels or via metal-induced displacement of redox metal ions (Stohs and Bagghi, 1995), also increased towards lower levels. Therefore, there was a strong potential for measured metals along the Llobregat River to increase ROS production and cause oxidative tissue damage in Hydropsyche larvae, and to antioxidant enzymes to respond to them. Indeed, a correlation analysis denoted positive relationships between the concentration of Fe, Pd and Cd in hydropsiche larvae and the activities of GST, CAT and levels of lipid peroxides.