RESUMEN
The early life stages of freshwater mussels are particularly sensitive to copper (Cu) contamination. We measured the acute toxicity, bioaccumulation, and sublethal effects of Cu in glochidia. In addition, we used radiolabeled Cu (64 Cu) to examine the time-dependent kinetics of uptake over 24 h. Uptake of 64 Cu by live and dead glochidia exposed to 0.11 µmol/L exhibited similar hyperbolic patterns over the first 40 min, indicating an adsorptive phase independent of larval metabolism. Thereafter, uptake was linear with time, with a 10-fold lower bioaccumulation rate in live than in dead animals, representing a close to steady state of Cu regulation. In contrast, dead glochidia exhibited a progressively increasing uptake, possibly attributable to the fact that metal-binding sites become more accessible. Mortality was strongly correlated with bioaccumulation (48 h); live glochidia exposed to Cu concentrations >0.27 µmol/L lost their regulatory ability and accumulated Cu to an even greater extent than dead animals. Exposure to Cu induced significant decreases in whole-body Na+ and Mg2+ concentrations; increases in reactive oxygen species concentration, lipid peroxidation, and protein carbonylation; and a decrease in antioxidant capacity against peroxyl radicals. Overall, these results clarify the patterns of Cu uptake and regulation, emphasize the importance of distinguishing between live and dead larvae, and indicate that toxicity is associated with Cu bioaccumulation, involving both ionoregulatory disturbance and oxidative stress. Environ Toxicol Chem 2018;37:1092-1103. © 2017 SETAC.
Asunto(s)
Bivalvos/efectos de los fármacos , Cobre/metabolismo , Cobre/toxicidad , Agua Dulce/química , Animales , Antioxidantes/metabolismo , Radioisótopos de Cobre , Iones , Cinética , Larva/efectos de los fármacos , Peroxidación de Lípido/efectos de los fármacos , Oxidación-Reducción , Carbonilación Proteica/efectos de los fármacos , Especies Reactivas de Oxígeno/metabolismo , Factores de Tiempo , Pruebas de Toxicidad Aguda , Contaminantes Químicos del Agua/toxicidadRESUMEN
Several studies have indicated that the early life stages of freshwater mussels are among the most sensitive aquatic organisms to inorganic chemicals, including copper. However, little is known about the toxic mode of action and sub-lethal effects of copper exposure in this group of imperiled animals. In this study, the physiological effects of long-term copper exposure (survival, growth, copper bioaccumulation, whole-body ion content, oxygen consumption, filtration rate, ATPase activities, and biomarkers of oxidative stress) were evaluated in juvenile (6 month old) mussels (Lampsilis siliquoidea). The mussels' recovery capacity and their ability to withstand further acute copper challenge were also evaluated in secondary experiments following the 28 day exposure by assessing survival, copper bioaccumulation and whole-body ion content. Mussels chronically exposed to 2 and 12 µg Cu/L showed significantly higher mortality than those held under control conditions (mortality 20.9, 69.9 and 12.5%, respectively), indicating that juvenile L. siliquoidea is underprotected by the U.S. Environmental Protection Agency (USEPA) biotic ligand model (BLM)-derived chronic water quality criteria (WQC) (2.18 µg Cu/L) and the hardness-derived USEPA WQC (12.16 µg Cu/L). Soft tissue copper burden increased equally for both copper exposures, suggesting that chronic toxicity is not associated with copper bioaccumulation. Several physiological disturbances were also observed during chronic copper exposure. Most relevant was a decrease in whole-body sodium content paralleled by an inhibition of Na(+) K(+)-ATPase activity, indicating a metal-induced ionoregulatory disturbance. Filtration and oxygen consumption rates were also affected. Redox parameters (reactive oxygen production, antioxidant capacity against peroxyl radicals, glutathione-S-transferase (GST) activity, and glutathione (GSH) concentration) did not show clear responses, but membrane damage as lipid peroxidation (LPO) was observed in both copper exposures. Mussels previously held in control conditions or pre-exposed to 2 µg dissolved Cu/L were able to maintain their ionic homeostasis and did not experience mortality after the 4-d recovery period. In contrast, those previously exposed to 12 µg dissolved Cu/L exhibited 50% mortality indicating that they had already reached a 'point of no return'. Pre-exposure to copper did not influence mussel response to the copper challenge test. As observed for the chronic exposure, mortality of mussels held in the absence of copper and submitted to the challenge test was also associated with an ionoregulatory disturbance. These results indicate that ionoregulatory disruption in freshwater mussels chronically exposed to copper is the main mechanism of toxicity and that redox parameters do not appear to be useful as indicators of sub-lethal copper toxicity in these animals.