Interactive effects of copper and dissolved organic matter on sodium uptake, copper bioaccumulation, and oxidative stress in juvenile freshwater mussels (Lampsilis siliquoidea)

Freshwater mussels are exceptionally sensitive to many contaminants including metals, but the mechanisms of toxicity are not fully understood. Similarly, our understanding of the protective effects of dissolved organic matter (DOM) is also undergoing revision, since recent studies have found that DOM may also directly affect organism physiology, in addition to its well known capability in complexing and reducing bioavailability of metals. In the present study, these issues were investigated in juvenile (6-12 months old) freshwater mussels (Lampsilis siliquoidea) in moderately-hard reconstituted water (Ca²⁺=0.406mmol/L; Mg²⁺=0.537mmol/L; Na⁺=1.261mmol/L;K⁺=0.077mmol/L; hardness=80-100mg/L CaCO₃; pH=8.02 and DOM=0.3mgC/L). Mussels were acutely exposed (24 and 96h) to Cu (0, 2 or 12μgCu/L) combined with three concentrations (0, 3 or 6mgC/L) of DOM of terrigenous origin (Luther Marsh). We analyzed unidirectional Na⁺ influx, whole-body ion content (Na⁺, K⁺, Ca²⁺ and Mg²⁺), enzyme (Na⁺/K⁺-ATPase, H⁺-ATPase and carbonic anhydrase) activities, copper bioaccumulation and oxidative stress-related parameters. Exposure to DOM alone caused a marked increase in the unidirectional Na⁺ influx rate and a decrease in v-type H⁺-ATPase activity, suggesting that DOM alone can cause alterations in membrane transport functions and therefore, whole-body Na⁺ metabolism. Unidirectional Na⁺ influx rate and Na⁺K⁺-ATPase activity were inhibited when mussels were exposed to the higher Cu concentration tested (12μg Cu/L). The influx inhibition was ameliorated by the simultaneous presence of DOM. At this same Cu concentration, DOM also significantly protected mussels against whole-body Na⁺ and K⁺ losses associated with Cu exposure, as well as against Cu bioaccumulation. Oxidative stress parameters did not show clear trends across treatments. Overall, our results indicate that Cu is a potent ionoregulatory toxicant to freshwater mussels. They also demonstrate that natural DOM protects against both Cu bioaccumulation and ionoregulatory toxicity, and that at least part of this protection results from direct positive effects of DOM on Na⁺ metabolism.