Effects of Dietary Copper and Escherichia coli Challenge on the Immune Response and Gill Oxidative Balance in the Freshwater Mussel Diplodon chilensis.

Copper is a water and sediment pollutant that can be biomagnified by phytoplankton, and it often co-occurs with fecal bacteria. We addressed the combined effects of copper and Escherichia coli on the immune response and gill oxidative balance of the freshwater mussel Diplodon chilensis. Bivalves were sorted into four groups fed with 1) control algae, 2) bacteria (E. coli), 3) copper-enriched algae (Cu 2+ ) algae, and 4) copper-enriched algae followed by bacteria (Cu 2+  + E. coli). Cellular and humoral immune and cytotoxic variables were analyzed in hemolymph, and detoxifying/antioxidant enzyme activities (glutathione S-transferase [GST] and catalase [CAT]) and lipid peroxidation (thiobarbituric acid reactive substances [TBARS]) were studied in gill tissue. The total hemocyte number increased after Cu 2+ exposure, independently of the E. coli challenge. The proportion of hyalinocytes significantly diminished in the E. coli and Cu 2+ groups but not in Cu 2+  + E. coli groups; granulocytes significantly increased with E. coli but not with Cu 2+  + E. coli treatments. Phagocytic activity was higher in all treatments than in control mussels. Acid phosphatase activity was increased by E. coli and inhibited by Cu 2+ and Cu 2+  + E. coli. Both E. coli and Cu 2+ but not Cu 2+  + E. coli augmented alkaline phosphatase activity. The Cu 2+ and Cu 2+  + E. coli treatments reduced the lysosomal membrane stability and cell viability. Humoral bacteriolytic and phenol oxidase activities were not affected by any treatment. The Cu 2+ treatment induced gill CAT and GST activities and increased TBARS levels. The Cu 2+  + E. coli treatment reversed this CAT and GST stimulation and increased the Cu 2+ effect on TBARS. Dietary Cu 2+ affects bivalves' immunological and oxidative status and impairs defensive responses against bacteria. In turn, E. coli potentiates the gill oxidative effects of Cu 2+ . Environ Toxicol Chem 2022;00:1-12. © 2022 SETAC.