The oxidative stress response in freshwater-acclimated killifish (Fundulus heteroclitus) to acute copper and hypoxia exposure

Aquatic organisms face multiple stressors in natural ecosystems. Here we examine the effects of moderate hypoxia and low-level copper (Cu) on freshwater (FW)-acclimated killifish. Both Cu and hypoxia can affect oxidative stress in fish, but it is unclear if in combination these two stressors would act synergistically. We exposed killifish for 96 h to Cu in normoxia (total 23.4 ± 0.9 μg Cu L− 1), or either no Cu (2.33 ± 0.01 mg O2 L− 1) or with Cu in hypoxia (23.6 ± 0.8 μg Cu L− 1; 2.51 ± 0.04 mg O2 L− 1), and compared them to normoxic controls with no added Cu (0.7 ± 0.1 μg Cu L− 1; 9.10 ± 0.00 mg O2 L− 1) at a hardness of 140 mg L− 1 as CaCO3 equivalents. Gills showed significant Cu accumulation with both excess waterborne Cu in normoxia and in hypoxia. This was accompanied by increases in gill catalase (CAT) activity but with no significant changes in either protein carbonyls or lipid peroxidation (TBARS). Hypoxia alone decreased gill protein carbonyls. Liver showed no change in Cu load, but a significant decline in CAT activity occurred with Cu in normoxia. Liver showed an increase in TBARS with Cu in normoxia. Cu when combined with hypoxia caused a significant decline in cytochrome c oxidase (COX) and citrate synthase (CS) activity in gill and liver. Thus, low waterborne levels of Cu and moderate hypoxia both affected gill and liver phenotypes. However, killifish are tolerant of Cu and hypoxia, and there was no evidence of a synergistic response to exposure to the two stressors combined compared to each stressor alone.