Copper accumulation and oxidative stress in the sea anemone, Aiptasia pallida, after waterborne copper exposure

Copper is a common marine pollutant yet its effects on symbiotic cnidarians are largely understudied. To further understand the impact of elevated copper concentrations on marine symbiotic organisms, toxicity tests were conducted using the model sea anemone, Aiptasia pallida, with and without its zooxanthellae symbiont. Symbiotic and aposymbiotic A. pallida were exposed to sublethal copper concentrations (0, 5, 15, and 50 µg/L) for 7 d and copper accumulation, behavior, and the activity of the oxidative stress enzymes, superoxide dismutase (SOD), and catalase (CAT) were measured. Additionally, acute 96-h toxicity tests were conducted to determine LC50 values of the organisms after copper exposure. Both symbiotic and aposymbiotic A. pallida rapidly accumulated copper in a time and dose dependent manner. However, higher copper concentrations accumulated in the aposymbiotic as compared to the symbiotic A. pallida. In response to the highest two copper exposures (15 and 50 µg/L) symbiotic A. pallida upregulated CAT activity to combat the damaging effects of hydrogen peroxide. Contrary to these results, SOD activity significantly decreased during the highest copper exposure, when compared to controls. CAT activity was not detected and SOD was substantially (> 10 fold) reduced in aposymbiotic A. pallida, suggesting that the zooxanthellae are associated with the oxidative stress response. Copper exposure as low as 5 µg/L caused tentacle retraction and increased mucus production in both symbiotic and aposymbiotic anemones. The LC50 values for symbiotic and aposymbiotic A. pallida exposed to copper for 96 h were 148 µg/L (95% confidence interval = 126.4, 173.8) and 206 µg/L (95% confidence interval = 175.2, 242.2), respectively. Understanding the varying responses of symbiotic and aposymbiotic A. pallida to copper stress may advance our comprehension of the functional roles of zooxanthellae and host. Although the mechanism of copper toxicity has not been fully elucidated, it is clear that A. pallida accumulate copper and are sensitive, as effects were detected at environmentally relevant copper concentrations. Likewise, A. pallida may be useful in biomonitoring copper polluted environments.