Effects of temperature and trace metal exposure on physiology, bioenergetics, and oxidative stress in juvenile Ostrea edulis.

The European flat oyster (Ostrea edulis) is an ecosystem engineer targeted by coastal restoration initiatives. Juveniles released to establish biogenic reefs may simultaneously encounter warming and trace-metal pollution. To investigate these combined effects, juvenile O. edulis were maintained for 21 days at 5, 15, and 22 °C and exposed in separate experiments to nominal zinc (Zn) concentrations of 100 and 1000 μg L-1 or copper (Cu) concentrations of 10 and 100 μg L-1, alongside controls without metal addition. Whole-organism performance, bioenergetics, and lipid peroxidation were assessed. Bioenergetic endpoints included protein, carbohydrate, and lipid contents, electron transport system (ETS) activity, and cellular energy allocation (CEA), while malondialdehyde (MDA) served as a marker of lipid peroxidation. Condition index (CI) declined progressively with warming, oxygen consumption rate (ṀO2) increased at 22 °C, and shell closure duration decreased at 15 and 22 °C. None of these whole-organism endpoints was significantly affected by metal exposure. Protein content and ETS activity declined at 22 °C in both experiments. Total energy reserves decreased from 5 to 22 °C only in the Cu experiment, while high Zn exposure suppressed ETS activity and increased CEA without changing energy reserves. High Zn exposure led to overall elevated MDA levels, whereas high Cu exposure increased MDA only at 22 °C. These findings identify temperature as the dominant driver of physiological responses under the tested conditions and demonstrate that an absence of detectable metal effects on whole-organism performance can coexist with cellular impairment. The occurrence of Cu-induced lipid peroxidation specifically at 22 °C further highlights the relevance of combined thermal and metal stress. Longer-term experiments with environmentally realistic metal mixtures are needed to determine whether the observed cellular responses translate into impaired juvenile performance, with implications for oyster restoration.