Climate Change and Contaminants of Emerging Concern: Enrofloxacin Effects on Manila Clam ( Ruditapes philippinarum ) Under Simulated Ocean Warming

In this study, we investigated the combined effects of enrofloxacin (ENR; 5 and 500 ng/L) and temperature (17°C, ambient; 21°C, warming) on physiological and biochemical responses of clams. Biomarkers of antioxidant and biotransformation defense (CAT, TAC, GSTs, and CbEs), oxidative damage (LPO and protein carbonyls), metabolic activity (respiration rate and ETS), and energy reserves (proteins, carbohydrates, and lipids) were measured, and results were integrated through multivariate analysis (principal coordinate analysis, PCoA) and the IBR index. At 17°C, ENR exposure elicited moderate adjustments, including GSTs induction and ETS stimulation at high concentration, indicating that clams could sustain the additional metabolic demand associated with detoxification. At 21°C, however, antioxidant defenses were destabilized: GSTs activity was markedly reduced while CAT was strongly induced, pointing to a compensatory but unbalanced response. This was accompanied by a mismatch in energy metabolism, as respiration rate increased without ETS stimulation, leading to the mobilization of carbohydrate and lipid reserves. Although oxidative damage remained limited, with only a significant rise in LPO at low ENR concentration under warming, the IBR index confirmed higher biological stress at 21°C across treatments. Overall, our findings show that elevated temperatures amplify the physiological impact of ENR exposure by impairing detoxification pathways and accelerating energetic costs. These results highlight temperature as a critical factor modulating the resilience of bivalves to pharmaceutical pollutants, with implications for coastal ecosystems under climate change scenarios.