Diel-cycling hypoxia and hypercapnia interact with the physiological and redox response of the bivalve Mytilus edulis to heat-wave

Climate change and eutrophication intensify marine heat-waves (MHWs) and cause fluctuations in oxygen and pH levels (FH) in coastal ecosystems. A temperate zone ecosystem engineer, the blue mussel (Mytilus edulis), provides a valuable model to investigate the cumulative effects of these stressors on the performance of keystone benthic organisms. To assess whether FH exposure alters mussels' responses to MHWs, mussels were exposed to either well-oxygenated or FH conditions (daytime ∼8.5 mg L^-1 O2, pH 8.0, night-time ∼0.6 mg L^-1 O2, pH 7.3) for 4-weeks before temperature increases (18 °C-26 °C). Physiological responses including respiration (RR), clearance (CR) and ammonia excretion rate (ER), byssus production, biomarkers of energy status and of oxidative stress were measured before and after 1-4, 7-10, and 14-17 days of MHW. Energy consumption rate increased and mussels' CR decreased but cellular energy reserves remained stable during MHW. Total antioxidant capacity overall increased and lipid peroxidation (LPO) decreased after one day of MHW, except in FH digestive gland, where LPO remained at baseline, suggesting redox balance adjustments post-FH acclimation and warming. RR increased and ER transiently increased during the MHW, with FH exposure preventing ER return to baseline and RR stabilization after two weeks post-MHW. A parallel increase of the protein concentrations between 4 and 10 days of MHW in gills indicate protein turnover shifts linked to the thermal acclimation. These findings suggest that FH acclimation modifies mussels' physiological responses to MHW, potentially reducing their resilience to the intensifying coastal hypoxia and extreme weather events predicted for future Baltic Sea coasts.