Future Ocean warming may prove beneficial for the northern population of European seabass, but ocean acidification does not

1. Abstract The world’s oceans are acidifying and warming due to increasing amounts of atmospheric CO2. Thermal tolerance of fish much depends on the cardiovascular ability to supply the tissues with oxygen. The heart itself is highly dependent on oxygen and heart mitochondria thus might play a key role in shaping an organism’s tolerance to temperature. The present study aimed to investigate the effects of acute and chronic warming on respiratory capacities of European sea bass (Dicentrarchus labrax L.) heart mitochondria. We hypothesized that acute warming would impair mitochondrial respiratory capacities, but be compensated after long-term. Increasing PCO2 may cause intracellular changes, likely further constricting cellular energy metabolism. We found increased leak respiration rates in acutely warmed heart mitochondria of cold-conditioned fish in comparison to measurements at their rearing temperature, suggesting a lower aerobic capacity to synthesize ATP. However, thermal acclimation led to increased mitochondrial functionality, e.g. higher RCRo in heart mitochondria of warm-conditioned compared to cold-conditioned fish. Exposure to high PCO2 synergistically amplified the effects of acute and long-term warming, but did not result in changes by itself. We explained the high ability to maintain mitochondrial function under OA with the fact that seabass are moving between various environmental conditions. Improved mitochondrial capacities after warm conditioning could be due to the origin of this species in the warm waters of the Mediterranean. Our results also indicate that seabass are not yet fully adapted to the colder temperatures in their northern distribution range and might benefit from warmer temperatures. Summary statement Mitochondria of juvenile European sea bass hearts are impaired by acute warming, but seem to benefit from acclimation to warmer temperatures, they are only marginally impacted by ocean acidification. 2. List of abbreviations Δ500: Acidification condition (ambient PCO2 + 500 µatm) Δ1000: Acidification condition (ambient PCO2 + 1000 µatm) A: Ambient PCO2 condition C: Cold life conditioned group CI: Complex I of the electron transport system CII: Complex II of the electron transport system CIV: Complex IV of the electron transport system dph: Days post hatch IPCC: Intergovernmental Panel on Climate Change Leak-CI: State 4 respiration of complex I MS-222: Tricaine methane sulfonate OA: Ocean acidification OAW: Ocean acidification and warming OW: Ocean warming OXPHOS: Full state 3 respiration of the electron transport system OXPHOS-CI: State 3 respiration of complex I OXPHOS-CII: State 3 respiration of complex II OXPHOS-cytC: State 3 respiration after addition of cytochrome c PCO2: Partial pressure of CO2 RCP: Representative concentration pathway RCRo: Respiratory control ratio State 4o: State 4 respiration after addition of oligomycin W: Warm life conditioned group