Aerobic metabolism of aquatic ectotherms is highly sensitive to fluctuating climates. Many mitochondrial traits exhibit phenotypic plasticity in response to acute variations in temperature and oxygen availability. These responses are critical for understanding the effects of environmental variations on aquatic ectotherms performance. Using the European seabass, Dicentrarchus labrax, we determined the effects of acute warming and deoxygenation in vitro on mitochondrial respiratory capacities and mitochondrial efficiency to produce ATP (ATP/O ratio). We show that acute warming reduced ATP/O ratio but deoxygenation marginally raised ATP/O ratio, leading to compensatory effect of low oxygen availability on mitochondrial ATP/O ratio at high temperature. The acute effect of warming and deoxygenation on mitochondrial efficiency might be related to the leak of protons across the mitochondrial inner membrane, as the mitochondrial respiration required to counteract the proton leak increased with warming and decreased with deoxygenation. Our study underlines the importance of integrating the combined effects of temperature and oxygen availability on mitochondrial metabolism. Predictions on decline in performance of aquatic ectotherms due to climate change may not be accurate, since these predictions typically look at respiratory capacity and ignore efficiency of ATP production.

The mitochondrial respiration rates (LEAK respiration, OXPHOS respiration, COX activity) and the mitochondrial ATP synthesis (ATP production) were measured with high resolution respirometers eqquiped with fluorescent sensors (Oroboros Instruments, Innsbruck, Austria) and recorded using DatLab software. The mitochondrial efficiency to produce ATP (ATP/O ratio) was calculated as the ratio of ATP production to OXPHOS respiration.

The assays were performed in red muscle of seabass at 13°C and 16°C. The animals have been fasted 24h. The mitochondria preparation was a homogenate. There is no missing value.