A multi-scaling approach showing a transient metabolic mismatch in a freshwater fish (Zingel asper) during an acute heat stress.

Heat stress events will be more frequent and intense in the future. These events will challenge the capacity of organisms to exhibit sufficient metabolic flexibility to adapt to such variations. To better understand the acclimation processes implemented in response to acute warming, with an integrative approach we examined in vivo metabolic rate and cardiac mitochondrial respiration in the Rhône streber, during and after a heat stress on a precise timeline. The temperature was raised from 13°C to 18°C (+ 1°C per hour) and maintained at 18°C for 5 days, before returning to 13°C at the same rate. We repeatedly measured, during the heat stress and 5 days after the end of the event, in vivo metabolic rate in the same individuals and cardiac mitochondrial respiration from different individuals. At the organismal level, oxygen consumption increased in line with warming, and was subsequently followed by a return to pre-acclimated levels just after the end of the heat stress. At the opposite, cardiac mitochondrial respiration decreased during the heat stress, especially 24 hours in, and was recovered at the end of the event. Our results suggest that the heat stress was responsible for a metabolic mismatch in the strebers. Indeed, we observed (i) a strong thermodynamic effect without any acclimation process, suggesting that the range of temperatures chosen was not stressful for the fish, and (ii) the establishment of a transitory energy saving process. Our results underline the need for more integrative studies to understand how organisms will adapt to climate change.