Trade-offs in a reef-building coral after six years of thermal acclimation.

There is growing evidence that reef-building corals can acclimate to novel and challenging thermal conditions. However, potential trade-offs that accompany acclimation remain largely unexplored. We investigated physiological trade-offs in colonies of a globally abundant coral species (Pocillopora acuta) that were acclimated ex situ to an elevated temperature of 31 °C (i.e., 1 °C above their bleaching threshold) for six years. By comparing them to conspecifics maintained at a cooler temperature, we found that the energy storage of corals was prioritized over skeletal growth at the elevated temperature. This was associated with the formation of higher density skeletons, lower calcification rates and consequently lower skeletal extension rates, which entails ramifications for future reef-building processes, structural complexity and reef community composition. Furthermore, symbionts were physiologically compromised at 31 °C and had overall lower energy reserves, likely due to increased exploitation by their host, resulting in an overall lower stress resilience of the holobiont. Our study shows how biological trade-offs of thermal acclimation unfold, helping to refine our picture of future coral reef trajectories. Importantly, our observations in this six-year study do not align with observations of short-term studies, where elevated temperatures were often associated with the depletion of energy reserves, highlighting the importance of studying acclimation of organisms at relevant biological scales.