Thermal extremes likely trigger metabolic imbalance in coral holobionts

Rising ocean temperatures are constraining the availability of dissolved oxygen and simultaneously increasing the respiratory oxygen requirements of marine organisms. This is particularly relevant for tropical corals, as periods of anomalously high temperature destabilize the symbiosis between corals and Symbiodiniaceae, resulting in coral bleaching. These observations point towards a possible role of mismatched rates of photosynthetic oxygen production and consumption in contributing to the breakdown of the holobiont under heat stress. Here we use a global dataset comprising experimentally derived relationships between coral metabolic rates and temperature to investigate this hypothesis. Across all available relationships, we calculated and analysed the activation energy (E), optimum temperature (T_opt) of respiration, net productivity, gross productivity and where possible, P: R ratio. Despite known variations in the thermal tolerances among corals in our database, we resolved composite thermal performance curves for scleractinian corals and provide insight into differences between tropical and temperate corals and among selected genera. We show that after the theoretical T_opt is exceeded, photosynthesis declines at a faster rate than respiration. At temperatures exceeding the theoretical T_opt for net productivity, this metabolic mismatch could possibly contribute to the destabilization of the coral-symbiont association. Specifically, we postulate that a lack of symbiont oxygen production and heightened holobiont respiratory demand at peak temperatures represents a burden on the oxygen budget of the holobiont.