Ex Situ Thermal Preconditioning Modulates Coral Physiology and Enhances Heat Tolerance: A Multispecies Perspective for Active Restoration

Global warming threatens reef-building corals by challenging their adaptive capacity. Therefore, interventions such as stress-hardening by thermal preconditioning could become crucial for their survival. This study aimed to systematically assess the effects of distinct thermal preconditioning regimes (stable-high at 29 °C, variable-high at 29 ± 1.5 °C, and stable-ambient control at 26 °C) on the baseline physiology and thermal tolerance of six stony coral species (<i>Galaxea fascicularis</i>, <i>Porites rus</i>, <i>Acropora muricata</i>, <i>Montipora digitata</i>, <i>Pocillopora verrucosa</i>, and <i>Stylophora pistillata</i>) to determine commonalities in the stress-hardening responses that transcend species-specific signatures. For this, we quantified changes in photosynthetic efficiency and bleaching intensity before and after a short-term heat stress assay and up to 30 days later. Stress-hardening was successful in all preconditioned corals, with the variable-high regime slightly outperforming the stable-high regime. Preconditioning reduced the heat stress response by up to 90%, yet species differed in receptiveness. It also improved resilience (survival and recovery), and corals with high inherent thermal tolerance recovered better than susceptible species. Notably, both preconditioning regimes affected baseline physiology, exclusively of the branching species, causing tissue paling and decreased photosynthetic efficiency. We conclude that implementing thermal stress-hardening protocols requires consideration of the species-specific receptiveness and potential physiological trade-offs.