Thermal preconditioning modulates coral physiology and heat tolerance: A multi-species perspective

Global warming threatens reef-building corals by challenging their natural adaptive capacity. Therefore, interventions such as stress hardening by thermal preconditioning could become crucial for their survival. Stress-hardening approaches recognize that organisms living in thermally variable environments are better able to withstand marine heat waves. However, a systematic assessment of preconditioning effects on the baseline physiology and thermal tolerance across coral species is lacking. We assessed the changes of thermal tolerance in six stony coral species (Galaxea fascicularis, Porites rus, Acropora muricata, Montipora digitata, and Stylophora pistillata) in three thermal preconditioning treatments of stable-high 29 {degrees}C and variable-high 29 {degrees}C with a daily oscillation of {+/-} 1.5 {degrees}C, compared to corals in stable-ambient 26 {degrees}C. We quantified changes in photosynthetic efficiency and coral bleaching intensity before and after a short-term heat stress assay and up to 30 days later. Stress-hardening success after preconditioning was observed in nearly all preconditioned corals, but the increases in thermal tolerance were species-specific. The greatest increase was recorded in G. fascicularis and A. muricata, with stress responses reduced by over 80 %. In contrast, preconditioning regimes had minor effects on stress tolerance of S. pistillata, making it least receptive to this intervention. After 30 days, most stress-hardened species demonstrated higher survival and recovery rates than their conspecifics from the stable-ambient regime. Notably, both preconditioning regimes affected baseline physiology, especially in the branching species, as indicated by minor tissue paling and decreased photosynthetic efficiency. We conclude that implementing thermal stress hardening protocols will require careful consideration of the species-specific receptiveness and evaluation of the potential trade-offs that can be inflicted with the post-conditioning shifts in physiological baselines.