Maximal coral thermal tolerance is found at intermediate diel temperature variability

O_LIIt has become critically important to identify environmental drivers of enhanced thermal tolerance in coral populations as ocean warming threatens the persistence of coral reef ecosystems globally. Variable temperature regimes that expose corals to sub-lethal heat stress have been recognized as a mechanism to increase coral thermotolerance and lessen coral bleaching; however, there is a need to better understand which thermal regimes are best for promoting coral stress hardening, and if thermal priming results in consistent benefits across species with distinct life-history strategies. C_LIO_LIStandardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (Acropora, Pocillopora and Porites) originating from six reef sites fluctuating in temperature by up to 7.7{degrees}C day-1, with an annual mean diel variability of 1-3{degrees}C day-1. Bleaching severity and dark-acclimated photochemical yield (Fv/Fm) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3{degrees}C -- up to 9{degrees}C above the regional maximum monthly mean. C_LIO_LIThe greatest thermal tolerance across all species was found at the site with intermediate mean diel temperature variability (2.2{degrees}C day-1), suggesting there is an optimal priming exposure that leads to maximal thermotolerance. Interestingly, Acropora and Pocillopora originating from the least thermally variable regimes (i.e., 2.8{degrees}C day-1), whereas the opposite was true for Porites, suggesting divergent responses to priming across taxa. C_LIO_LIWe highlight that fine-scale heterogeneity in temperature dynamics across habitats can increase coral thermal tolerance in diverse coral lineages, although in a non-linear manner. Remarkably, comparisons across global studies revealed that the range in coral thermotolerance uncovered in this study across a single reef system (300 km). This important finding indicates that local gene flow could improve thermal tolerance between habitats. However, as climate change continues, exposure to intensifying marine heatwaves is already compromising thermal priming as a mechanism to enhance coral thermal tolerance and bleaching resistance. C_LI