Local habitat heterogeneity rivals regional differences in coral thermal tolerance

Variable temperature regimes that expose corals to sublethal heat stress have been recognized as a mechanism to increase coral thermal tolerance and lessen coral bleaching. However, there is a need to better understand which thermal regimes maximize coral stress hardening. Here, standardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (Acropora, Pocillopora, Porites) originating from six reef sites representing an increasing gradient of annual mean diel temperature fluctuations of 1–3 °C day⁻¹. Bleaching severity and dark-acclimated photochemical yield (i.e., F_v/F_m) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3 °C. The greatest thermal tolerance (i.e., F_v/F_m effective dose 50) was found at the site with intermediate mean diel temperature variability (2.2 °C day⁻¹), suggesting there is an optimal priming exposure that leads to maximal thermal tolerance. Interestingly, Acropora and Pocillopora originating from the least thermally variable regimes (< 1.3 °C day⁻¹) had lower thermal tolerance than corals from the most variable sites (> 2.8 °C day⁻¹), whereas the opposite was true for Porites, suggesting divergent responses across taxa. Remarkably, comparisons across global studies revealed that the range in coral thermal tolerance uncovered in this study across a single reef (< 5 km) were as large as differences observed across vast latitudinal gradients (300–900 km). This 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.