The roles of heating rate, intensity, and duration on the response of corals and their endosymbiotic algae to thermal stress

Anthropogenic ocean warming is one of the biggest threats to marine organisms worldwide. However, it remains unclear how the duration and intensity of thermal anomalies affect organismal stress responses and thermal thresholds. We used detailed tracking of coral endosymbiont and host physiology and dose-response analyses to compare the effects of multiple heating rates, intensities, and exposure durations on two reef-building corals, Acropora hemprichii and Porites lobata, from adjacent sides of a reef (protected vs. exposed) in the Central Red Sea known to differ in high-frequency (< 24 h) temperature variability. Corals were exposed to acute heat exposures (18 h) with four target temperatures (32 °C, 35 °C, 36.5 °C, and 38 °C), versus prolonged heat exposures lasting 7–15 days where temperatures were raised 0.5 and 1.5 °C day⁻¹ to four target temperatures (32 °C, 33.5 °C, 35 °C, and 36.5 °C). In the prolonged experiment, dose-response curves assessing algal endosymbiont F_v/F_m revealed little initial effect of temperature, before an exponential decline above 34 °C for both species. Temperature at time of measurement and degree heating hours above 34 °C (DHH₃₄) were the variables most strongly associated with declines in F_v/F_m. The F_v/F_m thermal thresholds for P. lobata from the high-variability protected site were higher than the exposed site in the faster heating, prolonged heat stress experiment despite minimal differences in endosymbiont density, chlorophyll-a, and host protein between sites. Together, our dose-response analysis revealed complex effects of DHH₃₄, heating rate, and species-specific differences in the influence of local thermal histories shaping thermotolerance limits for these corals.