Coral heat tolerance under variable temperatures: effects of different variability regimes and past environmental history vs. current exposure

Exposure to high-frequency temperature variability often but not always enhances coral heat tolerance, raising the question of whether this depends on the type of variability regime and past vs. current exposure. We collected corals from a macrotidal, highly fluctuating temperature environment and preconditioned them to either constant or variable daily temperatures for ~ 1.5 yr. Corals were then exposed to three new temperature variability regimes for ~ 1 month (constant control, symmetric variability, and tidal variability) to assess the effect of short-term environmental history, followed by a 12-d heat stress test. Measurements of visual coral health, photophysiology, photosynthesis, respiration, and calcification rates showed that preconditioning to constant vs. variable temperatures for 1.5 yr did not significantly impact coral physiology and heat tolerance. In contrast, environmental history experienced in the month prior to the heat stress test significantly influenced the physiological responses, with corals exposed to both types of variability having lower heat tolerance. Interestingly, corals in the tidal variability regime suffered greater health declines than in the symmetric variability regime although both treatments had the same cumulative heat exposure. Since heating rate and temperature amplitude were higher in the tidal variability regime (but time spent above the bleaching threshold was shorter), this suggests that short, extreme heat pulses may be more deleterious than longer but more moderate ones, though other factors likely also played a role. Overall, our findings demonstrate that daily temperature variability has significant potential to alter coral heat tolerance but only certain types of variability may enhance coral adaptive capacity.