Long-term thermal history, not short-term exposure to different variability regimes, drives heatwave tolerance in key reef calcifiers

Exposure to fluctuating seawater temperatures can increase tolerance of some tropical corals to marine heatwaves. Whether similar variability can influence tolerance in other key calcifying taxa, such as reef-building algae, remains unknown. We experimentally tested whether diurnal temperature variability enhances heatwave tolerance in four species of tropical calcifying algae, and whether responses differ depending on the occurrence of peak temperatures during the day versus the night. Heatwave exposure suppressed calcification and photophysiological performance in three of the four species, and neither temperature variability, nor the timing of peak thermal exposure, significantly altered this response. These results suggest that the thermal tolerance of the study species is controlled by long-term acclimatization rather than by short-term exposure to different variability regimes. Nonetheless, the metabolic suppression induced by the heatwave demonstrates a poorly understood degree of vulnerability in this functional group. Despite these impacts, the algal species demonstrated greater resilience to heat stress than many reef-building corals likely would under equivalent conditions (7–10 Degree Heating Weeks). Differential thermal susceptibility among reef-building taxa, including potentially greater resilience in some algal species, may strongly influence the ecological trajectories of tropical reef communities as heatwaves increase in frequency and intensity.