Short-term exposure to different thermal variability regimes does not shift heatwave tolerance in tropical calcifying macroalgae

Exposure to fluctuating seawater temperatures can increase the physiological tolerance of some marine organisms to heatwaves. Yet, whether thermal variability similarly influences the tolerances of key calcifying taxa, such as reef-building algae, remains unresolved. We experimentally tested whether diurnal temperature variability enhances heatwave tolerance in four species of tropical calcifying algae ( Halimeda opuntia , Halimeda discoidea , Neogoniolithon sp. and Lithophyllum sp.), 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. The thermal tolerances of the study species may, therefore, be rooted in their long-term environmental histories, rather than short-term exposure to different variability regimes. 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.