Abstract 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.
