Susceptibility of Acropora tenuis to consecutive thermal stress

Elevated temperatures cause mass coral bleaching, leading to reef degradation. The frequency of bleaching events is increasing, and severe bleaching events have been predicted to occur annually in the next few decades. However, the ability of corals to acclimate and adapt to these unprecedented stresses remains unknown. In this study, we investigated how three years of consecutive thermal stress affect the adult fragments of the coral Acropora tenuis. The fragments were exposed to temperature treatments of ~ 28 °C (control) and ~ 31 °C (heat stress) until they began to bleach. We measured the survival rate, maximum quantum yield of photosystem II (Fv/Fm) of the symbiotic algae, and algal density of the fragments. The survival rate of the fragments under thermal stress decreased over the three-year period, reaching 20% by the end. Additionally, we observed a decrease in Fv/Fm and a reduction in algal density in the stressed fragments compared to those in the control fragments during all three years of the thermal stress period. These findings collectively suggested that consecutive bleaching-level thermal stress increases the susceptibility of corals to heat.