Short-term recovery responses in Acropora hyacinthus exposed to moderate-term thermal stress

Coral bleaching, driven by rising ocean temperatures, poses a significant threat to coral reefs worldwide. This study investigates the short-term recovery dynamics of the common Indo-Pacific coral Acropora hyacinthus following a simulated moderate-term bleaching event. Coral fragments were collected from Nishidomari, Kochi, Japan, and subjected to a controlled heat stress experiment in aquaria. Symbiodiniaceae physiology, including cell density, chlorophyll a concentration, and photosynthetic efficiency, was monitored throughout the bleaching and recovery periods. Coral host transcriptomic analysis was conducted to identify differentially expressed genes and to assess the functional enrichment of biological processes. During the bleaching period, significant declines in Symbiodiniaceae physiology were observed, consistent with prior bleaching studies. Unexpectedly, during the short-term recovery period, Symbiodiniaceae physiology continued to decline. However, the host transcriptome revealed the expression of recovery-specific genes and a shift from stress response to metabolic status reprogramming, like responses observed in other coral species. This upregulation of genes associated with metabolic reprogramming suggests that the coral host was actively working to repair and restore homeostasis despite the continued decline in Symbiodiniaceae cell density. This highlights the importance of integrating molecular data with physiological measurements to effectively evaluate coral recovery status. Further research is needed to determine if this rapid transcriptomic response translates into long-term resilience, particularly in the face of increasingly frequent bleaching events.