Heat-tolerant subtropical Porites lutea may be better adapted to future climate change than tropical one in the South China Sea.

Coral reefs are degrading at an accelerating rate owing to climate change. Understanding the heat stress tolerance of corals is vital for their sustainability. However, this tolerance varies substantially geographically, and information regarding coral responses across latitudes is lacking. In this study, we conducted a high temperature (34 °C) stress experiment on Porites lutea from tropical Xisha Islands (XS) and subtropical Daya Bay (DY) in the South China Sea (SCS). We compared physiological levels, antioxidant activities, and transcriptome sequencing to explore heat tolerance mechanisms and adaptive potential. At 34 °C, both XS and DY corals experienced significant bleaching and the physiological/biochemical index decreased, with XS corals exhibiting greater changes than DY corals. Transcriptome analysis revealed that coral hosts respond to heat stress mainly by boosting metabolic activity. The subtle transcriptional responses of zooxanthellae C15 underscored the host's pivotal role in thermal stress responses. DY coral hosts showed lower bleaching, stronger physiological plasticity, and higher temperature tolerance thresholds than XS, indicating superior heat tolerance. This superiority is linked to negative feedback transcriptional regulation strategies, including active environmental stress response and genetic information damage repair. The differences in thermal adaptability between tropical and subtropical P. lutea in the SCS may be attributed to their genetic differences and native habitat environments, suggesting that subtropical P. lutea may have the potential to adapt to future climate change. This study provides novel insights for predicting the fate of corals at different latitudes in terms of global warming and provides instructive guidance for coral reef ecological restoration.