Resolving coral temperature vulnerability through heat and cold bleaching thresholds.

Coral bleaching is most commonly associated with heat stress, while cold-water bleaching remains an underrecognized threat. Building upon the widely used ED50 metric for standardized heat tolerance, we introduce a new metric, cold ED50, to quantify cold bleaching thresholds. By comparing cold and heat ED50s, we define the temperature variability range of coral species. To achieve this, we used Coral Bleaching Automated Stress System (CBASS) assays to assess heat and cold temperature tolerance across three Red Sea scleractinian corals (Acropora sp., Pocillopora favosa, Stylophora pistillata) during peak summer and winter along with microbial profiling. Acropora sp. exhibited the highest heat ED50 (38.68 ± 0.39 °C) in summer, while S. pistillata had the lowest cold ED50 (15.63 ± 0.26 °C) in winter. Our results revealed species-specific bacterial communities, with Endozoicomonadaceae dominating across seasons. We show that bleaching thresholds are negatively correlated with the abundance of Endozoicomonadaceae in Acropora sp. during summer. Notably, coral recovery capabilities after extreme temperatures also vary between species. This dual temperature tolerance framework offers a more comprehensive assessment of coral resilience and vulnerability in a rapidly changing climate.