Empirically derived thermal thresholds of four coral species along the Red Sea using a portable and standardized experimental approach

Abstract Global warming is causing an unprecedented loss of species and habitats worldwide. This particularly apparent for tropical coral reefs, with increasing number reefs experiencing mass bleaching mortality on annual basis. As such, there a growing need standardized experimental approach to rapidly assess the thermal limits corals predict survival across regions. Using portable system, Coral Bleaching Automated Stress System (CBASS), we conducted 18 h acute stress assays quantitively determine upper four length Red Sea coastline, from Gulf Aqaba (GoA) Djibouti (~ 2100 km). We measured dark-acclimated photosynthetic efficiency ( F v /F m ), algal symbiont density, chlorophyll a, visual intensity following heat stress. was most precise response variable assessed, advancing effective dose 50 (ED50, i.e., temperature at which 50% initial measured) as empirically derived proxy tolerance. ED50 thresholds central/southern populations were consistently higher Acropora hemprichii, Pocillopora verrucosa, Stylophora pistillata (0.1–1.8 °C above GoA corals, respectively), in line prevailing warmer maximum monthly means (MMMs), though lower than relative site MMMs (1.5–3.0 °C). P. verrucosa had lowest overall. Despite coming hottest site, Porites lobata southern Sea, suggesting long-term physiological damage or ongoing recovery severe, prior event. Altogether, CBASS resolved historical, taxonomic, possibly recent environmental drivers variation thresholds, highlighting potential standardized, short-term assay universal assessing ecological evolutionary corals.