Gulf of Aqaba as a thermal refuge: Insights from four years of intensifying marine heatwaves

Marine heatwaves (MHWs) are intensifying, posing an increasing threat to coral reefs globally. Despite warming faster than the global average, the Gulf of Aqaba (GoA, Northern Red Sea) has been regarded as a climate refuge for corals. In this study, we analyzed coral-algae symbiosis integrity based on algal densities, chlorophyll content, and host and symbiont energy reserves (proteins, carbohydrates, lipids) of tagged colonies of Stylophora pistillata and Pocillopora damicornis throughout four consecutive summers characterized by moderate (2022), strong (2021, 2023), and severe (2024) MHWs. The 2024 MHW, lasting 113 days, was exceptional, with sea surface temperatures reaching 32.6 °C, 3.4 °C above the long-term climatology, and accumulating 30 Degree Heating Weeks, the highest recorded globally during 2024. Nonetheless, S. pistillata and P. damicornis persisted without bleaching, while maintaining stable host energy reserves and higher symbiont carbohydrates in 2024 compared to previous summers. To assess potential species-specific responses, we additionally monitored Seriatopora hystrix, Porites spp., and Cyphastrea spp. colonies before, during, and after the severe 2024 MHW. All monitored colonies endured the unprecedented thermal stress without bleaching or photosynthetic impairment. Porites spp. displayed metabolic tolerance with stable values throughout the event. Only Cyphastrea spp. showed significant symbiont reduction (-55 %) during peak stress, recovering by January 2025. As climate change places 44 % of reef-building corals at risk of extinction, our findings highlight the resilience of GoA corals. However, emerging shallow bleaching suggests that this refuge may be approaching its limit, underscoring the urgent need for regional conservation efforts.