Reef building corals show resilience to the hottest marine heatwave on record in the Gulf of Aqaba

Coral reefs are facing rapid deterioration, primarily due to a global rise in seawater temperature. In conjunction, the frequency and intensity of extreme high temperature events, known as marine heatwaves (MHWs), are increasing. The Gulf of Aqaba (GoA) in the northern Red Sea is home to corals known for their thermal resilience, yet concerns have been raised regarding the potential for MHWs to put this coral refuge at risk. In summer of 2021, the hottest MHW so far occurred in the GoA, with sea surface temperatures peaking at 31 degrees C and persisting above the local summer maximum for 34 days. To assess the physiological response of the corals Stylophora pistillata and Pocillopora damicornis to this event, we analyzed the monthly content across a year of host and symbiont proteins, carbohydrates, and lipids, pre-, during, and post the MHW, as a proxy for metabolic stress. We found that the MHW was not fatal to either species and did not induce bleaching, based on algal densities and chlorophyll content. Species-specific responses were detected. In S. pistillata, host protein content decreased (33%) at the onset of the MHW (August) compared to pre-MHW levels (July). Algal symbionts of S. pistillata were unaffected by the MHW in their maximal photosynthetic efficiency (Fv/Fm) and exhibited higher carbohydrate levels (+34%) at the end of the MHW (September) compared to its onset. In contrast, no significant catabolic response was detected in P. damicornis host or symbionts, and the maximal relative electron transport rate (rETRmax) of symbionts was 37% higher during the MHW than the annual average. These results highlight the remarkable ability of common GoA corals to withstand extreme thermal anomalies, underscoring the global significance of this coral refuge.