Beyond peak summer temperatures, branching corals in the Gulf of Aqaba are resilient to thermal stress but sensitive to high light

Despite rapidly rising sea surface temperatures and recurrent positive temperature anomalies, corals in the Gulf of Aqaba (GoA) rarely experience thermal bleaching. Elsewhere, mass coral bleaching has been observed in corals when the water temperature exceeds 1–2 °C above the local maximum monthly mean (MMM). This threshold value or “bleaching rule” has been used to create predictive models of bleaching from satellite sea surface temperature observations, namely the “degree heating week” index. This study aimed to characterize the physiological changes of dominant reef building corals from the GoA in response to a temperature and light stress gradient. Coral collection and experiments began after a period of 14 consecutive days above MMM in the field. Stylophora pistillata showed negligible changes in symbiont and host physiology parameters after accumulating up to 9.4 degree heating weeks during peak summer temperatures, for which the index predicts widespread bleaching and some mortality. This result demonstrates acute thermal tolerance in S. pistillata from the GoA and deviation from the bleaching rule. In a second experiment after 4 weeks at 4 °C above peak summer temperatures, S. pistillata and Acropora eurystoma in the high-light treatment visibly paled and suffered greater midday and afternoon photoinhibition compared to corals under low-light conditions (35% of high-light treatment). However, light, not temperature (alone or in synergy with light), was the dominant factor in causing paling and the effective quantum yield of corals at 4 °C above ambient was indistinguishable from those in the ambient control. This result highlights the exceptional, atypical thermal tolerance of dominant GoA branching corals. Concomitantly, it validates the efficacy of protecting GoA reefs from local stressors if they are to serve as a coral refuge in the face of global sea temperature rise.