Assessment of temperature optimum signatures of corals at both latitudinal extremes of the Red Sea

Abstract

Rising ocean temperatures are pushing reef-building corals beyond their temperature optima (Topt), resulting in reduced physiological performances and increased risk of bleaching. Identifying refugia with thermally resistant corals and understanding their thermal adaptation strategy is therefore urgent to guide conservation actions. The Gulf of Aqaba (GoA, northern Red Sea) is considered a climate refuge, hosting corals that may originate from populations selected for thermal resistance in the warmer waters of the Gulf of Tadjoura (GoT, entrance to the Red Sea and 2000 km south of the GoA). To better understand the thermal adaptation strategy of GoA corals, we compared the temperature optima (Topt) of six common reef-building coral species from the GoA and the GoT by measuring oxygen production and consumption rates as well as photophysiological performance (i.e. chlorophyll fluorescence) in response to a short heat stress. Most species displayed similar Topt between the two locations, highlighting an exceptional continuity in their respective physiological performances across such a large latitudinal range, supporting the GoA refuge theory. Stylophora pistillata showed a significantly lower Topt in the GoA, which may suggest an ongoing population-level selection (i.e. adaptation) to the cooler waters of the GoA and subsequent loss of thermal resistance. Interestingly, all Topt were significantly above the local maximum monthly mean seawater temperatures in the GoA (27.1°C) and close or below in the GoT (30.9°C), indicating that GoA corals, unlike those in the GoT, may survive ocean warming in the next few decades. Finally, Acropora muricata and Porites lobata displayed higher photophysiological performance than most species, which may translate to dominance in local reef communities under future thermal scenarios. Overall, this study is the first to compare the Topt of common reef-building coral species over such a latitudinal range and provides insights into their thermal adaptation in the Red Sea.

High variability and enhanced nocturnal oxygen uptake in coral reef sponges

“…To gauge the presence of photosymbionts and estimate the photosynthetic potential of each sponge species we obtained several fresh representative samples of ~ 5 square cm from each sponge taxa and transferred them in seawater to the lab, where they were qualitatively examined using an Imaging‐PAM (Walz GmbH, Effeltrich, Germany) for the presence of photosynthetic activity by measuring the maximum photochemical efficiency (Fv/fm) as described in Banc‐Prandi et al (2022).…”

Higher daily temperature range at depth is linked with higher thermotolerance in antipatharians from the canary islands

“…levels of response of a physiological variable) to thermal stress across populations, could be modulated by thermal history, in particular by temperature variability. Studies showed that thermal performances of scleractinians can vary across latitudes (Astrangia poculata, Aichelman et al, 2019; Orbicella franksi, Silbiger et al, 2019;Stylophora pistillata, Acropora muricata, Porites lobata, Seriatopora hystrix, and Pocillopora verrucosa;Banc-Prandi et al, 2022) and seasons (Porites cylindrical and Acropora valenciennesi; Jurriaans et al, 2019), due to thermal adaptation/acclimatization. Studies that have compared thermal sensitivities of scleractinians across depths either found no differences in thermal sensitivity (instantaneous acute heat stress, 5-10 vs. 30-35m;Gould et al, 2021), higher sensitivity (ramping of 1°C/day; 20 vs. 43m; Frates et al, 2021) or lower sensitivity (ramping of 1°C/hour; 25 vs. 85m; Godefroid et al, 2023) in mesophotic colonies.…”