Global warming is resulting in unprecedented levels of coral mortality due to mass bleaching events and, more recently, marine heatwaves, where rapid increases in seawater temperature cause mortality within days. Here, we compare the response of a ubiquitous scleractinian coral, Stylophora pistillata, from the northern Red Sea to acute (7 h) and chronic (7-11 d) thermal stress events that include temperature treatments of 27 C (i.e., the local maximum monthly mean), 29.5 C, 32 C, and 34.5 C, and assess recovery of the corals following exposure. Overall, S. pistillata exhibited remarkably similar responses to acute and chronic thermal stress, responding primarily to the temperature treatment rather than duration or heating rate. Additionally, corals displayed an exceptionally high thermal tolerance, maintaining their physiological performance and suffering little to no loss of algal symbionts or chlorophyll a up to 32 C, before the host suffered from rapid tissue necrosis and mortality at 34.5 C. While there was some variability in physiological response metrics, photosynthetic efficiency measurements (i.e., maximum quantum yield Fv/Fm) accurately reflected the overall physiological response patterns, with these measurements used to produce the Fv/Fm effective dose (ED 50) metric as a proxy for the thermal tolerance of corals. This approach produced similar ED 50 values for the acute and chronic experiments (34.47 C vs. 33.81 C), highlighting the potential for acute thermal assays with measurements of Fv/Fm as a systematic and standardized approach to quantitively compare the upper thermal limits of reef-building corals using a portable experimental system.

Reef coral calcification depends on regulation of pH in the internal calcifying fluid (CF) in which the coral skeleton forms. However, little is known about calcifying fluid pH (pH) regulation, despite its importance in determining the response of corals to ocean acidification. Here, we investigate pH in the coral Stylophora pistillata in seawater maintained at constant pH with manipulated carbonate chemistry to alter dissolved inorganic carbon (DIC) concentration, and therefore total alkalinity (A). We also investigate the intracellular pH of calcifying cells, photosynthesis, respiration and calcification rates under the same conditions. Our results show that despite constant pH in the surrounding seawater, pH is sensitive to shifts in carbonate chemistry associated with changes in [DIC] and [A], revealing that seawater pH is not the sole driver of pH Notably, when we synthesize our results with published data, we identify linear relationships of pH with the seawater [DIC]/[H] ratio, [A]/ [H] ratio and [[Formula: see text]]. Our findings contribute new insights into the mechanisms determining the sensitivity of coral calcification to changes in seawater carbonate chemistry, which are needed for predicting effects of environmental change on coral reefs and for robust interpretations of isotopic palaeoenvironmental records in coral skeletons.

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