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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. pistillataexhibited 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 chlorophyllaup 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 yieldFv/Fm) accurately reflected the overall physiological response patterns, with these measurements used to produce theFv/Fmeffective dose (ED₅₀) metric as a proxy for the thermal tolerance of corals. This approach produced similar ED₅₀values for the acute and chronic experiments (34.47°C vs. 33.81°C), highlighting the potential for acute thermal assays with measurements ofFv/Fmas a systematic and standardized approach to quantitively compare the upper thermal limits of reef‐building corals using a portable experimental system.

 

Corals from the northern Red Sea and Gulf of Aqaba exhibit extreme thermal tolerance. To examine the underlying gene expression dynamics, we exposed Stylophora pistillata from the Gulf of Aqaba to short-term (hours) and long-term (weeks) heat stress with peak seawater temperatures ranging from their maximum monthly mean of 27 °C (baseline) to 29.5 °C, 32 °C, and 34.5 °C. Corals were sampled at the end of the heat stress as well as after a recovery period at baseline temperature. Changes in coral host and symbiotic algal gene expression were determined via RNA-sequencing (RNA-Seq). Shifts in coral microbiome composition were detected by complementary DNA (cDNA)-based 16S ribosomal RNA (rRNA) gene sequencing. In all experiments up to 32 °C, RNA-Seq revealed fast and pervasive changes in gene expression, primarily in the coral host, followed by a return to baseline gene expression for the majority of coral (>94%) and algal (>71%) genes during recovery. At 34.5 °C, large differences in gene expression were observed with minimal recovery, high coral mortality, and a microbiome dominated by opportunistic bacteria (including Vibrio species), indicating that a lethal temperature threshold had been crossed. Our results show that the S. pistillata holobiont can mount a rapid and pervasive gene expression response contingent on the amplitude and duration of the thermal stress. We propose that the transcriptomic resilience and transcriptomic acclimation observed are key to the extraordinary thermal tolerance of this holobiont and, by inference, of other northern Red Sea coral holobionts, up to seawater temperatures of at least 32 °C, that is, 5 °C above their current maximum monthly mean.