Corals from the northern Red Sea, in particular the Gulf of Aqaba (GoA), have exceptionally high bleaching thresholds approaching >5℃ above their maximum monthly mean (MMM) temperatures. These elevated thresholds are thought to be due to historical selection, as corals passed through the warmer Southern Red Sea during recolonization from the Arabian Sea. To test this hypothesis, we determined thermal tolerance thresholds of GoA versus central Red Sea (CRS) Stylophora pistillata corals using multi-temperature acute thermal stress assays to determine thermal thresholds. Relative thermal thresholds of GoA and CRS corals were indeed similar and exceptionally high (~7℃ above MMM). However, absolute thermal thresholds of CRS corals were on average 3℃ above those of GoA corals. To explore the molecular underpinnings, we determined gene expression and microbiome response of the coral holobiont. Transcriptomic responses differed markedly, with a strong response to the thermal stress in GoA corals and their symbiotic algae versus a remarkably muted response in CRS colonies. Concomitant to this, coral and algal genes showed temperature-induced expression in GoA corals, while exhibiting fixed high expression (front-loading) in CRS corals. Bacterial community composition of GoA corals changed dramatically under heat stress, whereas CRS corals displayed stable assemblages. We interpret the response of GoA corals as that of a resilient population approaching a tipping point in contrast to a pattern of consistently elevated thermal resistance in CRS corals that cannot further attune. Such response differences suggest distinct thermal tolerance mechanisms that may affect the response of coral populations to ocean warming.

中文翻译：

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对比整个红海珊瑚全生物的热应激反应模式表明不同的热耐受机制

来自红海北部，尤其是亚喀巴湾 (GoA) 的珊瑚具有异常高的白化阈值，比其最高月平均 (MMM) 温度高出 5℃ 以上。这些升高的阈值被认为是由于历史选择造成的，因为珊瑚在从阿拉伯海重新殖民期间穿过了温暖的南红海。为了验证这一假设，我们确定了 GoA 与中央红海 (CRS) Stylophora pistillata 的热耐受阈值珊瑚使用多温度急性热应力测定来确定热阈值。GoA 和 CRS 珊瑚的相对热阈值确实相似且异常高（高于 MMM 约 7℃）。然而，CRS珊瑚的绝对热阈值平均比GoA珊瑚高3℃。为了探索分子基础，我们确定了珊瑚全生物的基因表达和微生物组反应。转录组学反应显着不同，对 GoA 珊瑚及其共生藻类的热应激反应强烈，而在 CRS 菌落中反应明显减弱。与此同时，珊瑚和藻类基因在 GoA 珊瑚中表现出温度诱导的表达，同时在 CRS 珊瑚中表现出固定的高表达（前加载）。GoA珊瑚的细菌群落组成在热应激下发生了巨大变化，而 CRS 珊瑚则表现出稳定的组合。我们将 GoA 珊瑚的反应解释为接近临界点的弹性种群的反应，这与 CRS 珊瑚中无法进一步协调的热阻持续升高的模式形成对比。这种反应差异表明不同的热耐受机制可能会影响珊瑚种群对海洋变暖的反应。