Reef-building corals are at risk of extinction from ocean warming. While some corals can enhance their thermal limits by associating with dinoflagellate photosymbionts of superior stress tolerance, the extent to which symbiont communities will reorganize under increased warming pressure remains unclear. Here we show that corals in the hottest reefs in the world in the Persian Gulf maintain associations with the same symbionts across 1.5 years despite extreme seasonal warming and acute heat stress (≥35°C). Persian Gulf corals predominantly associated with Cladocopium (clade C) and most also hosted Symbiodinium (clade A) and/or Durusdinium (clade D). This is in contrast to the neighbouring and milder Oman Sea, where corals associated with Durusdinium and only a minority hosted background levels of Cladocopium. During acute heat stress, the higher prevalence of Symbiodinium and Durusdinium in bleached versus nonbleached Persian Gulf corals indicates that genotypes of these background genera did not confer bleaching resistance. Within symbiont genera, the majority of ITS2 rDNA type profiles were unique to their respective coral species, confirming the existence of host-specific symbiont lineages. Notably, further differentiation among Persian Gulf sites demonstrates that symbiont populations are either isolated or specialized over tens to hundreds of kilometres. Thermal tolerance across coral species was associated with the prevalence of a single ITS2 intragenomic sequence variant (C3gulf), definitive of the Cladocopium thermophilum group. The abundance of C3gulf was highest in bleaching-resistant corals and at warmer sites, potentially indicating a specific symbiont genotype (or set of genotypes) that may play a role in thermal tolerance that warrants further investigation. Together, our findings indicate that co-evolution of host-Symbiodiniaceae partnerships favours fidelity rather than flexibility in extreme environments and under future warming.

中文翻译：

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世界上最热的珊瑚礁表现出共生的忠诚度而不是灵活性。

造礁珊瑚有因海洋变暖而灭绝的危险。尽管某些珊瑚可以通过与具有较高胁迫耐受性的鞭毛鞭毛光生物结合，从而提高其温度范围，但尚不清楚共生生物群落在变暖压力下的重组程度。在这里，我们表明，尽管极端季节性变暖和急性高温（≥35°C），但在波斯湾世界上最热的珊瑚礁中，珊瑚在1.5年内仍具有相同的共生关系。波斯湾珊瑚主要与克拉多菌属（C类）相关，大多数还与共生菌属（A类）和/或Durusdinium（D类）有关。这与邻近和较温和的阿曼海形成了鲜明对比，阿曼海中的珊瑚与杜鲁斯丁尼姆有关，只有极少数的珊瑚具有本底的背景。在急性高温下 在漂白的和未漂白的波斯湾珊瑚中，共生菌和硬孢菌素的较高发生率表明这些背景属的基因型没有赋予抗漂白性。在共生菌属中，大多数ITS2 rDNA类型谱对于它们各自的珊瑚物种而言是独特的，从而证实了宿主特异性共生菌谱系的存在。值得注意的是，波斯湾站点之间的进一步区分表明，共生种群要么是孤立的，要么是数十到数百公里内的专门化种群。跨珊瑚物种的热耐受性与单个ITS2基因组内序列变异体（C3gulf）的流行有关，这是嗜热克拉科菌属的定义。在抗漂白的珊瑚和较暖的地方，C3gulf的含量最高，可能表明特定的共生基因型（或一组基因型）可能在耐热性中起作用，有待进一步研究。总之，我们的发现表明，寄主-共生植物科的伙伴关系的共同发展有利于保真而不是在极端环境和未来变暖的情况下的灵活性。