Black corals (Hexacorallia: Antipatharia) are a major component of mesophotic and deep marine ecosystems. Due to their preference for light deprived environments, black corals have historically been considered azooxanthellate, yet recent works have found them in association with dinoflagellates of the family Symbiodiniaceae down to 396 m depth. While corals and Symbiodiniaceae generally establish a symbiotic relationship in shallow water environments, the implications of this association is less well understood at deeper depths, where low light penetration cannot sustain efficient photosynthetic activity for the algae. However, Symbiodinaceae are not obligate autotrophs, and their capacity for heterotrophic feeding categorizes them as mixotrophs. In this study, we investigated the presence and diversity of Symbiodiniaceae associated with the deep-sea black coral Bathypathes thermophila (Antipatharia: Schizopathidae), collected from 204 to 655 m depth in the Saudi Arabian Red Sea. Using high-throughput sequencing of the ITS2 region, we report (1) the deepest record to date of Symbiodiniaceae associated with an anthozoan from 655 m, and (2) the first Red Sea record of Antipatharia in association with Symbiodiniaceae. Our analyses revealed that 14 out of 27 colonies of B. thermophila were associated with Symbiodiniaceae of the genera Cladocopium and Durusdinium. We unveiled 16 novel ITS2 type profiles, possibly unique to black corals and/or to these depths, along with seven profiles that were already known from shallow-water hard corals. No significant pattern was detected in terms of community diversity in relation to depth or sampling locality. Our study supports the existence of black corals-Symbiodiniaceae association and warrants further research to better understand the evolutionary processes and physiological mechanisms driving this association, specifically in light deprived environments.

中文翻译：

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迷失在黑暗中：红海深水中的Antipatharia-Symbiodiniaceae联合体

黑珊瑚（Hexacorallia：Antipatharia）是中光和深海生态系统的主要组成部分。由于黑珊瑚偏爱光照不足的环境，历来被认为是偶氮黄藻类，但最近的研究发现它们与共生藻科的甲藻类共存，深达 396 m。虽然珊瑚和共生藻科通常在浅水环境中建立共生关系，但这种关联的影响在更深的深度却不太清楚，因为低光穿透性无法维持藻类有效的光合作用活动。然而，共生科并不是专性自养生物，它们的异养摄食能力将它们归类为混合营养生物。在这项研究中，我们调查了与深海黑珊瑚相关的共生科的存在和多样性嗜热深水螟（Antipatharia：Schizopathidae），采集于沙特阿拉伯红海 204 至 655 m 深处。利用 ITS2 区域的高通量测序，我们报告了 (1) 迄今为止与距 655 m 的珊瑚虫相关的最深共生体记录，以及 (2) 红海第一个与共生体相关的 Antipatharia 记录。我们的分析表明，27 个菌落中有 14 个嗜热双歧杆菌与共生科属有关枝叶属和杜鲁斯丁属。我们公布了 16 种新颖的 ITS2 类型剖面，可能是黑珊瑚和/或这些深度所独有的，以及浅水硬珊瑚中已知的 7 种剖面。就与深度或采样地点相关的群落多样性而言，没有检测到显着的模式。我们的研究支持黑珊瑚-共生科关联的存在，并需要进一步研究，以更好地了解驱动这种关联的进化过程和生理机制，特别是在光照不足的环境中。