Table_1_Lost in the dark: Antipatharia-Symbiodiniaceae association in the deep waters of the Red Sea.xlsx

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.

黑珊瑚（Hexacorallia: Antipatharia）是中深层与深海海洋生态系统的关键组成部分。鉴于其偏好无光环境，黑珊瑚长期以来被认为是无共生虫黄藻（azooxanthellate）类群，但近期研究发现它们可与Symbiodiniaceae（虫黄藻科）甲藻共生，最深记录可达396米水深。尽管珊瑚与Symbiodiniaceae通常在浅水环境中建立稳定共生关系，但在深层海域，光照穿透度极低，无法维持藻类高效光合活性，因此该共生关系在深水环境中的生态学意义仍不甚明晰。不过，Symbiodiniaceae并非专性自养生物，其兼具异养营养的能力使其被归类为混合营养型生物。本研究针对采集自沙特阿拉伯红海204至655米水深的深海黑珊瑚嗜热深黑珊瑚（Bathypathes thermophila, Antipatharia: Schizopathidae），探究了其共生Symbiodiniaceae的存在与多样性。通过对ITS2（内转录间隔区2）区域的高通量测序，本研究获得两项重要发现：（1）截至目前，655米水深中共生有Symbiodiniaceae的六放珊瑚（anthozoan）最深记录；（2）红海地区首次记录到黑珊瑚目类群与Symbiodiniaceae共生。分析结果显示，27株嗜热深黑珊瑚群体中，有14株与Cladocopium属和Durusdinium属的Symbiodiniaceae类群共生。本研究揭示了16种全新的ITS2型谱，推测可能为黑珊瑚或该深度环境所特有，同时还发现7种已知的浅水硬珊瑚ITS2型谱。未检测到共生群落多样性与采样深度或采样地点存在显著关联模式。本研究证实了黑珊瑚与Symbiodiniaceae共生关系的存在，亟需开展进一步研究以深入阐明驱动该共生关系的演化过程与生理机制，尤其是在无光环境下的相关机制。