The green alga Ostreobium is an important coral holobiont member, playing key roles in skeletal decalcification and providing photosynthate to bleached corals that have lost their dinoflagellate endosymbionts. Ostreobium lives in the coral's skeleton, a low-light environment with variable pH and O₂ availability. We present the Ostreobium nuclear genome and a metatranscriptomic analysis of healthy and bleached corals to improve our understanding of Ostreobium's adaptations to its extreme environment and its roles as a coral holobiont member. The Ostreobium genome has 10,663 predicted protein-coding genes and shows adaptations for life in low and variable light conditions and other stressors in the endolithic environment. This alga presents a rich repertoire of light-harvesting complex proteins but lacks many genes for photoprotection and photoreceptors. It also has a large arsenal of genes for oxidative stress response. An expansion of extracellular peptidases suggests that Ostreobium may supplement its energy needs by feeding on the organic skeletal matrix, and a diverse set of fermentation pathways allow it to live in the anoxic skeleton at night. Ostreobium depends on other holobiont members for vitamin B12, and our metatranscriptomes identify potential bacterial sources. Metatranscriptomes showed Ostreobium becoming a dominant agent of photosynthesis in bleached corals and provided evidence for variable responses among coral samples and different Ostreobium genotypes. Our work provides a comprehensive understanding of the adaptations of Ostreobium to its extreme environment and an important genomic resource to improve our comprehension of coral holobiont resilience, bleaching and recovery.

中文翻译：

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基因组适应珊瑚相关藻类Ostreobium中的内生石生活方式

绿藻Ostreobium是重要的珊瑚全虹膜成员，在骨骼脱钙中发挥关键作用，并为失去了鞭毛鞭毛内共生体的漂白珊瑚提供光合产物。Ostreobium生活在珊瑚的骨骼中，这是一个弱光环境，具有可变的pH值和O 2可用性。我们介绍了Ostreobium的核基因组，并对健康和漂白的珊瑚进行了转录组分析，以增进我们对Ostreobium适应其极端环境的适应性及其作为珊瑚全生命周期成员的作用的了解。Ostreobium基因组具有10,663个预测的蛋白质编码基因，显示出在弱光和可变光照条件下的生命适应性以及内石环境中的其他应激源。这种藻类具有丰富的光捕获复合蛋白，但是缺乏许多用于光保护和光感受器的基因。它还具有大量的氧化应激反应基因库。细胞外肽酶的扩增表明，骨灰质霉菌可能通过以有机骨架基质为食来补充其能量需求，并且多样化的发酵途径使其能够在夜间生活在缺氧骨骼中。Ostreobium依赖于其他整体维生素成员获取维生素B12，我们的转录组可以识别潜在的细菌来源。元转录组显示，Ostreobium成为了漂白珊瑚中光合作用的主要媒介，并为珊瑚样本和不同Ostreobium基因型之间的可变反应提供了证据。