Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.07.21.211367v1?rss=1 Authors: Iha, C., Varela, J. A., Avila, V., Jackson, C. J., Bogaert, K. A., Chen, Y., Judd, L. M., Wick, R. R., Holt, K. E., Pasella, M. M., Ricci, F., Repetti, S. I., Medina, M., Marcelino, V. R., Chan, C. X., Verbruggen, H. Abstract: 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 corals 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 Ostreobiums adaptations to its extreme environment and its roles as a coral holobiont member. The Ostreobium genome has 11,735 predicted protein-coding genes and shows adaptations for life in low and variable light conditions and other stressors in the endolithic environment. It presents an unusually rich repertoire of light harvesting complex proteins, lacks many genes for photoprotection and photoreceptors, and has a large arsenal of genes for oxidative stress response. An expansion of extracellular peptidases suggests that Ostreobium may supplement its energy needs and compensate for its lysine auxotrophy 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 biotin and 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. Copy rights belong to original authors. Visit the link for more info