Adaptive mechanisms of the deep-sea coral Polymyces wellsi (Flabellidae, Scleractinia) illuminate strategies for global climate change

The oxygen minimum zone (OMZ) typically occurs in the tropical western Pacific with rather low oxygen, unfavorably low pH and extreme food limitation. Understanding how the deep-sea corals survive these rough conditions, especially how they operate calcification in the depth near the aragonite saturation horizon, is of great scientific interest. In this study, we obtained a deep-sea solitary coral Polymyces wellsi living in the OMZ of the Caroline Ridge and analyzed its mitochondrial genome and transcriptome. Phylogenetic analysis based on mitochondrial genomes suggested that the solitary character and deep-sea adaptation evolved at least twice in Scleractinia. Compared with the shallow water relatives, the genes associated with biomineralization, mitochondrial components and cilia motion were both positively selected and expanded in P. wellsi, indicating their roles in the adaptations to low pH, insufficient oxygen and scare food supply in the OMZ, respectively. The involvement of the positively selective genes and expanded domains in biomineralization can be further supported by the high expression of their homologues in the calicoblastic cells that participate in the process of calcification. An interesting finding in this study is that the positively selected amino acids in P. wellsi increased the isoelectric points of its skeleton organic matrix proteins, presenting an energy-saving strategy for the deep-sea coral to maintain a suitable pH in its calcifying fluid and normally precipitate CaCO3 under the stress of external acidified seawater. Overall, this study provides valuable genetic resource for studying the adaptive mechanisms of deep-sea scleractinian corals inhabiting the OMZ.