| Summary: | Effects of ocean acidification (OA) on deep-sea coral habitats in Alaska could be pronounced given the particularly shallow and rapidly shoaling calcite and aragonite saturation horizons in the region. The magnitude of potential effects could partially depend on the corals' calcium carbonate mineralogy. We used X-ray diffraction and powerful full-pattern Rietveld data refinement to precisely determine the skeletal composition of 62 species of Alaskan corals-the most comprehensive cold-water coral dataset for any region of the world. Alaskan corals have complex mineralogy, including a high percentage of slightly polymorphic taxa. Scleractinians and octocorals were principally aragonite and calcite, respectively. A few octocorals were composed of the most soluble form of calcium carbonate (high-Mg calcite). Hydrocorals have the most complex mineralogy with many polymorphic taxa, and some genera have both aragonite and calcite species. Most coral taxa live at least partially below the current saturation horizons so may be physiologically compensating for the effects of OA via important life-history trade-offs. We found evidence of mineral-switching related to depth distribution or broad-scale biogeography. All Alaskan corals are protected by organic tissue and may have the ability to up-regulate the pH of internal calcifying fluid relative to ambient seawater. No Alaskan corals are at risk for skeletal dissolution based on present-day carbonate chemistry conditions in the North Pacific Ocean although the carbonate mineralogy of a few taxa may approach estimated dissolution points. Alaska's ecologically most important corals (Primnoa pacifica and Stylaster spp.) are most at risk to potential effects of OA given their highly soluble skeletons, depth distribution, and observed propensity for tissue loss from contact with fishing gear and predation. Laboratory experiments are currently underway to determine if Primnoa pacifica can tolerate carbonate chemistry conditions predicted for year 2100 and maintain important life-history functions.
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