Seawater carbonate chemistry and 4D insights into coral growth under ocean acidification

We focus on primary polyps, the initial coral calcifying stage, of Stylophora pistillata, an ubiquitous Indo-Pacific stony coral species, grown under ambient and under acidic conditions (pH 8.2 and 7.6, respectively; NBS scale) predicted to occur by the end of the 21st century under the high greenhouse gas emission scenario RCP8.5. Here, we bring together results from scanning electron microscopy (SEM), synchrotron X-ray μCT, X-ray diffraction (XRD), and X-ray fluorescence (XRF) enhanced with Monte-Carlo simulations, to understand the behavior of backscattered electrons within coral skeleton material. This made it possible to determine the mineral phase composition, while revealing new sub-micrometer insights into the 3D mineral growth dynamics. Our results provide insights into newly formed coral skeletons including mineral characteristics, growth patterns and geometric relations of rapid accretion deposits (RADs) and thickening deposits (TDs) under normal and acidified oceans.