Full in vivo characterization of carbonate chemistry at the site of calcification in corals

Reef-building corals form their calciumcarbonate skeletonswithin an extracellular calcifying medium(ECM). Despite the critical role of the ECM in coral calcification, ECM carbonate chemistry is poorly constrained in vivo, and full ECM carbonate chemistry has never been characterized based solely on direct in vivo measurements. Here, we measure pH(ECM) in the growing edge of Stylophora pistillata by simultaneously using microsensors and the fluorescent dye SNARF-1, showing that, when measured at the same time and place, the results agree. We then conduct microscopeguided microsensor measurements of pH, [Ca2+], and [CO32-] in the ECM and, from this, determine [DIC](ECM) and aragonite saturation state (Omega(arag)), showing that all parameters are elevatedwith respect to the surrounding seawater. Our study provides the most complete in vivo characterization of ECM carbonate chemistry parameters in a coral species to date, pointing to the key role of calcium-and carbon-concentrating mechanisms in coral calcification.