Coral calcification

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The process by which the polyp precipitates aragonite from seawater at the base of its tissue, in a thin calcifying fluid space that it actively controls. The coral raises the pH and the carbonate ion concentration of that space above ambient seawater by pumping protons out, which is what makes precipitation thermodynamically favourable in water that would not spontaneously produce it. The energy for that pumping comes substantially from the symbionts, which is why calcification is light-enhanced: rates measured in daylight are several times those measured in the dark, and a bleached colony calcifies very little. Two consequences follow that a reader should hold together. Calcification is not merely correlated with the symbiosis, it is powered by it. And because the process depends on the ambient carbonate chemistry the coral is working against, it becomes more expensive as that chemistry changes.

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