Figure 4.

Schematics of the (a) coral calcification mechanisms [18,21,48], and (b) calcification processes. Scleractinian corals create their calcium carbonate skeletons within an extracellular calcifying fluid located between the sub-calioblastic cells and the skeleton with fluid supplied from the seawater [48]. Coral pH upregulation occurs via the pumping of H⁺ out of the calcifying fluid, promoting the diffusion of metabolic CO₂ from the mitochondria (M) into the calcifying fluid. CO₂ is converted into bicarbonate using carbonic anhydrase (CA) producing additional H⁺, and active transport using bicarbonate transporters (BAT) also occurs [49]. Metabolic CO₂ is also supplied to the symbionts (Z) [49,50]. pH upregulation shifts the equilibrium of DIC in favour of carbonate relative to bicarbonate , producing additional H⁺. Calcification occurs and Ca²⁺ is depleted from the calcifying fluid, causing a decrease in Ω_cf. Multiple mechanisms of calcium transport may operate (e.g. Ca-ATPase, Ca-channels, and Ca²⁺/Na⁺ exchange), combined with seawater renewal, to re-supply Ca²⁺ for calcification.