Figure 1. Mechanisms involved in coral calcification.

Calcification occurs within the subcalicoblastic space from an initial seawater-derived fluid with additional metabolic sourced supply of DIC⁵,⁶,⁷. Elevation of calcifying fluid pH_cf occurs via removal of protons from the calcification site by Ca²⁺-ATPase exchangers. The carbonic anhydrases (CA) catalyse the forward reactions converting CO₂ into HCO₃⁻ ions⁷,³⁴. Transfer of DIC into the subcalicoblastic space may occur via diffusion of CO₂ and/or by HCO₃⁻ pumping via bicarbonate anion transporters (BAT)⁵,⁶,⁷. The link between the activity of zooxanthellae located in the oral coral endoderm tissue to the generation of metabolic DIC within the aboral endoderm and calicoblastic cells (orange) and transport to the calcifying fluid remains uncertain⁵,⁶,⁷ (Figure modified from McCulloch et al.³¹).