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 pHcf 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).