The process by which corals secrete calcium carbonate to form their skeletons is calcification.
Stony corals use specialized cells near the skeleton to control the chemistry of a tiny calcifying space. Calcium ions and dissolved inorganic carbon are concentrated there, while proteins and other organic molecules help guide the nucleation and growth of calcium carbonate crystals. Most reef-building corals form skeletons primarily from aragonite, a crystal form of calcium carbonate.
Calcification builds the hard cups, walls, and branches that give coral colonies their structure. Over many generations, the accumulated skeletons of countless colonies can create reefs, islands, and other carbonate formations. Coral polyps add new skeletal material as they grow, while older skeleton remains beneath them.
Photosynthesis is closely linked to the process because the corals’ algal symbionts supply energy and influence carbon chemistry, but photosynthesis itself is not calcification. Ocean acidification can make calcification more energetically demanding by reducing carbonate-ion availability.