How do coral polyps build the limestone framework of a coral reef?

Coral polyps build the limestone framework of a reef by secreting a rigid external skeleton made of calcium carbonate. Most reef-building corals, known as stony corals or Scleractinia, are colonial organisms. They begin as individual polyps that reproduce through budding, creating colonies where new polyps grow atop the skeletons of their predecessors.12

This process is significantly enhanced by a symbiotic relationship with zooxanthellae, which are dinoflagellate algae living within the coral tissues. These algae perform photosynthesis, which helps the coral remove metabolic waste products like carbon dioxide. By accelerating the removal of this carbon dioxide, the zooxanthellae increase the speed at which the coral can deposit its calcium carbonate skeleton.3

As these colonies grow side by side and over one another, they form a continuous, massive structure. Other organisms, such as calcareous algae, further contribute to the reef by binding and cementing these coral skeletons together, while fragments from various shelly organisms fill the gaps to solidify the framework.1