In some species of red algae called coralline algae, the cell walls become hardened with calcium carbonate. These algae hence important for the formation of coral reefs.
Coral reefs are formed through the accumulation of calcareous exoskeletons of coral animals, calcareous red algae and molluses. They form the foundation of reefs by secreting a calcium carbonate skeleton that provides protection for the coral polyps. Calcium carbonate is secreted continuously by the coral colony.