Most people think that the stony corals are the only organisms that build the structure of the coral reefs. However, coralline algae also make significant contributions to some reefs. Coralline algae are plants that build a calcium carbonate support, called a thallus, which is covered by a thin layer of plant tissue.
Coralline algae occur in pink and green, patchy, crustose, and branching photosynthesizing forms.
They help cement the reef together. The algal surfaces attract coral larvae that settle and begin developing into coral colonies. Two diseases of coralline algae have been described thus far. Both involve the loss of algal tissue that exposes the underlying white carbonate support in the process. The two diseases can be distinguished by color and form.