The Coral's Colored Engine The bright colors of the Great Barrier Reef come from microscopic algae called zooxanthellae, which live inside the tissues of coral polyps. This relationship is symbiotic. The coral provides the algae with a protected environment and the compounds they need for photosynthesis. In return, the algae, a dinoflagellate from the family Symbiodiniaceae, produce energy through photosynthesis, supplying the coral with up to 90% of its food. This energy exchange allows the stony corals to build their calcium carbonate skeletons, which accumulate over centuries to form the massive structures of the reef.
When ocean temperatures rise, this partnership breaks down. A temperature increase of just one degree Celsius above the usual summer maximum, if sustained for four weeks, can cause the coral to become stressed. Under this stress, the zooxanthellae begin to produce reactive oxygen species that are toxic to the coral host. To protect itself, the coral expels the algae from its tissues. Without the pigmented algae, the coral's tissues become transparent, revealing the white calcium carbonate skeleton beneath. This phenomenon is coral bleaching. A bleached coral is not dead, but it is starving and more susceptible to disease.