Corals contain photosynthetic algae called zooxanthellae in their tissues. These algae provide the coral with food in return for protection in a symbiotic relationship and give the corals their distinctive colour.

Prolonged stressful environmental conditions cause a breakdown in this symbiotic relationship. These conditions include unusually high or low sea temperature, high or low light levels, freshwater or pollutants.

As corals are stressed they lose the zooxanthellae from their tissues, leaving the white calcium carbonate skeleton visible through the coral tissue. This process is known as coral bleaching.

There have been several major bleaching events worldwide in recent decades.

The extent of coral bleaching is dependent on both above average temperatures and the length of time that the water temperature remains high. Bleached corals have the ability to recover as conditions return to normal but if the conditions remain unfavourable for an extended time they will die. The threat to corals increases as the bleaching events become more frequent because they have no time to recover.