Why Does Coral Bleaching Occur

Sixty genera of coral have a symbiotic relationship with the algae zooxanthellae. Only some deepwater and cold water corals can survive without them. This symbiotic relationship benefits both organisms,the zooxanthellae provides the coral with glycerol, glucose and alanine and the coral provides the zooxanthellae with nitrogen and protection against predators. The degree to which the organisms depend on one another vary between species. The zooxanthellae algae gives the coral a yellow-brown or dark brown colour. (Rupert, Barnes, 1994)
Coral bleaching occurs when the zooxanthellae dies or the coral expels it, causing the coral to lose its colour. Coral bleaching can occur for a variety of different reasons such as a change in sea temperature