Scientists are now warning that the 2015-2016 El Nino may cause a mass bleaching event of coral reefs next year.
Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation. The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like unicellular flagellate protozoa that are photosyntheticand live within their tissues.
Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Some scientists consider bleaching a poorly-understood type of “stress” related to high irradiance; environmental factors like sediments, harmful chemicals and freshwater; and high or low water temperatures. This “stress” causes corals to expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term “bleached”.
Bleaching has been attributed to a defense mechanism in corals; this is called the “adaptive bleaching hypothesis,” from a 1993 paper by Robert Buddemeier and Daphne Fautin. Bleached corals continue to live, but growth is limited until the protozoa return.