What is "coral bleaching"

n. (context ecology English) The loss of colour in a coral reef due to the loss of symbiotic algae as a result of stress.

Coral bleaching is the loss of intracellular endosymbionts ( Symbiodinium, also known as zooxanthellae) from coral either through 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 photosynthetic and 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.

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