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 photosynthetic and

live within their tissues. Zooxanthellae give coral its coloration, with

the specific color depending on the particular clade. Under stress, corals

may expel their zooxanthellae, which leads to a lighter or completely

white appearance, hence the term "bleached"