Coral bleaching results when the symbiotic zooxanthellae (single celled algae) are expelled from the host coral organism due to stress. The corals that form the structure of the great reef ecosystems of tropical seas depend on these symbiotic photosynthesizing unicellular algae called 'zooxanthellae' that live within their tissues. When the zooxantheallae are expelled, the coral looses its pigment, leading to a bleached or completely white appearance.

Coral bleaching is a vivid symptom of the stresses already being caused by global warming, but coral may be stressed by factors other than elevated sea surface temperatures, such as solar irradiance (photosyntheticaly active radiation and ultraviolet band light). Other anthropogenic factors include pollution and smothering silt runoff from coastal development. Stressors expose coral to diseases like white band disease and black band disease .

If the stressors leading to the coral bleaching subside quickly, the coral host can repopulate its symbiotic algae within weeks or months.

Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.

The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed.

Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.

External links

• Great Barrier Reef Marine Park Authority coral bleaching information page.
• ReefBase: a global information system on coral reefs.
• More details on coral bleaching, causes and effects.