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The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".

Coral bleaching is a vivid sign of corals responding to stress, primarily increased water temperatures. Higher rates of bleaching are being linked to global warming, but bleaching may occur from other stress factors, such as solar irradiance (photosyntheticaly active radiation and ultraviolet band light), changes in water chemistry, and silt runoff. Some of these factors are anthropogenic, while others occur naturally.

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survies, it often requires weeks to months for the remaining symbiont population to reach a normal density. Following bleaching, corals may reuptake the same zooxanthellae clade, or may exchange the clade for one that is more appropriate for conditions. Some corals are known to host multiple clades of zooxanthellae in separated colonies ^[1]. Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to dying following a bleaching event. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zoozanthellae can influence the risk of bleaching.^[2]

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 and Congo.

In 1996, Kushumaro, et al. ^[4] reported that the agent for the coral bleaching in the red sea was an infectious bacteria attacking the symbiotic algae. The agent has been later identified as Vibrio shiloi. The pathogen is infectious only during warm period ; therefore, global warming spurs the spread of infection.

During the summer of 2003, red sea coral reef's gained resistance to the pathogen and cannot be infected any more ^[5]. The main hypothesis for the emerged resistance is probiotic : inside the symbionic commuinities of bacteria living with the corals, one species capable of lysing V. Shiloi has gained prominence. The hypothetic bacteria is not yet found.

• Great Barrier Reef Marine Park Authority information on bleaching.
• ReefBase: a global information system on coral reefs.
• More details on coral bleaching, causes and effects.
• Reeffutures.org: causes, effects and monitoring of bleaching on Great Barrier Reef
• Travellers Impressions