Coral bleaching is a phenomenon in which corals lose their distinctive color because they expel the one-celled organisms which live with them in a symbiotic relationship. If a coral is fully bleached, it will turn entirely white, and it will die unless the stress on the coral is reduced, allowing the symbiotic organisms to return to the coral. Worldwide, coral bleaching began to increase at a rate which some scientists viewed as alarming in the 1990s, with researchers fearing that massive losses in the world's coral population could be highly likely.
Research on coral bleaching has shown that it is caused by stress to coral. Coral is in fact a very delicate organism, adapted to survive in nutrient-poor waters, but it does not tolerate environmental changes well. A healthy coral reef can sustain a wide assortment of organisms, promoting biodiversity, and if the corals die off, the creatures which surround the reef may die off as well. Therefore, coral bleaching isn't just a threat to coral, but to marine health in general in some regions of the world.
A number of things can stress corals. Rising water temperatures and increased ultraviolet radiation are suspected to be two major stressors for corals. In addition, coral can be disturbed by nutrient runoff from the land, heavy sedimentation, pollution, increasing salinity, acidification of the surrounding water, and infection with various organisms. If the coral bleaching is recognized early and people act quickly, it can sometimes be arrested, allowing the coral to recover, but in other cases biologists are powerless to help the coral.
During a period of bleaching, coral is still alive. However, as it loses its one-celled buddies, it can become unhealthy. Corals rely on zooxanthellae algae to photosynthesize, providing energy from the sun which can be utilized by the coral. When these algae are expelled for a prolonged period of time, a coral can start to die. It is common for bleached reefs to start to break apart, which can lead to problems in the future, even if the bleaching is stopped and the reef is coaxed into recovering. The more damage there is, the longer it will take for the reef to be restored.
Some researchers believe that coral bleaching may actually be an adaptation on the part of the coral to cope with environmental changes. By expelling the zooxanthellae algae, the coral can make room for algae which might be better adapted to the changed environment, potentially allowing the coral to survive. Historical evidence from the geological record seems to support this idea, but biologists have sounded a note of caution; while coral may be able to cope with rising temperatures on its own, it cannot handle pollution and changes to the chemical composition of the water.