Coral bleaching refers to the loss of color that occurs when coral expel their symbiotic algae, called zooxanthellae. This expulsion occurs when the corals are under extreme stress. Corals are fairly resilient animals, able to recover from numerous assaults from natural disasters, predators such as the crown-of-thorns starfish and even human activity. However, reef-forming corals require a relatively strict range of temperatures for optimum health, and sea surface temperatures have been rising.
When water temperatures remain above about 28-degrees Celsius for more than a day or two, the corals become stressed and eventually respond by expelling their colorful algae. Normally, these brightly colored, single-celled plants live within the tissues of the coral, using photosynthesis to provide food to the coral. The corals' wastes, in return, provide nutrients to the algae.
Coral tissues are transparent, so the color of the resident algae is visible through the tissue. Likewise, when the algae are no longer present, the white skeleton shows through, and the coral formation appears "bleached."
Unfortunately, coral cannot survive long without its symbiotic algae. Although polyps do trap prey with their stinging tentacles, they are unable to get sufficient nutrients in this manner. The corals are able to survive for a short time on the food they can draw in from the water, but without their algae, they slowly starve to death. The coral can recover if the algae are able to return within a few weeks, but when sea surface temperatures remain high, the algae are unable to recolonize the coral.
Bleaching events have become more common in recent years, and scientists are concerned that if temperatures continue to rise, the rate of coral bleaching will also accelerate, eventually killing off most, if not all, of the reefs worldwide. The situation may not be quite this dire, as a few species of coral are able to live in warmer waters. It is possible that these warm-water tolerant corals could replace the corals that die off on reefs, but whether this would happen and how long it would take are not known.
High sea surface temperatures are the main cause of coral bleaching, but infections, increased UV radiation exposure and water pollution can also stress corals enough to cause bleaching. The bleaching process is the same regardless of the cause; sustained stress results in corals expelling their symbionts and dying of starvation.
That an animal as small as a coral can build a structure as massive as a coral reef is an amazing thing. Even more amazing is the untold diversity of marine life, much of it still unexplored, contained within a single reef. Scientists believe that coral reefs may support as many as a million species of animals, many not yet discovered. The loss of the world's coral reefs, to bleaching or any other disaster, would mean the loss of this diverse habitat and its inhabitants. Their loss would be a great tragedy and must not occur.