Coral tissue that has turned white or pale is described as ‘bleached’ and can occur in small patches on individual colonies or across whole tracts of reefs. Bleaching is a symptom of stress in coral and/or the coral’s zooxanthellae. Much of the natural patchy bleaching occasionally seen in otherwise healthy corals is unimportant as it causes little or no long-term problems for a colony. Such patches may be due to a temporary loss of photosynthetic pigment from the zooxanthellae, predators of many sorts, or attacks from neighbouring colonies in the battle for growing space. Colony-wide bleaching is clearly of more concern as this can result in the death of the coral. Such bleaching has a number of causes including disease, lowered salinity, high temperature or increased light (as a result of turbidity decrease). In each case, bleaching indicates that tissue integrity has been compromised. Although any of these causes can result in widespread mortality, by far the most common and most conspicuous cause is a combination of high light and elevated temperature. Where this occurs across whole reef tracts it is known as ‘mass bleaching’ which often leads to mass death.
Such temperature/light induced bleaching normally begins with excessively elevated levels of photosynthesis which leads zooxanthellae to produce toxic levels of oxygen. This occurs most commonly in shallow reef waters where corals are exposed to a combination of high sea-water temperatures and high levels of sunlight, for corals can regulate the quantity of zooxanthellae in their tissues, but not the rate at which these produce oxygen. Under conditions of high temperature and excess light, zooxanthellae become poisonous, actually deactivating the sunscreens corals produce to protect themselves. After studying this process in detail, coral physiologists have discovered it to be a general phenomenon found in terrestrial plants as a mechanism for protecting leaf tissue from excess oxygen through over-exposure to sunlight. In corals exposed to extreme levels of solar radiation in general and ultra-violet light in particular, together with high water temperatures (normally associated with shallow ponded water), zooxanthellae produce oxygen 4-8 times faster than the coral host can take it up. When this happens, some oxygen ceases to play a normal role in photosynthesis and becomes chemically active as oxygen ‘radicals’ causing cellular distress. As oxygen radicals become toxic, the zooxanthellae that produce them are expelled by the corals even though this action too puts the corals at risk. Expulsion most commonly takes place by sloughing-off the gastrodermal cells in which the algae live, however there are also other mechanisms like resorption and tissue death. Such discoveries explain why bleaching is light- as well as temperature-dependent. They also explain a host of experimental variations correlated with environment, colony characteristics (shape and species) and the place and depth where experimental corals were originally collected. Additionally they help to explain why some corals die after they bleach: even when other nutrients are present and there is plenty of food in the form of zooplankton they may simply not have enough gastrodermal cells to function.
The temperature/light conditions which cause corals to expel their zooxanthellae are normally not localised and commonly affect a significant portion of the reef experiencing the temperature pulse. This can result in widespread mass bleaching of coral colonies, particularly at shallow depth.