Coral reef bleaching, the ecological phenomenon of whitening of reefs, results from the loss of symbiotic zooxanthellae and/or a reduction in photosynthetic pigment concentrations in zooxanthellae residing within the gastrodermal tissues of host animals. Bleaching can be triggered by a variety of environmental factors including extremes of temperature (heat shock and cold shock), ultraviolet radiation, pollution, disease, etc.. Although any of these processes may act on a local scale, large-scale bleaching (or mass bleaching) events have generally been linked to elevated sea surface temperature (or high irradiance) as a result of recent global warming, accompanied by intensified El Ni o-Southern Oscillation(ENSO) events. Mass bleaching events often result in large-scale coral mortality, such as the 1998 widespread bleaching event that led to massive mortality of global reefs. Furthermore, such coral reef bleaching has increased in frequency, intensity, and geographical extent over the past decade. It has been suggested that bleaching will become even more common in the next 30-50 years.
There are two mechanisms that are responsible for coral reef bleaching: one is cellular mechanism, involving a variety of processes such as degeneration of zooxanthellae in situ, apoptosis and necrosis, release of zooxanthellae from gastrodermal tissues and release of algae within host cells; the other is photoinhibition of photosynthesis which involves photodamage to photosystem Ⅱ (PSⅡ) of the zooxanthellae, with the sequent increase in the production of damaging reactive oxygen species (ROS).
Although bleached coral reefs may recover, their full recovery from a mass bleaching event will take several years to decades. Approximately 40% of the 16% of the world’s reefs that were seriously damaged in 1998 are either recovering well or have recovered in 2004, and others are continue to recover, with stronger recovery in well-managed and remote reefs. However, such recovery could be reversed if anthropogenic pressures continue to increase on coral reefs. Coral reef bleaching will have impact on reef ecology in terms of ecological system degradation, coral growth rate reduction, coral community structure change, reef fish community change, as well as reef skeletal structure loss.
There is an adaptive bleaching hypothesis, in which stressed corals first lose their symbionts (bleaching) and then regain a new mix of symbionts that are better suited to the stressed environment. Accordingly, coral reef may survive in further warming environment through adapting to new algal symbionts (such as clade D-Zooxanthellae).