What is coral bleaching? by Prof. Ove Hoegh-Guldberg, Chairman

Reef-building corals are central to coral reefs and are symbiotic with critical dinoflagellate algal symbionts commonly known as zooxanthellae. The latter provide the coral with most of its energy needs. Coral bleaching occurs when stressed corals expel their symbiotic algae, loosing their brown colour. In the past 20 years, there have been six major bleaching events that have corals reefs stark white across thousands of kilometres.

The dinoflagellate algae provide the coral with energy and nutrients from their photosynthetic processes. The animal host is supplies inorganic nutrients such as ammonia and phosphate from their waste metabolism. These compounds are in short supply in the nutrient impoverished waters of coral reefs. Coral bleaching leads to mortality if the stress is prolonged or intense. In 1998, an estimated 16% of the world's reef-building corals died.

Coral bleaching can be triggered by a range of environmental and physiological stresses, such as warmer than normal seas, elevated solar radiation, pollution, reduced salinity and anoxia. The global episodes of mass bleaching have been tracked to short periods of elevated sea temperature. Corals live close to their upper thermal limit and when ocean temperatures rise ~ 1ºC or higher above long-term monthly averages for the warmest months, bleaching begins. These conditions occur when skies are clear, water motion is minimal and wind are at their lowest

A range of cellular phenomena is thought to occur within the cell. It is still unclear as to whether coral cells deliberately expel the symbiotic algae or whether bleaching is a response to host cell death in response to the algae becoming toxic to the coral at higher temperatures. Thermal stress has been shown to cause dysfunction of photosynthetic metabolism in the algae, leading to the production of toxic free oxygen radicals. Researchers still debate whether it is primarily the light or dark reactions of photosynthesis that are affected. Studies have shown that corals may survive bleaching and recover. However, other studies have shown that subchronic damage may occur and result in reduced reproductive capacity and growth. These aspects, while little studied, may have great implications for the long-term health of coral ecosystems.