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A reef-building coral is in a mutually beneficial partnership (or symbiosis) between the coral animal (polyps) and plants (millions of tiny, single-celled algae called zooxanthellae). The zooxanthellae are packed into every cell of one layer of coral tissue and usually occur at densities of more than one million per square centimetre. Like land plants, zooxanthellae use light and dissolved carbon dioxide in the process of photosynthesis to produce energy-rich compounds, some of which are transferred to the host coral. In return, the coral protects the zooxanthellae. Because most corals rely on the zooxanthellae to supply much of their food, they are usually restricted to shallow seas where sufficient light can reach the coral and its zooxanthellae. Like all living things, corals and zooxanthellae survive and grow well within a preferred range of environmental conditions. Temperatures outside their operating range are detrimental to them. Unlike fish that can move to habitats with more suitable environmental conditions, corals cannot escape areas of high temperature because they are attached to the reef structure. If temperatures are too hot for too long, the symbiotic relationship between the coral animal and its tenants, the zooxanthellae, collapses. Reef-building corals appear to be living only 1-2°C below their upper thermal tolerance limit. Under these conditions, the Sun's energy which is normally used to produce food for the algae and coral, is diverted into the production of oxygen radicals. These oxygen radicals, similar to the chemicals implicated in human aging, are highly corrosive. They damage the parts of the zooxanthellae (the chloroplasts) where photosynthesis takes place. The coral polyps can also be damaged as a result of heat stress. The result is that large numbers of zooxanthellae are expelled from the coral. The photosynthetic pigments in the zooxanthellae give the coral its distinctive colour, so when the number of algae in the tissues are greatly reduced due to stressful conditions, the corals appear pale. If stressful conditions persist, most of the algae can be lost which leaves the tissues transparent and reveals the white skeleton beneath so that the coral looks 'bleached'. If high temperatures are relatively short-lived, the zooxanthellae that remain within the coral tissue divide rapidly once the stress diminishes and the coral gradually regains its colour and survives. However, even corals that survive bleaching can suffer long-term effects such as reduced growth and reproduction, and they can be more susceptible to disease. If the stressful conditions are prolonged or particularly severe, the density of zooxanthellae remains low, and many corals will die. There is great variation in the susceptibility to bleaching events between different coral species, and even colonies within a species.
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