<Tr> <Td> </Td> <Td> This article's factual accuracy may be compromised due to out - of - date information . Please update this article to reflect recent events or newly available information . (May 2017) </Td> </Tr> <P> Coral bleaching occurs when coral polyps expel algae that live inside their tissues . Normally, coral polyps live in an endosymbiotic relationship with the algae and that relationship is crucial for the coral and hence for the health of the whole reef . Bleached corals might continue to live for some time . But as the algae provide the coral with up to 90% of its energy, after expelling the algae the coral begins to starve . After bleaching, some corals may recover, while some die . Above - average sea water temperatures caused by global warming have been identified as a leading cause for coral bleaching worldwide . Between 2014 and 2016, the longest global bleaching events ever were recorded . According to the United Nations Environment Programme, these bleaching events killed coral on an unprecedented scale . In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral . In 2017, the bleaching further expanded to areas of the reef that were previously spared, such as the central one . The average interval between bleaching events has halved between 1980 and 2016 . </P> <P> The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae - like single - celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration . The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient - poor tropical waters . In exchange, the coral provide the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis . Negative environmental conditions thwart the coral's ability to provide for the zooxanthellae's needs . To ensure short - term survival, the coral - polyp then expels the zooxanthellae . This leads to a lighter or completely white appearance, hence the term "bleached". As the zooxanthellae provide up to 90% of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve . </P> <P> Coral can survive short - term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish . In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it . If the coral polyps die of starvation after bleaching, they will decay . The hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth . Eventually, the coral skeletons will erode, causing the reef structure to collapse . </P>

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