page_title: Coral bleaching
section_title: Process
hierarchical_section_title: Coral bleaching / Process
caption_attribution_description: English: Symbiodinium, colloquially called "zooxanthellae". Corals contain dense populations of round micro-algae commonly referred to as zooxanthellae. A typical coral will have one to several million symbiont cells in an area of tissue the size of a thumbnail.
context_page_description: Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with these algae, which are crucial for the health of the coral and the reef. The algae provides up to 90 percent of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. Some corals recover.
  The leading cause of coral bleaching is rising water temperatures. A temperature about 1 °C above average can cause bleaching. According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global 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 extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016.
context_section_description: 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 provides the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis. Negative environmental conditions, such as abnormally warm or cool temperatures, high light, and even some microbial diseases, can lead to the breakdown of the coral/zooxanthellae symbiosis. To ensure short-term survival, the coral-polyp then consumes or expels the zooxanthellae. This leads to a lighter or completely white appearance, hence the term "bleached". As the zooxanthellae provide up to 90 percent of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve.
  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.
caption: Zooxanthellae, the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis
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