• Coral bleaching occurs when coral polyps expel algae that lives 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 continue to live. But as the algae provide the coral with 90% of its energy, after expelling the algae the coral begins to starve.
  •  Above-average sea water temperatures caused by global warming have been identified as a leading cause for coral bleaching worldwide.
  • In 2016, bleaching hit 90 percent of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef’s coral.


  • 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 for up to 90% of the coral’s energy needs through photosynthesis, after expelling, the coral begins to starve.