Coral bleaching

Coral bleaching is the loss of endosymbiotic algae from the coral, either through expulsion or loss of algal pigmentation. Above-average sea water temperatures caused by global warming have been identified as a leading cause for coral bleaching worldwide. 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. 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 hit 90 per cent of coral on the Great Barrier Reef and killed more than 20 per cent of the reef's coral.

Bleaching occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained. The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like unicellular flagellate protozoa that are photosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion. This process is a "downward spiral", whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae. Bleaching has been attributed to a defense mechanism in corals; this is called the "adaptive bleaching hypothesis", from a 1993 paper by Robert Buddemeier and Daphne Fautin.

...
Wikipedia