Coral bleaching

Coral bleaching occurs when stony corals turn white.
Stony corals are simple animals that form large reefs. The animals live in a endosymbiotic relationship with single-celled algae. To get sunlight the corals live just below sea level. The algae make food products for the coral polyp by photosynthesis.
Coral polyps are sensitive to changes in their environment. This includes the temperature of the water they live in. Under stress, coral polyps may expel the algae which live inside their tissues. The algae provide up to 90% of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. Some corals recover.
Warmer sea water temperatures caused by global warming is the leading cause of coral bleaching.
The United Nations Environment Programme says that the longest recorded global bleaching events happened between 2014 and 2016. Coral was killed 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 got into the central region of the reef. The interval between bleaching events has halved between 1980 and 2016.
Recent research showed that the coral-algae relationship is much older than was thought. This suggests it has survived many climate changes.
Some corals change color instead of turning white when they are bleached. The colors can be very bright. In May 2020, scientists from the University of Southampton said that the colors act like sunscreen and protect the coral cells from light. The scientists said this means some corals can lose their algae for a while and might have developed bright colors as a way to make it easier for the algae to come back and to stay alive until then.