Coral bleaching

Bleached corals

Healthy corals

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.^[1]

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.^[2] Some corals recover.

Warmer sea water temperatures caused by global warming is the leading cause of coral bleaching.^[2]

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.^[3]^[4]^[5] In 2017, the bleaching got into the central region of the reef.^[6]^[7] The interval between bleaching events has halved between 1980 and 2016.^[8]

Recent research showed that the coral-algae relationship is much older than was thought. This suggests it has survived many climate changes.^[9]

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.^[10]^[11]

Coral Bleaching Media

Coral and microscopic algae have a symbiotic relationship. When water temperatures get too high, the algae leave the coral tissue and the coral begins to starve.
Zooxanthellae, the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis
Healthy coral at left, and bleached, but still living, coral at right
A colourful bleaching event photographed in Palawan, Philippines, in 2010. The colours derive from high concentrations of sun-screening pigments produced by the coral host.
Bleached coral—partially overgrown with algae
Bleached Acropora coral with normal coral in the background
Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching.
A major coral bleaching event took place on this part of the Great Barrier Reef in Australia.
This schematic shows how bleaching can trigger the production of sun-screening pigments that are responsible for the bright colours observed during some bleaching events. In case of a mild or short episode of stress, the protective pigments may help the algal symbionts return to the coral after the stress episode has ended, helping the coral recover and survive the bleaching event.
The lemon damselfish (Pomacentrus moluccensis) is a coral-associated species that has been shown to decline dramatically following coral bleaching.