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Why do corals bleach?

Coral reefs are colorful places. Bright-colored fish, anemones, and crustaceans dart, flutter, and creep among patches of coral. Corals themselves can be brightly colored. More often, they are muted browns and greens. Although a coral may seem somewhat rock-like, the hard structure that many people think of as coral is just a skeleton. In living coral, that skeleton is covered with tiny polyps. These tentacled polyps are all extensions of the same animal. Over time, they build up the skeleton, which provides them with a safe place to live. Corals in turn provide the base for coral reefs, which are home to countless marine animals.

Corals polyps, themselves, don’t have much color. Their body tissues are nearly transparent. But they form a symbiotic relationship with tiny algae. It’s these algae, called zooxanthellae (zoo-zan-THEL-ee), that give corals their color. They also provide corals with food. Like plants, the algae use sunlight to make food from oxygen and water in a process called photosynthesis. Most of the food they make feeds the corals. In return, corals provide the algae with a place to live.

When corals become stressed, their zooxanthellae struggle to process sunlight. They begin to make molecules that can harm the coral. To protect themselves from those molecules, the corals force the algae to leave. Without color from the zooxanthellae, the skeleton shines through the clear polyps. The coral turns a ghostly white, and we say that it has bleached.

Bleaching evident on coral in the South China Sea. (Photo by Tom DeCarlo, © Woods Hole Oceanographic Institution)
MIT-WHOI Joint Program student Tom DeCarlo inspecting a bleached coral in the South China Sea. (Photo by Pat Lohmann, © Woods Hole Oceanographic Institution)
Incidence of coral bleaching due to warming seawater. (Photo by Hannah Barkley, © Woods Hole Oceanographic Institution)
Coral bleaching indicative of disease and stress. (Photo courtesy of Anne Cohen, © Woods Hole Oceanographic Institution)
Graduate student Hanny Rivera takes a tissue sample from a bleached coral. When ocean waters warm, corals lose their colorful endosymbiotic algae, and their white skeletons become visible beneath their transparent tissue. (Photo by Tom DeCarlo, © Woods Hole Oceanographic Institution))
Bleached Acropora colony photographed in July 2015. A new study finds that a 2°C rise in the sea surface temperature of the South China Sea in June 2015 was amplified to produce a 6°C rise on the Dongsha Atoll, killing approximately 40 percent of the resident coral community. (Photo by Tom DeCarlo, © Woods Hole Oceanographic Institution)

Bleaching can happen when the water gets too warm. Or when it becomes polluted. Or even when the corals are exposed to too much sunlight. Under warm temperatures, even normal levels of sunlight can cause bleaching. Or if something changes in the environment to expose a coral to more light than before, it can also bleach.

Corals aren’t dead when they bleach. They’ve simply lost their color. But when the zooxanthellae leave, they take with them the coral’s main source of food. Corals can snag tiny bits of food from the water with their tentacles. But most corals don’t get enough to eat this way. If the problem that caused bleaching lasts more than four weeks, they will likely die.

If the temperature returns to normal, or another problem that caused bleaching goes away, zooxanthellae will return to their coral hosts. When this happens, corals can begin to grow again. But once they have bleached, corals need time to fully recover. It may take up to three years before the coral is completely healed from the event.

Coral bleaching is a common problem and one that scientists are working to better understand. They hope to find ways to help corals recover from bleaching events. And they’re working to make corals more resistant to the stressors that cause bleaching. This would protect the reefs that are home to so much marine life.

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Great Barrier Reef Foundation. Coral Bleaching. https://www.barrierreef.org/the-reef/coral-bleaching

NOAA. Coral Bleaching—A Review of the Causes and Consequences. A Reef Manager’s Guide to Coral Bleaching, chapter 4. https://www.coris.noaa.gov/activities/reef_managers_guide/reef_managers_guide_ch4.pdf

NOAA. Coral Bleaching—Background. https://www.coral.noaa.gov/education/bleaching-background.html

NOAA. What is coral bleaching? https://oceanservice.noaa.gov/facts/coral_bleach.html

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