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Why are corals turning ghostly white? Scientists unravel mystery.

Coral bleaching, a process by which reef-building corals lose their algae and turn white, has long thought to be a result of faulty photosynthesis caused by high temperatures. But new research shows that bleaching can occur at night, too. 

By Joseph CastroLiveScience Contributor / September 9, 2013

Corals appear near the back reef of Ofu, a national park in American Samoa.

Dan Barshis


Across the globe, reef-building corals live in symbiosis with algae, which provide the animals with food and their iconic brilliant color. But environmental stress — high temperatures, in particular — can kill corals by causing them to "bleach," a process in which they lose their vital algal friends and turn ghostly white.

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Scientists have long thought that faulty algal photosynthesis (the process that uses light to make food) ultimately triggers coral bleaching, but new research now shows that substantial bleaching can also occur when heat-stressed corals are not exposed to light (such as at night).

The study, published today (Sept. 5) in the journal Current Biology, suggests that different molecular mechanisms may spark coral bleaching and that certain strategies proposed to prevent bleaching, such as shielding corals from sunlight when water temperatures are high, may need to be re-evaluated.

"The results make us rethink how coral remediation might be achieved," said study lead author Arthur Grossman, an algal physiologist at the Carnegie Institution for Science in California. "As we learn more about the mechanisms involved in coral bleaching, we may be able to ameliorate the situation a bit more." [In Images: A Trip to the Coral Triangle]

Coral reefs in danger

Coral reefs are sometimes called "rainforests of the ocean," as they are an important part of the aquatic ecosystem, providing food and shelter for countless marine species. But coral reefs around the world are in decline due to a number of different issues, including overfishing, water pollution and coastal development.

A greater problem, however, may be atmospheric carbon dioxide. Since the industrial revolution, humans have increasingly piped more carbon dioxide into the atmosphere, much of which the ocean then absorbs. The resulting chemical reactions reduce seawater pH, making it more acidic. "If the water gets more acidic, it's harder for corals to make calcium carbonate for their skeletons," Grossman told LiveScienceOcean acidification slows coral growth and weakens the reef infrastructure, making it more vulnerable to erosion and predators.

Increased atmospheric carbon dioxide also raises global temperatures, which leads to coral bleaching — the breakdown of the symbiotic relationship between coral polyps and single-celled algae called zooxanthellae. Normally, algae supply corals with oxygen, glucose, glycerol, amino acids and other nutrients, while corals protect algae and feed them the compounds they need for photosynthesis.

Until now, the prevailing theory behind coral bleaching explained that when water temperatures are too high, the photosynthetic apparatus of algae — the chloroplast — is unable to efficiently process incoming light. The algae begin producing toxic, reactive oxygen molecules during photosynthesis, which interact with and disrupt algal membranes and proteins. The excess oxygen can also react with the water to produce hydrogen peroxide, which damages coral tissue.

After a while, the algae separate from the corals, though scientists aren't sure if the corals expel the algae or if the algae abandon the corals. Without the algae, the corals become bleached and will die if they don't quickly take up zooxanthellae again.

Heat-stressed algae

Grossman and his colleagues wondered if coral bleaching can still occur if algae are heat-stressed and in the dark, when the photosynthetic machinery is turned off. To find out, they first tested how a model system — the sea anemone, Aiptasia, and its algal symbiont, Symbiodinium — responds to heat stress.

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