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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. 

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

Dan Barshis

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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.

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.

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