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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They found that the sea anemone loses its algae in both the light and dark at 93.2 degrees Fahrenheit (34 degrees Celsius), and that the heat damages the algae's photosynthetic abilities; that is, they saw that the remaining algae fluoresce less than normal (fluorescence has previously been indicated as a way to test the health of coral). When the team returned the sea anemones to their normal temperature of 80.6 degrees F (27 degrees C), the animals continued to bleach for several days, but their algal populations eventually returned to their pre-stress levels.Skip to next paragraph
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The researchers then heated up nine reef-building corals from the genus Acropora, which came from Ofu Island in American Samoa and from the Monterey Bay Aquarium in California. At 93.2 degrees F, seven of the coral species bleached (the team isn't sure if the two other species would have bleached under higher temperatures). [Images: Colorful Corals of the Great Barrier Reef]
"The surprising thing is that in many cases, the bleaching was just as strong in the dark as it was in the light," Grossman said. "Photosynthesis is not necessary for bleaching to occur, though it may exacerbate bleaching."
A lingering mystery
The researchers suggest that other mechanisms may also trigger coral bleaching, such as nitric oxide molecules released during heat stress or reactive oxygen molecules that don't come from photosynthesis.
Another possibility is that the heat disrupts the functions of the algae and coral membranes, which allow the symbionts to pass nutrients between each other. In this case, the coral or algae realize they are not getting what they need, so they separate. There is some validity to this idea, Grossman said — in another experiment, the team discovered that they could get sea anemones to spit out their algae if they stopped photosynthesis with a drug.
Grossman also notes that the research shows that corals change color during bleaching because of the loss of algae. Some scientists have previously suggested that corals may turn white because the algae lose their pigmentation, but Grossman and his colleagues found that the expelled algae were still pigmented.
The researchers think that ejecting the algae in the dark during heat-stress may actually be beneficial to the coral. "When the light comes up the following day, if you still have algae in there you will get more reactive oxygen species and eventually destroy yourself," Grossman explained, adding that future work will tease out any advantages there may be to coral bleaching and elucidate the role that gene expression plays in the matter.
"We want to continue probing it on the molecular level and trying to pinpoint those specific mechanisms that will let us understand this whole process," Grossman said. "Then maybe we can do something about coral bleaching."
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