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Spring 2000

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Sunburned Reefs
Paul Karr

Global warming has been a controversial theory almost since the moment it was first advanced. But now hard evidence of "sunburned" Caribbean coral seems to confirm not only the gradual warming of the world's oceans, but also its effect on ocean ecology.

For years, biologists had reported uncommon white corals in various pockets of the Caribbean. However, during the late 1980s, three UGA researchers began to piece together the larger picture: the phenomenon was Caribbean-wide. This was especially alarming because, as reefs die, the number of fish and lobsters that hide in them also diminish - as do the number of tourists who visit them.

Tropical corals are extremely thin-tissued creatures that make up reefs in tropical and subtropical seas. They actually are made up of algae, living inside of a coral animal. If the temperature of the seawater during summer remains too high for too long, photosynthesis in the coral's algae breaks down, leaving the coral with less food; the animal starves, and its white skeleton becomes visible - hence the "bleached" white color.

A key protein in photosynthesis, known as the D1 protein, is extremely sensitive to high temperatures and is a critical step in the demise of these helpful algae. UGA associate ecology professor William Fitt, postdoctoral botany student Mark Warner and botany professor Gregg Schmidt began observing this phenomenon in the Caribbean as early as 1987, in conjunction with research being carried out by UGA ecologist Jim Porter. But they needed a larger sample, and an unusually warm summer, to confirm their hunches.

Fast-forward to late summer 1997. That, as it developed, was an El Ni-o year; water temperatures hovered at or above 86 F for more than six weeks - warmer water, for a longer time, than is usually recorded in that part of the world in late summer and fall. (Caribbean summer water temperatures are usually closer to 84 F, and they drop considerably with the first autumn cold fronts.)

A coral and its algae can protect themselves to a point during normal summers. They can shunt electrons from midday sunlight away from their photosystems - where the D1 and other proteins reside - to places where the radiation won't harm the coral; they don "chemical sunglasses" in a sense.

But such widespread coral bleaching, occurring routinely in so many places around the tropics, indicated something was going wrong with this process; Fitt, Schmidt and Warner wanted to know what, and also whether it was affecting normal functions between the coral and its algae.

First they dove off Key Largo National Marine Sanctuary in Florida, then at the Caribbean Marine Research Center in the Bahamas. They carted tiny reef samples back to the field lab and examined them with a fluorometer, which assesses damage to photosynthesis. Their question was simple: Was photosynthesis working properly in the bleached coral?

It wasn't.

"Coral is very photo- and temperature-sensitive," Fitt said. "We now know that, if water temperature is too high for too long, everything goes wrong very quickly - like throwing a screwdriver into a running engine."

It was an important enough finding that, in August 1999, the National Science Foundation awarded the team a $400,000 grant to continue studying causes of coral reef destruction.

In addition to excessively warm temperature, a number of other factors, including pollution, may be contributing to this coral bleaching. But Fitt remains certain his team "caught the bandit in the act."

"Global warming is hard to see in the short term, but it's undeniably happening," he said. "Tropical corals are already on the edge of their 'temperature envelope' of life during most summers. Some say they are the canary in the coal mine: Global warming pushes them that little bit higher or longer, tipping the scales, and they begin to suffer."

E-mail mwarner@botany.uga.edu for more information.

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Sunburned Reefs