By Bill Chameides

Why have some corals weathered bleaching events better than others? The answer may hold the key to their long-term survival.

Coral reefs are one of the more fascinating communities found on the planet -- at least that's what my scuba-diving friends tell me. In addition to their beautiful colors and shapes and the wonderful array of fish that find sanctuary in them, there is coral biology.

Many people think of that hard, sharp-edged, funny-shaped stuff made up of calcium carbonate as the "coral," but it's actually not; that's the structure. Corals are an assemblage of sedentary, clear animals called polyps, some of which build the reef's substrate. (See our coral reef slide show.)

Much of the coral reef's beauty comes from single-celled protozoa or algae called zooxanthellae that live within the polyps and lend the coral its colorful splendor. Symbiosis couples the lives of the coral and the zooxanthellae together. The zooxanthellae carry out photosynthesis, which provides food for the polyps, and their waste products provide the nutrients the zooxanthellae need for photosynthesis.

A Host of Stresses, Including Warmer Ocean Temperatures, Is Putting Corals at Risk

Unfortunately, the future of coral reefs is in question.

A process called coral bleaching has decimated many coral reefs. Bleaching can be caused by a variety of environmental stresses, including higher than normal ocean temperatures, which cause the coral to expel the zooxanthellae, thereby cutting the polyps off from their food supply and depriving them of their color. If the bleaching is severe enough and lasts long enough, it can spell death for the entire reef.

During the 1998 El Nino event, about 16 percent of corals worldwide died from bleaching events. In the Indian Ocean alone between 50 and 90 percent of corals died during that same period. Since 1977, a combination of bleaching and disease has reduced the Caribbean's bottom cover of coral by more than 80 percent. (More on the state of coral reefs.)

Higher than normal ocean temperatures can cause coral bleaching, an event which, if severe and lengthy enough, can spell death for the entire reef. (View different kinds of corals in our slide show.)

But not all coral has fallen to severe bleaching.

Why do some survive bouts of stress and others do not? Answering that question could hold the key to developing a survival strategy for the world's coral as the globe continues to warm.

And so, marine biologists have set out with new and powerful instruments capable of mapping out the genetic makeup of coral communities to better understand how they respond to temperature changes.

Looking for Clues in Coral Survivors

The early results have been tantalizing. It appears that some forms of zooxanthellae, specifically the clade D type, appear to be relatively resistant to coral bleaching.

Following this thread of research, Todd LaJeunesse of Penn State University and co-authors studied the genetic makeup of zooxanthellae in two contrasting coral communities: one in the northeastern Indian Ocean, where coral have been resistant to bleaching, and the other in the western Indian Ocean, where coral bleaching was extensive during the El Nino/ocean warming events of 1998. They reported their results in the Journal of Biogeography.

Consistent with previous investigators` work, the clade D zooxanthellae in the northeastern Indian Ocean studied by LaJeunesse et al were abundant in the resistant corals. They were not the dominant type in corals from the western Indian Ocean.

An enticing aspect of their findings is that the northeastern Indian Ocean, where the bleaching-resistant coral are found, tends to be significantly warmer than the region where the bleaching-susceptible coral are found.

If Adaptation Is Possible, Will It Need a Helping Hand?

The implication: coral reefs have been able to adapt to warmer ocean temperatures where they have had to. So, if they have adapted in the past, perhaps they can adapt again -- this time to climate change. And if they can't adapt fast enough to the rapid temperature change we are currently experiencing, perhaps we can help them along by expanding the number of clade D zooxanthellae into threatened coral communities where current clade D types are few and far between.

Yeah, but...

This is welcome news -- a host of environmental threats on top of global warming is threatening coral reefs, an important link in the chain of marine life. We can ill afford their demise.

But before we celebrate, there are some issues to ponder. I don't think we yet know how to introduce clade D zooxanthellae in reef communities where they are not abundant nor could we know if there might not be some unintended consequences from such reef `seeding' activities.  There are other complications as well. And even if we figure out how to do this and succeed in saving coral reefs from bleaching, we still have another serious problem to deal with: the threat to coral reefs from ocean acidification.

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