Is Bleaching Coral's Way of Making the Best of a Bad Situation?

Ben Harder
for National Geographic News
July 25, 2001

For an organism that can't move, coral turns out to be pretty nimble.

Coral has a critical partnership with certain algae that absorb sunlight and convert it to energy needed to feed the complex array of life found in a reef ecosystem. The loss of these algae, a common consequence of pollution or climate change, leaves a reef "bleached" and unable to produce energy from sunlight.

Coral bleaching has increased widely in recent decades. Because it often precedes coral death and the loss of the reef itself, conservationists are naturally concerned that many of the world's reefs are in trouble.

But new findings suggest that when coral is threatened, bleaching may be part of the solution.

It now appears that coral colonies, when confronted with dramatic environmental changes, may purge themselves of existing algae to make room for other algae more capable of thriving in the challenging conditions. Bleaching, then, may not signify coral's imminent demise, but its ability to tough out new conditions.

In one set of experiments, marine scientist Andrew C. Baker of the New York Aquarium found that corals that undergo bleaching after being exposed to sudden environmental change are more—not less—likely to survive in the long run.

"This counters conventional wisdom that bleaching is detrimental from all perspectives," Baker said.

Many Threats

Although many corals look bony and durable, reefs are highly fragile ecosystems, sensitive to human disturbance and environmental stress. Coral reefs all around the world are threatened by water pollution, soil erosion, fertilizers, fishing with explosives, careless diving, and other assaults.

Rising global air and water temperatures are another threat. Although warm tropical waters are ideal for reef development, excessively high sea temperatures can harm or kill algae.

Global warming may accelerate the melting of polar ice caps, causing sea levels to rise. Sunlight doesn't penetrate deep water, so reefs—and their algae—may be deprived of a critical source of energy.

"The frequency of coral bleaching will increase dramatically during this century as a result of increased global warming," said Malcolm McCulloch, a geochemist at Australian National University in Canberra.

Continued on Next Page >>


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