Clues to Coral Disease

A new study shows that warming temperatures threaten coral reefs.

By Liz Williams

Humans have long depended on coral reefs. Incubators for marine life, they provide us with food and natural, offshore protection from storms. But a new study suggests that rising tropical ocean temperatures exacerbate a disease that threatens these fragile structures, especially in areas where they are particularly abundant.

The building blocks of coral reefs—tiny, flower-shaped creatures called coral polyps—are highly sensitive to changes in ocean temperature, and warmer waters might also drive disease outbreaks, according to a study released this month in PLoS Biology.

“We’ve long suspected that temperature played a role,” says John Bruno, an assistant professor at UNC Chapel Hill and lead author of the study, “[but] we didn’t really have the data until recently.”

Other researchers agree that the data in the study shed more light on coral death.

“I certainly think it was timely and very useful,” says Ove Hoegh-Guldberg, director of the Centre for Marine Studies at the University of Queensland, who was not involved with the study.

Researchers reviewed data collected over a six-year period by the Australian Institute of Marine Science (AIMS). Through the institute’s Long Term Monitoring Program, scientists conducted surveys on coral coverage and the prevalence of a coral disease called white syndrome at 48 different sites across the Great Barrier Reef off the coast of Australia.

After looking at sea surface temperatures taken weekly via satellite and annual surveys for white syndrome and coral coverage, they compared the number of weekly temperature fluctuations of 1°C or more at each site in the year prior to each survey. The scientists found that white syndrome outbreaks occurred more often after warm years, but only in reefs with more than 50 percent coral cover.

“The effect of temperature and the effect of coral cover are equally significant,” says Bruno. “People often argue that coral disease often occurs when the reef decays or becomes unhealthy, but our research found the opposite.”

Scientists know that warmer waters increase bleaching events, which are generally widespread, affecting up to 55 percent of the coral colonies in the Great Barrier Reef in particularly warm years, according to AIMS.

In bleaching events, tiny, brown algae usually inhabit the tissues of living coral, giving them color and providing food through photosynthesis. But when corals are exposed to a stressor—such as abnormally warm water—this symbiosis breaks down and the coral are left, pale and starving, to fend for themselves.

Though white syndrome is not as widespread as coral bleaching, warmer ocean temperatures also affect the disease. White syndrome, however, kills some of the coral while bleaching releases the algae living on it.

White syndrome typically starts with a telltale white band of newly killed exoskeleton. That band progresses gradually, eventually engulfing the remaining, healthy coral population. In its wake, turf algae take root, darkening the dead white areas and marking the regions that die first.

But as with many other coral diseases, the cause of white syndrome is unknown.

Some scientists say the culprit is bacteria while others have found evidence of programmed cell death near white syndrome lesions. Some cell death is natural, but it may also be triggered by other influences, including physiological stress or unknown pathogens.

“You need to go down to the sub-cellular level to understand these diseases,” says Hoegh-Guldberg, “We don’t think it’s all about being attacked by bacteria.”

Regardless of the cause of the disease, the recent findings highlight one way in which a temperature increase of 1 to 3° C—the expected change due to global warming by 2100, as reported by AIMS—could affect this delicate marine ecosystem.

For Bruno, the effect of warmer waters on coral emphasizes the importance of combating global warming. “Governments have to do a better job of mitigating the impacts of climate change,” he says. “It’s something we’ve got to do globally.”

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