Coral bleaching and death could be early warning of environmental change, Cornell ecologists warn

ANAHEIM, CALIF. -- The dying corals of the Florida Keys could be an early warning of tough times ahead for the planet's environment, Cornell University ecologists worry. The reason: Hundred-year-old corals are succumbing to diseases they previously survived.

Increasing global temperatures and worsening pollution, the ecologists say, could place so much stress on ecosystems that organisms of all kinds will face new challenges.

"When we see corals that have persisted for hundreds of years suddenly die from opportunistic infections, we have to wonder what has changed in their environment," says C. Drew Harvell, associate professor of ecology at Cornell.

Harvell organized a session,"Diseases of the Ocean: A New Environmental Challenge," at the annual meeting of the American Association for the Advancement of Science (AAAS) today (Jan. 22) to bring together leading microbiologists, ecologists and pathologists to evaluate the environmental threats from disease in the ocean. Speaking in the session was Kiho Kim, a postdoctoral research associate with Harvell at Cornell, who reported on an unusual disease in Florida Keys corals.

Kim said that monitoring of sea fan corals in the Keys, where up to 40 percent of sea fans are infected by a fungal disease and many have already died, suggests that lower water quality and higher ocean temperatures stress corals and increase their susceptibility to disease. He said the Florida findings support a growing consensus among scientists worldwide that as ocean ecosystems become degraded they will offer more favorable places for disease outbreaks and the emergence of new pathogens.

"We didn't begin our study of sea fans to monitor death and destruction," Harvell said. "Originally, we were interested in the natural disease-resistance properties of corals, such as the anti-bacterial and anti-fungal chemicals they produce, because some of those compounds may be useful in human medicine. That disease resistance normally keeps a coral alive for hundreds of years, despite living in an ocean full of potential pathogens."

She said Garrett Smith of the University of South Carolina at Aiken was responsible for tracing the sea fan disease to a common, soil-dwelling fungus. A type ofAspergillus fungus, washed out to sea by land erosion, collects on the flexible, fan-shaped surface of the corals and promotes an aspergillosis infection that first discolors and eventually causes lesions and tumors as it destroys some corals, the researchers said. Sea fans, which position themselves perpendicular to water currents are especially vulnerable to any pathogenic organisms in the passing water, Harvell noted.

"Somehow, a soil pathogen that was best known for infecting aged and immune-compromised humans has crossed the land-sea barrier," Harvell said. "Now, one of our jobs is to discover what has compromised the resistance of the corals at some sites. Although a significant number of sea fans have died at a few sites, at many locales they recover from infections, pointing to the success of their natural resistance."

While coral disease is reported throughout the Caribbean, the reef ecosystems of the Florida Keys may be particularly vulnerable because they are close to what ecologists call "natural stressors," such as fluctuating water temperatures and substantial freshwater runoff, Harvell said. The situation has worsened in recent years, the Cornell ecologist observed, with multiple "anthropogenic stressors," such as eutrophication, siltation and other effects of intensive human use of the land and off-shore waters.

Symptoms and effects of Aspergillus infection
Kiho Kim
Symptoms and effects of Aspergillus infection on the sea fan Gorgonia ventalina. Infected sea fans are charaterized by lesions of necrotic tissue surrounded by a dark purple halo.

"Then you have rising water temperatures of the oceans," Harvell added. "Whether you believe that global warming is a function of human activity and whether last year's El Nino was a symptom of global warming, the fact is that sea temperatures globally in 1998 were high. And 1998 was the worst year ever recorded globally for coral bleaching."

Corals bleach (or lose their symbiotic algae) when stressed by high temperatures, Harvell explained, adding: "I think we have to question the relationship between temperature stresses and diseases of the oceans."

Lately in the Florida Keys, coral death has been occurring so suddenly and rapidly that Harvell and kim must monitor their research sites three times a year. She credits the assistance of Cornell undergraduate researchers, including Alisa Alker, who dive from NOAA vessels and return to the laboratory to perform biological assays of coral samples.

"With a very few exceptions, we know so little about the pathogenic organisms that are affecting the coral reefs," Harvell said. "We don't know if new diseases are emerging, if the hosts are becoming more susceptible or both. We need to identify these new diseases and we should do it now while we have the chance. Disease ecology is poorly understood in the ocean because diseases are like lightning strikes -- they hit unexpedly, burn through a population, and then they are often gone."

Harvell and Kim conduct their studies from the Keys Marine Laboratory in Long Key, with the assistance of Reef Relief in Key West. Their research is supported by the National Science Foundation, National Oceanic and Atmospheric Administration (NOAA) and the New England Bio Labs Foundation.


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