Raw Human Waste Killing Off Coral Reefs?

Ben Harder
for National Geographic News
June 27, 2002
In many parts of the world where sewage treatment is lacking, raw human
waste spills into the open sea. Now, research suggests that certain
microbes in the human gut are responsible for two coral diseases that
have devastated large patches of reef.

Billions of bacteria live in the intestines of each person. Many of these microbes, including Serratia marcescens, don't typically cause illness in people. Others, such as Arcobacter, can cause diarrhea in children when too abundant.

But these two intestinal bugs and many others are much less benign when they meet coral, according to scientists who have linked the bacteria with white pox and black band disease, which have decimated two types of coral in recent years.

White pox has nearly wiped out hornlike elkhorn coral in some reefs in the Caribbean and elsewhere. Black band disease has stricken round coral in many reefs throughout the seas.

"The one-two punch from those two diseases alone could explain the vast majority of disease impact on reefs," said Bruce Fouke of the University of Illinois at Urbana-Champaign.

A Killer Consortium

Fouke and his colleagues have been researching black band disease in Curacao, a Caribbean island in the Netherlands Antilles.

Black band disease consists of a dense mat of microbes. The disease begins as a small concentric ring that expands in circumference "like a rubber band" as it moves around the coral, and then contracts as it eats its way across the opposite side, Fouke said. "It leaves nothing but bare skeleton behind it," he said.

Fouke initially thought that three or four types of bacteria might be involved in black band disease. Instead, he found a legion of microbes. "Our research shows there are over 40 organisms that live" in the disease mat, Fouke said.

In the May issue of Applied and Environmental Microbiology, Fouke's team reported the results of extensive laboratory and genetic work on the numerous bacteria involved in black band disease.

No single pathogen seems wholly responsible for the disease, they've found. Instead, different types of bacteria in the cluster interact in ways that ultimately destroy the coral. Some live on the surface of the mat and block oxygen-rich seawater from penetrating the lower layers, where bacteria that need an oxygen-free environment "go about their dirty work," Fouke said.

Single Target

Meanwhile, many miles away from Fouke's research station in the Netherlands Antilles, marine ecologist James Porter of the University of Georgia and his colleagues have been working to better understand the coral disease white pox.

The researchers began monitoring the Eastern Dry Rocks Reef in the Florida Keys National Marine Sanctuary, off the shore of Key West, in 1994. There, in 1996, they documented the first case of white pox.

Although later research showed it to be contagious, white pox appears to strike only one kind of coral—"the magnificent branching elkhorn coral," known as the "giant redwoods of the reef," said Porter.

Within four years after the white pox was found, the population of elkhorn coral in Eastern Dry Rocks Reef had decreased by 82 percent.

In lab analysis and experiments, the researchers concluded that the cause of white pox is a jellybean-shaped bacterium called Serratia marcescens. It is common in the human gut but is not known to be typical in healthy reefs.

Emerging Diseases

When he learned about Porter's study of white pox, Fouke sent an e-mail message to his students in Curacao asking them to make a note of every time they came across white pox during their ongoing research on black band disease. Any geographical pattern seen in the locations where the two diseases are found might offer clues about the factors that play a role in both the diseases, or in one but not the other.

New coral diseases have been seen more and more since the mid-1990s. Now that scientists know coral is susceptible to human pathogens, that trend may be partially explained by the growing populations of people living along tropical coasts—and greater volumes of human waste spilling into coastal waters. Among their observations, Porter and his colleagues found that white pox destroys coral more quickly in late summer, when seawater is the warmest. In a report June 18 in Proceedings of the National Academy of Sciences, they suggested that global warming could be a factor in the loss of coral because it may "lengthen the disease season."

As scientists work to understand how water temperature and other factors contribute to coral diseases, or even why intestinal bacteria attack coral in the first place, the immediate damage is impossible to ignore. Referring to elhorn coral, Porter said: "What used to be the most common coral in the Caribbean has now been recommended for inclusion on the Endangered Species List."

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