Chemical Pollution, Human Sewage Killing Corals

BOSTON, Massachusetts, November 6, 2001 (ENS) - A combination of human sewage and shipyard discharge may be responsible for the development and spread of deadly black band disease in corals, researchers at the University of Illinois say. The scientists say pollution may be major contributor to the worldwide decline of coral reefs, playing as big a role as global warming.

black band

Black band disease is characterized by a black band appearing between the living and the dead portions of coral (Photo courtesy National Marine Fisheries Service)
"Black band disease is characterized by a ring shaped bacterial mat that migrates across a coral colony, leaving dead tissue in its wake," said University of Illinois (UI) geologist Bruce Fouke. "Like a tropical rainforest, a coral reef system is a cradle of biodiversity. If we destroy the reefs, we destroy the ocean's ability to reproduce."

To better understand the disease, Fouke and his colleagues, UI microbiologist Abigail Salyers and postdoctoral researchers George Bonheyo and Jorge Frias-Lopez studied corals off the island of Curacao in the Netherlands Antilles, near the Venezuelan coast.

First, the researchers mapped outbreaks of the disease along the reef. Then they looked for metals such as aluminum, cadmium and zinc that are common pollutants from shipyards and oil refineries.

"The highest number of infected corals, as well as the highest concentration of dissolved metals, occurred near the city of St. Annabaai, which has a major harbor and one of the largest oil refineries in the Caribbean," Fouke said. "This suggests that diseased coral may be experiencing increased environmental stress due to pollution, which in turn decreases the coral's resistance to bacterial infection."

fish & corals

Coral reefs cover less than one percent of the Earth's surface (Photo courtesy U.S. Coral Reef Task Force)
Healthy corals contain a natural population of bacteria within a mucous rich biofilm that provides protection from light, exposure and sedimentation, Fouke said.

"Environmental stresses cause corals to secrete more of this mucous to coat their outer tissues," he explained. "This leads to elevated levels of natural microbial populations, as well as the introduction of new, potentially harmful bacterial populations."

To identify the microbes inhabiting the black band bacterial mat, the researchers examined the microbes' DNA. They found several organisms, including Arcobacter and Campylobacter, which are human pathogens and could be a direct link to raw sewage.

Also present in the bacterial mat was a ropy network of cyanobacteria, a unique group of photosynthetic bacteria that cannot live without light. In field experiments, the researchers used shields to block light from infected corals. Black band disease disappeared from the regions that were not exposed to light.

brain coral

A brain coral infected with black band disease (Photo courtesy U.S. Coral Reef Task Force)
"This indicates that cyanobacteria are an important part of the disease development, but may not be the pathogen," Fouke said. "Perhaps the cyanobacteria form an apartment complex, allowing a variety of destructive anaerobic bacteria to take up residence in the low oxygen microenvironment."

Many more tests are needed to identify what is killing the coral, Fouke said. "But, the present trilogy of disease distribution, high metal concentrations and presence of human pathogens creates a signpost, at least, that human pollution is playing a role."

Pollution, combined with global warming, could wipe out many of the world's coral reefs, scientists warn. Both pollution and global warming stress the tiny polyps which build reefs, making them more vulnerable to disease.

Diseases of stony corals have skyrocketed over the last decade, according to the National Marine Fisheries Service. Studies have shown that coral diseases are affecting greater numbers of coral species, are increasing in frequency and distribution, and are spreading to new regions faster than ever recorded in the past.

In recent years, a number of new coral diseases have emerged, with new types of symptoms not observed in the past. Several of these diseases kill coral tissue at rates much faster than ever observed before.


About 10 percent of the world's coral reefs have already vanished (Photo courtesy U.S. Coral Reef Task Force)
Global warming is also believed to be behind an epidemic of coral bleaching. Bleaching occurs when microscopic plants inhabiting corals are damaged by slight temperature rises.

The plants, or zooxanthellae, which color the coral and provide food are expelled. If they do not return, the coral will die.

Most coral bleaching can be explained by a one degree Celsius rise in water temperature above the normal summer maximum temperature, experts say.