Chemical Pollution And Human Sewage Could Be Killing Corals

“Black band disease is characterized by a ring-shaped bacterial mat that migrates across a coral colony, leaving dead tissue in its wake,” said 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.” 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. 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 biomat, the researchers extracted the microbes’ DNA, amplified and sequenced it. 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 biomat 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.

“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.”

Fouke presented the team’s findings at the annual meeting of the Geological Society of America, held Nov. 5-8 in Boston. The Office of Naval Research funded the work.