Global warming not sole cause of coral reef decline

Researchers at the University of Illinois at Urbana-Champaign (UIUC) have

found, contrary to the majority of studies pointing to

global warming as the principal cause of coral reef

decline (see related

story), black band disease caused by human

activity. Characterised by a ring-shaped bacterial

mat that migrates across a coral colony, leaving dead

tissue in its wake, the disease is also responsible for the rapid

decline in coral reefs worldwide. A recently-published UN atlas of coral reefs found that

they occupy an area 10 times smaller than previously

estimated (see related

story).

To better understand the disease, UIUC geologist Bruce

Fouke and his colleagues, 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 and then they

looked for metals such as aluminium, 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 harbour 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, but

environmental stresses cause corals to secrete more of

this mucous to coat their outer tissues, leading to

elevated levels of natural microbial populations, as

well as the introduction of new, potentially harmful

bacterial populations, the researcher explains.

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, as well as a 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 and 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 concludes. “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.”