Israeli scientists say they have studied a type of coral whose bleaching is triggered by a bacterium, and found that this is carried and transmitted by a coral-feeding worm. If the same is true for other corals, scientists may be able to prevent the disease by targeting the worm vector, just as they control malaria by targeting mosquitoes.
Corals depend on algae inside their bodies to provide carbohydrates and oxygen through photosynthesis. This partnership allows corals to thrive in infertile areas of ocean, and explains why reef communities are so rich. But when water temperature rises, the algae either die or the corals eject them in response to the stress of the heat. Before too long the entire reef turns white. It is unclear how well reefs later recover, and the sharp rise in coral bleaching since the 1980s has sparked dire predictions for their future (see Box).
Most marine biologists agree that bleaching is caused by rising sea temperatures, and the best way to stop it is to cut carbon emissions. But that is an indirect, long-term strategy that may not save reefs in time. Now there may be a more immediate remedy.
In the late 1990s, Yossi Loya and Eugene Rosenberg led a team from Tel Aviv University that found a bacterium that causes coral bleaching. Crucially, it turns out to be more virulent at higher temperatures. "Coral biologists are not microbiologists," says Rosenberg. "They saw the association of bleaching with temperature, but they did not see the bacterium."
The microbe, named Vibrio shiloi, was responsible for bleaching the stony coral Oculina patagonica in the Mediterranean Sea. But researchers were mystified because the bacterium was only present in the summer. "We had to find out what happened to it in the winter," Rosenberg explains.
To reveal its hideout, they added a fluorescent chemical tag to the water around the coral. This tag attaches specifically to the bacterium's DNA, and they were surprised to see where the fluorescence later showed up. "I thought we would find it in the water, or at least in the sediment," says Rosenberg. But the strongest glow came from the marine fireworm, Hermodice carunculata, they will report in an upcoming issue of Environmental Microbiology. The researchers believe the worm harbours the bacterium, and transmits it to the coral while feeding.
It is the first time a vector has been found for a coral disease. Rosenberg says his team is now developing a technology that could help prevent the disease's spread, although he wouldn't reveal details. He suspects much of the world's coral bleaching is caused by similar pathogens, and some of them may have different vectors. This year, he showed that another new bacterial species, Vibrio coralliilyticus, causes bleaching in a Red Sea reef coral. Pathogenic strains of Vibrio have also been isolated from reefs off the coasts of England and Brazil.
Ove Hoegh-Guldberg, a coral bleaching expert and director of the Centre for Marine Studies at the University of Queensland, thinks mass bleaching events move too quickly to be caused by an infectious disease. "But it is possible that this microbe naturally inhabits corals, a bit like Escherichia coli in the human gut, and temperature increases cause it to trigger bleaching," he says.
Written by LYNN DICKS
New Scientist issue: 12 April 2003
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- Mass bleaching events have occurred around the world in 11 different years since 1983, including last year and five consecutive years from 1987. The most serious events occur during El Ni-o years.
- In 1998, bleaching spread from the Eastern Pacific to the Indian Ocean in less than three weeks, and went on to affect 16 per cent of the world's corals.
- The UN Environment Programme estimates that between 30 and 50 per cent of healthy coral reefs will be gone by 2100.
- Mean global sea temperature has risen by 0.7 degrees C over the past 100 years.
- In the Mediterranean, 80 to 90 per cent of Oculina patagonica has bleached every summer for the past 10 years.
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