Bacteria cause plague in coral reefby S. Milius
Just in time for the underwater version of flu season, researchers have identified the bacterium responsible for an unusually virulent coral disease.
Plague type II first appeared on the Florida Reef Tract in June 1995 and had spread along 200 kilometers of the Florida Keys by October. It advanced through healthy tissue at rates of up to 2 centimeters in 24 hours, killing corals in a few days. Plague type I, which appeared in the 1970s, spread only a few millimeters a day and took 3 to 4 months to kill.
Type II flared up again in 1996, and reef biologists fear that this summer will bring a new wave of the disease.
Identifying the bacterium "is the first step in deciding how to treat these diseases," says Laurie L. Richardson of Florida International University in Miami. Type II plague comes from a previously unknown species of Sphingomonas, Richardson and her colleagues report in the April 9 Nature. They do not know where the species comes from, why it attacked suddenly, or whether it's related to the first plague.
The new study represents the most rigorous application so far of classic mammalian pathology to a coral disease, Richardson says. Her team used Koch's postulates, named after a 19th-century founder of microbiology, Robert Koch. The criteria are applied to determine that a microorganism is actually causing a disease rather than just thriving in sick tissue. Koch's procedure involves isolating the suspected villain from diseased tissue, using that microorganism to infect a healthy creature, and then isolating the organism from the sick test animal.
Success with Koch's postulates has been rare in the world of coral disease, Richardson says. Twenty years' worth of effort by other researchers has failed to yield consistent identification of a pathogen in another affliction, white band disease. The organism behind plague type I also remains unknown.
In one disease, an ailment called black band, Richardson and a group of colleagues have identified a complex cause. The dark band that creeps over coral and destroys tissue comes from a consortium of blue-green algae, a deadly layer cake of cooperating organisms from three genera that secretes toxic sulfides from its bottom tier.
John C. Ogden, director of the Florida Institute of Oceanography in Saint Petersburg, praises the Richardson team's approach to disease identification as "exactly what's needed." Without pathogen research, reef biologists are stuck trying to diagnose diseases by blatant symptoms like color changes.
The problem is like a doctor trying to diagnose human disease just "by counting spots," he says.
Little research has explored treatments for coral maladies, he observes. Sometimes black band eases when divers enclose a sick coral in a bag of antibiotic solution or just vacuum away the killer goo. This time-consuming treatment "only makes sense on very old, highly beloved corals on trails that are visited by thousands of people each year," Ogden says.
Another researcher studying coral diseases, James M. Cervino of the Global Coral Reef Alliance in New York City, points out that plague type II is "the fastest-spreading disease, and it affects the most species [of coral]." At the end of 1997, he saw it hit sites in the southern and central Caribbean, beyond its original Florida range.
He's convinced that reef diseases are growing more common, possibly as a result of increased stress on the organisms. He does offer some good news, however. A disease he's been tracking, rapid-wasting syndrome, seems to be slowing. He's exploring the hypothesis that it came from a rash of unusually deep parrot fish bites that became infected by a pathogen not yet described in a publication.
Richardson, L.L., et al. Floridas mystery coral-killer identified. Nature 392(April 9):557.
Additional information can be found at: http://ourworld.compuserv.com/homepages/mccarty_and_peters/dis-list.htm.
Pennisi, E. 1994. Killer CLOD strikes Indo-Pacific reefs. Science News 146(Aug. 27):135.
Travis, J. 1996. Bleaching power. Science News 149(June 15):379.
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