by Michael Marshall
Three bacteria seem to be responsible for a disease that has killed most of the Caribbean's reef-building corals.
White band disease causes the outer layer of corals to turn white and peel off. First noticed in the 1970s, it has swept through the Caribbean's staghornand elkhorn corals. Both are now critically endangered. They are the region's only branched corals, creating most of the complex habitat that animals like fish rely on, so their loss has depleted the entire ecosystem.
"This disease has taken out the key ecosystem architects," says David Smith of the University of Essex in Colchester, UK.
Now some likely culprits for white band disease have been identified. Michael Sweet of the University of Derby in the UK and colleagues treated diseased corals with antibiotics and tracked whether they recovered. They also monitored changes in the microorganisms present. If an antibiotic killed a microbe, but the coral did not recover, that microbe was not responsible for the disease.
Three suspectsThree bacteria were consistently present in diseased corals, but not healthy ones: Vibrio charchariae – a long-suspected offender – Lactobacillus suebicus and an unidentified species of Bacillus. One or more seems to be responsible for white band disease.
Other bacteria have been tentatively identified in earlier studies, and Smith says they might also trigger the disease. "The likelihood is that at different times, different bacteria cause an infection," he says.
The team also found that the tissue damage that gives the disease its name is caused by a single-celled organism called a ciliate. Killing the ciliate did not stop the disease, but it reduced the tissue damage, suggesting the ciliate was taking advantage of the bacterial infection.
Both the bacteria and the ciliate seem to be essential to create white band disease. "You wouldn't get a disease without the bacteria," says Sweet. "But without the ciliate, you get a different disease."
Smoking gunCould the discovery help save the staghorn and elkhorn corals? Possibly. "Getting this smoking gun really provides you with conservation options," says Smith. It might be possible to identify where the bacteria are coming from and target the site – human activity, such as sewage dumping, may have boosted the bacteria, for example.
Tempting as it might be, we can't treat white band disease with antibiotics, at least not in the wild, says Smith. For one thing, there is already a global epidemic of antibiotic-resistant bacteria, and spraying reefs with antibiotics would only make it worse. Also, corals are home to "good" bacteria and cannot survive without them. "Antibiotics could knock out the bacteria that are required by the coral," says Smith.
However, Sweet wants to try boosting the coral's good bacteria, which help it fight off infections. "We could dose it with the coral's natural microflora," he says, and see if that boosts the coral's immune response. He calls this approach "Yakult for coral".