Harmful microbes may be hitchhiking across the Atlantic Ocean on motes of dust, according to a new report. The presence of live pathogens on particles long thought to be sterile raises the possibility that diseases could ride from continent to continent on the wind.
Every year, churning storms in North Africa hoist as much as a billion tons of dust into the atmosphere, and prevailing winds drive the particles westward across the Atlantic Ocean. During the summer months, this dust stream sweeps over the Caribbean and the southeastern United States, bringing Technicolor sunsets to Florida. Dust itself can be harmful--it makes life miserable for people with asthma and can cause or exacerbate other lung diseases. But most scientists believed that ultraviolet rays in sunlight would sterilize the particles during the 5- to 7-day journey, says microbiologist Dale Griffin of the United States Geological Survey's Center for Coastal Geology in St. Petersburg, Florida.
Suspecting that microorganisms could in fact survive the trip, Griffin and colleagues collected air samples on dusty and clear days on St. John, one of the U.S. Virgin Islands. The researchers incubated some samples in culture and scanned others with a microscope for bacteria, fungi, and viruses.
On dusty days, the number of microbes in the air surged by up to 10 times, the researchers report in the June issue of Aerobiologia. Though no germs deadly to humans turned up in the dust, about 10% of the captured microbes can cause illness in people with weakened immune systems, and about 25% can attack plants. Griffin wasn't surprised: "To think that [microbes] couldn't survive 5 to 7 days in the atmosphere is beyond me." The authors speculate that the bugs escape ultraviolet radiation because dust at higher altitudes may shield the lower layers or because the microbes take shelter within cracks on the particles.
The study "shows that microorganisms are dispersible all over the world," says atmospheric microbiologist Bruce Lighthart of University of Montana in Missoula. But to prove that the microbes really hail from Africa and not from somewhere closer to home, the researchers need to sample dust along the route and trace the bugs to their origin, he says.