Stowaways. Zebra mussels probably hitched a ride to the Great Lakes in ballast water.

Stopping Microbial Stowaways

ORLANDO, FLORIDA--Empty cargo ships take in ballast water to stay stable, but in doing so, they also load their hulls with billions of microorganisms. These critters can wreak ecological havoc when they're set loose at a foreign port. Irradiating ballast water with ultraviolet (UV) light may be a simple way to stop invasive species, according to a study presented here on 21 May at a meeting of the American Society for Microbiology.

Animal and plant stowaways in ballast water have caused huge headaches. In the United States alone, the zebra mussel clogged waterways to the tune of billions of dollars. But microbes, too, may upset the ecological balance at their destination, in addition to creating public health risks. Last year, for instance, a team led by aquatic ecologist Gregory Ruiz of the Smithsonian Environmental Research Center in Edgewater, Maryland, found that ballast water from ships entering the Chesapeake Bay contained Vibrio cholerae, the bacterium that causes cholera.

To prevent maritime invasions, some countries require ships to exchange their ballast water in midocean. In addition, researchers are looking at ways to heat (Science, 14 July 2000, p. 241) or filter ballast water. But neither technique gets rid of most bacteria--and UV light might.

On a barge floating in Lake Superior, a team led by Ivor Knight of James Madison University in Harrisonburg, Virginia, tested an experimental UV light installation. Engines on the ship pumped 5500 liters of lake water per minute, sent it through a filter, and then sent it through a vat in which it was zapped with 254-nanometer-wavelength UV light. If UV light transmission through the water was low, possibly because of iron or other compounds, the system killed 40% to 50% of the bacteria. If the water let more UV light through, however, the system killed 95% of the bugs. Ideally, 99.99% of the microbes would be killed, Knight says, but boosting the UV intensity or tinkering with the system in other ways will improve upon the prototype, he says.

Ruiz sounds a cautious note: Researchers currently don't know how effective an irradiation system has to be to reduce the risk of invasions significantly. Perhaps bacteria are so efficient at replicating that even a very small number slipping through can cause problems, he says: "How many is too many? That's the challenge."

Related sites

To find the presentation abstract, search here for "ballast"
Ivor Knight's home page
Gregory Ruiz's home page, with information about marine invasions and the National Ballast Water Information Clearinghouse