One of the oldest forms of transportation, ships have sailed the worlds oceans for centuries, transporting every conceivable kind of freight. But they also carry unwanted cargo that is both invisible and potentially dangerous.
New research by scientists at the Smithsonian Environmental Research Center shows that commercial ships traveling among ports scattered throughout the globe transfer large numbers of microbes in their ballast water. And while the task of identifying these uninvited guests has only just begun, at least two of them are known to cause disease.
As ships travel around the world moving cargo and people, they stabilize themselves by taking on ballast water from bays and estuaries rich with marine life. When they deliver their loads at various ports of call, they need to discharge this water. The U.S. receives more than 79 million tons of ballast water each year from overseas ships. Until now, very little was known about how many microorganisms are being moved to a new home in this process.
But a recent study published by researchers from the Smithsonian Environmental Research Center (SERC) in the journal Nature shows that the numbers are huge. They found nearly one billion bacteria and seven billion viruses in each liter of ballast water in ships in the Chesapeake Bay in Maryland.
"The significance of our work here is that it really underscores the magnitude of transfer of those organisms," says Greg Ruiz, a research scientist at SERC and lead author of the study. "We know nothing about their invasions and potential consequences in our marine estuarine systems."
|Newly developed staining techniques with fluorescent antibodies helped locate bacteria more accurately.|
image: Smithsonian Environmental Research Center
Ruiz and his team were permitted to board 15 ships and take samples from the ballast tanks, which are large tanks used to stabilize ships. The study was funded by Maryland Sea Grant and National Sea Grant.
Although Ruiz says that not much is known about the species of the organisms found, the team did identify Vibrio cholerae, the bacterium that causes cholera. Past studies showed that the microbe was present on five percent of ships, but using newly developed staining techniques with fluorescent antibodies, Ruiz and his team found it on all of the ships they sampled. By directly counting the bacteria using stains, researchers were able to detect many organisms that they wouldnt have been able to isolate by growing cultures in the lab. This explains the unexpectedly high prevalence of V. cholerae found in this study, Ruiz says.
While the detection of cholera in the Chesapeake Bay is not new, the SERC team isolated a type of v. cholerae known as 0139, a new serological strain which was responsible for a large cholera epidemic in Bangladesh in 1992. "Its probably not a significant health threat to the American public," Ruiz says. "If we look around to see where these epidemics are occurring, those arent in North America, so the risk [here] is fairly low."
|Ship emptying ballast.|
image: Maryland Sea Grant
However, Ruiz does think that theres a significant risk to the marine community, since marine bacteria and viruses colonize and have impacts on commercial fisheries and marine ecosystems.
Such impacts can be seen, for example, in the case of the parasite MSX, which has virtually wiped out oysters in the Chesapeake Bay. In 1959, MSX killed over one million bushels of oysters and its effects continue to ravage the Chesapeakes oyster population. Until recently, no one was sure whether it was introduced or flourished due to environmental changes. But a recent study led by Gene Burreson of the Virginia Institute of Marine Science has shown that the DNA of MSX in Atlantic oysters is the same as that in Pacific oysters, which more or less proves that the parasite was introduced from the west coast.
Its not just bacteria and viruses that are harmful when transported to new waters. Examples of the toll taken by recent invasions of other non-native species include the Asian swamp eel found in Florida, the giant salvinia (an invasive aquatic plant) found in the Colorado River, and zebra mussels that clog the Great Lakes. The effects of invasive species can, among other things, decrease the biological diversity of native ecosystems, impact water quality and crops, and poison wildlife, according to the U.S. Fish & Wildlife Service.
In response to widespread concern over invasive species, President Clinton signed legislation last year to create an Invasive Species Council, which is coordinating efforts to combat the problem.
Although not all invasive species are introduced by ballast water from ships, the potential for damage by the transfer of non-native microorganisms from ballast water is real, due to the large numbers of viruses and bacteria found in the vessels.
Ruiz says most ships still follow old and inadequate protocols, such as dumping their ballast water in the open ocean, where it will be diluted by salty water thats supposed to discourage the growth of microbes. However, even this measure is voluntary for all but a handful of U.S. ports. Even if its 90 percent effective, as some say, a 10 percent residual can have large implications in the case of microbes, according to Ruiz. "Microorganisms are going to be the most difficult to control
due to their size and their sheer abundance," he says.
Newer methods of treating ballast water are beginning to be used, including ozone treatment, heat treatment and UV radiation. "Its a huge challenge, both understanding the invasion process and what the risk factors are, and to address the wide spectrum of microorganisms that are coming onto ships," says Ruiz. His team plans to expand its research to test for other microbes, including pathogens that could affect not only shellfish, but the people that eat them.
Elsewhere on the Web
Ballast water management (U.S. Coast Guard)
Ballast water treatment feasability study (ANS)
Disinfection of ballast water with chemical disinfectants (NOAA)
Purging ships of aquatic invaders (EPA)
Non-indigenous species 2000-2001 (Maryland Sea Grant)
National Ballast Clearinghouse (SERC)
Biennial Report on exotic species (EPA)
Restoring Oysters to U.S. Coastal Waters (Maryland Sea Grant)
Marine Invasions Program (SERC)
U.S. Ports Endorse Ballast Water Management Legislation (AAPA)