Transfer of microorganism facilitated by ballast water

Microbial organisms are major constituents of shipboard ballast water. Researchers find large volumes of bacteria and viruses discharged into ports worldwide.

provided by National Sea Grant College Program

esearchers at the Smithsonian Environmental Research Center (SERC), the Center of Marine Biotechnology (COMB) of the University of Maryland Biotechnology Institute and Old Dominion University reported in the November 2 issue of the British journal Nature that ballast water discharges by the world's ocean-going ships "create a long-distance dispersal mechanism for human pathogens, and may be important in the worldwide distribution of microorganisms as well as the epidemiology of waterborne diseases affecting plants and animals." It has long been known that such discharges have unintentionally spread plankton, shellfish and fish from one body of water to another, sometimes with major impacts to the receiving ecosystem.

Lead researcher Gregory Ruiz, from SERC, reports in Nature that he and his colleagues have found high concentrations of discharged microbes in the ballast water of ships arriving in Chesapeake Bay from foreign ports. Until these studies by Greg Ruiz and his colleagues, Tonya Rawlings, Fred Dobbs, Lisa Drake, Timothy Mullady, Anwarul Huq and Rita Colwell, the global movement of microorganisms via ballast water discharges was virtually ignored. The research was funded largely by the Maryland Sea Grant and the National Sea Grant College programs.

Ruiz says that although there is no reported evidence of outbreaks of human diseases from non-indigenous microbes in ballast water, the findings indicate the need for much greater concern than has heretofore been shown.

"Despite growing concern about biological invasions and emergent diseases, the extent and effects of the transfer of microorganisms in ballast water are virtually unexplored," writes Ruiz and his colleagues. "We know of no published estimates of microbial genetic diversity in ballast water, and the fate of microorganisms discharged from ballast tanks remains unknown. Given the magnitude of ongoing transfer, and its potential consequences for ecological and disease processes, large-scale movement of microorganisms by ships merits attention from both invasion biologists and epidemiologists."

Vibrio cholerae, for example, the bacterium that causes human epidemic cholera, was detected in all ships tested, and included the recently emerged serotype 0139, which was recently isolated in Bangladesh. While V. cholerae and other potential pathogens are normal constituents of coastal waters in the United States, they do not ordinarily occur in high enough concentrations to cause human health problems. However, with expanding world trade and increasingly larger vessels moving among international ports, the impact of non-indigenous microorganisms could be profound. The study authors write that laboratory observations have "revealed that some bacteria are viable upon arrival, and that their release creates an opportunity for the colonization of coastal ecosystems."

The impact of invasive species on aquatic ecosystems over the last 25 years has in some cases been overwhelming. Zebra mussels may be the most infamous aquatic invader of the United States in recent years. A native of the Caspian Sea, it was most likely released by a freighter into the Great Lakes in the mid-80s. Its spread has been astounding. Overwhelming native species, it has caused extensive environmental and economic damage throughout the Great Lakes and has spread to ecosystems in 22 states throughout the Great Lakes, and the Hudson, Ohio and Mississippi river drainage basins.

Numerous examples of harmful invasive species abound throughout the world: the Baltic Sea's green crab is now ranging the northeast and west coasts of the United States, aquatic plants such as phragmites, water milfoil and hydrilla are taking over many near shore areas, the rapa whelk from the Sea of Japan is in the lower Chesapeake, the comb jelly from the Chesapeake arrived in the Black Sea and has grown uncontrollably without any natural predators.

The transfer of microbes could well be the most insidious problem in ballast water discharges. Management efforts are underway to limit the risk of non-indigenous species in such discharges, says Ruiz. These include exchange of ballast water at sea, the installation of filtration systems, and heat and chemical treatments. However, the transfer of microorganisms may be especially difficult to control with such options because of their small size, their tolerance to high temperatures and their ability to form resting stages. "Our research," he adds, "has clearly demonstrated that a potential problem exists and that we must concentrate on clarifying the impact and developing effective strategies to address this issue."