While in port, shipping vessels often suck huge quantities of water into their ballast tanks to replace the stabilizing weight of cargo they've off-loaded. Along with this water comes abundant aquatic life, such as mussels and crabs, which journey with the ships—often crossing entire oceans—until the ballast is dumped in preparation for loading new goods.

Largely ignored in this lively exchange are microbes. A new study finds that releases of ballast water dump huge quantities of bacteria and viruses into ports around the globe.

Gregory M. Ruiz of the Smithsonian Environmental Research Center in Edgewater, Md., and his colleagues sampled ballast water from 15 ships entering the Chesapeake Bay. In the Nov. 2 Nature, they report an average of 830 million bacteria and 7.4 billion viruses in each liter of water. Though most of the microbial vagabonds have yet to be identified, Ruiz' team did find in every sample at least one of the two bacterial strains that can cause epidemic cholera.

Though other studies had found microbes in ballast, Ruiz says that his study was the first to quantify them and to assay for those that were in a resting phase but able to reproduce when conditions turned favorable.

The new data also indicate that, counter to previous studies, most of the cholera bacteria were not attached to fine debris in the water. That's potentially bad news, Ruiz explains, because it suggests that filters won't work as a removal strategy for these potentially deadly microbes.

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