Bacteria and other microorganisms that could harm humans and marine life are being spread around the world in the ballast water of ships, according to a study in today’s issue of the journal Nature. The researchers found significant concentrations of a potentially fatal form of cholera in ballast water released from ships in Maryland’s Chesapeake Bay, and although conditions in the bay do not seem conducive for the cholera to flourish, its presence is cause for concern. “Each day, it’s a game of ecological roulette, and when we’ll lose, we’re not sure,” said Ann Swanson of the Chesapeake Bay Commission. Previous studies have documented the spread of exotic species such as zebra mussels and jellyfish through ballast water, but this is the first research into the spread of microorganisms. About 87 million tons of ballast water are discharged into U.S. ports each year, the study found.