Double-acting bacteria immobilize toxic nanoparticles.

Bacteria found lurking in the bowels of an abandoned Wisconsin mine might have a role in cleaning up toxic metals. A new study shows that these bacteria make compounds that cement minute metallic particles into balls that naturally drop out of contaminated water.

As part of their metabolic cycles, certain bacteria that live in watery, oxygenfree environments take up one type of sulfur-containing chemical, a sulfate, and transform it into another type, a sulfide, that they then release. The sulfide binds to metals dissolved in water to form nanoparticles.

John Moreau of the U.S. Geological Survey in Middleton, Wis., and his colleagues studied the activity of such bacteria in a flooded lead-and-zinc mine. They discovered that zinc sulfide nanoparticles "were being scooped up and glommed together into spheroids," he says.

The larger spheroids tend to settle out long before they get into the water supply.

The researchers found that, by weight, proteinlike material formed 10 to 15 percent of the metal spheroids. In lab tests, zinc sulfide nanoparticles clumped when placed in contact with the amino acid cysteine, a protein component. The team reports its findings in the June 15 Science.

Scientists had previously come across metal spheroids in other oxygen-deprived environments and had proposed that heat, pressure, or magnetism might have formed the balls. Moreau and his colleagues now say that the bacteria can do double duty, creating a sulfide that leads to nanoparticle formation as well as making the proteinlike compounds that appear to promote nanoparticle dumping.--S.W.