Bacteria Form Uranium Dioxide 'Pearls'

One prospect for bioremediation of groundwater contaminated with heavy metals is capitalizing on the ability of microbes to convert soluble metal ions into insoluble minerals that are immobilized in soil. A research team led by Matthew J. Marshall and James K. Fredrickson of Pacific Northwest National Laboratory has added to understanding of this process by partially unraveling the mechanism by which Shewanella oneidensis bacteria convert U⁶⁺ ions into 5-nm UO₂ nanoparticles (PLoS Biol. 2006, 4, e268).

S. oneidensis was known to reduce uranium, iron, and other metals under anaerobic conditions by electron transfer via certain cytochrome enzymes. The bacterium plays an important role in the weathering of rocks and global cycling of soil nutrients.

The researchers developed mutant strains of the bacterium that enabled them to observe that UO₂ nanoparticles are formed both inside and outside of the cells. They were able to pinpoint a specific cytochrome in the outer membrane of the cells that is responsible for electron transfer outside the cells. The cells release the cytochrome into an extracellular polymeric substance, a kind of slime produced by the bacterium, where the reduction primarily takes place.

The sticky slime serves not only as a medium for the reduction but also as a glue to bind the extracellular nanoparticles and possibly prevent them from reoxidizing back to U⁶⁺, Fredrickson says. This property could be the basis of a remediation process to halt underground migration of uranium and other heavy metals at contaminated industrial and nuclear waste sites, he adds.