Even today, the legacy of the Cold War leaches into the waters of Colorado. Uranium, freed from the earth and destined for nuclear weapons, now contaminates the groundwater beneath several Colorado mines. But at some of these mines, a most unusual clean-up crew is at work. Lashing about with long electric cables connected to their own bodies, they remove dissolved uranium from the water. Each one of these janitors is just a thousandth of a millimetre across. They’re called Geobacter. They’re bacteria.
The handful of Geobacter species are recent discoveries. The first one, G.metallireducens, was discovered in the Potomac River in 1987. Another, G.sulfurreducens, was later found in oil-soaked Oklahoman soils. The group has the remarkable and useful ability to break down a range of contaminating chemicals, such as petroleum compounds. While humans use oxygen to rend carbon compounds into carbon dioxide and water, Geobacter can use iron oxides and other metals for the same purpose. Roughly speaking, it breathes metal and rock.
This ability has made Geobacter into an obvious candidate for cleaning up environmental mess – a process known as bioremediation. For the last decade, for example, scientists have been using G.sulfurreducens to clean up uranium-contaminated groundwater in Colorado mines. The bacteria add electrons to uranium ions, converting them from a form that easily dissolves in water into one that doesn’t. The uranium drops out of the water, and it can be more easily removed.
This has been a case of technology running ahead of the science. We knew a fair bit about what the bacteria were doing, but less about how they do it. But that gap is being filled. Earlier this year, Derek Lolvey from the University of Massachussetts found that G.sulfurreducens channels electrons along its own home-grown electric wires. It literally plugs itself into its environment.
These wires, known as pili, are just a few nanometres wide, but can be much longer than the bacteria themselves. Although they are made of protein, they can conduct electricity as well as materials used in the electronics industry. Different species can use their pili to wire up to one another, and scientists can even harvest electricity from the bacteria (albeit inefficiently) by growing them on electrodes.
Now, Dena Cologgi from Michigan State University has found that the pili are essential for Geobacter’s uranium-removing abilities. Geobacter uses the pili to offload electrons onto uranium particles, covering a far greater area than it could otherwise reach. Cologgi found that pili-wielding Geobacter removed substantially more uranium from contaminated water than strains that lacked the gene responsible for creating these wires.
Geobacter also uses its pili to protect itself, by breaking down uranium at a distance. Dense deposits of uranium built up around the wires and far away from the bacterium itself. However, the pili-less mutants accumulated uranium in their own membranes and were the worse for it. Their vital functions slowed, they grew less quickly, and they took more time to recover from uranium exposure. Geobacter clearly benefits from keeping its reactions at wire’s length.
Cologgi hopes that her study, by showing how Geobacter acts upon uranium, will help scientists to design better ways of using the bacterium to clean up our messes. It might also apply to other species. Another uranium-removing bacterium called Sheanella oneidensis also has conductive wires. There could be an entire network of conducting bacteria waiting to be tapped into.
Reference: Cologgi, Pastirk, Speers, Kelly & Reguera. 2011. Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism. PNAS http://dx.doi.org/10.1073/pnas.1108616108
Image by Anna Klimes & Ernie Carbone
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