Bacteria have long been fighting on the front lines of uranium-contaminated groundwater. Their ability to take uranium out of a solution and mineralize it has proven invaluable at abandoned uranium mines. The mechanism by which they accomplish this fortunate feat has remained a mystery—until now. A paper in the Proceedings of the National Academy of Sciences this week reveals the details.

Bacteria of the genus Geobacter can help remediate a number of different groundwater contaminants. Besides uranium, they can also take on petroleum compounds and chlorinated solvents—two of the most widespread contaminants. They are anaerobic bacteria, meaning that they live in the absence of dissolved oxygen. Instead, they utilize a variety of elements that includes nitrogen, manganese, iron, sulfur, and, yes, uranium.

It comes down to chemical oxidation/reduction reactions, which are basically electron transfers. The things the bacteria are munching on—petroleum compounds, chlorinated solvents, and other organic compounds—are electron donors. The elements used for respiration (i.e. oxygen, iron, etc.) are electron acceptors. Energy is released to the cell as part of this reaction, which is, of course, why living things go through all the trouble to do it.

It’s well known that some members of Geobacter can utilize uranium as an electron acceptor, similar to how humans use oxygen. As part of that electron transfer reaction, hexavalent uranium (which has a charge of +6) becomes tetravalent uranium (which has a charge of +4). Hexavalent uranium is easily dissolved in water, meaning it is mobile in the subsurface and moves with groundwater flow. Tetravalent uranium, however, quickly precipitates in mineral form, locking it up in the sediment or bedrock. This is what makes the bacteria so helpful at uranium contamination sites—they prevent contamination from spreading in groundwater.

So, do these bacteria produce an enzyme that helps them process uranium in this way, or is there something else going on? That’s been the question for some time now. A group of microbiologists suspected that pili—tiny, thread-like appendages on the surface of many bacteria—might have something to do with it. To test this, they compared mutants that could not develop pili to normal bacteria. They found that the pili were indeed central to the mineralization of uranium. To understand why, we’ve got to include a study that was published in Nature Nanotechnology just last month.

The second study discovered that the pili of that same Geobacter species are electrically conductive. In fact, they are just as conductive as metallic nanowires. As if the first example of metal-like conductivity in a biological structure wasn’t enough, the researchers were even able to get films of these bacteria to function as transistors.

It makes sense, then, that the pili would act as electron conduits to facilitate oxidation/reduction reactions. But why go to such lengths when the uranium could just be brought into the cell? There are a couple reasons. First, the long pili greatly increase the bacteria’s surface area, making more of it available for reactions. Second, it protects the cell. Mineralization of the uranium inside the cell could really mess with its vital functions. The pili-deficient mutants were not only less effective at utilizing (and mineralizing) uranium, their respiratory activity also slowed as they took in ever more uranium to accept electrons.

So what’s the practical upshot of this knowledge? If we understand how these bacteria go about their work of attenuating groundwater contamination, we’re better equipped to facilitate the conditions required to optimize uranium mineralization. What’s more, we may even work out how to engineer synthetic or biological improvements to the process.

PNAS, 2011. DOI: 10.1073/pnas.1108616108, Nature Nanotechnology, 2011. DOI: 10.1038/nnano.2011.119 (About DOIs).