A Novel Strategy in the War on Waste
by Charlotte Overby, Illumination, Spring 2002
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In addition to hydrogen, D. desulfuricans pulls electrons from metals, such as iron. These bacteria first caught the attention of scientists because they oxidize iron atoms, making the iron water-soluble and leading to metal corrosion. "It's been known for years that sulfate-reducing bacteria are suspected to be the primary culprits in ferrous metal corrosion. They are the ones that cause your gas tank to corrode, or create the pits you see in corroded metal," she explains. "So this ‘bug,' as I call it, has a history of being able to metabolize metals. We know that it interacts with metals on a routine basis." Researchers began to wonder if the same bacteria that can cause gas tanks to leak might one day be harnessed to "corrode," alter or reduce more toxic metallic elements such as uranium.

By studying this electron transfer process in D. desulfuricans, researchers learned that when D. desulfuricans comes into contact with uranium, it pushes electrons onto uranium VI, a type of uranium that is water-soluble. Type VI's oxidation state changes and converts to uranium IV, which is less dangerous. The bacteria don't eliminate the uranium's radioactivity, but they do convert it to a form--type IV--that is not water-soluble, and therefore can be contained, collected and stored more easily.

Wall and her research team have taken a genetic approach to studying the "bug," working to understand which of its 3,000 to 4,000 genes actually contribute to the reduction of uranium VI to uranium IV. The structure of the candidate proteins makes them act like electrical wires, says Wall, and they bristle in the exterior wall of the bacteria, ready to receive electrons and shoot them out. If uranium is available, it will give the electrons to the uranium and trigger the reduction. "There was already quite a bit known about the flow or push of electrons, but the interaction with uranium hadn't been very well analyzed yet," says Wall. "We initiated genetic analysis of this uranium reduction."

Wall was asked by the Los Alamos National Laboratory to collaborate on research aimed at isolating the uranium-reducing protein. "If we could block one of these proteins responsible for pushing electrons, then we should be able to say ok, we have stopped uranium reduction, therefore this must be an important protein in that process."

In the lab, researchers grow the bacteria in small bottles and then use water and a centrifuge to clean them. The clean bacteria are resuspended--or stirred up again after centrifuging--and placed into a tube containing specific amounts of concentrated uranium. The interaction begins. Samples are then drawn from the tube throughout the test period--say, every hour--and placed in a machine called a Kinetic Phosphorescence Analyzer. This machine can measure the reduction of uranium VI to IV, and researchers can then establish rates at which the reduction takes place and under what conditions.

The work is difficult because D. desulfuricans' ability to "perform," so-to-speak, is affected by changes in temperature, the presence or absence of other microorganisms and oxygen, pH values and nutrient ratios, to name a few. All of these conditions must be understood and accounted for during testing and this is, in part, where Wall's expertise lies. Construction of MU's new 231,000 square feet, $60 million Life Sciences Center will bring researchers on campus together, facilitate interaction among them and, Wall anticipates, the new labs will help further stimulate her own research efforts.

Wall's work to decipher how D. desulfuricans works on uranium is fundamental research that will support the newest applications of bioremediation technology. In the last 15 years, the list of contaminants to which bioremediation technology has been applied has grown. This technology has been used to remediate ground water contaminated by underground storage leaks, treat industrial wastewater and reclaim a variety of hazardous waste sites. One bioremediation strategy, referred to as biostimulation, was used to clean oil from Alaska's shorelines after the Exxon-Valdez disaster. Microbiologists discovered that a lack of nutrients was limiting microbial degradation of the oil. Workers applied oil-soluble fertilizer to the coastline, which stimulated the growth of natural oil-degrading microorganisms. Other sites with radioactive and metallic contaminants, such as uranium, plutonium, mercury and lead, to name a few, are among the most difficult to contend with.

In the U.S., the job of containing or treating hazardous sites belongs to the DOE, which is faced with cleaning up hazardous and radioactive waste at more than 120 sites in 36 states. The scope of the problem is staggering: 475 billion gallons of polluted water, 75 million cubic meters of contaminated sediments and 3 million cubic meters of leaking waste buried in landfills, trenches and spill areas. The work of simply characterizing and cataloging these sites is enormous and on going; to find technologies to clean them up is monumental. Estimates of the cost to complete the job range from $300 billion to $1.7 trillion.