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A unique microbial fuel cell
Scientists report the first successes in overcoming a major limitation of microbial fuel cells (MFCs)—the need to add and replenish compounds that transport electrons to the electrode—in a system described in this issue of ES&T (pp 3401–3408). In the paper, researchers from Ghent University (Belgium) show that Pseudomonas aeruginosa produces its own electron transporters, which can be used by other bacteria. Scientists and engineers in the field are excited by the results, which they say have the potential to increase the power output of MFCs.
Although MFCs are still in the proof-of-concept stage and generate very little power, environmental engineers are increasingly interested in them as an alternative energy source that could use, for example, organic wastes in wastewater to generate electricity (Environ. Sci. Technol. 2004, 38, 160A–167A). The existence of soluble electron carriers, produced by one bacterium and usable by other bacteria, “could have really profound practical applications in terms of increasing the power output,” says Bruce Rittmann of Arizona State University. Uwe Schröder of the Ernst Moritz Arndt University of Greifswald (Germany) agrees, adding that a soluble shuttle allows for more microbes per surface area unit and greater power generation.
Pseudomonas and other bacteria produce electrons as the result of consuming substrates, such as glucose, acetate, or the “leftovers” in wastewater. In order to power the MFC under anaerobic conditions, these electrons must be transported to the cell’s anode. Until now, researchers have needed to frequently replenish redox mediators in the MFCs in order to shuttle the electrons from the bacteria to the anode.
The key component of the MFC design that Korneel Rabaey and his colleagues describe in the new paper is P. aeruginosa, a well-characterized bacterium and a human pathogen. What is distinctive about this strain of P. aeruginosa is that it naturally produces pyocyanin and other phenazines, which can function over and over as electron-carrying redox mediators. Moveover, pyocyanin synthesis is induced by the presence of the anode. In their experimental MFC, the Ghent scientists found that pyocyanin can be recycled at least 11 times. The researchers are on the lookout for nonpathogenic bacteria that produce similar redox mediators.
“More important is that these shuttles that we were able to purify could also be used by other bacteria,” says Rabaey, the paper’s lead author. “If there are mediators present in, let’s say, a biofilm, multiple layers of bacteria can participate in the electricity generation, which means that the power output can be multiplied with a certain factor.”
Why were they able to isolate a strain of Pseudomonas that grows better in the presence of an anode and that produces significant amounts of pyocyanin only in the presence of the anode? “It’s Darwin’s law of natural selection,” says Rabaey. “The reason why the electrode stimulates the production [of pyocyanin] is because the electrode keeps a selective pressure in the system towards bacteria that can produce a lot of these phenazines.”
The laboratory-scale MFCs used in these experiments produce tiny amounts of power—less than 4 milliwatts per square meter. However, Rabaey and corresponding author Willy Verstraete believe that MFCs yielding an energy density of one kilowatt per cubic meter of reactor—enough power to run your clothes washer while you watch your MFC-powered television-will be achieved within a year or two.
Rittmann and Schröder both think commercial application of the technology will take about a decade. Although pyocyanin substantially increased the power output of this new MFC, the output from these laboratory-scale reactors is still low. “We need to see this same phenomenon occurring and these same kinds of increases occurring when we are starting at a much higher base, like maybe 1000 times higher,” says Rittmann. Nonetheless, both think there is a lot of promise. “Microbial fuel cell research is still in its infancy, and quite new concepts and developments can be expected,” says Schröder.
