The Berkeley Science Review: Articles

Poo Power
Harnessing the energy of waste (view PDF)
by Susan Young

Like spinning straw into gold, microbial fuel cells (MFCs) can create electricity from seemingly useless organic wastes like sewage or farm runoff. Poo power may sound like the answer to our current energy crisis, especially given that the wastewaters that fuel it are abundant and essentially free, but current devices aren't efficient enough to suit most energy needs. In MFCs, bacteria act as living catalysts to drive energy production, but despite this, most efforts toward improving the efficiency of MFCs have not focused on the biological details of the waste to wattage conversion. A recent report from Professor John Coates' group in the Department of Plant and Microbial Biology, however, demonstrates that focusing on the bacteria that power MFCs can not only bring to light the biological mechanisms behind MFC energy production but can also lead to the discovery and isolation of bacteria that are especially proficient at energy production.

Fuel cells are devices that convert chemical energy into electrical energy. In a conventional hydrogen fuel cell, a negatively charged electrode, the anode, is the site of a chemical reaction that splits hydrogen into hydrogen ions and electrons. The released electrons flow through an external circuit that lies between the anode and the positively charged electrode, the cathode, creating an electric current. In MFCs, on the other hand, bacterial metabolism acts as the electron source: bacteria release electrons from organic food sources and transfer them externally to the anode of the fuel cell, thereby creating the electric flow.

All organisms must get rid of the electrons generated by metabolism, but not every organism does it the same way. "We breathe in air and oxygen is our electron acceptor. These bacteria breathe iron and it is their electron acceptor," says Coates.

Iron-breathing bacteria may sound exotic, but you would probably find some of these microbes in soil from your own back yard, as did Coates' son for a science fair project. The bacteria in MFCs often come from sewage treatment "sludge," so most MFCs are powered by a complicated mix of unidentified bacterial species. "We wanted to make [the bacterial community] as simple as possible so we'd have the best chance of isolating those key players responsible for the electron transfer on the electrode," says Kelly Wrighton, a graduate student in the Coates lab and lead author in this study.

To cultivate a simplified community, Wrighton and coworkers set up a restrictive growing environment within their MFCs. First, they maintained their fuel cells at 130û F, a higher temperature than most, which favored thermophilic (heat-loving) life while preventing the growth of bacteria that prefer more moderate temperatures. Second, they established an oxygen-free environment within the MFC to select for microbes capable of "iron-breathing" respiration. Finally, a non-fermentable carbon source facilitated external electron transfer to the anode by the microbes. After 100 days of monitoring the current produced by the MFCs, Wrighton and coworkers removed the anode to see who was growing on its surface.

With the help of recent innovations in DNA-based "fingerprinting" of bacteria, they found the reactors that produced electricity contained a different collection of bacteria than the control reactors that did not. Overall, the absolute numbers of bacteria cells in current-producing systems had decreased as compared to the more dense starting culture; however, certain members of the starting community had actually grown in number— and their identities were surprising.

The cellular proteins we know to be capable of transporting electrons are found in cell membranes. Before this study, all bacteria known to be capable of external electron donation belonged to a single major branch of bacteria that have an outer membrane surrounding their cell wall that plays an integral role in electron transfer. Surprisingly, many of the bacteria enriched in the thermophilic MFCs were from the branch of the family tree that lacks an outer membrane. Exactly how these bacteria are transferring electrons to the anode without an outer membrane is somewhat of a mystery. Wrighton speculates that perhaps these bacteria have a conductive cell wall or their electron transporting proteins might be attached to the cell wall via some molecular tether.

Wrighton and coworkers next set out to isolate a single member of the bacterial community growing on the anode so they could learn more about the biological processes governing external electron transfer. From a scraping of the anode, they developed a pure culture of a strain of Thermincola bacteria they dubbed "JR." Strain JR produced more current than any organism previously studied in traditional MFCs. A Thermincola genome project is underway that will allow Wrighton and coworkers to look for clues as to which genes make external electron transfer possible. Once determined, the molecular details of the cell-to-anode electron transfer could guide improvements in the design of anode materials that are better suited for MFCs.

Strain JR is the first member of its phylum we know to be capable of directly transferring electrons to an anode. Given that strain JR is so proficient at converting organic materials into electricity, this certainly suggests that there may be a number of other bacteria not only capable of this electrical alchemy, but perhaps even more adept.

Susan Young is a graduate student in molecular and cell biology.

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