Biology Becomes Electric
Tapping the Power of Microbes
Biomedical engineer Timothy S. Gardner and MD/PhD candidate Josh Thaden sterilize an inoculation loop in preparation for sampling and analysis of cultures of Shewanella oneidensis and Pseudomonas aeruginosa. Both organisms have the ability to generate small amounts of electrical current when grown in a microbial fuel cell, though Shewanella is considered the more likely candidate for practical devices.
Alternative energy proponents tout the hydrogen fuel cell as a new technology that could help reduce global warming and dependence on foreign oil. While hydrogen shows great promise as a cleaner, greener fuel source, refining it into a usable state can be expensive, and accessing supplies may be difficult in some geographic areas. There is, however, a much cheaper and more ubiquitous source of energy for a fuel cell: bacteria.
A microbial fuel cell resembles any other electrochemical fuel cell, except the conversion of the fuel source into electricity is transacted by one of several common species of bacteria. Feed a microbial fuel cell anything the bacteria can grow on, and the bacteria will split it into electrons and protons. As the protons flow through the cell on one side and recombine with oxygen on the other, an electric current will flow across the device and power whatever is attached to it. Such bacteria are called electrogens.
One promising microbial electrogen, a novel species of bacteria called Shewanella oneidensis, can convert glucose and other sugars into electrons during respiration and transfer those electrons to metals or electrodes. Unfortunately, the current produced is insufficient for practical applications beyond a single lightbulb. The maximum current output by electrogenic bacteria in the fuel cell is approximately 100 watts per cubic meter. That's about one-tenth the input electrical energy contained in the sugars upon which the bacteria feed. But biomedical engineer Timothy S. Gardner believes this energy can yet be captured and exploited.
A microfluidic microbial fuel cell developed by the Gardner Lab in collaboration with researchers at Oak Ridge National Laboratory. Inside the fluid chamber, electricity-generating bacteria grow and adhere to gold electrodes, which are only a few microns across, enabling real-time current measurement and microscopy. The prototype shown above may lead to bacterial power sources for microelectronic devices.
“We're trying to tweak the genetics of these organisms to direct as close as possible to 100 percent of the fuel (sugar sources) to respiration and therefore to current production, rather than to generate more cells or go to waste,” says Gardner, whose research is supported by the U.S. Department of Energy. “If you can produce power outputs at 1,000 watts per cubic meter, you can light up a house or power a cell phone. Our goal is to generate as much power as we can from the smallest possible device.”
To achieve that goal, Gardner and eight graduate research assistants mapped the metabolic gene regulatory circuits of Shewanella representing about 800 to 1,000 genes, and identified some of the most important controlling “valves” of those circuits. “The cell is essentially a series of pipes,” he explains. “You push fuel in one side, it flows through these pipes and reacts and either becomes waste products or electricity. If you delete or mutate certain genes (valves), you can shut down pipes leading to waste products or maximize the number of pipes that lead to electricity production, and thus boost electrical output.”
Gardner is now trying to identify more genes involved in regulating Shewanella metabolism and to develop a genetic engineering strategy for key valves in the entire cellular “circuit board.” If he succeeds in boosting the bacteria's electrical energy production tenfold, the microbial fuel cells that result could be used, for instance, to feed off of—and detoxify—sewage at a sewage treatment plant or waste at an agricultural dump while powering nearby houses or household appliances.
For more information, see http://gardnerlab.bu.edu.
