By Carol Potera, American Society for Microbiology

Microbial fuel cells (MFC) can generate electricity from renewable sources— in some prototype models, using sunlight via photosynthetic bacteria for power, according to microbiologist Tim Donohue at the University of Wisconsin-Madison (UWM), and his collaborators. Although Rhodobacter sphaeroides-driven MFCs operate in the milliwatt range, they do so harnessing sunlight and waste materials while transiently producing hydrogen and fixing carbon dioxide.

The UWM prototype MFC relies on photosynthetic R. sphaeroides to generate hydrogen that, in turn, is converted to electricity. In light, the maximum power density reaches 790 milliwattsm-2, which falls to 0.5 milliwatts m-2 in the dark. Additional details appear in the October 10, 2007 Journal of Applied Bacteriology. (Read the paper here). In addition to scaling up the MFC and improving its production of hydrogen, Donohue and his collaborators are exploring ways to harness the carbon dioxide that is fixed but then released when R. sphaeroides cells revert to a nonphotosynthetic mode of growth. If fixed carbon dioxide can be converted into high-value byproducts, “we can get two bangs for one buck,” he says.

Last June, these MFC development efforts became part of a larger U.S. Department of Energy (DOE) program, when department officials established the Great Lakes Bioenergy Research Center (GLBRC), which Donohue directs. DOE funded GLBRC with a five-year, $125-million grant. Based at UWM, the research center includes investigators at other sites, such as Michigan State University in East Lansing, the University of Florida, Gainesville, and Oak Ridge National Laboratory in Oak Ridge, Tenn. Other energy development efforts within this program focus, for example, on making ethanol or other fuels more efficiently from biomass. One of those research efforts, led by Cameron Currie, focuses on colonies of leaf-cutting ants (Attini) that cultivate microbially based, energy-producing biomass gardens and residual compost heaps. Fungi and bacteria form symbiotic relationships within the gardens, which the ant colonies pack with grass, leaves, and other cellulosic biomass. The bacteria partly break down those materials, providing nutrients for fungi that, in turn, serve as food for the ants. The ants haul off residual biomass to compost dumps.

“We’re particularly interested in the dumps,” Currie says, alluding to sites where cellulose and lignin are decomposed. The dumps reach temperatures exceeding 70°C, suggesting that they contain thermophiles with potentially valuable properties for humans interested in harvesting biomass for energy. “Our work will use the ant-fungal system as a source of novel enzymes and as a model for understanding the conversion of plant biomass to energy,” he says. Currie, Donohue, and other GLBRC researchers are seeking ways to overcome the biomass bottleneck— that is, to unlock and efficiently harness energy stored in plant stalks, wood chips, and agricultural or other cellulosic wastes. GLBRC is situated in the $120-million, 330,000-squarefoot UWM Microbial Sciences Building, which opened last September and also houses the departments of bacteriology and of medical microbiology and immunology as well as the Food Research Institute.