Millions of tiny microbes infest the water that carries the detritus of human life and society. Some of them steadily break down the organic material in waste streams and produce electrons in the process. By harvesting these electrons, scientists have created microbial fuel cells. New research shows how such biological power plants can be stacked to create usable current.

Willy Verstraete and his colleagues at Ghent University in Belgium tested the fuel cells in an array of configurations: in a series, in parallel and individually. Over the course of more than 200 days, the researchers fed the microbes a diet of anaerobic and aerobic sludge, as well as hospital and potato processing factory wastewater. By the end of the experimental time frame, the short-term power densities--a measure of power produced per unit of mass--of the fuel cells had tripled. The team also found that the parallel stack was most efficient at producing an electric charge, consistently creating stronger current.

The scientists main discovery, however, had to do with the co-evolution of the electrochemical properties of the fuel cell and the actual microbial community. At the start of the experiment, the tiny power plants relied on a diverse community of proteobacteria, including several species of Geobacter and Shewanella, and produced power somewhat inefficiently. But by the end of the experiment--when performance was at its peak--one species, Brevibacillus agri, made up the majority of the electron-producing microbes.

This microbial evolution calls for further research into the electron-producing properties of various species and their interaction, the authors write. A paper presenting the findings will be published in the May 15 issue of Environmental Science & Technology.