30 July 2009

Mud to electricity, new microbe strain powers up

There is a new strain of electricity producing microbes that can dramatically increase power output per cell and overall bulk power.

This microbe also works with a thinner biofilm than earlier strains, cutting the time to reach electricity-producing concentrations on the electrode. This development comes from experiments with Geobacter, the sediment-loving microbe whose hairlike filaments help it to produce electric current from mud and wastewater.

Dr. Hana Yi takes a reading from microbe fuel cell.

“This new study shows that output can be boosted, and it gives us good insights into what it will take to genetically select a higher-power organism,” said Derek Lovley, who is working with a team at the University of Massachusetts Amherst and supported by the Office of Naval Research and the U.S. Department of Energy.

This latest development opens the door to improved microbial fuel cell architecture and should lead to “new applications that extend well beyond extracting electricity from mud,” Lovley said. In the new experiments, the UMass Amherst researchers adapted the microbe’s environment, which pushed it to adapt more efficient electric current transfer methods.

“In very short order, we increased the power output by eight-fold, as a conservative estimate,” Lovley said. “With this, we’ve broken through the plateau in power production that’s been holding us back in recent years.” Now, planning can move forward to design microbial fuel cells that convert waste water and renewable biomass to electricity, treat a single home’s waste while producing localized power, power mobile electronics, vehicles, and implanted medical devices, and drive bioremediation of contaminated environments.

Geobacter’s hairlike pili are extremely fine, only 3 to 5 nanometers in diameter or about 20,000 times finer than a human hair, and more than a thousand times longer than they are wide. Nevertheless, they are strong. Nicknamed nanowires for their role in moving electrons, pili are the secret to this particular microbe’s ability to produce electric current from organic waste and sediment. Geobacter’s pili seem critical for forming the biofilm, which aids transfer of the electron products to iron in soil and sediment. In nature, bacteria colonies form gluey biofilms to anchor to a surface such as a tooth or an underwater rock, providing a living environment near a food source.

The Geobacter biofilm’s “fortuitous” electron-transferring skill, the product of natural selection, presented a pathway Lovley could leverage. He and colleagues grew Geobacter as usual on a graphite electrode, providing acetate as food and allowing a colony to form the biologically active slime, or biofilm where electron transfer takes place across the nanowires. But for this new experiment, they added a tiny, 400-millivolt “pushback” current in the electrode that forced Geobacter to press harder to get rid of its electrons.

The result of providing a more challenging environment, within five short months, Lovley said, was evolution of a beefed-up microorganism that can press at least eight times more electric current across the electrode than the original strain.

“I’m really happy with this outcome,” Lovley said. “It’s exceptionally fast feedback to us and a very satisfying result. I’m still a little amazed that they make electricity, but I’m happy to be exploring how to harness that ability. I’m sure there’ll be applications developed in the future that we can’t even envision right now.”

Microbial fuel cells, which convert fuel to electricity without combustion, consist of an electrode known as an anode that accepts electrons from the microorganisms, and another electrode known as a cathode, which transfers electrons onto oxygen. Electrons flow between the anode and the cathode to provide the current that electronic devices can harvest for power.

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