25 February 2010—Experimental microbial fuel cells could turn bacteria into batteries that generate electricity from biomass. The key to this technology is the ability of bacteria to transfer electrons to their surroundings—for example, to the anode of a microbial fuel cell. But if the organisms have to be in direct contact with the anode, such devices would have to have extremely large surface areas.
Researchers from Aarhus University, in Denmark, report today in the journal Nature that bacteria appear to conduct electricity while separated by several millimeters, at least a thousand times as far apart than previously demonstrated. The naturally occurring electric currents, if confirmed, would allow bacteria spaced at least 12 millimeters apart to communicate electrically. The discovery might lead to new paths to treating infection and a better understanding of microbial ecosystems.
”It’s exciting to realize that organisms are connected in electric networks, cooperating through them, affecting the cycling of nature,” say Lars Peter Nielsen, an associate professor in the department of biological sciences at Aarhus, who led the work.
Yuri Gorby, an associate professor at the J. Craig Venter Institute, in San Diego, who was not involved in the research, is excited about the implications of the work because it supports the hypothesis that bacteria construct conductive nanowires. Though he says there is still much to do—including showing that the nanowires are indeed the cause of the conduction—”the fire is starting to catch” for this type of research, which he calls ”electromicrobiology.”
The Danish researchers made their discovery by accident when they found that mud samples from their local Aarhus Harbor started acting strangely after being left in beakers for a few weeks.
According to Nielsen, his team knew oxygen wouldn’t penetrate more than a millimeter into the mud. But over a centimeter down in the beakers, ”there were processes going on as if oxygen were there,” Nielsen says. Hydrogen sulfide—a compound that smells like rotten eggs—was disappearing at the bottom of the samples as if it were reacting with oxygen, but no oxygen was present.
”We were mystified,” says Nielsen. ”It was a real paradox to us.”
They prepared more mud samples and used microsensors to measure the distribution of oxygen and hydrogen sulfide from the surface through to the bottom of the samples under different conditions over several weeks, with varying amounts of oxygen at the top layer.
Their experiments confirmed that the bacteria at the bottom of the samples were interacting electrically with oxygenated mud at the top. The bugs at the bottom break down hydrogen sulfide to produce energy, but they need oxygen to do it, because it absorbs the excess electrons generated in the process. The bacteria would ordinarily use oxygen nearby, but in the Aarhus experiment, the electrons traveled nearly a centimeter to get to the oxygen to complete the reaction.
By process of elimination, the researchers concluded that the mechanism for this electron transport was a network of naturally occurring nanowires.
Microbial nanowires have been seen before. In fact, the Venter Institute’s Gorby was involved in their discovery in 2004. But the nanowires have been proven to carry electrons only across their diameter—at the most 30 nanometers—not from one end of the wire to the other.
Gorby is optimistic that the research will show that nanowire structures are conducting electrons over long distances. But he cautions that while the Danish data are highly suggestive, more work needs to be done. The researchers need to identify the structures, measure their conductivity directly, then construct experiments that conclusively demonstrate the role of the wires in the sediment, Gorby says. It will involve continuing the complex experiments that his group has been working on for the past six years.
If researchers can prove that nanowires contribute to moving electrons through microbial systems, Gorby says, it might lead to better microbial fuel cells, by allowing the bacteria to harvest electrons tens or hundreds of thousands of cell lengths away.
The work could even shed light on bacterial infections in the inner ear. ”There could be an electrical communication system under the ground, or in our inner ear, or in a cystic fibrosis lung,” Gorby says. If bacterial infections are indeed hardwired, ”that might suggest novel treatment strategies.”