The technique won't combat global warming directly, since both CO2 and methane are potent greenhouse gases, but it could help store alternative energies such as wind and solar more efficiently.
It works like this: giving small jolts of electricity to single-celled microorganisms known as archea prompts them to remove C02 from the air and turn it into methane, released as tiny "farts." The methane, in turn, can be used to power fuel cells or to store the electrical energy chemically until its needed.
"We found that we can directly convert electrical current into methane using a very specific microorganism," said Bruce Logan, a professor at Pennsylvania State University, who details his discovery in the journal Environmental Science and Technology.
We envision this as a way to store electrical energy, to convert electricity into a biofuel," he said.
Archea are older, and more primitive, than bacteria, lacking a nucleus and other cellular machinery.
Most archea are still a mystery to scientists, but methane-producing archea, known as methanogens, are well known. They team up with termites to digest wood pulp. With other microorganisms, they help decompose organic matter.
Now scientists hope to use methanogens to create microbial fuel cells, which is where Logan's team found Methanobacterium palustre, the electricity-drinking, methane-emitting archea, clustered around the cathode.
In the natural environment, various bacteria emit electrons that the archea use as fuel. The archea are 80 percent efficient at conserving the electrical energy into the chemical bonds of methane, good enough that Logan and his team want to use the methanogen to store energy generated by intermittent power sources, like wind, solar or tidal energy.
If a wind turbine already generated electrical energy, and energy is lost converting it to methane and then back again into electricity, why not just stick the electricity into a battery and save more of it?
"How big a battery do you have?" answered Logan. It would take a large, expensive battery to store all that electricity. A fuel cell would be an easier and cheaper way to store and transport it.
Yet the single-step archea model is suspiciously simple, said Bruce Rittmann, a microbiologist at Arizona State University. Rittman isn't convinced that Logan's team found archea capable of converting CO2 to methane in one step.
"It just doesn't have the cellular machinery to turn electrons directly into methane," said Rittman, who is drafting a paper arguing against Logan's technique.
"It's an interesting discovery, producing methane with electricity from the cathode," said Rittman. It just might be too good to be true.