The method, called electrochemical cultivation, supplies these bacteria with a steady supply of electrons that the bacteria use to respire, or "breathe." It opens the possibility that one day electricity generated from renewable sources like wind or solar could be funneled to iron oxidizing bacteria that combine it with carbon dioxide to create biofuels, capturing the energy as a useful, storable substance.
"It's a new way to cultivate a microorganism that's been very difficult to study. But the fact that these organisms can synthesize everything they need using only electricity makes us very interested in their abilities," says Daniel Bond of the BioTechnology Institute at the University of Minnesota -- Twin Cities, who co-authored the paper with Zarath Summers and Jeffrey Gralnick.
To "breathe," iron oxidizers take electrons off of dissolved iron, called Fe(II) -- a process that produces copious amounts of rust, called Fe(III). Iron-oxidizing bacteria are found around the world, almost anywhere an aerobic environment (with plenty of oxygen) meets an anaerobic environment (which lacks oxygen). They play a big role in the global cycling of iron and contribute to the corrosion of steel pipelines, bridges, piers, and ships, but their lifestyle at the interface of two very different habitats and the accumulation of cell-trapping Fe(III) makes iron oxidizers difficult to grow and study in the lab. Bond and his colleagues added the marine iron oxidizer Mariprofundus ferrooxydans PV-1, along with some nutrient medium, to an electrode carefully tuned to provide electrons at the same energy level, or potential, as Fe(II) would provide. The idea, says Bond, was to "fool the bacteria into thinking they're at the world's best buffet of Fe(II) atoms."
It worked. The bacteria multiplied and formed a film on the electrode, Bond says, and eventually they were able to grow M. ferrooxydans with no iron in the medium, proof that the bacteria were living off the electrons they absorbed from the electrode to capture carbon dioxide and replicate. And since the electron donor is a solid surface, say the authors, it's pretty likely that the bacterial electron-harvesting machinery is exposed on the outer membrane of the cell.