|Scientists have developed a way to grow iron-oxidizing bacteria using electricity instead of iron, an advance that will allow them to better study the organisms and could one day be used to turn electricity into fuel and and other energy conversion technologies.|
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.
Using iron oxidizing bacteria such as in this study brings interesting possibilities for energy storage, energy conversion and carbon sequestration for the future.
Bacteria able to transfer electrons to metals can be harnessed in technologies that require the precision of enzymes and the adaptability of living organisms. It may be achievable to produce electricity directly from waste water, specific compounds can be synthesized at electrode surfaces, and chemicals can be detected in complex mixtures.
Scientists think these bacteria must carry out the iron oxidation step on their surfaces. If that's true, Bond reasoned, the outsides of the organisms should be covered with proteins that interact with Fe(II), so you should be able to provide a stream of pure electrons to the outsides of the bacteria and get them to grow.
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.
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.
It's this capture of carbon dioxide that could enable electrochemical cultivation to create biofuels or other useful products one day, Bond says.
"Bacteria are experts at the capture of carbon dioxide. They build cells and compounds" with the carbon, he says. They might one day be exploited as microscopic energy packagers: bacteria like M. ferrooxydans could capture electricity from an electrode, combine it with carbon dioxide, and package it as a carbon-rich compound we could use as fuel. This would take the energy in electricity, which is ephemeral, and convert it into a tangible product that could be stored in a tank. But that kind of work is a long way off, cautions Bond."If there are 100 steps to making this work – this is step one," he says.
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