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Bacteria Can Transform Minerals Electrically

17 December 2009 (All day)
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Laura Liermann/Penn State University

Mmmm, rust! Shewanella bacteria shed their excess electrons on a bed of iron oxide.

Got a messy cleanup problem that requires a molecule-by-molecule fix? Instead of nanotech, how about deploying an array of ready-made, versatile bacteria? Scientists studying a genus of the rock-dwelling bacteria called Shewanella have found out how the organisms can transform minerals by zapping them with tiny electrical currents. The discovery could lead to new types of fuel cells to generate electricity, to better environmental-cleanup techniques, and possibly even to a new generation of organically made materials.

Bacteria live in almost every environment on Earth, from the ocean's deepest trenches to the Himalayas' highest peaks. Perhaps the main reason is their supreme adaptability. Animals use oxygen as part of metabolism. But some microbes can thrive in the absence of oxygen, something that has puzzled scientists for nearly half a century. Even when scientists finally discovered that the organisms were using rocks instead of oxygen to purge electrons, they still couldn't figure out the exact molecular mechanism that made such metabolism possible.

Now, after 5 years of studies in laboratories in the United States and the United Kingdom, a team has discovered the elusive process. It turns out that Shewanella use a class of proteins on their surface that functions like an electrical wire between the bacteria's interior and exterior. The proteins--called deca-heme c-class cytochromes--bond with the rock molecules and convey electrons out through the cell membrane, the composition of which normally functions as an insulator. The process also chemically alters the rock, releasing elements such as iron and manganese, the team reports online this week in the Proceedings of the National Academy of Sciences.

"As a geochemist, I was surprised to see just how much 'machinery' the microbe builds to move electrons," says co-author Susan Brantley of Pennsylvania State University, University Park. She says that mechanism could be the key to using Shewanella and related bacteria in activities such as electricity production and oil-spill cleanup. Lab experiments have shown that other kinds of bacteria can generate an electrical current. Likewise, bacteria over time will clean up oil spills. The new research, Brantley explains, could lead to cheaper and more efficient ways to do both by tweaking the bacteria's metabolism.

The fact that Shewanella live underground naturally makes them ideal candidates for environmental-cleanup tasks, says biochemist and lead author David Richardson of the University of East Anglia in the United Kingdom. "Understanding their biochemistry could help to develop strategies to stimulate their activities [at the cleanup sites]," he says.

The findings provide, “finally, the hard-core biochemical information that explains how these kinds of metabolic reactions can take place,” says geochemist Eric Roden of the University of Wisconsin, Madison. It is "the full explanation that people in many disciplines have been waiting for," he says.

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