domingo, 31 de marzo de 2013

Bio-Nanowires Conduct Electricity

ORIGINAL: Raijini Rao

Imagine a conducting nanowire, only 3-5 nm wide but many thousand times longer, connecting a microbial community to form mini-power grids. Naturally occurring soil bacteria, such as Geobacter, use these conductivepili for long-range electron transport. How and why do they do this?

▶ All living organisms respire. Our cells break down sugars to obtain energy by extracting electrons that are handed down a relay chain to oxygen, which becomes water. The proteins (cytochromes) that conduct electrons are aided by special metallic centers, studded with iron, so they can cycle between Fe2+ (ferrous) and Fe3+ (ferric) states that differ by one electron. Geobacter uses these cytochromes too, just as our cells do. But oxygen only made its debut a mere 2.4 billion years ago. Before that, ancient bacteria shuttled the electrons to other acceptors, such as sulfides, nitrates and Fe3+. When Geobacter is deprived of oxygen, it grows out long pili into mineral rich rocks and "breathes" iron (drawing on top right). The current is believed to pass between layers of bacteria (middle right image) across a distance of 12 millimetres, which may not seem large, but is 10,000 body lengths to bacteria!

But can proteins conduct current? Researchers knew that the pili were conductive, behaving like ohmic devices (image at bottom right). Although the pili were decorated with cytochromes, they were spaced too far apart to transfer electrons between their metallic centers. When the protein chains were mutated to replace a type of amino acid, the pili lost conductivity. These "aromatic" amino acids have pi-pi orbitals that may be conducting electrons.

Live Wires: Bacterial nanowires can be used in generating microbial energy cells, bioremediation of pollutants (like uranium), and in nano-manufacturing of a variety of devices. The main image shows bacteria growing on metal electrodes.

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