Bacteria 'nano-wires' clean up uranium contamination

By Leila Battison
Science reporter

image captionThe bacterium accumulates uranium outside its cell, protecting itself from poisoning

Tiny filaments growing from the surface of bacteria could help to improve the efficiency of uranium removal from contaminated waters.

Uranium mining and atomic weapons testing during the Cold War era led to the contamination of sediments and groundwaters by toxic soluble uranium.

Some clean-up methods use bacteria to solidify the uranium in sediments.

But the mechanisms by which this occurs are not well understood, the US team reports in PNAS journal.

As a result, control of uranium contamination is still problematic.

A study by US researchers at Michigan State University has identified a group of bacteria known as Geobacter, which produces tiny protein filaments, or nano-wires, that remove the dissolved uranium from waters and precipitate it outside the cell.

The filaments alter the soluble form of uranium into a less-soluble form, which can be more easily removed from sediments. This reaction is a by-product of the bacteria's normal metabolism, which generates energy by altering the chemistry other metals, such as iron.

Greater efficiency

Dr Gemma Reguera and her team at MSU have found a way to purify the nano-wires in the natural population of Geobacter, and to genetically increase their concentration. The amount of solid uranium deposited around a cell is directly proportional to the number of filaments it has.

Dr Reguera said that she "envisions these nano-wires being incorporated into devices, for use in places like Chernobyl and Fukushima where the radiation is too high for the bacteria to survive."

Individual filaments are just four nanometres across, but create a network many times the size of the cell itself. This extensive network increases the surface available for converting the uranium, and keeps it a safe distance from the cell. Thus the harmful uranium is deposited outside the bacterial cell and the bacterium itself is not poisoned.

As a result, the filament-producing Geobacter strains grow faster and recover more quickly in uranium-contaminated waters than other species of bacteria.

With this network of filaments a single cell can precipitate at least three times more uranium than could be otherwise produced, with obvious advantages for the bioremediation industry.

The US Department of Energy estimates that around two million acres of land across 30 states in the US are contaminated with uranium, mostly from exhaustive mining. Groundwater contamination in the UK is mostly limited to weapons testing sites, where levels can reach up to 18,000 mg per kg of sediment.

Geobacter are currently used in a variety of industrial applications, owing to their ability to convert metals outside their cells using these filaments.

The discovery of the microbial nano-wires' contribution to the immobilisation of uranium can help to optimise bioremediation strategies, which have in the past been limited by insufficient knowledge.

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