Nature cleans up metal nanoparticle pollution

Deep inside a flooded mine in Wisconsin, US scientists have discovered anaerobic bacteria emitting proteins that sweep up metal nanoparticles into immobile clumps. Exactly how and why the bacteria accomplish their housecleaning task remains a mystery, but the finding may lead to effective bioremediation strategies designed to clean up toxic metals.

Ion image map of carbon (blue), nitrogen (green) and sulphur (red) distributions in biofilm made by sulphate-reducing bacteria. Sulphur is from zinc-sulphide nanocrystals aggregated into larger clumps; nitrogen is from proteins and polypeptides.

Former University of California, Berkeley, graduate student John Moreau and his colleagues analysed the zinc sulphide metabolic waste product of bacteria that thrive in the oxygen-free mine, and found an association between sulphate-reducing bacteria and aggregates of biogenic zinc sulphide nanocrystals formed in the watery environment.

To their surprise, the researchers found proteins and polypeptides embedded within the nanoparticles. These were arranged like tree rings, with the proteins coating the particle surfaces and filling the gaps between them.

Biomineralisation has traditionally been thought of as something that occurs within cells, or at least in contact with them. But in this case the association between proteins and minerals takes place hundreds of microns away from the cells.

Moreau is now attempting to isolate and characterise the proteins of interest. ‘Using mass spectrometry, it may be possible to arrive at a characterisation of the protein composition from first principles,’ he says. It may be also be possible to stimulate sulphate-reducing bacteria in-vitro to produce the same or similar proteins in response to the presence of metals such as copper, zinc or cadmium.

With an understanding of what causes the nanoparticles to aggregate, there is the potential to control their mobility by adding constituents that drive aggregation, says Moreau’s PhD supervisor Jill Banfield. The Berkeley group is now carrying out laboratory experiments with sulphate-reducing bacteria, and looking at how the organisms respond to metals.

‘The suggestion that some amino acids are more effective than others in binding metal-rich particles is exciting,’ says University of Wisconsin geochemist John Valley. ‘It’s astonishing that microbes living in this environment can scavenge metals and purify the water to the point that it is actually drinkable.’

Figure: Ion image map of carbon (blue), nitrogen (green) and sulphur (red) distributions in biofilm made by sulphate-reducing bacteria. Sulphur is from zinc-sulphide nanocrystals aggregated into larger clumps; nitrogen is from proteins and polypeptides (© John Moreau/UC Berkeley).

Further reading: Extracellular Proteins Limit the Dispersal of Biogenic Nanoparticles, Moreau et al., Science 316, 5831 (2007).

Article first published in Nanomaterials News.