Yet more excited geology
September 28, 2007, 6:39 am
Filed under: Warning: contains molecules

About a day after I posted on excited geology my esteemed colleague Phil Ball pointed out this paper in GRL to me about the possibility that soil bacteria share electrons with each other through networks of nanowires — an idea that would always seem extremely cool and in the circumstances seemed steeped in syncronicity too. Phil looked into the work and wrote us a fine news story for this week’s Nature. Excerpt:

Last year, Gorby and his colleagues discovered that Shewanella oneidensis bacteria can grow long filaments, just 100 nanometres (a hundred millionths of a millimetre) thick, which conduct electricity (Y. A. Gorby et al. Proc. Natl Acad. Sci. USA 103, 11358–11363; 2006). The researchers presented evidence that the microbes use these ‘nanowires’ to shunt electrons produced during metabolic reactions onto the surface of mineral grains in the soil, to be taken up by metal ions. Without an electron acceptor, the bacteria cannot function properly and die. The researchers found that several other bacterial species also produce such nanowires.

Oxygen molecules act as convenient electron dumps for bacteria that lie near the soil surface. But little air penetrates to some environments, such as deep lake sediments or waterlogged soils. Now, Gorby and his team think they have found evidence that the bacterial nanowires can link up into a network, conducting electrons to the aerated surface. The researchers filled plastic columns with wet sand infiltrated with a nutrient compound (lactate), and allowed S. oneidensis to grow in this ‘fake soil’. Only the top of the column was in contact with air.

Electrodes inserted at various heights up the columns revealed that, after about ten days, electrical charge was coursing up the column. Gorby’s team examined the sand under a microscope and found that it was threaded by a web of filaments between the bacterial cells. These are wires that provide the pathways for electron transport up to the surface, they suggest.

In contrast, when the team grew a colony of mutant cells that could spawn only very thin, frail and non-conducting filaments, the electrodes in the column remained uncharged.

Phil goes on to note some caveats about the work, notably from Derek Lovley at University of Massachusetts, Amherst, and it does seem quite possible that this sort of wiring is not a major feature of the real world. Redox shuttles in biofilms may be a much more central phenomenon. But it’s definitely thought provoking. For some context to that thought, try “Microbial ecology meets electrochemistry: electricity-driven and driving communities“, a recent review in the ISME journal by many hands, including that of Ken Nealson, quoted in Phil’s piece. And if this wired-up stuff is for real, what are the implications, not just for natural phenomena, but for technologies like the microbial fuel cells (subscription) my colleague Charlotte Schubert wrote about last year? (This blog is not devoted to bigging up Nature; but we do do a pretty good job.)


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