Nanowires growing from bacteria might link up distant chemical reactions in sediments.
Bacteria lurking in sediment at the bottom of the sea are pulling off a clever trick — using an electric current to link together the chemical reactions of oxygen in water with those of sediment nutrients deeper down.
Lars Peter Nielsen at Aarhus University in Denmark and his colleagues suggest in work published in this week's Nature1 that a chain of bacteria work together to transport electrons from a marine sediment to the overlying water up to two centimetres away. The electrons are produced by reactions between organic matter and hydrogen sulphide in the sediment, and transported to the sediment surface where they react with oxygen.
This means that throughout the entire system, the top layers of sediment 'breathe' for the whole, and those at the bottom 'eat' for the whole.
The research helps to add weight to a suggestion within the geophysics and microbiology communities that bacteria can grow tiny 'wires' and hook up to form a biogeobattery — a giant natural battery that generates electrical currents.
Researchers already knew that processes in marine sediments use reduction and oxidation — or redox — reactions with oxygen to turn over organic matter and hydrogen sulphide, but how it occurred was poorly understood. "I've never seen a complete explanation of how oxygen is consumed in the sea bottom," says Nielsen.
In their experiments using sediment from Aarhus Bay, Nielsen and his team changed the amount of oxygen in the water at the sediment surface. In samples starved of surface oxygen, the hydrogen sulphide sitting deeper down was consumed at a slower rate and so built up. When oxygen was returned to the water, hydrogen sulphide levels fell. These changes happened in less than an hour.
That was puzzling. An hour isn't long enough for molecular diffusion or conventional chemical reactions to bring about these changes, so Nielsen decided that something else must be responsible. "You have processes going on deep down in the sediment that are connected to the oxygen consumption at the very surface of the sediment. This is evidently mediated by bacteria," says Nielsen.
Bacteria at the top will consume the oxygen and bacteria at the bottom will consume all the food they can find in the sediment, Nielsen says. To do this, Nielsen suggests that certain bacteria capable of transporting electrons to their surroundings are connecting through tiny protruding wires that they grow. So far he has no direct evidence that these wires exist. But they would enable single electrons to move across huge distances, relative to bacterial size: "We're talking about a couple of centimetres, which for bacteria is 20,000 times their body size," says Nielsen.
Andre Revil, of the Colorado School of Mines in Golden, has supported the notion of a biogeobattery for some years but says that the idea has met with scepticism2. The mysteries of the redox reactions going on in sediments were not being put into a wider context previously, Revil suggests. "When people were looking at redox chemistry, they were looking at it at the local scale. They would never imagine you could have transport of electrons over centimetre scales."
Nielsen's work helps strengthen the arguments for this long-distance 'communication', says Revil, and will complement his own work to measure electric fields generated by this tiny electron flow. But Nielsen still needs to prove his theory that bacteria are forming nanowires by providing direct evidence of their existence, Revil adds.
Yuri Gorby at the J. Craig Venter Institute in San Diego, California, agrees that some direct evidence for nanowires should be sought — probably by freezing a sediment sample and taking images using an electron microscope. But Gorby is nevertheless excited by Nielsen's work. "The implications for nanowires in sedimentary environments are profound," says Gorby.
For example, says Revil, the tiny currents produced by sediments could be harnessed by placing a large network of graphite electrodes on the seabed to generate enough current to power a monitoring buoy. The ability to perform redox chemistry deep in the sediments could also be used to turn toxic pollutants into a non-toxic form; for instance, by adding bacteria to sediment that is contaminated with radioactive or oil-spill waste, he suggests.
Gorby and his colleagues have found evidence of possible nanowires in many natural systems, he says, including in hydrothermal vents in Yellowstone National Park, Wyoming. "It is just a matter of time before the full implications of bacterial nanowires from diverse environments and organisms are recognized," says Gorby.
Nielsen says that there is more work to be done. "There's so much to learn about this now. We need better tools to go out and study this in nature; we need to know how this network is constructed. We need to review a lot of old data and try to see if it makes better sense now."
Nielsen, L. P., Risgaard-Petersen, N., Fossing, H., Christensen, P. B. & Sayama, M. Nature 463, 1071-1074 (2010).
Revil, A. et al. J. Geophys. Res. 115, G00G02 doi:10.1029/2009JG001065 (2010).
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Sanderson, K. Bacteria buzzing in the seabed. Nature (2010). https://doi.org/10.1038/news.2010.90