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Living cables explain enigmatic electric currents

The enigma of electric currents in the seabed is solved. Scientists from Aarhus University have sensationally discovered bacteria that function as living electrical cables. Each of the centimetre-long 'cable bacteria' contains a bundle of insulated wires leading an electric current from one end to the other.

2012.10.24 | Peter Bondo Christensen, Christina Troelsen

1. Cable bacteria in the mud of the sea bottom (Credit: Mingdong Dong, Jie Song and Nils Risgaard-Petersen)

2. Cross-section of four cable bacteria each with a circle of 15 wires just below the cell surface. (Credit: Karen E. Thomsen)

3. A small cavity in the seabed reveals a number of cable bacteria that conduct electric currents between the red surface and the deep, black, anaerobic sediment layers of the seabed. (Credit: Nils Risgaard-Petersen)

4. In a teaspoonful of mud, there may be one kilometre of living electric cables and bundles of them can be pulled up with the finger (Credit: Nils Risgaard-Petersen)

5. Cable bacteria in sediment. The wires become apparent on the bacterial outer surface as distinct ribs running across the cable bacteria cells. (Credit: Jie Song and Nils Risgaard-Petersen)

Cable bacteria explain electric currents in the seabed

Electricity and seawater are usually a bad mix. And it was thus a very big surprise when scientists from Aarhus University a few years ago discovered electric currents between biological processes in the seabed. Since then they have been searching for an explanation and together with partners from the University of Southern California, USA, they now present sensational results in Nature.

"Our experiments showed that the electric connections in the seabed must be solid structures built by bacteria," says PhD student Christian Pfeffer, Aarhus University.

He could interrupt the electric currents by pulling a thin wire horizontally through the seafloor. Just as when an excavator cuts our electric cables.

In microscopes, scientists found a hitherto unknown type of long, multi-cellular bacteria that was always present when scientists measured the electric currents.

"The incredible idea that these bacteria should be electric cables really fell into place when, inside the bacteria, we saw wire-like strings enclosed by a membrane," says Nils Risgaard-Petersen, Aarhus University.

Kilometers of living cables

The bacterium is one hundred times thinner than a hair and the whole bacterium functions as an electric cable with a number of insulated wires within it. Quite similar to the electric cables we know from our daily lives.

“Such unique insulated biological wires seem simple but with incredible complexity at nanoscale,” says PhD student Jie Song, Aarhus University, who used nanotools to map the electrical properties of the cable bacteria.

In an undisturbed seabed more than tens of thousands kilometers cable bacteria live under a single square meter seabed. The ability to conduct an electric current gives cable bacteria such large benefits that it conquers much of the energy from decomposition processes in the seabed.

Unlike all other known forms of life, cable bacteria maintain an efficient combustion down in the oxygen-free part of the seabed. It only requires that one end of the individual reaches the oxygen which the seawater provides to the top millimeters of the seabed. The combustion is a transfer of the electrons of the food to oxygen which the bacterial inner wires manage over centimeter-long distances. However, small disturbances can lead to fatal "cable breakage" in the fragile bacteria.

Biological innovation

"On the one hand, it is still very unreal and fantastic. On the other hand, it is also very tangible," says Professor at Aarhus University, Lars Peter Nielsen, who is in charge of exploring the natural electrical currents.

Along with a number of international cooperation partners, several scientists at Aarhus University already address the new and exciting questions that arise. Right from the understanding of bioelectronics at the molecular level to the role of cable bacteria in the history of Earth.

The future will tell whether this wondrous result of the biological evolution can also be used in new types of electronics.

The article in Nature:

“Filamentous bacteria transport electrons over centimetre distances” by Christian Pfeffer, Steffen Larsen, Jie Song, Mingdong Dong, Flemming Besenbacher, Rikke Louise Meyer, Kasper Urup Kjeldsen, Lars Schreiber, Yuri A. Gorby, Mohamed Y. El-Naggar, Kar Man Leung, Andreas Schramm, Nils Risgaard-Petersen & Lars Peter Nielsen. DOI:10.1038/nature11586.

Links:

Download press release in German (PDF)

Download press release in Chinese (PDF)

See Danish press release

Contact:

Professor Lars Peter Nielsen, Department of Bioscience, Aarhus University.
E-mail: lars.peter.nielsen@biology.au.dk.
Tel: +45 871 56542 or +45 6020 2654.

Senior Scientist Nils Risgaard-Petersen, Center for Geomicrobiology, Aarhus University.
E-mail: nils.risgaard-petersen@biology.au.dk.
Tel: +45 871 56508 or +45 2965 6325.

More images:

6. Power outage: PhD student Christian Pfeffer pulls a thin wire through the seabed and thereby cutting the cable bacteria. (Credit: Nils Risgaard-Petersen)

7. Yet another cable bacterium identified. PhD student Steffen Larsen examines bacteria from the seabed in the microscope (Credit: Nils Risgaard-Petersen)

8. PhD student Jie Song has captured a close-up of cable bacteria through a scanning electron microscope (Credit: Nils Risgaard-Petersen)

9. A cable bacterium consists of a long chain of cells with a common ring of insulated wires (Credit: Mingdong Dong)

Center for Geomicrobiology, Knowledge exchange
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Revised 2014.03.06

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