Self-Constructed Electrically Conductive Bacterial Networks Could Contribute to Microbial Fuel Cells
Researchers in Japan have experimentally shown that the metal-reducing bacteria Shewanella loihica PV-4 and semi-conducting nanominerals (iron (III) oxide nanoparticles) aggregate to form electrically conducting networks.
Measuring and establishing the existence of a bacterial extracellular ET [electron transfer] is a research subject that is not only relevant to the understanding of microbial activities in subsurface environments, but also for designing and fabricating bio-anode materials for microbial fuel cells.—Nakamura et al. (2009)
Biological fuel cells use enzymes or whole microorganisms as biocatalysts for the direct conversion of chemical energy to electrical energy. One type of microbial fuel cell uses anodes coated with a bacterial film. The fuel consists of a substrate that the bacteria can break down. The electrons released in this process must be transferred to the anode in order to be drawn off as current.
Metal-reducing bacteria that live in subterranean sediments transfer electrons to the iron oxide minerals on which they dwell as the last step of their metabolism. In this process, trivalent iron ions are reduced to divalent ions.
A team led by Prof. Kazuhito Hashimoto at the University of Tokyo has investigated how this transfer is carried out in Shewanella loihica. A paper on the work appears in the journal Angewandte Chemie International Edition.
They added the cells to a solution containing very finely divided nanoscopic iron(III) oxide particles and poured the solution into a chamber containing electrodes. A layer of bacteria and iron oxide particles was rapidly deposited onto the indium tin oxide electrodes at the bottom of the chamber. When the cells were “fed” lactate, a current was detected. Electrons from the metabolism of the lactate are thus transferred from the bacteria to the electrode.
Scanning electron microscope images show a thick layer of cells and nanoparticles on the electrode; the surfaces of the cells are completely coated with iron oxide particles. The researchers were able to show that the semiconducting properties of the iron oxide nanoparticles, which are linked to each other by the cells, contribute to the surprisingly high current. The cells act as an electrical connection between the individual iron oxide particles.
Cytochromes, enzymes in the outer cell membrane of these bacteria, transfer electrons between the cells and the iron oxide particles without having to overcome much of an energy barrier. The result is a conducting network that even allows cells located far from the electrode to participate in the generation of current.
Ryuhei Nakamura, Fumiyoshi Kai, Akihiro Okamoto, Greg J. Newton, and Kazuhito Hashimoto (2009) Self-Constructed Electrically Conductive Bacterial Networks. Angewandte Chemie International Edition 48, No. 3, 508-511, doi: 10.1002/anie.200804750