Spectroscopic Slicing to Reveal Internal Redox Gradients in Electricity‐Producing Biofilms

Exocellular electron conduction is one of the most fascinating discoveries of microbiology in the last decade. In natural and man-made environments it is thought to play a fundamental role, allowing the exchange of electrons between bacterial cells and between these and external electronic acceptors. Moreover, it is one of the fundamental processes behind the production of electricity by electroactive biofilms grown on electrodes, which are of paramount importance in such emerging technologies as microbial fuel cells,microbial electrolysis cells, the microbial electrosynthesis process, and whole cell biosensors. Electroactive biofilms grown on polarized electrodes are in fact a very appropriate system for exploring electron conduction because researchers have learnt to grow them in fully controlled conditions with excellent reproducibility. In the typical case, biofilms of Geobacter sulfurreducens, the most efficient current producer described to date, are grown on a polarized electrode during 4-6 days until obtaining a constant current. During this time, cells reproduce actively at maximal rate, using the electrode as the electron acceptor, until reaching a thickness of about 40-60m m. At this stage, cells within the entire population seem to operate at an uniform rate, as evidenced by the constant increase in the accumulation of proteins. A remarkable feature in these biofilms is that a uniform distribution of cell activity can only be possible if cells at a distance from the electrode are electrically connected to it by a low-resistance pathway that prevents electron acceptor limitations. There is debate as to whether such electron conduction is based on one of two alternative mechanisms : metallic-like conduction through nanowires, or electron hopping (superexchange) through cytochromes.