When grown on the surface of anode electrode, Geobacter sulfurreducens forms a multi-cell thick biofilm in which all cells appear to couple oxidation of acetate with electron transport to the anode which serves as the terminal metabolic electron acceptor. Just how electrons are transported through such a biofilm from cells to the underlying anode surface over distances that can exceed 20 microns remains unresolved. Current evidence suggests it may occur by electron hopping through a proposed network of redox cofactors comprised of immobile outer membrane and/or extracellular multi-heme c-type cytochromes. In the present work we apply spatially resolved Confocal Resonant Raman (CRR) microscopic analysis to investigate anode-grown Geobacter biofilms. The results confirm presence of an intra-biofilm redox gradient whereby the probability that a heme is in the reduced state increases with increasing distance it resides from the anode surface. Such a gradient is required to drive electron transport toward the anode surface by electron hopping via cytochromes. The results also indicate that at open circuit when electrons are expected to accumulate in redox cofactors involved in electron transport due to inability of the anode to accept electrons, nearly all c-type cytochrome hemes detected in the biofilm are oxidized. The same outcome occurs when a comparable potential to that measured at open circuit (-0.30 V vs. SHE) is applied to the anode, whereas nearly all hemes are reduced when an exceedingly negative potential (-0.50 V vs. SHE) is applied to the anode.  These results suggest that nearly all c-type cytochromes hemes detected in the biofilm can be electrochemically accessed by the electrode, but most have oxidation potentials too negative to transport electrons originating from acetate metabolism. The results also reveal lateral heterogeneity (x-y dimensions) in type of c-type cytochromes within the biofilm that may affect electron transport to the electrode.

[1] Lebedev, N., Griva, I. & Blom A. Internal Control of Electron Transfer through a Single Iron Atom by Chelating Porphyrin. J. Phys. Chem. C 117, 6933−6939, 2013.

[2] Lebedev, N., Strycharz-Glaven, S. M. & Tender L. M. Spatially Resolved Confocal Resonant Raman Microscopic Analysis of Electron Transfer in Geobacter sulforreducens Biofilm. ChemPhysChem, 15, 320-327, 2014.

[3] Lebedev, N., Strycharz-Glaven, S. M., & Tender L. M. High resolution AFM and single-cell resonance Raman spectroscopy of Geobacter sulfurreducens biofilms early in growth. Frontiers in Energy Research, 2:34, doi: 10.3389, 2014.