Negative Faradaic Resistance in Extracellular Electron Transfer by Anode-Respiring Geobacter sulfurreducens Cells

Geobacter sulfurreducens is a Gram-negative delta-proteobacterium that is capable of transferring respiratory electrons to solid-state extracellular acceptors, including anodes, via abundant c-type cytochromes expressed in the outer membrane. As extracellular electron transfer (EET) to solid-state acceptors can proceed without the addition of exogenous mediators, this microbe has attracted significant attention with respect to energy conversion between electrical and chemical energy. Here, we revealed that clear redox peaks were present in the whole-cell cyclic voltammogram of G. sulfurreducens at around -0.15 V vs Ag vertical bar AgCl when tin-doped In2O3 (ITO) glass was used as an anode. Interestingly, the EET current decreased in the potential region more positive than the midpoint potential (E-m) of the redox peak. Thus, EET of anode-respiring G. sulfurreducens cells exhibited a negative Faradaic resistance (NFR) characteristic at the E-m of the redox peak, in contrast to general electrochemical systems where the Faradaic current typically increases with increasing overpotential.