Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of <i>Geobacter sulfurreducens</i> Biofilms

Abstract

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of <i>Geobacter sulfurreducens</i> has been extensively studied and has been revealed to be facilitated through <i>c</i>-type cytochromes, which mediate charge between the electrode and <i>G. sulfurreducens</i> in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a <i>G. sulfurreducens</i> biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of <i>G. sulfurreducens</i> biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.