Lack of cytochrome involvement in long-range electron transport through conductive biofilms and nanowires of Geobacter sulfurreducens

Two competing models for long-range electron transport through the conductive biofilms and nanowires of Geobacter sulfurreducens exist. In one model electrons are transported via pili that possess delocalized electronic states to function as protein wires with metallic-like conductivity. In the other model electrons are transported by more traditional electron transfer via electron hopping/tunneling between the c-type cytochromes in G. sulfurreducens biofilms and pili. The cytochrome hypothesis was further examined. Quantifying c-type cytochromes in G. sulfurreducens biofilms and pili indicated that there are insufficient cytochromes to account for electron transport through the bulk of the biofilm or pili and demonstrated that there is a negative correlation between cytochrome abundance and biofilm conductivity. Direct imaging using atomic force microscopy revealed that cytochromes were not packed close enough on pili to permit electron hopping/tunneling along the pili. Inactivating cytochromes had no impact on biofilm conductivity. The results of electrochemical gating studies were inconsistent with electron transport via cytochromes. Theoretical considerations suggest that a cytochrome model cannot explain the previously reported response of biofilm conductivity to temperature changes. These multiple lines of evidence, which rely on approaches with different sets of assumptions, demonstrate that the hypothesis that long-range electron transport through G. sulfurreducens biofilms and nanowires can be attributed to electron hopping/tunneling between c-type cytochromes is incorrect. In contrast, these multiple lines of evidence are consistent with long-range electron transport through the biofilms via networks of pili that possess metallic-like conductivity.