Exogenous Electron Shuttle-Mediated Extracellular Electron Transfer: Electrochemical Parameters and Thermodynamics.

Abstract

Despite the importance of exogenous electron shuttles (ESs) in extracellular electron transfer (EET), a lack of understanding of the key properties of ESs is a concern given their different influences on EET processes. Here, the ES-mediated EET capacity of four types of iron reducing bacteria (FeRB) was evaluated by examining both the iron reducing capacity and the electricity generated in a microbial fuel cell. The results indicated that all the ESs substantially accelerated the iron reduction and the current generation compared to only FeRB. The iron reduction rates and the generated currents were linearly correlated with both the standard redox potential (E_h^o) and the electron transfer capacity (ETC) of the ESs. A thermodynamic analysis of the electron transfer from the electron donor to the iron oxides or the electrode suggested that the EET from c-type cytochromes (c-Cyts) to ESs is a crucial step causing the differences in EET capacities among various ESs. Based on the derived equations, both E_h^o and ETC can quantitatively determine potential losses (DE) that reflect the potential loss of the ES-mediated EET. In situ spectral kinetic analysis of ES reduction by c-Cyts in a living FeRB suspension was first investigated with the E_ES, E_c-Cyt, and DE values being calculated. This study can provide a comprehensive understanding of the role of ESs in EET.