Electrochemical Characterization Reveals Multiple Distinct Electron Transport Pathways in Anode Biofilms of Geobacter Sulfurreducens

Geobacter sulfurreducens has become a model microorganism for anode respiration in the field of microbial electrochemistry.  Our group has studied the sigmoidal behavior of the current/potential response of G. sulfurreducens, modeled through the Nernst-Monod relationship.  In its current form (Fig. 1), the Nernst-Monod relationship assumes that there is a single limiting redox process, which governs the energy that is gained by G. sulfurreducens from anode respiration.

Fig. 1 shows the Nernst-Monod equation.  Here, j is the current produced at the anode potential E, j_max is the maximum current, n is the number of electrons transferred, F is the Faraday constant, R is the gas constant, T is the temperature, and E_KA is the anode potential that results in j as half of j_max. 

Our previous studies have used the Nernst-Monod relationship to model slow-scan cyclic voltammograms (CV) of G. sulfurreducens grown at -0.4 V vs. Ag/AgCl with n=1, and an E_KA = -0.42 V vs. Ag/AgCl. Our ability to fit the data further corroborated the assumption of a single redox processes controlling the metabolic response of G. sulfurreducens. However, several published studies and our own recent work also suggested that the Nernst-Monod relationship did not always fit the CV data. Thus, we used electrochemical impedance spectroscopy (EIS) as a function of anode potential in a G. sulfurreducens biofilm, with the aim of getting further insight on the electron transport processes for anode respiration. EIS can theoretically allow separating the contribution of various processes governing electron transfer rates, as long as they are separated in their characteristic frequency (i.e. have different time constants).

We have performed EIS analyses, at various anode potentials, on G. sulfurreducens biofilms grown at -0.3 V vs. Ag/AgCl. These have shown the existence of more than one charge transfer process governing its electron transport, each with a different E_KA. This suggests that there are multiple governing redox cofactors in G. sulfurreducens that are used in combination to perform anode respiration.  Through chronoamperometry experiments we have also further shown that G. sulfurreducens has the ability to change which cofactors are used depending on the poised anode potential.  

We hypothesize that the distinct redox cofactors possibly help G. sulfurreducens respire different forms of iron, whose oxide forms present in nature have different redox potentials. Our results also suggests that the Nernst-Monod relationship needs to be expanded to include the presence of multiple redox cofactors with different E_KA, and these may differ for different anode-respiring bacteria.