Spotlight on the Energy Harvest of Electroactive Microorganisms: The Impact of the Applied Anode Potential

Abstract

Electroactive microorganisms (EAM) harvest energy by reducing insoluble terminal electron acceptors (TEA) including electrodes via extracellular electron transfer (EET). Therefore, compared to microorganisms respiring soluble TEA, an adapted approach is required for thermodynamic analyses. In EAM, the thermodynamic frame (i.e., maximum available energy) is restricted as only a share of the energy difference between electron donor and TEA is exploited via the electron-transport chain to generate proton-motive force being subsequently utilized for ATP synthesis. However, according to a common misconception, the anode potential is suggested to co-determine the thermodynamic frame of EAM. By comparing the model organism Geobacter spp. and microorganisms respiring soluble TEA, we reason that a considerable part of the electron-transport chain of EAM performing direct EET does not contribute to the build-up of proton-motive force and thus, the anode potential does not co-determine the thermodynamic frame. Furthermore, using a modeling platform demonstrates that the influence of anode potential on energy harvest is solely a kinetic effect. When facing low anode potentials, NADH is accumulating due to a slow direct EET rate leading to a restricted exploitation of the thermodynamic frame. For anode potentials ≥ 0.2 V (vs. SHE), EET kinetics, NAD⁺/NADH ratio as well as exploitation of the thermodynamic frame are maximized, and a further potential increase does not result in higher energy harvest. Considering the limited influence of the anode potential on energy harvest of EAM is a prerequisite to improve thermodynamic analyses, microbial resource mining, and to transfer microbial electrochemical technologies (MET) into practice.

Keywords: electroactive microorganisms; electron-transport chain; extracellular electron transfer; microbial energy harvest; microbial thermodynamics; modeling.

Figure 1

Schematic canonical electron-transport chains of Geobacter species, Escherichia coli, and Paracoccus denitrificans. (A) In Geobacter spp., two electrons per NADH are transferred to a TEA via NADH dehydrogenase (NDH), menaquinone pool (MQ), inner membrane cytochrome (IMC), periplasmic cytochrome (PPC), and outer membrane cytochrome (OMC). The generated proton-motive force (pmf) is 1.5–3 H⁺/e⁻. Pale-colored protons indicate current uncertainties in the generated pmf. The redox-Bohr effect leads to the translocation of 1 proton per transferred electron to the extracellular space. (B) In E. coli, two electrons per NADH are transferred to oxygen via NADH dehydrogenase (NDH), ubiquinone pool (UQ), and complex IV (CIV). All reactions occur at the inner membrane resulting in a pmf of 5 H⁺/e⁻. (C) In P. denitrificans, two electrons per NADH are transferred via NADH dehydrogenase (NDH), ubiquinone pool (UQ), complex III (CIII), cytochrome c (Cc), nitrate reductase (Nar), nitrite reductase (Nir), nitric oxide reductase (Nor), and nitrous oxide reductase (Nos) to nitrogen. NarK: nitrate/nitrite transporter. Nir, Nor, and Nos consume protons from periplasm. The generated pmf is 3 H⁺/e⁻. OM: outer membrane; PP: periplasm; IM: inner membrane.

Figure 2

Schematic illustration of the used model and model results for Geobacter spp. biofilms growing on anodes set to −0.1 V (black line), 0 V (red line), 0.1 V (blue line), 0.2 V (green line), 0.4 V (purple line), and 0.6 V (yellow line). (A) Schematic model representation: Acetate oxidation is coupled to NAD⁺ reduction resulting in energy harvest (ΔG_cat) subsequently used for the build-up of biomass (ΔG_an) and for providing driving force for growth (ΔG_diss). Electrons are then transferred to intracellular cytochromes and further to a conductive biofilm matrix. Finally, electrons are donated to the anode. All reactions occur at individually calculated rates (r_bio, r_intra, r_matrix, r_anode) (Korth et al., 2015). (B) Current density. (C) Biofilm thickness. (D) Microbial energy harvest. (E) Acetate concentration. With anode potentials ≤ 0.1 V, the thermodynamic frame defined by acetate, NAD⁺/NADH ratio, and other reactants is not fully exploited. Slow EET kinetics result in thermodynamically unfavorable reaction conditions for catabolic reaction (i.e., low NAD⁺/NADH ratio) leading to lower current density, biofilm thickness, and microbial energy harvest at comparable acetate consumption. For anode potentials ≥ 0.2 V, direct EET is not limiting catabolism and reaction conditions are thermodynamically improved. Consequently, current density, biofilm thickness, and microbial energy harvest do not further increase with higher potentials. The model is based on separated anodic compartment (volume = 250 mL, anode area = 10 cm²) and cathodic compartment via a membrane (membrane area = 10 cm²). Acetate concentration is 3 mM, phosphate buffer concentration is 50 mM, and initial pH is 6.95. Further model parameters are detailed in Supplementary Table S1 and Korth et al. (2015).