Extracellular Electron Transport Research Articles

Bioelectricity (electromicrobiology) and sustainability.

SummaryElectromicrobiology is the domain of those prokaryotes able to interact with charged electrodes, using them as electron donors and/or electron acceptors. This is performed via a process called extracellular electron transport, in which outer membrane cytochromes are used to oxidize and/or reduce otherwise unavailable insoluble electron acceptors. EET‐capable bacteria can thus be used for a variety of purposes, ranging from small power sources, water reclamation, to pollution remediation and electrosynthesis. Because the study of EET‐capable bacteria is in its nascent phase, the applications are mostly in developmental stages, but the potential for significant contributions to environmental quality is high and moving forward.

Redox conduction in biofilms: From respiration to living electronics

Summary Recent studies of electrochemically active bacteria paint a mechanistic picture where multistep hopping of electrons can be driven by redox gradients, allowing electron conduction over length scales previously thought impossible in biological systems. This review explores these observations from an electrochemical and biophysical perspective, with emphasis on the role of metalloproteins such as the multiheme extracellular electron transport conduits thought to mediate this transport. Since these conduits naturally evolved to interact with external surfaces, a fundamental understanding has special implications for a new generation of bioelectrochemical technologies and living electronics that wire bacteria to electrodes.

Proton Transport in the Outer‐Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

AbstractThe microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D2O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D2O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.

Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport.

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D2O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D2O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.

Engineering a Native Inducible Expression System in Shewanella oneidensis to Control Extracellular Electron Transfer.

Shewanella oneidensis MR-1 is a model organism for understanding extracellular electron transport, in which cells transfer intracellular electrons to an extracellular terminal electron acceptor such as insoluble minerals or poised electrodes. Biotechnological applications exploiting the respiratory capabilities of Shewanella species have led to their proposed use in wastewater treatment, bioremediation, and remote sensors. Transcriptional regulation tools can be used to rationally engineer S.oneidensis, optimizing performance in biotechnological applications, introducing new capabilities, or investigating physiology. Engineered gene expression in S.oneidensis has primarily involved the use of foreign regulatory systems from Escherichia coli. Here we characterize a native S.oneidensis pathway that can be used to induce gene expression with trimethylamine N-oxide, then engineer strains in which extracellular electron transfer is controlled by this compound. The ability to induce this pathway was assessed by measuring iron reduction over time and by analyzing anodic current produced by cells grown in bioreactors.

Novel resolution-contrast method employed for investigating electron transfer mechanism of the mixed bacteria microbial fuel cell

Due to the synergy existing in mixed bacteria, electricity-generation performances of the mixed bacteria microbial fuel cell (m-MFC) are better than that of the pure bacteria microbial fuel cell (p-MFC). It is necessary to explore the electron transfer mechanisms of the m-MFC to further improve electricity generation. In this study, a facile “resolution-contrast” method was employed in the two-chamber m-MFC to investigate the electron transfer mechanism using mixed bacteria as the anodic inoculums. The anode was wrapped with a mixed cellulose esters membrane to eliminate the effects of direct extracellular electron transport, while the mediator electron transport was investigated by electrochemical methods. The results showed that both transfer methods existed simultaneously in the m-MFC. The main exoelectrogens were Escherichia coli, Pseudomonas aeruginosa, and Brevundimonas diminuta, and the main redox mediators included 1-hydroxyanthracene-9,10-dione, phenazine-1-carboxylic acid and 2,4-diacetyl phloroglucinol.

Pioneer the Physical Chemistry of Biological Electron Transfer based on Bacterial Ext racellular Electron Transport

Pioneer the Physical Chemistry of Biological Electron Transfer based on Bacterial Ext racellular Electron Transport

Soil microorganisms can overcome respiration inhibition by coupling intra- and extracellular metabolism: 13C metabolic tracing reveals the mechanisms.

CO2 release from soil is commonly used to estimate toxicity of various substances on microorganisms. However, the mechanisms underlying persistent CO2 release from soil exposed to toxicants inhibiting microbial respiration, for example, sodium azide (NaN3) or heavy metals (Cd, Hg, Cu), remain unclear. To unravel these mechanisms, NaN3-amended soil was incubated with position-specifically 13C-labeled glucose and 13C was quantified in CO2, bulk soil, microbial biomass and phospholipid fatty acids (PLFAs). High 13C recovery from C-1 in CO2 indicates that glucose was predominantly metabolized via the pentose phosphate pathway irrespective of inhibition. Although NaN3 prevented 13C incorporation into PLFA and decreased total CO2 release, 13C in CO2 increased by 12% compared with control soils due to an increased use of glucose for energy production. The allocation of glucose-derived carbon towards extracellular compounds, demonstrated by a fivefold higher 13C recovery in bulk soil than in microbial biomass, suggests the synthesis of redox active substances for extracellular disposal of electrons to bypass inhibited electron transport chains within the cells. PLFA content doubled within 10 days of inhibition, demonstrating recovery of the microbial community. This growth was largely based on recycling of cost-intensive biomass compounds, for example, alkyl chains, from microbial necromass. The bypass of intracellular toxicity by extracellular electron transport permits the fast recovery of the microbial community. Such efficient strategies to overcome exposure to respiration-inhibiting toxicants may be exclusive to habitats containing redox-sensitive substances. Therefore, the toxic effects of respiration inhibitors on microorganisms are much less intensive in soils than in pure cultures.

A Microenvironment for Shewanella oneidensis MR-1 Exists within Graphite Felt Electrodes

Shewanella oneidensis MR-1 employs various methods of Extracellular Electron Transport (EET) to insoluble terminal electron acceptors, including graphite felt anodes of BioElectrochemical Systems (BESs). Fibers in the felt form a three dimensional meshwork within which microbes can form biofilms, as well as occupy the interstitial spaces as planktonic cells. Our results indicate that these interstices generated by the meshwork create a novel microenvironment where planktonic cells grow to higher density under certain conditions. When incubated anaerobically with 18 mM lactate and 30 mM fumarate, planktonic cell counts within the electrodes are ∼10-fold higher than bulk planktonic cell counts; a phenomenon we termed the “interstitial felt effect”. Upon lowering both lactate and fumarate concentrations 10-fold, while bulk planktonic cell counts are stable, the interstitial felt effect disappears. This effect reappears when lactate and fumarate concentrations are lowered another 10-fold. Cyclic voltammetry experiments did not reveal any modification of the graphite fibers within the electrodes. A mutant strain lacking the primary flavin transporter gene, bfe, also expresses the interstitial felt effect. The interstitial planktonic microenvironment of electrically inert polyurethane sponge did not demonstrate this phenomenon. This study provides new insights into interactions of microbes with electrode materials that may help improve overall BES performance.

Nanoscale Secondary Ion Mass Spectrometry Analysis of Individual Bacterial Cells Reveals Feedback from Extracellular Electron Transport to Upstream Reactions

Nanoscale Secondary Ion Mass Spectrometry Analysis of Individual Bacterial Cells Reveals Feedback from Extracellular Electron Transport to Upstream Reactions

A Synthetic Biology Approach to Engineering Living Photovoltaics.

The ability to electronically interface living cells with electron accepting scaffolds is crucial for the development of next-generation biophotovoltaic technologies. Although recent studies have focused on engineering synthetic interfaces that can maximize electronic communication between the cell and scaffold, the efficiency of such devices is limited by the low conductivity of the cell membrane. This review provides a materials science perspective on applying a complementary, synthetic biology approach to engineering membrane-electrode interfaces. It focuses on the technical challenges behind the introduction of foreign extracellular electron transfer pathways in bacterial host cells and the past and future efforts to engineer photosynthetic organisms with artificial electron-export capabilities for biophotovoltaic applications. The article highlights advances in engineering protein-based, electron-exporting conduits in a model host organism, E. coli, before reviewing state-of-the-art biophotovoltaic technologies that use both unmodified and bioengineered photosynthetic bacteria with improved electron transport capabilities. A thermodynamic analysis is used to propose an energetically feasible pathway for extracellular electron transport in engineered cyanobacteria and identify metabolic bottlenecks amenable to protein engineering techniques. Based on this analysis, an engineered photosynthetic organism expressing a foreign, protein-based electron conduit yields a maximum theoretical solar conversion efficiency of 6-10% without accounting for additional bioengineering optimizations for light-harvesting.

Extracellular Electron Transfer and Biosensors.

This chapter summarizes in the beginning our current understanding of extracellular electron transport processes in organisms belonging to the genera Shewanella and Geobacter. Organisms belonging to these genera developed strategies to transport respiratory electrons to the cell surface that are defined by modules of which some seem to be rather unique for one or the other genus while others are similar. We use this overview regarding our current knowledge of extracellular electron transfer to explain the physiological interaction of microorganisms in direct interspecies electron transfer, a process in which one organism basically comprises the electron acceptor for another microbe and that depends also on extended electron transport chains. This analysis of mechanisms for the transport of respiratory electrons to insoluble electron acceptors ends with an overview of questions that remain so far unanswered. Moreover, we use the description of the biochemistry of extracellular electron transport to explain the fundamentals of biosensors based on this process and give an overview regarding their status of development and applicability. Graphical Abstract.

NapB in excess inhibits growth of Shewanella oneidensis by dissipating electrons of the quinol pool

Shewanella, a group of ubiquitous bacteria renowned for respiratory versatility, thrive in environments where various electron acceptors (EAs) of different chemical and physiological characteristics coexist. Despite being extensively studied, we still know surprisingly little about strategies by which multiple EAs and their interaction define ecophysiology of these bacteria. Previously, we showed that nitrite inhibits growth of the genus representative Shewanella oneidensis on fumarate and presumably some other CymA (quinol dehydrogenase)-dependent EAs by reducing cAMP production, which in turn leads to lowered expression of nitrite and fumarate reductases. In this study, we demonstrated that inhibition of fumarate growth by nitrite is also attributable to overproduction of NapB, the cytochrome c subunit of nitrate reductase. Further investigations revealed that excessive NapB per se inhibits growth on all EAs tested, including oxygen. When overproduced, NapB acts as an electron shuttle to dissipate electrons of the quinol pool, likely to extracellullar EAs, because the Mtr system, the major electron transport pathway for extracellular electron transport, is implicated. The study not only sheds light on mechanisms by which certain EAs, especially toxic ones, impact the bacterial ecophysiology, but also provides new insights into how electron shuttle c-type cytochromes regulate multi-branched respiratory networks.

Nanoelectronic Investigation Reveals the Electrochemical Basis of Electrical Conductivity in Shewanella and Geobacter.

The electrical conductivity measured in Shewanella and Geobacter spp. is an intriguing physical property that is the fundamental basis for possible extracellular electron transport (EET) pathways. There is considerable debate regarding the origins of the electrical conductivity reported in these microbial cellular structures, which is essential for deciphering the EET mechanism. Here, we report systematic on-chip nanoelectronic investigations of both Shewanella and Geobacter spp. under physiological conditions to elucidate the complex basis of electrical conductivity of both individual microbial cells and biofilms. Concurrent electrical and electrochemical measurements of living Shewanella at both few-cell and the biofilm levels indicate that the apparent electrical conductivity can be traced to electrochemical-based electron transfer at the cell/electrode interface. We further show that similar results and conclusions apply to the Geobacter spp. Taken together, our study offers important insights into previously proposed physical models regarding microbial conductivities as well as EET pathways for Shewanella and Geobacter spp.

Evolution of Cell Size Homeostasis and Growth Rate Diversity during Initial Surface Colonization of Shewanella oneidensis.

Cell size control and homeostasis are fundamental features of bacterial metabolism. Recent work suggests that cells add a constant size between birth and division ("adder" model). However, it is not known how cell size homeostasis is influenced by the existence of heterogeneous microenvironments, such as those during biofilm formation. Shewanella oneidensis MR-1 can use diverse energy sources on a range of surfaces via extracellular electron transport (EET), which can impact growth, metabolism, and size diversity. Here, we track bacterial surface communities at single-cell resolution to show that not only do bacterial motility appendages influence the transition from two- to three-dimensional biofilm growth and control postdivisional cell fates, they strongly impact cell size homeostasis. For every generation, we find that the average growth rate for cells that stay on the surface and continue to divide (nondetaching population) and that for cells that detach before their next division (detaching population) are roughly constant. However, the growth rate distribution is narrow for the nondetaching population, but broad for the detaching population in each generation. Interestingly, the appendage deletion mutants (ΔpilA, ΔmshA-D, Δflg) have significantly broader growth rate distributions than that of the wild type for both detaching and nondetaching populations, which suggests that Shewanella appendages are important for sensing and integrating environmental inputs that contribute to size homeostasis. Moreover, our results suggest multiplexing of appendages for sensing and motility functions contributes to cell size dysregulation. These results can potentially provide a framework for generating metabolic diversity in S. oneidensis populations to optimize EET in heterogeneous environments.

Flavin as an Indicator of the Rate-Limiting Factor for Microbial Current Production in Shewanella oneidensis MR-1

Microbial electrode catalysis such as microbial fuel cells or electrosynthesis involves electron exchange with the electrodes located at the cell exterior; i.e., extracellular electron transport (EET). Despite the vast amount of research on the kinetics of EET to optimize the catalysis rate, the relevance of other factors, including upstream metabolic reactions, has scarcely been investigated. Herein, we report an in vivo electrochemical assay to confirm whether EET limits anodic current production (j) for the lactate oxidation of Shewanella oneidensis MR-1. Addition of riboflavin, which specifically enhances the EET rate, increased j only in the early phase before j saturation. In contrast, when we removed a trace metal ion necessary for upstream reactions from the electrolyte, a significant decrease in j and the lactate consumption rate was observed only after j saturation. These data suggest that the limiting factor for j shifted from EET to upstream reactions, highlighting the general importance of enhancing, for example, microbial metabolism, especially for long-standing practical applications. Our concept to specifically control the rate of EET could be applicable to other bioelectrode catalysis systems as a strategy to monitor their rate-limiting factors.

Kinetic Competition Between Extracellular Electron Transport and Upstream Reactions in Microbial Electrode Catalysis

Specific microbes are utilized for bioelectrode catalysis, at the anodic or cathodic electrode, in applications such as microbial fuel cells or electrosynthesis. In the anode chamber of microbial fuel cells (MFCs), microbial metabolism oxidizes organic compounds, and the electrons are transferred to the anode by extracellular electron transport (EET) processes. Despite numerous studies on the kinetics of EET to enhance the catalysis rate, the relevance with upstream reactions, including metabolism and intracellular electron transport chain, has scarcely been investigated. Herein, we investigate the rate-determining step for anodic current production (j) for the lactate oxidation of Shewanella oneidensis MR-1 by using riboflavin to specifically enhance the rate of EET via outer membrane c-type cytochromes (OM c-Cyts).1 Microbial current production was measured in an anaerobic, three-electrode system poised at +0.4 V vs. SHE and kept at 303 K with 10 mM sodium lactate as a sole electron donor. Without the addition of flavin, j increases, saturates, and decreases as illustrated in Figure 1a. Upon the addition of 5µM riboflavin, j showed immediate increase in increasing and saturating current phases, though it increased sharper in the former (Figure 1b and c). In contrast, there was little effect of the riboflavin addition when j was decreasing (Figure 1d), suggesting that the RDS for j gradually shifted from EET to other upstream processes. Differential pulse (DP) voltammograms performed in each phases showed three peaks at approximately –230, –100, and +60 mV, which can be assigned to free riboflavin, bound riboflavin on OM c-Cyts, and the heme centers in OM c-Cyts, respectively.2 Compared with the DP voltammogram when j was increasing, the peak current of the bound flavin showed a 50% decrease after j saturation, though the peak current of the heme centers was almost identical, indicating the dissociation of bound flavin from the OM c-Cyts. Given that flavin dissociation is induced by heme oxidation in OM c-Cyts,2,3 the hemes in OM c-Cyts appear to become more oxidized after j saturation. These results further support the observation that the electron supply to OM c-Cyts from the upstream reactions gradually becomes slower than the EET rate. We will present the data for microbial growth, lactate consumption rate and voltammetric analysis in each phase, and the effect of trace metal removal from the medium to discuss the assignment of upstream reactions. Figure legend: Fig. 1. (a) A schematic image of the time course of current production. Area b, c, and d indicate the increasing, saturating, and decreasing current phase, respectively. (b–d) Current production in the presence (black lines) or absence (gray lines) of the addition of 5 µM riboflavin at the increasing (b), saturating (c), and decreasing (d) current phase, respectively. Arrows indicate the time points at which riboflavin was added. Microbes were added at t = 0 h. Reference 1. Dan C., Daniel B. B., Daniel R. B., and Jeffrey A. G.The Mtr Respiratory Pathway Is Essential for Reducing Flavins and Electrodes in Shewanella oneidensis. Journal of Bacteriology, 192, 2: 467–474 (2010). 2. Okamoto, A., Hashimoto, K., Nealson, K. H. and Nakamura, R. Rate enhancement of bacterial extracellular electron transport involves bound flavin semiquinones. P Natl Acad Sci USA 110, 7856-7861 (2013). 3. Okamoto, A. et al. Cell-secreted Flavins Bound to Membrane Cytochromes Dictate Electron Transfer Reactions to Surfaces with Diverse Charge and pH. Sci Rep-Uk 4, e5628 (2014) Figure 1

Molecular Design for Catalysts of Microbial Extracellular Electron Transfer: Redox Active Molecules Bound with Outer-Membrane Cytochrome C in Shewanella Oneidensis MR-1

1. Introduction Some microbial strains have an ability to transport respiratory electrons generated from cell inside to extracellular solid substrates via electron transfer proteins, c-type cytochromes, located at outer membrane (OM c-Cyts).[1] This interfacial electron transport between microbe and solid substrate is called extracellular electron transport, EET. Since EET can be coupled with intracellular enzymatic reactions, to understand and control the EET kinetics have impacts for development of bioelectrochemical technologies, such as microbial fuel cells[2] and electrode biosynthesis[3] as well as geochemical mineral cycling[4] and bioremediation. Recently, we found that in model EET microbe, Shewanella oneidensis MR-1, the rate of EET is largely enhanced by self-secreted flavin (Flavin mononucleotide: FMN, Riboflavin: RF) working as a binding redox active center, cofactor, in OM c-Cyts.[5] In other words, bound flavin cofactor in OM c-Cyts has catalytic activity for electron transfer from OM c-Cyts to electrode. In order to understand the EET acceleration mechanism and control the rate of EET, here, we identified and characterized alternative cofactor molecules.[6] Our data demonstrate that heterocyclic compounds containing nitrogen atom at 5-position in isoalloxazine ring (N(5)) (Figure) largely enhance current production from S. oneidensis MR-1 as efficient as the bound flavin, indicating that heterocyclic compounds with N(5) act as cofactors. Furthermore, we found that the pKa at N(5) site essentially affects the EET kinetics, providing us novel strategy for designing small molecule for regulating the EET rate to control pKa at N(5). 2. Experimental A single chamber, three electrode system was used for electrochemical measurement of intact S. oneidensis MR-1, which was inoculated for 12 hours in defined-media (DM) after 24 hours cultivation in LB. A tin-doped indium oxide (ITO) substrate, Ag/AgCl (KCl saturated) and a platinum wire were used as working, reference and counter electrode, respectively. 4.0 mL DM containing 2.0 µM flavin-like molecules (Figure) was deaerated by N2 bubbling. Cell suspensions with OD600 = 0.1 were inoculated in the reactor with the electrode poised at +0.4V vs. SHE in the presence of 10 mM lactate as a sole electron donor at 303K. To compare the EET rate for each heterocyclic compounds, the concentration of the bound cofactor was normalized by dissociation constant estimated from the data of differential pulse voltammetry.[6] 3. Results and Discussion To identify the molecules working as alternative cofactor in OM c-Cyts, we observed current generation from S. oneidensis MR-1 with several molecules possessing flavin-like polycyclic backbone (Figure). All the compounds containing N(5) as well as flavin increased current production in the range of 10-20 times compared with the absent of small molecules, while α-AQS, the molecule without N(5) in polycyclic backbone showed much less enhancement. These data strongly suggest that the N(5) compounds can operate as the cofactor to enhance the rate of EET as the flavin molecules. Differential pulse voltammetry revealed that the N(5) molecules have more positive redox potential in bound state compared with the bound flavin semiquinone, which is more favorable for receiving electrons from heme redox centers in OM c-Cyts. However, among these molecules, the larger enhancement factor did not follow more positive redox potential. On the other hand, the cofactors that have higher pKa at N(5) showed higher maximum current production. Given that the redox reaction of flavin semiquinone couples with the protonation/deprotonation reaction at N(5),[7] these results may indicate that protonation reaction at N(5) site associated with redox reaction in cofactors strongly affects the rate of EET. 4. Conclusion Our study revealed that heterocyclic compounds containing N(5) can enhance the EET rate as an alternative cofactor in OM c-Cyts. Furthermore, we found that pKa at N(5) strongly affects the rate of EET, which enables us to regulate the rate of EET by a structural modification of heterocyclic compounds containing N(5) and pKa control. These novel mechanistic insights would largely impact EET-utilizing bioelectrochemical technologies as well as general understanding for electrical interaction between microbe and extracellular environments. 5.

The electrically conductive pili of Geobacter species are a recently evolved feature for extracellular electron transfer.

The electrically conductive pili (e-pili) of Geobactersulfurreducens have environmental and practical significance because they can facilitate electron transfer to insoluble Fe(III) oxides; to other microbial species; and through electrically conductive biofilms. E-pili conductivity has been attributed to the truncated PilA monomer, which permits tight packing of aromatic amino acids to form a conductive path along the length of e-pili. In order to better understand the evolution and distribution of e-pili in the microbial world, type IVa PilA proteins from various Gram-negative and Gram-positive bacteria were examined with a particular emphasis on Fe(III)-respiring bacteria. E-pilin genes are primarily restricted to a tight phylogenetic group in the order Desulfuromonadales. The downstream gene in all but one of the Desulfuromonadales that possess an e-pilin gene is a gene previously annotated as 'pilA-C' that has characteristics suggesting that it may encode an outer-membrane protein. Other genes associated with pilin function are clustered with e-pilin and 'pilA-C' genes in the Desulfuromonadales. In contrast, in the few bacteria outside the Desulfuromonadales that contain e-pilin genes, the other genes required for pilin function may have been acquired through horizontal gene transfer. Of the 95 known Fe(III)-reducing micro-organisms for which genomes are available, 80 % lack e-pilin genes, suggesting that e-pili are just one of several mechanisms involved in extracellular electron transport. These studies provide insight into where and when e-pili are likely to contribute to extracellular electron transport processes that are biogeochemically important and involved in bioenergy conversions.

Sacrificing power for more cost-effective treatment: A techno-economic approach for engineering microbial fuel cells

Microbial fuel cells (MFCs) are a promising energy-positive wastewater treatment technology, however, the system’s cost-effectiveness has been overlooked. In this study, two new anode materials – hard felt (HF) and carbon foam (CF) – were evaluated against the standard graphite brush (GB) to determine if using inexpensive materials with less than ideal properties can achieve more cost-effective treatment than high-cost, high-performing materials. Using domestic wastewater as the substrate, power densities for the GB, HF and CF-MFCs were 393, 339 and 291 mW m−2 normalized by cathodic surface area, respectively. Higher power densities correlated with larger anodic surface areas and anodic current densities but not with electrical conductivity. Cyclic voltammetry revealed that redox systems used for extracellular electron transport in the GB, HF and CF-MFCs were similar (−0.143 ± 0.046, −0.158 ± 0.004 and −0.100 ± 0.014 V vs. Ag/AgCl) and that the electrochemical kinetics of the MFCs showed no correlation with their respective electrical conductivity. 16S rRNA sequencing showed the GB, HF and CF microbial community compositions were not statistically different while organic removal rates were nearly identical for all MFCs. The HF-MFC generated a power output to electrode cost (W $−1) 1.9 times greater than the GB-MFC, despite producing 14% less power and 15% less anodic current, while having 2.6 times less anodic surface area, 2.1 times larger charge transfer resistance and an electrical conductivity three orders of magnitude lower. The results demonstrate that inexpensive materials are capable of achieving more cost-effective treatment than high-performing materials despite generating lower power when treating real wastewater.