Cathodic Electron Transfer Research Articles

Bridging energy and microbes: How the anodic potential of microbial electrolysis cell improves anaerobic digestion

Enhanced methane production is typically attributed to cathodic electron transfer in microbial electrolysis cell coupled anaerobic digestion (MEC-AD). This study shifts focus to the overlooked anodic reactions, illuminating their integral role in methane production during MEC-AD. Experimentation at controlled anodic potentials established 0.6 V as optimal; this parameter yielded an 87 % increase in methane and a 53 % reduction in chemical oxygen demand. The anode enriched for electroactive bacteria, notably Pseudomonas, facilitating the oxidation of propionic and butyric acids into acetic acid, thus providing sufficient substrate for methanogens. The modulation of the NADH/NAD+ ratio by the anodic potential proved central to converting carbohydrates and volatile fatty acids into acetic acid. Selective potentials enriched microbes proficient in anaerobic digestion, corroborated by bioinformatics analyses indicating enzymatic upregulation in key pathways. This investigation provides actionable insights into optimizing MEC-AD systems, advancing bioenergy recovery technologies.

Revisiting the role of algal biocathodes in microbial fuel cells for bioremediation and value-addition

Algal-microbial fuel cells (A-MFC) offers a sustainable solution for wastewater treatment and energy recovery. The performance of a typical MFC is affected by the oxidative-reductive reactions occurring in it. The cathodic reduction is facilitated by electron acceptors such as oxygen and ferricyanide. However, higher operational cost is incurred from their application. Algae owing to its phototrophic metabolism, oxygenates the cathode photosynthetically and acts as a mediator for cathodic electron transfer. Mixotrophic metabolism of algae enable their adaptation and growth in pollutant-rich toxic environments, making them suitable for wastewater treatment and remediation. A-MFCs enable the generation of algal biomass, a rich source of carbohydrates, lipids, proteins, pigments, and many more for commercial applications. Algal-based CO2 sequestration, nutrient and heavy metal removal via assimilation and carbon capture pathways make A-MFC systems a promising approach for bioenergy generation and wastewater remediation. Hence, this review offers an overview on the principles and applications of A-MFC and their relevance in developing a waste-centered-circular economy.

Delineating cathodic extracellular electron transfer pathways in microbial electrosynthesis: Modulation of polarized potential and Pt@C addition

Microbial electrosynthesis (MES) is an innovative technology that employs microbes to synthesize chemicals by reducing CO2. A comprehensive understanding of cathodic extracellular electron transfer (CEET) is essential for the advancement of this technology. This study explores the impact of different cathodic potentials on CEET and its response to introduction of hydrogen evolution materials (Pt@C). Without the addition of Pt@C, H2-mediated CEET contributed up to 94.4 % at −1.05 V. With the addition of Pt@C, H2-mediated CEET contributions were 76.6 % (−1.05 V) and 19.9 % (−0.85 V), respectively. BRH-c20a was enriched as the dominated microbe (>80 %), and its relative abundance was largely affected by the addition of Pt@C NPs. This study highlights the tunability of MES performance through cathodic potential control and the addition of metal nanoparticles.

Selective colonization of multifunctional microbes that facilitates caproate production in microbial electrosynthesis system

As promising and value-added platform chemicals, the biosynthesis of medium-chain fatty acids (MCFAs, C6-C12) via chain elongation (CE) in microbial electrosynthesis systems (MES) has recently garnered significant attentions due to the unique bioelectrocatalytic behaviors and the reduction of exogenous electron donors (EDs). Caproate-synthesizing bacteria (CSB) played a crucial role in the generation of acetyl-CoA, which subsequently underwent reverse β oxidation to synthesize MCFAs. However, the presence of electrochemically active bacteria (EAB) was also pivotal for MES as they generated hydrogen in the cathodic electron transfer pathways, the specific colonization patterns of key symbionts remained unknown. The aim of this study was to investigate the selective colonization of EAB and CSB on the performance of caproate production in MES. The reactors colonized EAB and CSB achieved the maximum caproate concentration of 5332.5 mg COD/L, which was 7.9 and 1.3 times higher compared to that inoculated with either sole EAB or CSB, respectively. Moreover, the reactors colonized with mixed culture exhibited a clear redox peak and preferable electron transfer efficiency. CSB and EAB mutually interacted in metabolic activities, thereby resulting in increased substrate utilization and caproate production. The findings of this study provide a new strategy and insights for value-added chemicals recovery in microbial electrosynthesis platform.

Cathodic reduction of 2-hydroxy-1,4-naphthoquinone (lawsone) to generate 1,2,4-trihydroxynaphthalene as regenerable reducing agent for dispersed indigo

A central chemical step in the application of vat dyes is their reduction into the alkali soluble leuco-form. In denim production the indigo dye is reduced by catalytic hydrogenation, by use of a chemical stock vat or by cathodic electron transfer in presence of soluble mediator systems. In this study the potential of 2-hydroxy-1,4-anthraquinone (lawsone) to serve as reversible redox couple for indirect cathodic reduction of dispersed indigo was investigated. Results from cyclic voltammetry, voltammetry in flow cells equipped with different cathode material and batch electrolysis experiments in presence of dispersed indigo demonstrate successful electron transfer from cathodically reduced lawsone to dispersed indigo above pH 12.5. At this pH the di-anion of 1,2,4-trihydroxynaphthaline is present in solution. At lower pH e.g. pH 11.5 the mono-anion is present, which is not able to reduce indigo molecules. For the study synthetic chemicals have been used as a model, however when the chemically equivalent natural colorants henna and indigo are used instead a new route for a fully bio-based indigo dyeing becomes available.

Bioenergy Generation and Wastewater Purification with Li0.95Ta0.76Nb0.19Mg0.15O3 as New Air-Photocathode for MFCs

MFC is a promising technology that can be used for simultaneous electricity generation and wastewater treatment. Power energy generation of a ferroelectric cathodic ceramic, Li0.95Ta0.76Nb0.19Mg0.15O3 (LTNMg), has been measured in microbial fuel cells, integrating a single chamber fed by industrial wastewater (CODinitial = 471 mg L−1, and pHinitial = 7.24 at T = 27 °C). In this process, the mixed multicomponent oxide material has been prepared and characterized by XRD, PSD, TEM, and UV-Vis spectroscopy. The catalytic activity has been investigated by COD determination, analysis of heavy metals, and polarization measurement. The results show a high COD reduction efficiency, which reaches 95.70% after a working time of 168 h with a maximal power density of 228 mW m−2. In addition, the maximum value of generated voltage in the open-circuit potential (OCP) of this MFC configuration has been increased from 340 mV in the absence of a light source to 470 mV under irradiation, indicating the presence of a promoting photocatalytic effect of LTNMg, which improved the process of the cathodic electron transfer inside the MFC device.

Construction of bidirectional electron transfer biofilms via periodic polarity reversal

Bioelectrochemical denitrification has been widely applied to remove nitrate in water. Thereinto, bidirectional electron transfer biofilm removes nitrate effectively without nitrite accumulation. However, it is critical to maintain bidirectional electron transfer capability of biofilm. In this study, periodic polarity reversal strategy was applied to cultivate bidirectional electron transfer biofilm. The nitrate removal was observed at 93 ± 5.9 % in bidirectional electron transfer system (R1) cultivated by periodic polarity reversal strategy. High bio-community similarity was observed in anode and R1 biofilm with Geobacter sp. as dominant bacteria, indicating that the periodic polarity reversal strategy sustained anodic respiring bacteria. The exoelectrogens (Geobacter pickeringii) and denitrifiers (Chryseobacterium sp., Acidovorax sp. and Stenotrophomonas sp.) exhibited a positive correlation, implying interspecies mutualistic growth. Electroactive sites in R1 and anode biofilm derived from turnover CVs and DPVs were within the same range, indicating that the sustained anodic respiring bacteria also played essential role in cathodic electron transfer. Periodic polarity reversal utilized the “Bio-pseudocapacitance” feature of electroactive biofilm, making it possible to donate stored electrons (at anode period) for nitrate reduction in cathode period. The detailed contributions of direct electron transfer (DET) and mediated electron transfer (MET) were investigated to reveal the electron transfer mechanism at the bacteria-electrode interface. Different electron transfer pathway was suggested for electron donating and extracting process in this bidirectional electron transfer biofilm. Generally, periodic polarity reversal strategy provides an effective way to regulate bidirectional electron transfer biofilm and utilize the “Bio-pseudocapacitance” feature of biofilm to facilitate bioelectrochemical nitrate reduction.

Feedback current production by a ferrous mediator revealing the redox properties of Shewanella oneidensis MR-1

• FcMeOH mediated electron transfer between Shewanella cells and electrodes. • A positive feedback current demonstrates that electron may transfer among cells. • Cyclic voltammetry reveals redox properties of S. oneidensis MR-1. • Electrons may transfer by spreading among redox flavocytochromes. The redox properties of electroactive bacteria play an important role in biogeochemical cycling, environmental remediation and bioenergy generation. But the detailed mechanisms on the redox properties of living cells are still vague. Herein, we designed a series of electrochemical tests mediated by FcMeOH to investigate the bacterial electrode process. Cyclic voltammograms and a positive feedback current production of scanning electrochemical microscopy provided evidence on the process of electrochemical oxidation followed by biocatalytic reduction. Moreover, an irreversible process for the redox behaviours of Shewanella oneidensis MR-1 revealed that electrons may transfer by spreading among redox flavocytochromes at the cell membrane and that these proteins may form a conductive network to perpetuate cathodic electron transfer. These results provide an insight into redox reactions at the microbe-electrode interface.

Label-free detection of target proteins using peptide molecular wires as conductive supports

We report on the electrochemistry of a peptide molecular wire as a conductive support for ligand immobilization in biosensing assays. The helical 9-mer peptide, tagged at Nterminus with Methylene Blue (MB) and thiol-functionalized at Cterminus was anchored on a gold surface via gold/thiol chemistry. The helical peptide acts as molecular wire, mediating the two-step electron transfer (ET) from MB to the gold surface. The forward and backward square wave voltametric (SWV) components recorded in the presence of peptide wire were used to estimate the kinetic parameters of the electrode reaction. The simulated data matched the experimental ones for the two-step sequential surface mechanism (EE), with the rate constant of the first step ksur,1 = 20 s−1 and the cathodic ET coefficient α1 = 0.55. The kinetic parameters of the second step were ksur,2 > 1000 s−1, and α2 = 0.5. Small ligands for high-molecular weight targets can be grafted on the peptide wire between the MB tag and the surface through covalent bonding. The binding of the target to the peptide-anchored ligand hampers the ET transfer from MB to the electrode surface, causing a decrease of the peak current. These findings were used further to develop an electrochemical peptide-based biosensor with signaling-off interrogation. The biosensor was tested against two relevant targets for medical diagnosis: the anti-tumor-associated carbohydrate antigen (α-Tn) antibody and the growth hormone secretagogue receptor (GHS-R1a), after ligand grafting. Both targets were detected in the nanomolar range with an overall assay time of 10 min.

Delivering protons with electrons

Electrochemistry Many chemical reactions involve concurrent transfer of a proton and an electron. In electrochemical synthesis, this mechanism could prove useful in lowering the energy necessary for cathodic electron transfer alone, but it is hindered by competing direct coupling of the protons and electrons to make hydrogen instead. Chalkley et al. now report a molecular mediator consisting of a dimethylaniline base tethered to a cobaltocenium electron acceptor. This construct can deliver both a proton and an electron to a substrate from an acid and a cathode while skirting the hydrogen pathway. Science , this issue p. [850][1] [1]: /lookup/doi/10.1126/science.abc1607

Pin-point denitrification for groundwater purification without direct chemical dosing: Demonstration of a two-chamber sulfide-driven denitrifying microbial electrochemical system

The nitrate concentration in groundwater has been increasing over time due to the intensive use of nitrogen fertilizer. Current nitrate removal technologies are restricted by the high operational cost or the inevitable secondary contaminations. This study proposed a two-chamber sulfide-driven denitrifying microbial electrochemical system to denitrify nitrate in its cathode chamber. Instead of conventional organic substrates, sulfide is oxidized in the anode chamber to generate electrons for cathodic denitrification. Long-term performance of this novel system was evaluated over 200 days (100 cycles) of batch-fed operation. With the assistance of anodic microorganisms, sulfide can be directly oxidized to sulfate thus avoiding passivating the anode. Catalyzed by the cathodic microorganisms, complete denitrification was realized with neither nitrite nor nitrous oxide accumulation. Benefiting from the electroautotrophic behavior of the functional microorganisms, high electron utilization efficiencies were achieved, 80% and 85% for the anode (sulfide oxidation) and the cathode (denitrification) respectively. Both observed electrode potentials and microbial analyses revealed that cytochrome c is the crucial electron transfer mediator in the cathodic electron transfer for denitrification. Based on the analysis of planktonic and biofilm microbial samples, anodic and cathodic extracellular electron transfer bioprocesses are proposed, both the direct and mediated electron transfers involved, as were revealed by immobilized and planktonic functional microorganisms, respectively. This study demonstrates the feasibility of purifying nitrate-contaminated groundwater without sacrificing its water quality in a separate mode of treatment. This concept can be extended to a broader field, in which the water requires bio-polishing without introducing unwanted secondary pollution like the post-denitrification of wastewater effluents.

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms.

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV–vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.

The Ability of PVD Chromium (III) Oxide Films to Inhibit the Corrosion-Driven Cathodic Delamination of Organic Coatings from the Surface of Mild Steel

This paper describes a systematic study into the role of metallic chromium and chromium (III) oxide films of varying thickness in slowing or preventing the corrosion-driven cathodic disbondment of an organic overcoat applied to chromium/oxide coated steel. A graded wedge of chromium metal or chromium (III) oxide is applied to a steel substrate using physical vapour deposition (PVD) in conjunction with a stepping motor driven sliding shutter arrangement. An overcoat of electrically insulating polyvinyl butyral (PVB) lacquer is then applied and this acts as model organic coating. Corrosion is initiated from an artificially created coating defect, penetrating to the steel substrate, though the introduction of an aqueous sodium chloride electrolyte. Scanning Kelvin probe (SKP) potentiometry is then used to follow the resulting kinetics of PVB coating delamination in air at high relative humidity. The variation in layer thickness across the PVD wedge allows for a high-throughput investigation of the effect of the layer thickness for both metallic chromium and chromium (III) oxide on the rate of corrosion driven cathodic coating delamination, as shown schematically in the figure below. Varying chromium metal layer thickness between 100 nm and 1000 nm produced very little change in the rate of PVB film delamination and the delamination kinetics remained parabolic throughout. Parabollic kinetics are consistent with rate limitation by the mass transport of (sodium) cations through electrolyte layer forming beneath the delaminated coating. Conversely, increasing the thickness of chromium (III) oxide from 10nm to 200 nm (applied over a 500 nm metallic chromium layer), resulted in a significant decrease in rates of PVB delamination. At chromium oxide layer thicknesses ≥ 120 nm delamination kinetics became linear (zero order with respect to time). Linear kinetics are consistent with rate limitation by cathodic electron transfer to oxygen across the chromium (III) oxide layer. For chromium (III) oxide thicknesses of 200 nm PVB delamination was prevented over the entire 48 hour experimental time period. This level of inhibition is comparable to the protection provided by electroplated hexavalent chromium/ hydrated chromium oxide (ECCS) material used in packaging steel products. These finding indicate the ability of PVD to produce corrosion resistant coatings of controlled thickness and composition and provides relevant information regarding the likely relative importance of metallic metal and chromium (III) oxide in potential alternatives to coatings produced using hexavalent chromium. Figure I. Schematic representation of: a) the process by which a wedge of chromium metal or chromium (III) oxide is formed using a sliding shutter mechanism and b) how the PVB overcoated wedge sample is oriented with respect to the cathodic delamination cell. Figure 1

Power Generation by Selective Self-Assembly of Biocatalysts.

Through a careful chemical and bioelectronic design we have created a system that uses self-assembly of enzyme-nanoparticle hybrids to yield bioelectrocatalytic functionality and to enable the harnessing of electrical power from biomass. Here we show that mixed populations of hybrids acting as catalyst carriers for clean energy production can be efficiently stored, self-assembled on functionalized stationary surfaces, and magnetically re-collected to make the binding sites on the surfaces available again. The methodology is based on selective interactions occurring between chemically modified surfaces and ligand-functionalized hybrids. The design of a system with minimal cross-talk between the particles, outstanding selective binding of the hybrids at the electrode surfaces, and direct anodic and cathodic electron transfer pathways leads to mediator-less bioelectrocatalytic transformations which are implemented in the construction of a fast self-assembling, membrane-less fructose/O2 biofuel cell.

Fabrication and Performance Evaluation of Pd‐Cu Nanoparticles for Hydrogen Evolution Reaction

AbstractHerein, the effects of different types of surfactants including sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), and t‐octyl phenoxy polyethoxy ethanol (Triton X‐100, TX‐100) were presented on the hydrogen evolution reaction (HER) at the surface of carbon paste electrode (CPE) modified with Pd−Cu nanoparticles. Firstly, Cu nanoparticles with average diameters at about 80 nm were electro‐deposited at the CPE surface in H2SO4 solution containing CTAB and CuSO4 by potentiostatic method. Then, Pd−Cu bimetallic nanoparticles with diameters at about 75 nm were prepared through galvanic replacement reaction between PdII ions and Cu particles. The synthesized Cu nanoparticles were covered by CTAB in solution and surface tension of the nanoparticles were decreased, and consequently stabilized. Also, the surfactants play different roles on the HER depending on the electrode polarity and surface charges. The electro‐catalytic activity of the Pd−Cu/CPE and nano‐Cu/CPE towards HER was found to be highest in the presence of SDS. The effects of galvanic replacement reaction and type of surfactants on kinetic parameters of the HER such as cathodic electron transfer coefficient and exchange current density were comparatively investigated.

A Hybrid Material Combined Copper Oxide with Graphene for an Oxygen Reduction Reaction in an Alkaline Medium.

In this work, an electrode material based on CuO nanoparticles (NPs)/graphene (G) is developed for ORR in alkaline medium. According to the characterization of scanning electron microscope and transmission electron microscope, CuO NPs are uniformly distributed on the wrinkled G sheets. The X-ray diffraction test reveals that the phase of CuO is monoclinic. The CuO/G hybrid electrode exhibits a positive onset potential (0.8 V), high cathodic current density (3.79 × 10−5 mA/cm2) and high electron transfer number (four-electron from O2 to H2O) for ORR in alkaline media. Compared with commercial Pt/C electrocatalyst, the CuO/G electrode also shows superior fuel durability. The high electrocatalytic activity and durability are attribute to the strong coupling between CuO NPs and G nanosheets.

Repeated transfer enriches highly active electrotrophic microbial consortia on biocathodes in microbial fuel cells

Cathodic oxygen reduction catalyzed by autotrophic bacteria instead of a precious metal is a promising method to make use of microbial fuel cells (MFCs) in wastewater treatment with electricity production. However, the ecology of electrotrophic microbial consortia in wastewater systems that function as the catalyst for cathodic oxygen reduction is complicated and the electron transfer mechanisms are still unknown, which prevents further improvements of the biocathode performance. Enriched by the repeated transfer of a mature electrotrophic microbial consortia to new cathodes over 10 generations in 230 days, the start-up time was shortened from 21.4 to 7.6 days and the maximum current densities over the potential range of 0.5 to − 0.3 V increased by up to 112%, from 75 ± 5 A m−3 to 159 ± 3 A m−3, which was further confirmed in half-cell biocathode systems. The electrotrophic microbial consortia approached a relatively stable state after 8 generations. Acinetobacter, which is a member of Proteobacteria, was selectively enriched after 10 generations, which was closely related to the current production. Nitrospiraceae and Nitrosomonas may jointly perform a nitrogen cycling metabolic process and promote cathodic bioelectron transfer. Our findings confirmed that the electrotrophic microbial consortia on the cathode was able to be specifically evolved, leading to higher electroactivity, and also revealed which bacteria in fresh water are closely related to cathodic electron transfer.

PEDOT:PSS/MnO2/rGO ternary nanocomposite based anode catalyst for enhanced electrocatalytic activity of methanol oxidation for direct methanol fuel cell

In this work, a non-precious anode catalyst material PEDOT:PSS/MnO2/rGO ternary nanocomposite was synthesized by hydrothermal route followed by in situ oxidative polymerization. The morphology and structure of the synthesized samples were investigated by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). From the morphological investigations of the ternary nanocomposite, it is confirmed that PEDOT:PSS coated MnO2 nanorods are wrapped by rGO nanosheets. Brunauer-Emmett-Teller (BET) measurements confirm the porous structure and high surface area (190 m2/g) of the ternary nanocomposites. Electrochemical and electrocatalytic activities of PEDOT:PSS/MnO2/rGO coated ITO electrodes towards methanol oxidation were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) in 0.5 M NaOH as supporting electrolyte. Anodic and cathodic electron transfer coefficient (α and β) and heterogeneous rate constant (ks) of the ternary nanocomposite coated electrode were found to be 0.51, 0.45 and 0.055 s−1, respectively. The higher electrocatalytic activity i.e. higher oxidation current density (56.38 mA/cm2) and lower onset potential (0.32 V) of the ternary nanocomposite towards methanol oxidation may be due to synergistic effects of excellent conductivity of rGO nanosheets and porous nanostructure of PEDOT:PSS coated MnO2 nanorods. Long term stability holding of current density 50 mA/cm2 upto 1 h and higher cyclic stability (current retention factor 83%) upto 700th cycles imply that PEDOT:PSS/MnO2/rGO ternary nanocomposite can be the potential alternative of platinum based anode catalyst in direct methanol fuel cell.

An Electrochemist Perspective of Microbiologically Influenced Corrosion

Microbiologically influenced corrosion (MIC) is a major concern in a wide range of industries, with claims that it contributes 20% of the total annual corrosion cost. The focus of this present work is to review critically the most recent proposals for MIC mechanisms, with particular emphasis on whether or not these make sense in terms of their electrochemistry. It is determined that, despite the long history of investigating MIC, we are still a long way from really understanding its fundamental mechanisms, especially in relation to non-sulphate reducing bacterial (SRB) anaerobes. Nevertheless, we do know that both the cathodic polarization theory and direct electron transfer from the metal into the cell are incorrect. Electrically conducting pili also do not appear to play a role in direct electron transfer, although these could still play a role in aiding the mass transport of redox mediators. However, it is not clear if the microorganisms are just altering the local chemistry or if they are participating directly in the electrochemical corrosion process, albeit via the generation of redox mediators. The review finishes with suggestions on what needs to be done to further our understanding of MIC.

Anodic and Cathodic Extracellular Electron Transfer by the Filamentous Bacterium Ardenticatena maritima 110S.

Ardenticatena maritima strain 110S is a filamentous bacterium isolated from an iron-rich coastal hydrothermal field, and it is a unique isolate capable of dissimilatory iron or nitrate reduction among the members of the bacterial phylum Chloroflexi. Here, we report the ability of A. maritima strain 110S to utilize electrodes as a sole electron acceptor and donor when coupled with the oxidation of organic compounds and nitrate reduction, respectively. In addition, multicellular filaments with hundreds of cells arranged end-to-end increased the extracellular electron transfer (EET) ability to electrodes by organizing filaments into bundled structures, with the aid of microbially reduced iron oxide minerals on the cell surface of strain 110S. Based on these findings, together with the attempt to detect surface-localized cytochromes in the genome sequence and the demonstration of redox-dependent staining and immunostaining of the cell surface, we propose a model of bidirectional electron transport by A. maritima strain 110S, in which surface-localized multiheme cytochromes and surface-associated iron minerals serve as a conduit of bidirectional EET in multicellular filaments.