Extracellular Electron Transfer Process Research Articles

Cellular electron transfer in anaerobic photo-assisted biocathode microbial electrosynthesis systems for acetate production from inorganic carbon (HCO3–)

The crucial role played by the cellular electron transfer in the production of acetate from aqueous inorganic carbon (HCO3–, bicarbonate) in anaerobic photo-assisted microbial electrosynthesis (MES) biocathodes incorporating a WO3/MoO3/g-C3N4 Z-scheme heterojunction and either Serratia marcescens Q1 or Stenotrophomonas sp. JY6 electrotrophs is disclosed. The electron transfer inhibitor 2,4-dinitrophenol (DNP) diffusing through the bacterial cell membrane was used to alter the ratio of indirect (via H2) and direct extracellular electron transfer processes, resulting in lower physiological release of extracellular polymeric substances (EPS) and lower acetate production rates under a transitional period. DNP effectively cancelled the synergistic effects on acetate production produced by the photo-irradiation of the WO3/MoO3/g-C3N4 heterojunction biocathode. The physiological release of EPS with either a compositional diversity (Q1) or varying composition percentages (Q1 or JY6), was correlated to photo-irradiation and to the electrotrophic species, whereas the total amount of EPS was closely related with the electrotrophic DNP-based cellular electron transfer processes. Interestingly, the microorganisms adapted and overcame the impact of DNP after only 4 days of continuous operation and restored their normal physiological metabolism after 7.5 d operation with periodical replenishment of bicarbonate. This study provides a detailed theoretical basis to understand the impact of direct and indirect extracellular electron transfer in hybrid photo-assisted MES biocathodes and elucidates the behavior of non-photosynthetic electrotrophic bacteria favoring high conversion of aqueous inorganic carbon (HCO3–) to acetate.

Tailoring Surface Properties of Electrodes for Synchronous Enhanced Extracellular Electron Transfer and Enriched Exoelectrogens in Microbial Fuel Cells.

An extracellular electron transfer (EET) process between an electroactive biofilm and an electrode is a crucial step for the performance of microbial fuel cells (MFCs), which is highly related to the enrichment of exoelectrogens and the electrocatalytic activity of the electrode. Herein, an efficient N- and Fe-abundant carbon cloth (CC) electrode with the comodification of iron porphyrin (FePor) and polyquaternium-7 (PQ) was synthesized using a facile solvent evaporation and immersion method and developed as an anode (named FePor-PQ) in MFCs. The surface structural characterizations confirmed the successful introduction of N and Fe atoms, whereas FePor-PQ achieved the N content of 9.59 at %, which may offer various active sites for EET. The introduction of PQ contributed to improving the surface hydrophilicity, providing the composite electrode good biocompatibility for bacterial attachment and colonization as well as substrate diffusion. Based on the advantages, the MFC with the FePor-PQ anode produced a maximum power density of 2165.7 mW m-2, strikingly higher than those of CC (1124.0 mW m-2), PQ (1668.8 mW m-2), and FePor (1978.9 mW m-2). Furthermore, with the EET mediated by the binding of flavins and c-type cytochromes on the outer membrane was enhanced prominently, the typical exoelectrogen Geobacter was enriched up to 55.84% in the FePor-PQ anode biofilm. This work reveals a synergistic effect from heteroatom coating and surface properties tailoring to boost both the EET efficiency and exoelectrogen enrichment for enhancing MFC performance, which also provides valuable insights for designing electrodes in other bio-electrochemical systems.

Semiconducting Minerals Participated Extracellular Electron Transfer Process in High-Altitude Red Soil from Gansu, China

The microbial extracellular electron transfer (EET) process plays an important role in biogeochemistry. Here, semiconducting minerals that participated in EET in high-altitude red soil were investigated for the first time. Semiconducting minerals and microbial communities were analyzed by XRD and 16S rRNA, respectively. The photoresponse and semiconducting mineral properties of red soil were characterized by linear sweep voltammetry (LSV) and I–t curves, and well synchronous light-response properties were observed. Then, the EET process without light was explored in a dual-chamber system, the maximum power density of red soil cathode was nearly two times higher than graphite cathode, indicated that mineral serving as electron acceptor. Under light irradiation, the highest photo-current density (3.939 μA/cm2) was observed between red soil electrode and live bacteria, indicating the microbial EET process was significantly enhanced. Furthermore, electrochemical impedance spectroscopy (EIS) was employed to investigate the mechanism, results showed that the interface charge transfer resistance (Rct) of red soil electrode was obviously reduced from 396 to 316 Ω under light conditions. This study demonstrated that semiconducting minerals could participate microbial EET process via sunlight catalysis in high-altitude environments.

Silver nanoparticles boost charge-extraction efficiency in Shewanella microbial fuel cells.

Microbial fuel cells (MFCs) can directly convert the chemical energy stored in organic matter to electricity and are of considerable interest for power generation and wastewater treatment. However, the current MFCs typically exhibit unsatisfactorily low power densities that are largely limited by the sluggish transmembrane and extracellular electron-transfer processes. Here, we report a rational strategy to boost the charge-extraction efficiency in Shewanella MFCs substantially by introducing transmembrane and outer-membrane silver nanoparticles. The resulting Shewanella-silver MFCs deliver a maximum current density of 3.85 milliamperes per square centimeter, power density of 0.66 milliwatts per square centimeter, and single-cell turnover frequency of 8.6 × 105 per second, which are all considerably higher than those of the best MFCs reported to date. Additionally, the hybrid MFCs feature an excellent fuel-utilization efficiency, with a coulombic efficiency of 81%.

Different performance of pyrene biodegradation on metal-modified montmorillonite: Role of surface metal ions from a bioelectrochemical perspective

Microbial extracellular electron transfer (EET) at microbe-mineral interface has been reported to play a significant role in pollutant biotransformation. Different metals often co-exist with organic pollutants and are immobilized on mineral surfaces. However, little is known about the influence of mineral surface metal ions on organic pollutant biodegradation and the involved electron transfer mechanism. To address this knowledge gap, pyrene was used as a model compound to investigate the biodegradation of polycyclic aromatic hydrocarbon on montmorillonite mineral saturated with metal ions (Na(I), Ni(II), Co(II), Cu(II) and Fe(III)) by Mycobacteria strain NJS-1. Further, the possible underlying electron transfer mechanism by electrochemical approaches was investigated. The results show that pyrene biodegradation on montmorillonite was markedly influenced by surface metal ions, with degradation efficiency following the order Fe(III) > Na(I) ≈ Co(II) > Ni(II) ≈ Cu(II). Bioelectrochemical analysis showed that electron transfer activities (i.e., electron donating capacity and electron transport system activity) varied in different metal-modified montmorillonites and were closely related to pyrene biodegradation. Fe(III) modification greatly stimulated degrading enzyme activities (i.e., peroxidase and dioxygenase) and electron transfer activities resulting in enhanced pyrene biodegradation, which highlights its potential as a technique for pollutant bioremediation. The bacterial extracellular protein and humic substances played important roles in EET processes. Membrane-bound cytochrome C protein and extracellular riboflavin were identified as the electron shuttles responsible for transmembrane and cross extracellular matrix electron transfer, respectively. Additions of exogenetic electron mediators of riboflavin, humic acid and potassium ferricyanide accelerated pyrene biodegradation which further verified the critical role of EET in PAH transformation at bacteria-mineral interfaces. These results support the development of clay mineral based advanced bioremediation techniques through regulating the electron transfer processes at the microbe-mineral interfaces by mineral surface modification.

The performance of a cyanobacterial biomass-based microbial fuel cell (MFC) inoculated with Shewanella oneidensis MR-1

Cyanobacterial biomass is a rich renewable resource for direct bioelectricity generation in biomass-based microbial fuel cells (MFCs). In this study, the performance of a double chamber MFC with Anabaena vaginicola cyanobacterial biomass as the anodic substrate was investigated. The anode chamber was inoculated with a pure culture of Shewanella oneidensis MR-1, and different MFC experiments using pretreated and untreated cyanobacterial biomass substrate with 5 or 20 mg/L riboflavin were conducted. Pretreatment of the biomass and addition of 20 mg/L riboflavin led to the highest current density (366 mA/m3), maximum power density (144 mW/m3), chemical oxygen demand removal efficiency (65%), and coulombic efficiency (5.7%). Increasing the concentration of riboflavin improved bioelectricity generation (1.4–1.7-fold increase in maximum power density), confirming its effectiveness as a redox mediator, and its role in the extracellular electron transfer process of the S. oneidensis MR-1 biofilm. Moreover, pretreating the biomass also enhanced the conversion efficiency of biomass to bioelectricity. This indicated that pretreatment of the biomass influenced the utilization of biomass by S. oneidensis MR-1 cells. These findings can help increasing the potential of using renewable substrates for more efficient bioenergy generation in pure culture MFCs.

The role of extracellular polymeric substance matrix on Saccharomyces cerevisiae bioelectricity

Saccharomyces cerevisiae is one of the first microorganism established to be able to convert chemical energy into electricity, however its extracellular electron transfer (EET) mechanism was still unclear. Here, we show how yeast strains extracellularly transfer electron to an electrode surface. We observed yeast cells adsorption on solid surfaces occurs through the extracellular polymeric substance (EPS) network, and EET process takes place by redox species mostly confined on the electrode surface. We demonstrated that a flavoprotein is present in the Saccharomyces cerevisiae´s EPS and is responsible by non-diffusive extracellular electron transfer through the EPS electroactive microenvironment. It is proposed EPS acts as a polyelectrolyte for multistep electron-transfer between flavoproteins and consequently for the Saccharomyces cerevisiae EET and bioelectricity generation. The elucidation of this mechanism combined to electrode engineering can contribute to the advance of biosystems in green energy generation, such as microbial fuels.

Tuning Redox Potential of Anthraquinone-2-Sulfonate (AQS) by Chemical Modification to Facilitate Electron Transfer From Electrodes in Shewanella oneidensis.

Bioelectrochemical systems (BESs) are emerging as attractive routes for sustainable energy generation, environmental remediation, bio-based chemical production and beyond. Electron shuttles (ESs) can be reversibly oxidized and reduced among multiple redox reactions, thereby assisting extracellular electron transfer (EET) process in BESs. Here, we explored the effects of 14 ESs on EET in Shewanella oneidensis MR-1, and found that anthraquinone-2-sulfonate (AQS) led to the highest cathodic current density, total charge production and reduction product formation. Subsequently, we showed that the introduction of -OH or -NH2 group into AQS at position one obviously affected redox potentials. The AQS-1-NH2 exhibited a lower redox potential and a higher Coulombic efficiency compared to AQS, revealing that the ESs with a more negative potential are conducive to minimize energy losses and improve the reduction of electron acceptor. Additionally, the cytochromes MtrA and MtrB were required for optimal AQS-mediated EET of S. oneidensis MR-1. This study will provide new clues for rational design of efficient ESs in microbial electrosynthesis.

Electrochemically Active Biofilms as an Indicator of Soil Health

Soil health is a complex phenomenon that reflects the ability of soil to support both plant growth and other ecosystem functions. To our knowledge, research on extracellular electron transfer processes in soil environments is limited and could provide novel knowledge and new ways of monitoring soil health. Electrochemical activities in the soil can be studied by inserting inert electrodes. Once the electrode is polarized to a favorable potential, nearby microorganisms attach to the electrodes and grow as biofilms. Biofilms are a major part of the soil and play critical roles in microbial activity and community dynamics. Our work aims to investigate the electrochemical behavior of healthy and unhealthy soils using chronoamperometry and cyclic voltammetry. We developed a bioelectrochemical soil reactor for electrochemical measurements using healthy and unhealthy soils taken from the Cook Agronomy Farm Long-Term Agroecological Research site; the soils showed similar physical and chemical characteristics, but there was higher plant growth where the healthy soil was taken. Using carbon cloth electrodes installed in these soil reactors, we explored the electrochemical signals in these two soils. First, we measured redox variations by depth and found that reducing conditions were prevalent in healthy soils. Current measurements showed distinct differences between healthy and unhealthy soils. Scanning electron microscopy images showed the presence of microbes attached to the electrode for healthy soil but not for unhealthy soil. Glucose addition stimulated current in both soil types and caused differences in cyclic voltammograms between the two soil types to converge. Our work demonstrates that we can use current as a proxy for microbial metabolic activity to distinguish healthy and unhealthy soil.

Characterization of methanolic extract of seaweeds as environmentally benign corrosion inhibitors for mild steel corrosion in sodium chloride environment

Biocorrosion is an aggregation of microbes on the metal surface that induces ion degradation by oxidation and reduction of chemical and electrochemical reactions. Extracellular electron transfer (EET) process was a significant phenomenon in ion deterioration. To block the EET process, the inhibitor was used to destroy or prevent extracellular electron transfer during microbial metabolism. In the present study, methanolic extraction of two different seaweeds acts as a bio-inhibitor against the corrosive biofilm forming bacteria B.megaterium SKR7 on the mild steel. The optimized concentration of biofilm inhibition efficiency of methanolic extract P.pavonica (MEPP) and S.tenerrimum (MEST) was found to be 75% and 73% at 25 ppm respectively. It was confirmed by assay of antimicrobial activity and weight loss measurement which is represented the reduction of corrosion rate S3 (A) 0.355 mm/y and S3 (B) 0.442 mm/y of both seaweed extracts than control (1.598 mm/y). This observation was confirmed by the electrochemical studies by the significant decrease in charge transfer resistance (Rct) and the surface analysis of FTIR and SEM analysis were confirmed the seaweeds inhibit the biofilm. MEPP and MEST revealed the existence of organic and bioactive compounds like N, S, O blocks the electron transfers during the biofilm formation on a metal surface and thus inhibits corrosion rate. The study concluded seaweed extracts act as mixed-type of inhibitors, thus effectivley inhibit the biofilm formation and are easily accessible at low cost, eco-friendly.

MXene@Poly(diallyldimethylammonium chloride) Decorated Carbon Cloth for Highly Electrochemically Active Biofilms in Microbial Fuel Cells

AbstractThe anode characteristics are important to the performance of microbial fuel cells (MFCs) due to the influence on biofilm formation and extracellular electron transfer (EET). Herein, MXene was modified by the superhydrophilic cationic polymer poly(diallyldimethylammonium chloride) (PDDA) to prepare MXene@PDDA/carbon cloth (CC) as an anode for MFC. The unique anode combines the advantages of high conductivity, superhydrophilicity and biocompatibility. The anode facilitated the enrichment of electrochemical active bacterium and promoted the efficiency of EET process. MFCs based on MXene@PDDA/CC anodes can produce the remarkable power output and the satisfactory effluent removal efficiency through an external series resistance of 1 kΩ. The device can gain the maximum output voltage of 585 mV and the maximum power density of 811 mW m−2. The efficiencies of methyl orange decolorization and chemical oxygen demand were up to 79 % and 84 % after 12 h, respectively.

Enhancing the treatment of petrochemical wastewater using redox mediator suspended biofilm carriers

The treatment efficiency of petrochemical wastewater via the traditional activated sludge process is generally unsatisfactory due to the complex and toxic features of petroleum compounds. Redox mediator (RM) can improve the biodegradation of organic contaminants by participating in the microbial extracellular electron transfer process. However, the continuous dosing of the soluble redox mediator would lead to secondary pollution and rise operation cost. In this study, an insoluble and economical redox mediator material (biochar) was introduced into high-density polyethylene (HDPE) to prepare suspended biofilm carriers (RM-modified carriers), and applied in anaerobic/oxic (A/O) reactors. The reactor filled with RM-modified carriers (R3) showed the removal efficiency of chemical oxygen demand (COD) and phenol in the anaerobic zone were 34.6 % and 43.0 % respectively at organic loading rates (OLRs) of 22.2 kg COD/(m3·d), which increased by 6.8 % and 8.3 % respectively compared with that of the reactor filled with conventional HDPE carriers (R2), and increased by 12.4 % and 17.7 % respectively compared with that of the reactor without filled with carriers (R1). Microbial analysis proved that the relative abundance of Bacteroides that participated in electron transfer of R3 improved by approximately 6.3 % compared with that of R2. These findings demonstrated that RM-modified carriers improved the abundance of relevant functional bacteria and enhanced the degradation efficiency of the contaminants in petrochemical wastewater.

Deciphering Single-Bacterium Adhesion Behavior Modulated by Extracellular Electron Transfer.

For bacterial adhesion and biofilm formation, a thorough understanding of the mechanism and effective modulating is lacking due to the complex extracellular electron transfer (EET) at bacteria-surface interfaces. Here, we explore the adhesion behavior of a model electroactive bacteria under various metabolic conditions by an integrated electrochemical single-cell force microscopy system. A nonlinear model between bacterial adhesion force and electric field intensity is established, which provides a theoretical foundation for precise tuning of bacterial adhesion strength by the surface potential and the direction and flux of electron flow. In particular, based on quantitative analyses with equivalent charge distribution modeling and wormlike chain numerical simulations, it is demonstrated that the chain conformation and unfolding events of outer membrane appendages are dominantly impacted by the dynamic bacterial EET processes. This reveals how the anisotropy of bacterial conductive structure can translate into the desired adhesion behavior in different scenarios.

Effects and mechanisms of modified biochars on microbial iron reduction of Geobacter sulfurreducens

Biochar was proved as an electron shuttle to facilitate extracellular electron transfer (EET) of electrochemically active bacteria (EAB); however, its underlying mechanism was not fully understood. In this study, we aimed to further explore how the regulation of surface functional groups of biochar would affect the microbial iron reduction process of Geobacter sulfurreducens as a typical EAB. Two modified biochars were achieved after HNO3 (NBC) and NaBH4 (RBC) pretreatments, and a control biochar was produced after deionized water (WBC) washing. Results showed that WBC and RBC significantly accelerated microbial iron reduction of G. sulfurreducens PCA, while had no effect in the final Fe (II) minerals (e.g., vivianite and green rust (CO32−)). Besides, Brunauer-Emmett-Teller (BET) surface area, electron spin resonance (ESR) and electrochemical measurements showed that larger surface area, lower redox potential, and more redox-active groups (e.g., aromatic structures and quinone/hydroquinone moieties) in RBC explained its better electron transfer performance comparing to WBC. Interestingly, NBC completely suppressed the Fe (III) reduction process, mainly due to the production of reactive oxygen species which inhibited the growth of G. sulfurreducens PCA. Overall, this work paves a feasible way to regulate the surface functional groups for biochar, and comprehensively revealed its effect on EET process of microorganisms.

Enhanced mechanism of extracellular electron transfer between semiconducting minerals anatase and Pseudomonas aeruginosa PAO1 in euphotic zone

Focusing the marine euphotic zone, which is the pivotal region for interaction of solar light-mineral-microorganism and the elements cycle, we have conducted the research on the mechanism of semiconducting minerals promoting extracellular electron transfer with microorganisms in depth. Therein, anatase which is one of the most representative semiconducting minerals in marine euphotic zone was selected. The mineralogical characterization of anatase was identified by ESEM, AFM, EDS, Raman, XRD, and its semiconducting characteristics was determined by UV–Vis and Mott-Schottky plots. Determined by the electrochemical measurement of I-t curves, the photocurrent density of anatase was more prominent than dark current density. Pseudomonas aeruginosa PAO1 was widely distributed in the euphotic zone, and its mutants of operons deficient in biosynthesis pyocyanin (Δphz1Δphz2) and pili deficient (ΔpilA) were employed in this study. I-t curves indicated that both direct and indirect extracellular electron transfer processes occurred between anatase and PAO1. The indirect electron transfer depending on pyocyanin secreted by PAO1 was the main electron transfer mode. This work demonstrated the light-driven extracellular electron transfer and further revealed the photo-catalyzed mechanisms between anatase and PAO1 in marine euphotic zone.

Prompting the FDH/Hases-based electron transfers during Pt(IV) reduction mediated by bio-Pd(0)

Due to the excellent hydrogen affinity and high conductivity, palladium nanoparticles (Pd NPs) were considered as a potential strategy to regulate bacterial electron transfer and energy metabolism. Herein, Citrobacter freundii JH, capable of in-situ biosynthesizing Pd(0) NPs, was employed to promote Pt(IV) reduction. The results showed that the Pt(IV) reduction to Pt(II) was accomplished mainly via the flavins-mediated extracellular electron transfer (EET) process, while Pt(II) reduction to Pt(0) was limit step, and proceeded via two intracellular respiratory chains, including FDH/Hases-based short chain (S-chain) and typical CoQ-involved long respiratory chain (L-chain). Noteworthily, the incorporation of Pd(0) NPs mainly diverted the electrons to S-chain (as high as 71.7%−73.4%) by improving the hydrogenases (Hases) activity. Furthermore, Pd(0) NPs could stimulate the secreting of flavins and the combination between flavins and cytochrome c (c-Cyt), which converted electron transfer manner of L-chain. Additionally, Pd(0) NPs might also act as alternative proton channels to improve the energy metabolism. These findings provided significant insights into the promotion by Pd(0) NPs in terms of electron generation, electron consumption and proton translocation.

Crossing the Wall: Characterization of the Multiheme Cytochromes Involved in the Extracellular Electron Transfer Pathway of Thermincola ferriacetica.

Bioelectrochemical systems (BES) are emerging as a suite of versatile sustainable technologies to produce electricity and added-value compounds from renewable and carbon-neutral sources using electroactive organisms. The incomplete knowledge on the molecular processes that allow electroactive organisms to exchange electrons with electrodes has prevented their real-world implementation. In this manuscript we investigate the extracellular electron transfer processes performed by the thermophilic Gram-positive bacteria belonging to the Thermincola genus, which were found to produce higher levels of current and tolerate higher temperatures in BES than mesophilic Gram-negative bacteria. In our study, three multiheme c-type cytochromes, Tfer_0070, Tfer_0075, and Tfer_1887, proposed to be involved in the extracellular electron transfer pathway of T. ferriacetica, were cloned and over-expressed in E. coli. Tfer_0070 (ImdcA) and Tfer_1887 (PdcA) were purified and biochemically characterized. The electrochemical characterization of these proteins supports a pathway of extracellular electron transfer via these two proteins. By contrast, Tfer_0075 (CwcA) could not be stabilized in solution, in agreement with its proposed insertion in the peptidoglycan wall. However, based on the homology with the outer-membrane cytochrome OmcS, a structural model for CwcA was developed, providing a molecular perspective into the mechanisms of electron transfer across the peptidoglycan layer in Thermincola.

Production of bioelectricity may play an important role for the survival of Xanthomonas campestris pv. campestris (Xcc) under anaerobic conditions

The plant pathogen Xanthomonas is commonly found in biocontaminated bioreactors; however, few studies have evaluated the growth and impacts of this microorganism on bioreactors. In this study, we examined the characteristics of Xanthomonas campestris pv. campestris (Xcc). Our results showed that Xcc could reduce metal Fe (III) and decolorise methyl orange in vitro. Moreover, I-t and cyclic voltammetry curves showed that Xcc could generate bioelectricity and had two extracellular electron transfer pathways, similar to that of Shewanella. Based on the spectral analysis of intact cells and scanning electron microscopy analysis, one pathway was speculated to involve cytochrome C by direct contact with the pili or cell surface. The other pathway may involve indirect mediators, such as redox substrates, among extracellular polymeric substances. For the direct extracellular electron transfer process, the charge transfer coefficient α, electron number n, and the electron transfer rate constant ks were determined to be 0.49, 2.6, and 2.2 × 10−3 s−1, respectively. In the indirect extracellular electron transfer processes, the values of α, n, and ks were 0.52, 4, and 1.21 s−1, respectively. Of these two transfer methods, indirect electron transfer is dominant and faster than direct electron transfer. Moreover, after mutation of the dsbD gene, which is important for indirect electron transfer, the electrochemical parameters α, n, and ks decreased. Our findings reveal a new anaerobic mechanism mediating the survival of Xcc during wastewater treatment, and may help develop new strategies for preventing Xcc growth during wastewater treatment.

Mini-metagenome analysis of psychrophilic electroactive biofilms based on single cell sorting

Understanding the metabolic function of psychrophilic electroactive bacteria is important for the investigation of extracellular electron transfer (EET) mechanisms under low temperatures (4–15 °C). In this study, Raman activated cell ejection coupled high throughput sequencing was used to accurately generate a mini-metagenome of psychrophilic bacterial community. Hierarchical cluster analysis of the Raman spectrum could accurately select the target Geobacter cluster. The high relative abundance of the membrane transport functional genes ftsEX in the biofilm community indicated an adaptation to reduced temperature, which aided survival of the electroactive bacteria under low temperature. The basal metabolism such as citrate cycle and glycolytic pathway maintained the electron pool for the EET process. The identification of iron (III) transport system genes in high abundance indicated their presence in an active metabolic reaction for potential electron transfer process. It showed the potential involvement c-type cytochromes (coxA and cox1) activity in EET. These results indicated that psychrophilic Geobacter had effective EET mediated by c-type cytochromes at low temperatures.

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces.

Electrochemically active bacteria can transport their metabolically generated electrons to anodes, or accept electrons from cathodes to synthesize high-value chemicals and fuels, via a process known as extracellular electron transfer (EET). Harnessing of this microbial EET process has led to the development of microbial bio-electrochemical systems (BESs), which can achieve the interconversion of electrical and chemical energy and enable electricity generation, hydrogen production, electrosynthesis, wastewater treatment, desalination, water and soil remediation, and sensing. Here, the focus is on the current understanding of the microbial EET process occurring at both the bacteria-electrode interface and the biotic interface, as well as some attempts to improve the EET by using various nanomaterials. The behavior of nanomaterials in different EET routes and their influence on the performance of BESs are described. The inherent mechanisms will guide rational design of EET-related materials and lead to a better understanding of EET mechanisms.