Microbial nanowires: a new paradigm for biological electron transfer and bioelectronics

Electroactive biofilm communities in microbial fuel cells for the synergistic treatment of wastewater and bioelectricity generation

Increasing industrialization and urbanization have contributed to a significant rise in wastewater discharge and exerted extensive pressure on the existing natural energy resources. Microbial fuel cell (MFC) is a sustainable technology that utilizes wastewater for electricity generation. MFC comprises a bioelectrochemical system employing electroactive biofilms of several aerobic and anaerobic bacteria, such as Geobacter sulfurreducens, Shewanella oneidensis, Pseudomonas aeruginosa, and Ochrobacterum pseudiintermedium. Since the electroactive biofilms constitute a vital part of the MFC, it is crucial to understand the biofilm-mediated pollutant metabolism and electron transfer mechanisms. Engineering electroactive biofilm communities for improved biofilm formation and extracellular polymeric substances (EPS) secretion can positively impact the bioelectrochemical system and improve fuel cell performance. This review article summarizes the role of electroactive bacterial communities in MFC for wastewater treatment and bioelectricity generation. A significant focus has been laid on understanding the composition, structure, and function of electroactive biofilms in MFC. Various electron transport mechanisms, including direct electron transfer (DET), indirect electron transfer (IET), and long-distance electron transfer (LDET), have been discussed. A detailed summary of the optimization of process parameters and genetic engineering strategies for improving the performance of MFC has been provided. Lastly, the applications of MFC for wastewater treatment, bioelectricity generation, and biosensor development have been reviewed.

Microbial Nanotechnology for Precision Nanobiosynthesis: Innovations, Current Opportunities and Future Perspectives for Industrial Sustainability

A new area of biotechnology is nanotechnology. Nanotechnology is an emerging field that aims to develope various substances with nano-dimensions that have utilization in the various sectors of pharmaceuticals, bio prospecting, human activities and biomedical applications. An essential stage in the development of nanotechnology is the creation of nanoparticles. To increase their biological uses, eco-friendly material synthesis processes are becoming increasingly important. Recent years have shown a lot of interest in nanostructured materials due to their beneficial and unique characteristics compared to their polycrystalline counterparts. The fascinating performance of nanomaterials in electronics, optics, and photonics has generated a lot of interest. An eco-friendly approach of creating nanoparticles has emerged in order to get around the drawbacks of conventional techniques. Today, a wide range of nanoparticles have been created by employing various microbes, and their potential in numerous cutting-edge technological fields have been investigated. These particles have well-defined chemical compositions, sizes, and morphologies. The green production of nanoparticles mostly uses plants and microbes. Hence, the use of microbial nanotechnology in agriculture and plant science is the main emphasis of this review. The present review highlights the methods of biological synthesis of nanoparticles available with a major focus on microbially synthesized nanoparticles, parameters and biochemistry involved. Further, it takes into account the genetic engineering and synthetic biology involved in microbial nanobiosynthesis to the construction of microbial nanofactories.

To be or not to be a cytochrome: electrical characterizations are inconsistent with Geobacter cytochrome 'nanowires'

Geobacter sulfurreducens profoundly shapes Earth's biogeochemistry by discharging respiratory electrons to minerals and other microbes through filaments of a two-decades-long debated identity. Cryogenic electron microscopy has revealed filaments of redox-active cytochromes, but the same filaments have exhibited hallmarks of organic metal-like conductivity under cytochrome denaturing/inhibiting conditions. Prior structure-based calculations and kinetic analyses on multi-heme proteins are synthesized herein to propose that a minimum of ~7 cytochrome 'nanowires' can carry the respiratory flux of a Geobacter cell, which is known to express somewhat more (≥20) filaments to increase the likelihood of productive contacts. By contrast, prior electrical and spectroscopic structural characterizations are argued to be physiologically irrelevant or physically implausible for the known cytochrome filaments because of experimental artifacts and sample impurities. This perspective clarifies our mechanistic understanding of physiological metal-microbe interactions and advances synthetic biology efforts to optimize those interactions for bioremediation and energy or chemical production.

Low intensity magnetic separation of vivianite induced by iron reduction on the surface layer of Fe(III)[Fe(0)] iron scrap

Phosphorus (P) recovery through vivianite, which can be found in activated sludge, surplus sludge and digested sludge in the wastewater treatment plants (WWTPs), is a cutting-edge and efficient technology in recent years. However, how to generate and separate vivianite in an effective and economical way with natural iron oxide mineral was still the bottleneck to limit its application. Therefore, in this study, the P recovery efficiency (EP) and vivianite recovery efficiency (EV) of three kinds of iron oxides were investigated. We found that the EP of Akaganeite was 1.83 times and 4.88 times higher than that of Geothite and Hematite. Simultaneously, EV of Akaganeite was 1.64 times and 2.88 times higher than that of Geothite and Hematite. As Akaganeite is main component of rust on the surface of iron scrap, we used Fe(III)[Fe(0)] iron scrap with Fe(0) inside and Akaganeite outside as iron source and electron acceptor for vivianite production and magnetic separation. At the terminal stage (60 day), the P recovery efficiency with 20 g/L Fe(III)[Fe(0)] iron scrap was 36%. Applying a magnetical separator with magnetic field intensity of 0.3 T, vivianite was separated from the solution efficiently and immediately. Low intensity magnetic separation with iron scrap would recover P resources economically with the total cost to be $2.23/kg P, which was much lower than recovery via iron salts. Besides, it provided a significant insights into the P recovery and vivianite separation by reusing Fe waste during wastewater treatment.

Sludge source-redox mediators obtainment and availability for enhancing bioelectrogenesis and acidogenesis: Deciphering characteristics and mechanisms

Anaerobic biological treatment was regarded as one of promising options for realizing concurrent WAS reduction, stabilization and bioenergy/bioresource recycle. But the relatively low treatment efficiency limited its spreading application toward larger scale considerably in China. Aimed at such barrier, this study offered a novel enhancing strategy for achieving high-efficiency of bioenergy/bioresource recycle from WAS anaerobic treatment via improving bioelectrogenesis/acidogenesis using sludge source-redox mediators (SSRMs). SSRMs not only facilitated bioeletrogenesis with an increasing efficiency of 36% for voltage output and 39% for bioelectricity bioconversion, but also enhanced acidogenesis of WAS with a mean elevating efficiency of 37.5% of volatile fatty acids (VFAs) production within 5 d Mechanistic investigations indicated that SSRMs had a potential influence on improving the protein and carbohydrate metabolisms-related genes' expression for enhancing bioelectrogenesis and acidogenesis. Moreover, SSRMs exerted roles of electrochemical "catalysts" or as terminal electron acceptors with affecting functional proteins of complexes of Ⅰ and Ⅳ in electron transfer chains for improving electron transfer efficiency. Meanwhile, the core members' abundance, microbial diversity and community distributive evenness were prompted concurrently for carrying out superior bioelectrogenesis and acidogenesis. A schematic illustration was established for demonstrating the mechanism of SSRMs for enhancing bioelectrogenesis and acidogenesis via changing microbial metabolism functions, enhancing electron transfer efficiency, and regulating functional genes' expression of functional proteins (up-regulating cytochrome c oxidase and down-regulating-NADH dehydrogenase). This study provided an effective enhancing strategy for facilitating WAS bioconversion to bioenergy/bioresource with well-process sustainability.

Microbial nanowires: type IV pili or cytochrome filaments?

A dynamic field of study has emerged involving long-range electron transport by extracellular filaments in anaerobic bacteria, with Geobacter sulfurreducens being used as a model system. The interest in this topic stems from the potential uses of such systems in bioremediation, energy generation, and new bio-based nanotechnology for electronic devices. These conductive extracellular filaments were originally thought, based upon low-resolution observations of dried samples, to be type IV pili (T4P). However, the recently published atomic structure for the T4P from G. sulfurreducens, obtained by cryo-electron microscopy (cryo-EM), is incompatible with the numerous models that have been put forward for electron conduction. As with all high-resolution structures of T4P, the G. sulfurreducens T4P structure shows a partial melting of the α-helix that substantially impacts the aromatic residue positions such that they are incompatible with conductivity. Furthermore, new work using high-resolution cryo-EM shows that conductive filaments thought to be T4P are actually polymerized cytochromes, with stacked heme groups forming a continuous conductive wire, or extracellular DNA. Recent atomic structures of three different cytochrome filaments from G. sulfurreducens suggest that such polymers evolved independently on multiple occasions. The expectation is that such polymerized cytochromes may be found emanating from other anaerobic organisms.

The material-microorganism interface in microbial hybrid electrocatalysis systems

This review presents a comprehensive summary of the material-microorganism interface in microbial hybrid electrocatalysis systems. Microbial hybrid electrocatalysis has been developed to combine the advantages of inorganic electrocatalysis and microbial catalysis. However, electron transfer at the interfaces between microorganisms and materials is a very critical issue that affects the efficiency of the system. Therefore, this review focuses on the electron transfer at the material-microorganism interface and the strategies for building efficient microorganism and material interfaces. We begin with a brief introduction of the electron transfer mechanism in both the bioanode and biocathode of bioelectrochemical systems to understand the material-microorganism interface. Next, we summarise the strategies for constructing efficient material-microorganism interfaces including material design and modification and bacterial engineering. We also discuss emerging studies on the bio-inorganic hybrid electrocatalysis system. Understanding the interface between electrode/active materials and the microorganisms, especially the electron transfer processes, could help to drive the evolution of material-microorganism hybrid electrocatalysis systems towards maturity.

A laboratory experiment for science courses: Sedimentary microbial fuel cells

Nowadays there is a concern to improve the quality of education by including an interdisciplinary approach of concepts and their integration in the curriculum of scientific disciplines. The development of microbial fuel cells as a potential alternative for production of renewable energies gives undergraduate students the challenge of integrating interdisciplinary concepts in a hot topic of global interest as alternative energies. We present a laboratory experiment that has been part of a third-year undergraduate course in biology where students gained experience in assembling microbial fuel cells and the understanding of how they work. In this process, the students could integrate biological, biochemical, and electric concepts. In addition, the acquisition of manual skills and experimental design decisions are important for the development of future professionals.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Structure of Geobacter cytochrome OmcZ identifies mechanism of nanowire assembly and conductivity

OmcZ nanowires produced by Geobacter species have high electron conductivity (>30 S cm-1). Of 111 cytochromes present in G. sulfurreducens, OmcZ is the only known nanowire-forming cytochrome essential for the formation of high-current-density biofilms that require long-distance (>10 µm) extracellular electron transport. However, the mechanisms underlying OmcZ nanowire assembly and high conductivity are unknown. Here we report a 3.5-Å-resolution cryogenic electron microscopy structure for OmcZ nanowires. Our structure reveals linear and closely stacked haems that may account for conductivity. Surface-exposed haems and charge interactions explain how OmcZ nanowires bind to diverse extracellular electron acceptors and how organization of nanowire network re-arranges in different biochemical environments. In vitro studies explain how G. sulfurreducens employ a serine protease to control the assembly of OmcZ monomers into nanowires. We find that both OmcZ and serine protease are widespread in environmentally important bacteria and archaea, thus establishing a prevalence of nanowire biogenesis across diverse species and environments.

Ultrastrong Carbon Nanotubes-Copper Core-Shell Wires with Enhanced Electrical and Thermal Conductivities as High-Performance Power Transmission Cables

Demands for high-performance electrical power transmission cables continue to rise, especially for offshore power transmission, electric vehicles, portable electronics, and deployable military applications. Carbon nanotubes (CNTs)-Copper (Cu) core-shell wire is regarded as one of the best candidate material systems for transmitting electricity and thermal energy. In this study, a facile and robust approach was developed to enhance the CNT-Cu interfacial interactions. This approach consists of a substrate-enhanced electroless deposition step for Cu pre-seeding and thiol functionalization. Benefiting from the thiol-activated CNT surface and Cu seed deposit, the CNTs-Cu core-shell wire forms a densely packed Cu shell with a void-free CNT-Cu interface. Consequently, the CNTs-Cu core-shell wire possesses (1) superior specific strength (eightfold stronger), (2) 30% higher specific conductivity, (3) 120% higher specific ampacity, and (4) an impressive 110% higher thermal conductivity compared with pure Cu wires. Moreover, this composite wire still maintains its structural integrity and electrical properties over 600 cycles of the fatigue bending test, rendering this system an excellent candidate for high-performance electrical cable and conductor applications.

Sprayable biofilm – Agarose hydrogels as 3D matrix for enhanced productivity in bioelectrochemical systems

Bio-based energy production utilizing renewable resources can be realized by exoelectrogenic organisms and their application in bioelectrochemical systems (BES). These organisms catalyze the direct conversion of chemical into electrical energy and are already widely used in bioelectronics and biosensing. However, the biofilm-electrode interaction is a factor that limits sufficient space-time-yields for industrial applications. In this study, a hydrogel matrix consisting of agarose fibers was utilized as a scaffold for S. oneidensis cells to improve anodic processes in BES. This synthetic, scalable biofilm reached a higher current density in BES in comparison to naturally formed biofilms. Complemented with carbon nanofibers and riboflavin, the application of this functionalized hydrogel containing S. oneidensis cells led to an overall 9.1-fold increase in current density to 1324 mA m⁻² in comparison to 145 mA m⁻² for the planktonic control. In addition, the synthetic biofilm can be applied by spraying onto surfaces using a novel spray applicator. The latter allows to apply the biofilm effortless on large surfaces which will facilitate scalability and thus industrial application.

Enhancing Microbial Electron Transfer Through Synthetic Biology and Biohybrid Approaches: Part II : Combining approaches for clean energy

It is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa by harnessing the metabolic processes of bacteria to valorise a range of waste products including greenhouse gases (GHGs). Part I (1) introduced the EET pathways, their limitations and applications. Here in Part II, we outline the strategies researchers have used to modulate microbial electron transfer, through synthetic biology and biohybrid approaches and present the conclusions and future directions.

Microbial biofilms as living photoconductors due to ultrafast electron transfer in cytochrome OmcS nanowires

Light-induced microbial electron transfer has potential for efficient production of value-added chemicals, biofuels and biodegradable materials owing to diversified metabolic pathways. However, most microbes lack photoactive proteins and require synthetic photosensitizers that suffer from photocorrosion, photodegradation, cytotoxicity, and generation of photoexcited radicals that are harmful to cells, thus severely limiting the catalytic performance. Therefore, there is a pressing need for biocompatible photoconductive materials for efficient electronic interface between microbes and electrodes. Here we show that living biofilms of Geobacter sulfurreducens use nanowires of cytochrome OmcS as intrinsic photoconductors. Photoconductive atomic force microscopy shows up to 100-fold increase in photocurrent in purified individual nanowires. Photocurrents respond rapidly (<100 ms) to the excitation and persist reversibly for hours. Femtosecond transient absorption spectroscopy and quantum dynamics simulations reveal ultrafast (~200 fs) electron transfer between nanowire hemes upon photoexcitation, enhancing carrier density and mobility. Our work reveals a new class of natural photoconductors for whole-cell catalysis.

Despite enormous potential of solar-driven biocatalysis, most living systems lack photoactive proteins and require toxic and expensive synthetic materials limiting the performance. Here, a class of natural photoconductors is demonstrated through sub-picosecond heme-to-heme electron transfer in bacteria-produced protein nanowires.

Improvement of Direct Interspecies Electron Transfer via Adding Conductive Materials in Anaerobic Digestion: Mechanisms, Performances, and Challenges

Anaerobic digestion is an effective and sustainable technology for resource utilization of organic wastes. Recently, adding conductive materials in anaerobic digestion to promote direct interspecies electron transfer (DIET) has become a hot topic, which enhances the syntrophic conversion of various organics to methane. This review comprehensively summarizes the recent findings of DIET mechanisms with different mediating ways. Meanwhile, the influence of DIET on anaerobic digestion performance and the underlying mechanisms of how DIET mediated by conductive materials influences the lag phase, methane production, and system stability are systematically explored. Furthermore, current challenges such as the unclear biological mechanisms, influences of non-DIET mechanisms, limitations of organic matters syntrophically oxidized by way of DIET, and problems in practical application of DIET mediated by conductive materials are discussed in detail. Finally, the future research directions for practical application of DIET are outlined.

Effect of Contact Area and Shape of Anode Current Collectors on Bacterial Community Structure in Microbial Fuel Cells

Low electrical conductivity of carbon materials is a source of potential loss for large carbonaceous electrode surfaces of MFCs due to the long distance traveled by electrons to the collector. In this paper, different configurations of titanium current collectors were used to connect large surfaces of carbon cloth anodes. The current collectors had different distances and contact areas to the anode. For the same anode surface (490 cm²), increasing the contact area from 28 cm² to 70 cm² enhanced power output from 58 mW·m^-2 to 107 mW·m^-2. For the same contact area (28 cm²), decreasing the maximal distance of current collectors to anodes from 16.5 cm to 7.75 cm slightly increased power output from 50 mW·m^-2 to 58 mW·m^-2. Molecular biology characterization (qPCR and 16S rRNA gene sequencing) of anodic bacterial communities indicated that the Geobacter number was not correlated with power. Moreover, Geobacter and Desulfuromonas abundance increased with the drop in potential on the anode and with the presence of fermentative microorganisms. Electrochemical impedance spectroscopy (EIS) showed that biofilm resistance decreased with the abundance of electroactive bacteria. All these results showed that the electrical gradient arising from collectors shapes microbial communities. Consequently, current collectors influence the performance of carbon-based anodes for full-scale MFC applications.

Mass transfer and reaction simultaneously enhanced airlift microbial electrolytic cell system with high gaseous o-xylene removal capacity

To overcome the limitation of mass transfer and reaction rate involved in the biodegradation of gaseous o-xylene, the airlift reactor and microbial electrolysis cell were integrated to construct an airlift microbial electrolysis cell (AL-MEC) system for the first time, in which the bioanode was modified by polypyrrole to further improve biofilm attachment. The developed AL-MEC system achieved 95.4% o-xylene removal efficiency at optimized conditions, and maintained around 75% removal efficiency even while the inlet o-xylene load was as high as 684 g m^-3 h^-1. The existence of O₂ exhibited a competition in electrons with the bioanode but a positive effect on ring-opening process in the o-xylene oxidation. The limitation of mass transfer had been overcome as the empty bed resistance time in the range of 20-80 s did not influence the system performance significantly. The microbial community analysis confirmed the o-xylene degradation microbes and electroactive bacteria were the dominant, which could be further enriched at 0.3 V against standard hydrogen electrode. This work revealed the feasibility of the AL-MEC system for the degradation of o-xylene and similar compounds, and provided insights into bioelectrochemical system design with high gaseous pollution removal capacity.

Theoretical Modeling of Redox Potentials of Biomolecules

The estimation of the redox potentials of biologically relevant systems by means of theoretical-computational approaches still represents a challenge. In fact, the size of these systems typically does not allow a full quantum-mechanical treatment needed to describe electron loss/gain in such a complex environment, where the redox process takes place. Therefore, a number of different theoretical strategies have been developed so far to make the calculation of the redox free energy feasible with current computational resources. In this review, we provide a survey of such theoretical-computational approaches used in this context, highlighting their physical principles and discussing their advantages and limitations. Several examples of these approaches applied to the estimation of the redox potentials of both proteins and nucleic acids are described and critically discussed. Finally, general considerations on the most promising strategies are reported.

Bio-based carbon materials with multiple functional groups and graphene structure to boost methane production from ethanol anaerobic digestion

This study evaluated the effects of bio-based carbon materials on methane production by anaerobic digestion. The results showed that biochar and hydrochar can promote cumulative methane yield by 15% to 29%. However, there was no statistical significance (p > 0.05) between hydrochar and biochar produced at different temperature on methane production. 16S rRNA gene sequencing and bioinformatics analysis showed that biochar and hydrochar enriched microorganism that might participate in direct interspecies electron transfer (DIET) such as Pseudomonadaceae, Bacillaceae, and Clostridiaceae. The the surface properties of the modified biochar were characterized with BET, Raman, FTIR and XPS. Bio-based carbon materials with uniform dispersion provided a stable environment for the DIET of microorganisms and electrons are transferred through aromatic functional groups on the surface of materials. This study reveals bio-based carbon materials surface properties on methane production in anaerobic digestion and provides a new approach to recycling spent coffee grounds.

Bio-electrochemical frameworks governing microbial fuel cell performance: technical bottlenecks and proposed solutions

Microbial fuel cells (MFCs) are recognized as a future technology with a unique ability to exploit metabolic activities of living microorganisms for simultaneous conversion of chemical energy into electrical energy. This technology holds the promise to offer sustained innovations and continuous development towards many different applications and value-added production that extends beyond electricity generation, such as water desalination, wastewater treatment, heavy metal removal, bio-hydrogen production, volatile fatty acid production and biosensors. Despite these advantages, MFCs still face technical challenges in terms of low power and current density, limiting their use to powering only small-scale devices. Description of some of these challenges and their proposed solutions is demanded if MFCs are applied on a large or commercial scale. On the other hand, the slow oxygen reduction process (ORR) in the cathodic compartment is a major roadblock in the commercialization of fuel cells for energy conversion. Thus, the scope of this review article addresses the main technical challenges of MFC operation and provides different practical approaches based on different attempts reported over the years.

Sustainable operation requires addressing key MFC-bottleneck issues. Enhancing extracellular electron transfer is the key to elevated MFC performance.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Current Progress of Interfacing Organic Semiconducting Materials with Bacteria

Microbial bioelectronics require interfacing microorganisms with electrodes. The resulting abiotic/biotic platforms provide the basis of a range of technologies, including energy conversion and diagnostic assays. Organic semiconductors (OSCs) provide a unique strategy to modulate the interfaces between microbial systems and external electrodes, thereby improving the performance of these incipient technologies. In this review, we explore recent progress in the field on how OSCs, and related materials capable of charge transport, are being used within the context of microbial systems, and more specifically bacteria. We begin by examining the electrochemical communication modes in bacteria and the biological basis for charge transport. Different types of synthetic organic materials that have been designed and synthesized for interfacing and interrogating bacteria are discussed next, followed by the most commonly used characterization techniques for evaluating transport in microbial, synthetic, and hybrid systems. A range of applications is subsequently examined, including biological sensors and energy conversion systems. The review concludes by summarizing what has been accomplished so far and suggests future design approaches for OSC bioelectronics materials and technologies that hybridize characteristic properties of microbial and OSC systems.

High throughput techniques for the rapid identification of electroactive microorganisms

Electroactive microorganisms (EAM), capable of executing extracellular electron transfer (EET) in/out of a cell, are employed in microbial electrochemical technologies (MET) and bioelectronics for harnessing electricity from wastewater, bioremediation and as biosensors. Thus, investigation on EAM is becoming a topic of interest for multidisciplinary areas, such as environmental science, energy and health sectors. Though, EAM are widespread in three domains of life, nevertheless, only a few hundred EAM have been identified so far and hence, the rapid identification of EAM is imperative. In this review, the techniques that are developed for the direct identification of EAM, such as azo dye and WO₃ based techniques, dielectrophoresis, potentiostatic/galvanometric techniques, and other indirect methods, such as spectroscopy and molecular biology techniques, are highlighted with a special focus on time required for the detection of these EAM. The bottlenecks for identifying EAM and the knowledge gaps based on the present investigations are also discussed. Thus, this review is intended to encourage researchers for devolving high-throughput techniques for identifying EAM with more accuracy, while consuming less time.

External potential regulated biocathode for enhanced removal of gaseous chlorobenzene in bioelectrchemical system

Microbial electrolysis cell (MEC) with a biocathode could provide extra reaction driving force for gaseous chlorobenzene (CB) removal. In this work, external potentials (-0.1 to -0.7 V vs. SHE) were applied to regulate the biocathodic activity. Results showed -0.3 V was the optimum potential, while the removal efficiency, dechlorination efficiency and Coulombic efficiency achieved 94%, 65%, and 89%, respectively. Electrochemical stimulation enriched dechlorination microorganisms (Achromobacter and Gordonia), and significantly improved CB mineralization efficiency, which was twice higher than that without additional potential at 300 mg m^-3 inlet concentration. Furthermore, electron transfer between biocathode and microorganisms was mainly through direct electron transfer (DET). A new integrated redox pathway for CB anaerobic degradation was proposed, in which CB was sequentially converted into 2-chlorophenol and 3-chlorocatechol, then dechlorinated to catechol, and finally mineralized into CO₂. Overall, this work provided an insight into gaseous CB bioelectrochemical degradation through the potential regulation.

Bio-Electrochemical System Depollution Capabilities and Monitoring Applications: Models, Applicability, Advanced Bio-Based Concept for Predicting Pollutant Degradation and Microbial Growth Kinetics via Gene Regulation Modelling

Microbial fuel cells (MFC) are an emerging technology for waste, wastewater and polluted soil treatment. In this manuscript, pollutants that can be treated using MFC systems producing energy are presented. Furthermore, the applicability of MFC in environmental monitoring is described. Common microbial species used, release of genome sequences, and gene regulation mechanisms, are discussed. However, although scaling-up is the key to improving MFC systems, it is still a difficult challenge. Mathematical models for MFCs are used for their design, control and optimization. Such models representing the system are presented here. In such comprehensive models, microbial growth kinetic approaches are essential to designing and predicting a biosystem. The empirical and unstructured Monod and Monod-type models, which are traditionally used, are also described here. Understanding and modelling of the gene regulatory network could be a solution for enhancing knowledge and designing more efficient MFC processes, useful for scaling it up. An advanced bio-based modelling concept connecting gene regulation modelling of specific metabolic pathways to microbial growth kinetic models is presented here; it enables a more accurate prediction and estimation of substrate biodegradation, microbial growth kinetics, and necessary gene and enzyme expression. The gene and enzyme expression prediction can also be used in synthetic and systems biology for process optimization. Moreover, various MFC applications as a bioreactor and bioremediator, and in soil pollutant removal and monitoring, are explored.

Biosynthesis of vivianite from microbial extracellular electron transfer and environmental application

Vivianite (Fe₃(PO₄)₂·8H₂O) is a common hydrous ferrous phosphate mineral which often occurs in reductive conditions, especially anoxic non-sulfide environment containing high concentrations of ferrous iron (Fe²⁺) and orthophosphate (PO₄^3-). Vivianite is an important product of dissimilatory iron reduction and a promising route for phosphorus recovery from wastewater. Its formation is closely related to the extracellular electron transfer (EET), a key mechanism for microbial respiration and a crucial explanation for the reduction of metal oxides in soil and sediments. Despite of the natural ubiquity, easy accessibility and attractive economic value, the application value of vivianite has not received much attention. This review introduces the characteristics, occurrence and biosynthesis of vivianite from microbial EET, and systematically analyzes the application value of vivianite in the environmental field, including immobilization of heavy metals (HMs), dechlorination of carbon tetrachloride (CT), sedimentary phosphorus sequestration and eutrophication alleviation. Additionally, its potential functions as a slow-release fertilizer are discussed as well. In general, vivianite is expected to make more contributions to the future scientific research, especially the solution of environmental problems. Overcoming the lack of understanding and some technical limitations will be beneficial to the further application of vivianite in environmental field.

Endogenous Electric Signaling as a Blueprint for Conductive Materials in Tissue Engineering

Bioelectricity plays an important role in cell behavior and tissue modulation, but is understudied in tissue engineering research. Endogenous electrical signaling arises from the transmembrane potential inherent to all cells and contributes to many cell behaviors, including migration, adhesion, proliferation, and differentiation. Electrical signals are also involved in tissue development and repair. Synthetic and natural conductive materials are under investigation for leveraging endogenous electrical signaling cues in tissue engineering applications due to their ability to direct cell differentiation, aid in maturing electroactive cell types, and promote tissue functionality. In this review, we provide a brief overview of bioelectricity and its impact on cell behavior, report recent literature using conductive materials for tissue engineering, and discuss opportunities within the field to improve experimental design when using conductive substrates.

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.

Light exposure interferes with electroactive biofilm enrichment and reduces extracellular electron transfer efficiency

Light plays a vital role in shaping the structure of natural biofilms, but the effect of light on electroactive biofilm (EAB) has not been systematically studied. Herein, the influence of light on the formation of EAB was investigated. The EAB grown in darkness was more electroactive (EAB-0) with a peak current of ∼4.5 A/m², which was 196 and 5556 times higher than EABs formed under light intensities of 600 (EAB-600) and 1200 lux (EAB-1200). A thin EAB (30 μm) with spherical pink aggregates was obtained after 13 days in the darkness, comparing to a dense and flat biofilm grown under light conditions. Although the biomass in EAB-1200 (38.5 ± 1.6 mg/L) was 3 times larger than that in EAB-0 (11.4 ± 1.8 mg/L), the degradation of substrate was not sufficient. EAB-0 contained 85% Geobacter species, while the Rhodopseudomonas species made up 66% and 75% of EAB-600 and EAB-1200, respectively. The polysaccharides produced by EAB-1200 was 4801 ± 253 mg/m², which were 2.3 times higher than 2073 ± 160 mg/m² of EAB-0, resulting in lower electro-conductivity of the extracellular polymeric substances (EPS) under light conditions. Our findings confirmed that the light exposure affected EAB performance by altering the microbial components, electron transfer capacity, and biofilm morphology, which can be used in predictions of the formation and properties of engineered EAB in outdoor environments.

The porphyrin ring rather than the metal ion dictates long-range electron transport across proteins suggesting coherence-assisted mechanism

The fundamental biological process of electron transfer (ET) takes place across proteins with common ET pathways of several nanometers. Recent discoveries push this limit and show long-range extracellular ET over several micrometers. Here, we aim in deciphering how protein-bound intramolecular cofactors can facilitate such long-range ET. In contrast to natural systems, our protein-based platform enables us to modulate important factors associated with ET in a facile manner, such as the type of the cofactor and its quantity within the protein. We choose here the biologically relevant protoporphyrin molecule as the electron mediator. Unlike natural systems having only Fe-containing protoporphyrins, i.e., heme, as electron mediators, we use here porphyrins with different metal centers, or lacking a metal center. We show that the metal redox center has no role in ET and that ET is mediated solely by the conjugated backbone of the molecule. We further discuss several ET mechanisms, accounting to our observations with possible contribution of coherent processes. Our findings contribute to our understanding of the participation of heme molecules in long-range biological ET.

Simultaneous Genome Sequencing of Prosthecochloris ethylica and Desulfuromonas acetoxidans within a Syntrophic Mixture Reveals Unique Pili and Protein Interactions

Strains of Chloropseudomonas ethylica, 2-K, N2, and N3 are known to be composed of a syntrophic mixture of a green sulfur bacterium and a sulfur-reducing colorless component. Upon sequence analysis, the green sulfur photosynthetic bacterial component of strain N3 was dominant and was readily sequenced, but the less abundant sulfur-reducing bacterial component was apparent only when analyzed by metagenomic binning. Whole-genome comparison showed that the green bacterium belonged to the genus Prosthecochloris and apparently was a species for which there was no genome sequence on file. For comparison, we also sequenced the genome of Prosthecochloris sp. DSM 1685, which had previously been isolated from the 2-K mixture in pure culture and have shown that all three Prosthecochloris genomes belong to a new species, which we propose to be named Prosthecochloris ethylica comb. nov. Whole genomes were also sequenced for the isolated Desulfuromonas strains DSM 1675 (from strain 2-K) and DSM 1676 (from strain N2) and shown to be nearly identical to the genome found in the N3 mixture. The genome of the green sulfur bacterium contains large genes for agglutination proteins, similar to the ones proposed to be involved in larger photosynthetic consortia of Chlorochromatium aggregatum. In addition, we also identified several unique “tight adhesion (tad)” pili genes that are presumably involved in the formation of cell–cell interactions. The colorless component, on the other hand, contained a unique large multiheme cytochrome C and unique genes for e-pili (geopilin) formation, genetically clustered with a conserved ferredoxin gene, which are all expected to play an electron transfer role in the closed sulfur cycle in the syntrophic mixture. The findings from the simultaneous genome sequencing of the components of Cp. ethylica have implications for the phenomenon of direct interspecies interactions and coupled electron transfer in photosynthetic symbionts. The mechanisms for such interactions appear to be more common in the environment than originally anticipated.

Sulphonated polyhedral oligomeric silsesquioxane/sulphonated poly ether ether ketone nanocomposite membranes for microbial fuel cell: Insights to the miniatures involved

In this study we demonstrate Sulphonated Polyhedral oligomeric silsesquioxane (S-POSS) incorporated Sulphonated Poly Ether Ether Ketone (SPEEK) as an effective cation exchange membrane (CEM) for improving performance and sustainability in a fabricated tubular Microbial Fuel Cell (MFC). The organic-inorganic caged frame of S-POSS enables several ion conducting channels thereby resulting in better proton conductivity and water uptake in addition to hydroxide ions native in POSS. Among the membranes, SPEEK+ 5 wt% S-POSS exhibits a highest maximum performance of 162 ± 1.4 mW m^-2 with the highest IEC of 1.8 ± 0.05 meq g^-1. Microbial community analysis reveals the predominance of several bacterial strains contributing to wide range of mechanisms. Three phyla including Betaproteobacteria, Gammaproteobacteria and Firmicutes showed maximum predominance. In addition to a novel nanocomposite membrane, the present research introduces perceptions of two metabolic mechanisms of the microbial community available which opens pathway for future insights on how other miniatures involve in electron transfer mechanisms.

Engineering S. oneidensis for Performance Improvement of Microbial Fuel Cell-a Mini Review

Microbial fuel cell (MFC) is a promising technology that utilizes exoelectrogens cultivated in the form of biofilm to generate power from various types of sources supplied. A metal-reducing pathway is utilized by these organisms to transfer electrons obtained from the metabolism of substrate from anaerobic respiration extracellularly. A widely established model organism that is capable of extracellular electron transfer (EET) is Shewanella oneidensis. This review highlights the strategies used in the transformation of S. oneidensis and the recent development of MFC in terms of intervention through genetic modifications. S. oneidensis was genetically engineered for several aims including the study on the underlying mechanisms of EET, and the enhancement of power generation and wastewater treating potential when used in an MFC. Through engineering S. oneidensis, genes responsible for EET are identified and strategies on enhancing the EET efficiency are studied. Overexpressing genes related to EET to enhance biofilm formation, mediator biosynthesis, and respiration appears as one of the common approaches.

Bacterial nanotubes mediate bacterial growth on periodic nano-pillars

Surface topography designed to achieve spatial segregation has shown promise in delaying bacterial attachment and biofilm growth. However, the underlying mechanisms linking surface topography to the inhibition of microbial attachment and growth still remain unclear. Here, we investigated bacterial attachment, cell alignment and biofilm formation of Pseudomonas aeruginosa on periodic nano-pillar surfaces with different pillar spacing. Using fluorescence and scanning electron microscopy, bacteria were shown to align between the nanopillars. Threadlike structures ("bacterial nanotubes") protruded from the majority of bacterial cells and appeared to link cells directly with the nanopillars. Using ΔfliM and ΔpilA mutants lacking flagella or pili, respectively, we further demonstrated that cell alignment behavior within nano-pillars is independent of the flagella or pili. The presence of bacteria nanotubes was found in all cases, and is not linked to the expression of flagella or pili. We propose that bacterial nanotubes are produced to aid in cell-surface or cell-cell connections. Nano-pillars with smaller spacing appeared to enhance the extension and elongation of bacterial nanotube networks. Therefore, nano-pillars with narrow spacing can be easily overcome by nanotubes that connect isolated bacterial aggregates. Such nanotube networks may aid cell-cell communication, thereby promoting biofilm development.

Coherence-assisted electron diffusion across the multi-heme protein-based bacterial nanowire

Biological electron transfer (ET) is one of the most studied biochemical processes due to its immense importance in biology. For many years, biological ET was explained using the classical incoherent transport mechanism, i.e. sequential hopping. One of the relatively recent major observations in this field is long-range extracellular ET (EET), where some bacteria were shown to mediate electrons for extremely long distances on the micrometer length scales across individual nanowires. This fascinating finding has resulted in several suggested mechanisms that might explain this intriguing EET. More recently, the structure of a conductive G. sulfurreducens nanowire has been solved, which showed a highly ordered quasi-1D wire of a hexaheme cytochrome protein, named OmcS. Based on this new structure, we suggest here several electron diffusion models for EET, involving either purely hopping or several degrees of mixed hopping and coherent transport, in which the coherent part is due to a local rigidification of the protein structure, associated with a decrease in the local reorganization energy. The effect is demonstrated for two closely packed heme sites as well as for longer chains containing up to several dozens porphyrins. We show that the pure hopping model probably cannot explain the reported conductivity values of the G. sulfurreducens nanowire using conventional values of reorganization energy and electronic coupling. On the other hand, we show that for a wide range of the latter energy values, the mixed hopping-coherent model results in superior electron diffusion compared to the pure hopping model, and especially for long-range coherent transport, involving multiple porphyrin sites.

Acetate limitation selects Geobacter from mixed inoculum and reduces polysaccharide in electroactive biofilm

Bioelectrochemical systems (BESs) are widely investigated as a promising technology to recover bioenergy or synthesize value-added products from wastewaters. The performance of BES depends on the activity of electroactive biofilm (EAB). As the core of BES, it is still unclear how the EAB is formed from mixed inoculum, and how exoelectrogens compete with non-exoelectrogens. Here we confirmed that microbial community composition and the morphology of EAB on the electrode including the thickness and porosity of the biofilm are critical for the performance of BES, and these properties can be simply controlled by the substrate concentration during EAB formation. The EAB formed with 0.1 g/L of acetate (EAB-0.1) exhibited a 90% higher current density than that formed with 1.0 g/L acetate (EAB-1.0). EAB-0.1 had a 50% higher electroactivity per biomass and a 20% thinner thickness than EAB-1.0, which was partly due to the 54% decrease of insulative polysaccharide in biofilm. Limited acetate also imposed a selective pressure to enrich Geobacter up to 88% compared to 72% when acetate was abundant. Our findings demonstrate that a highly active EAB can be formed by limiting substrate concentration, providing a broader understanding of the EAB formation process, the ecology of interspecies competitions and potential applications for bioenergy recovery and trace toxicant detection in the future.

How does electron transfer occur in microbial fuel cells?

Microbial fuel cells (MFCs) have emerged as a promising technology for sustainable wastewater treatment coupled with electricity generation. A MFC is a device that uses microbes as catalysts to convert chemical energy present in biomass into electrical energy. Among the various mechanisms that drive the operation of a MFC, extracellular electron transfer (EET) to the anode is one of the most important. Exoelectrogenic bacteria can natively transfer electrons to a conducting surface like the anode. The mechanisms employed for electron transfer can either be direct transfer via conductive pili or nanowires, or mediated transfer that involves either naturally secreted redox mediators like flavins and pyocyanins or artificially added mediators like methylene blue and neutral red. EET is a mechanism wherein microorganisms extract energy for growth and maintenance from their surroundings and transfer the resulting electrons to the anode to generate current. The efficiency of these electron transfer mechanisms is dependent not only on the redox potentials of the species involved, but also on microbial oxidative metabolism that liberates electrons. Attempts at understanding the electron transfer mechanisms will boost efforts in giving rise to practical applications. This article covers the various electron transfer mechanisms involved between microbes and electrodes in microbial fuel cells and their applications.

Investigation and Improvement of Scalable Oxygen Reducing Cathodes for Microbial Fuel Cells by Spray Coating

This contribution describes the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). The increase in coating quality led to a strong increase in MFC performance in terms of peak power density and long-term stability. This more uniform coating was realized by an airbrush coating method for applying a self-developed polymeric solution containing different catalysts (MnO2, MoS2, Co3O4). In addition to the possible automation of the presented coating, this method did not require a calcination step. A cathode coated with catalysts, for instance, MnO2/MoS2 (weight ratio 2:1), by airbrush method reached a peak and long-term power density of 320 and 200–240 mW/m2, respectively, in a two-chamber MFC. The long-term performance was approximately three times higher than a cathode with the same catalyst system but coated with the former paintbrush method on a smaller cathode surface area. This extraordinary increase in MFC performance confirmed the high impact of catalyst coating quality, which could be stronger than variations in catalyst concentration and composition, as well as in cathode surface area.

Microbial Nanotechnology: Challenges and Prospects for Green Biocatalytic Synthesis of Nanoscale Materials for Sensoristic and Biomedical Applications

Nanomaterials are increasingly being used in new products and devices with a great impact on different fields from sensoristics to biomedicine. Biosynthesis of nanomaterials by microorganisms is recently attracting interest as a new, exciting approach towards the development of 'greener' nanomanufacturing compared to traditional chemical and physical approaches. This review provides an insight about microbial biosynthesis of nanomaterials by bacteria, yeast, molds, and microalgae for the manufacturing of sensoristic devices and therapeutic/diagnostic applications. The last ten-year literature was selected, focusing on scientific works where aspects like biosynthesis features, characterization, and applications have been described. The knowledge, challenges, and potentiality of microbial-mediated biosynthesis was also described. Bacteria and microalgae are the main microorganism used for nanobiosynthesis, principally for biomedical applications. Some bacteria and microalgae have showed the ability to synthetize unique nanostructures: bacterial nanocellulose, exopolysaccharides, bacterial nanowires, and biomineralized nanoscale materials (magnetosomes, frustules, and coccoliths). Yeasts and molds are characterized by extracellular synthesis, advantageous for possible reuse of cell cultures and reduced purification processes of nanomaterials. The intrinsic variability of the microbiological systems requires a greater protocols standardization to obtain nanomaterials with increasingly uniform and reproducible chemical-physical characteristics. A deeper knowledge about biosynthetic pathways and the opportunities from genetic engineering are stimulating the research towards a breakthrough development of microbial-based nanosynthesis for the future scaling-up and possible industrial exploitation of these promising 'nanofactories'.

Enhancing direct interspecies electron transfer in syntrophic-methanogenic associations with (semi)conductive iron oxides: Effects and mechanisms

Anaerobic digestion is an effective biological treatment process that produces methane by degrading organic compounds in waste/wastewater. It is a complicated microbial process by metabolic interactions among different types of microorganisms. In this process, efficient interspecies electron transfer between secondary fermenting bacteria and methanogens is the critical process for fast and effective methanogenesis. In syntrophic metabolism, hydrogen or formate has been considered as the conventional electron carrier transferring electrons from secondary fermenting bacteria to hydrogenotrophic methanogens. Recently, direct interspecies electron transfer (DIET) without the involvement of dissolved redox mediators is arousing great concerns and has been regarded as a more efficient and thermodynamically favorable interspecies electron transfer pathway for methanogenesis. Interspecies electron exchange through DIET is accomplished via the membrane-bound cytochromes or conductive pili. Several kinds of exogenously-added conductive or semiconductive iron oxides have been discovered to greatly enhance anaerobic methanogenesis through promoting DIET. Different (semi)conductive iron oxides give a boost to DIET through different mechanisms based on the physicochemical properties of the iron oxides and the reciprocal interactions between iron oxides and functional microorganisms. In this review, the current understanding of interspecies electron transfer in syntrophic-methanogenic consortions is summarized, the effects and deep-rooted mechanisms of (semi)conductive iron oxides on methanogenesis and DIET are discussed, and possible future perspectives and development directions are suggested for DIET via (semi)conductive iron oxides in anaerobic digestion.

Synergetic promotion of direct interspecies electron transfer for syntrophic metabolism of propionate and butyrate with graphite felt in anaerobic digestion

Direct interspecies electron transfer (DIET) was considered to replace interspecies H₂ transfer to enhance syntrophic metabolism during the anaerobic digestion. The graphite felt with excellent electrical conductivity was added into a semi-continuous reactor as a conductive material, strengthening DIET in enhancing the degradation of propionate and butyrate. The final average CH₄ production rate increased by 19.1% and 16.7%, respectively, during the propionate and butyrate degradation. Microbial community analysis revealed that the relative abundance of the microflora that could participate in DIET, like Geobacter, Methanosaeta and Methanosarcina species, was greatly improved in the presence of graphite felt. Further experiments showed that the high H₂ partial pressure had no obvious effect on the degradation of propionate and butyrate in the graphite felt-supplement reactors.

Soil solid-phase organic matter-mediated microbial reduction of iron minerals increases with land use change sequence from fallow to paddy fields

The microbial reduction of Fe(III) minerals (MRF) is an important process in paddy soil because it can affect the biogeochemical cycles of many major and trace elements. Natural organic matter (NOM) that mainly exists in the form of solid phase in soil can mediate MRF through electron shuttling functionality. However, whether a link exists between solid-phase NOM-mediated MRF in soil and the age of paddy field since the reclamation on fallow is unclear. Here, we use microbial reduction method to assess the solid-phase NOM-mediated MRF of paddy soils with different reclamation ages. The results show that solid-phase NOM-mediated MRF exhibits a positive response to land use change sequence from fallow to paddy field, indicating that the long-term natural development of paddy field favors the electron shuttling of NOM between cells and Fe(III) minerals. This increase in the electron shuttling of NOM is not due to the increase in the redox functional groups of NOM, but may be attributed to the formation of NOM-mineral complex through the synergistic increases in NOM content and transformation of soil texture from clay loam to loam. The decrease in the redox potential of Fe(III) minerals in soil caused by decreased pH and the increase in Fe content in the organic matter-complexed form may also partly facilitate electron transfer from NOM to Fe(III) minerals. Our work is useful in predicting the role of soil solid-phase NOM in mediating MRF in the context of long-term reclamation of paddy field and provides guidance for the environmental management of paddy fields.

Assessing Possible Mechanisms of Micrometer-Scale Electron Transfer in Heme-Free Geobacter sulfurreducens Pili

The electrically conductive pili of Geobacter sulfurreducens are of both fundamental and practical interest. They facilitate extracellular and interspecies electron transfer (ET) and also provide an electrical interface between living and nonliving systems. We examine the possible mechanisms of G. sulfurreducens electron transfer in regimes ranging from incoherent to coherent transport. For plausible ET parameters, electron transfer in G. sulfurreducens bacterial nanowires mediated only by the protein is predicted to be dominated by incoherent hopping between phenylalanine (Phe) and tyrosine (Tyr) residues that are 3 to 4 Å apart, where Phe residues in the hopping pathways may create delocalized "islands." This mechanism could be accessible in the presence of strong oxidants that are capable of oxidizing Phe and Tyr residues. We also examine the physical requirements needed to sustain biological respiration via nanowires. We find that the hopping regimes with ET rates on the order of 10⁸ s^-1 between Phe islands and Tyr residues, and conductivities on the order of mS/cm, can support ET fluxes that are compatible with cellular respiration rates, although sustaining this delocalization in the heterogeneous protein environment may be challenging. Computed values of fully coherent electron fluxes through the pili are orders of magnitude too low to support microbial respiration. We suggest experimental probes of the transport mechanism based on mutant studies to examine the roles of aromatic amino acids and yet to be identified redox cofactors.