Exploring the molecular mechanisms of electron shuttling across the microbe/metal space

Multi-dimensional perspectives into the pervasive role of microbial extracellular polymeric substances in electron transport processes

During the process of biological treatment, most microorganisms are encapsulated in extracellular polymeric substances (EPS), which protect the cell from adverse environments and aid in microbial attachment. Microorganisms utilize extracellular electron transfer (EET) for energy and information interchange with other cells and the outside environment. Understanding the role of steric EPS in EET is critical for studying microbiology and utilizing microorganisms in biogeochemical processes, pollutant transformation, and bioenergy generation. However, the current study shows that understanding the roles of EPS in the EET processes still needs a great deal of research. In view of recent research, this work aims to systematically summarize the production and functional group composition of microbial EPS. Additionally, EET pathways and the role of EPS in EET processes are detailed. Then factors impacting EET processes in EPS are then discussed, with a focus on the spatial structure and composition of EPS, conductive materials and environmental pollution, including antibiotics, pH and minerals. Finally, strategies to enhance EET, as well as current challenges and future prospects are outlined in detail. This review offers novel insights into the roles of EPS in biological electron transport and the application of microorganisms in pollutant transformation.

Colony-Based Electrochemistry Reveals Electron Conduction Mechanisms Mediated by Cytochromes and Flavins in Shewanella oneidensis

Bacteria utilize electron conduction in their communities to drive their metabolism, which has led to the development of various environmental technologies, such as electrochemical microbial systems and anaerobic digestion. It is challenging to measure the conductivity among bacterial cells when they hardly form stable biofilms on electrodes. This makes it difficult to identify the biomolecules involved in electron conduction. In the present study, we aimed to identify c-type cytochromes involved in electron conduction in Shewanella oneidensis MR-1 and examine the molecular mechanisms. We established a colony-based bioelectronic system that quantifies bacterial electrical conductivity, without the need for biofilm formation on electrodes. This system enabled the quantification of the conductivity of gene deletion mutants that scarcely form biofilms on electrodes, demonstrating that c-type cytochromes, MtrC and OmcA, are involved in electron conduction. Furthermore, the use of colonies of gene deletion mutants demonstrated that flavins participate in electron conduction by binding to OmcA, providing insight into the electron conduction pathways at the molecular level. Furthermore, phenazine-based electron transfer in Pseudomonas aeruginosa PAO1 and flavin-based electron transfer in Bacillus subtilis 3610 were confirmed, indicating that this colony-based system can be used for various bacteria, including weak electricigens.

Optimizing electrochemically active microorganisms as a key player in the bioelectrochemical system: Identification methods and pathways to large-scale implementation

The rapid global economic growth driven by industrialization and population expansion has resulted in significant issues, including reliance on fossil fuels, energy scarcity, water crises, and environmental emissions. To address these issues, bioelectrochemical systems (BES) have emerged as a dual-purpose solution, harnessing electrochemical processes and the capabilities of electrochemically active microorganisms (EAM) to simultaneously recover energy and treat wastewater. This review examines critical performance factors in BES, including inoculum selection, pretreatment methods, electrodes, and operational conditions. Further, authors explore innovative approaches to suppress methanogens and simultaneously enhance the EAM in mixed cultures. Additionally, advanced techniques for detecting EAM are discussed. The rapid detection of EAM facilitates the selection of suitable inoculum sources and optimization of enrichment strategies in BESs. This optimization is essential for facilitating the successful scaling up of BES applications, contributing substantially to the realization of clean energy and sustainable wastewater treatment. This analysis introduces a novel viewpoint by amalgamating contemporary research on the selective enrichment of EAM in mixed cultures. It encompasses identification and detection techniques, along with methodologies tailored for the selective enrichment of EAM, geared explicitly toward upscaling applications in BES.

Effects of Fe(III) (hydr)oxide mineralogy on the development of microbial communities originating from soil, surface water, groundwater, and aerosols

Microbial Fe(III) reduction is a key component of the iron cycle in natural environments. However, the susceptibility of Fe(III) (hydr)oxides to microbial reduction varies depending on the mineral's crystallinity, and the type of Fe(III) (hydr)oxide in turn will affect the composition of the microbial community. We created microcosm reactors with microbial communities from four different sources (soil, surface water, groundwater, and aerosols), three Fe(III) (hydr)oxides (lepidocrocite, goethite, and hematite) as electron acceptors, and acetate as an electron donor to investigate the shaping effect of Fe(III) mineral type on the development of microbial communities. During a 10-month incubation, changes in microbial community composition, Fe(III) reduction, and acetate utilization were monitored. Overall, there was greater reduction of lepidocrocite than of goethite and hematite, and the development of microbial communities originating from the same source diverged when supplied with different Fe(III) (hydr)oxides. Furthermore, each Fe(III) mineral was associated with unique taxa that emerged from different sources. This study illustrates the taxonomic diversity of Fe(III)-reducing microbes from a broad range of natural environments.

Optimizing the Metal Bioreduction Process in Recombinant Shewanella azerbaijanica Bacteria: A Novel Approach via mtrC Gene Cloning and Nitrate-Reducing Pathway Destruction

Environmental pollution is growing every day in terms of the increase in population, industrialization, and urbanization. Shewanella azerbaijanica is introduced as a highly potent bacterium in metal bioremediation. The mtrC gene was selected as a cloning target to improve electron flux chains in the EET (extracellular electron transfer) pathway. Using the SDM (site-directed mutagenesis) technique, the unique gene assembly featured the mtrC gene sandwiched between two napD/B genes to disrupt the nitrate reduction pathway, which serves as the primary metal reduction competitor. Shew-mtrC gene construction was transferred to expression plasmid pET28a (+) in the expression host bacteria (E. coli BL21 and S. azerbaijanica), in pUC57, cloning plasmid, which was transferred to the cloning host bacteria E. coli Top10 and S. azerbaijanica. All cloning procedures (i.e., synthesis, insertion, transformation, cloning, and protein expression) were verified and confirmed by precise tests. ATR-FTIR analysis, CD, western blotting, affinity chromatography, SDS-PAGE, and other techniques were used to confirm the expression and structure of the MtrC protein. The genome sequence and primers were designed according to the submitted Shewanella oneidensis MR-1 genome, the most similar bacteria to this native species. The performance of recombinant S. azerbaijanica bacterium in metal bioremediation, as sustainable strategy, has to be verified by more research.

Carbon source priority and availability limit bidirectional electron transfer in freshwater mixed culture electrochemically active bacterial biofilms

This study investigated, if a mixed electroactive bacterial (EAB) culture cultivated heterotrophically at a positive applied potential could be adapted from oxidative to reductive or bidirectional extracellular electron transfer (EET). To this end, a periodic potential reversal regime between - 0.5 and 0.2 V vs. Ag/AgCl was applied. This yielded biofilm detachment and mediated electroautotrophic EET in combination with carbonate, i.e., dissolved CO2, as the sole carbon source, whereby the emerged mixed culture (S1) contained previously unknown EAB. Using acetate (S2) as well as a mixture of acetate and carbonate (S3) as the main carbon sources yielded primarily alternating electrogenic organoheterotropic metabolism with the higher maximum oxidation current densities recorded for mixed carbon media, exceeding on average 1 mA cm-2. More frequent periodic polarization reversal resulted in the increase of maximum oxidative current densities by about 50% for S2-BES and 80% for S3-BES, in comparison to half-batch polarization. The EAB mixed cultures developed accordingly, with S1 represented by mostly aerobes (84.8%) and being very different in composition to S2 and S3, dominated by anaerobes (96.9 and 96.5%, respectively). S2 and S3 biofilms remained attached to the electrodes. There was only minor evidence of fully reversible bidirectional EET. In conclusion the three triplicates fed with organic and/or inorganic carbon sources demonstrated two forms of diauxie: Firstly, S1-BES showed a preference for the electrode as the electron donor via mediated EET. Secondly, S2-BES and S3-BES showed a preference for acetate as electron donor and c-source, as long as this was available, switching to CO2 reduction, when acetate was depleted.

Electron Transfer Beyond the Outer Membrane: Putting Electrons to Rest

Extracellular electron transfer (EET) is the physiological process that enables the reduction or oxidation of molecules and minerals beyond the surface of a microbial cell. The first bacteria characterized with this capability were Shewanella and Geobacter, both reported to couple their growth to the reduction of iron or manganese oxide minerals located extracellularly. A key difference between EET and nearly every other respiratory activity on Earth is the need to transfer electrons beyond the cell membrane. The past decade has resolved how well-conserved strategies conduct electrons from the inner membrane to the outer surface. However, recent data suggest a much wider and less well understood collection of mechanisms enabling electron transfer to distant acceptors. This review reflects the current state of knowledge from Shewanella and Geobacter, specifically focusing on transfer across the outer membrane and beyond-an activity that enables reduction of highly variable minerals, electrodes, and even other organisms.

Exploring substrate interaction in respiratory alternative complex III from Rhodothermus marinus

Rhodothermus marinus is a thermohalophilic organism that has optimized its microaerobic metabolism at 65 °C. We have been exploring its respiratory chain and observed the existence of a quinone:cytochrome c oxidoreductase complex, named Alternative Complex III, structurally different from the bc₁ complex. In the present work, we took profit from nanodiscs and liposomes technology to investigate ACIII activity in membrane-mimicking systems. In addition, we studied the interaction of ACIII with menaquinone, its potential electron acceptors (HiPIP and cytochrome c) and the caa₃ oxygen reductase.

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 Cysteine Pair Controls Flavin Reduction by Extracellular Cytochromes during Anoxic/Oxic Environmental Transitions

Many bacteria of the genus Shewanella are facultative anaerobes able to reduce a broad range of soluble and insoluble substrates, including Fe(III) mineral oxides. Under anoxic conditions, the bacterium Shewanella oneidensis MR-1 uses a porin-cytochrome complex (Mtr) to mediate extracellular electron transfer (EET) across the outer membrane to extracellular substrates. However, it is unclear how EET prevents generating harmful reactive oxygen species (ROS) when exposed to oxic environments. The Mtr complex is expressed under anoxic and oxygen-limited conditions and contains an extracellular MtrC subunit. This has a conserved CX8C motif that inhibits aerobic growth when removed. This inhibition is caused by an increase in ROS that kills the majority of S. oneidensis cells in culture. To better understand this effect, soluble MtrC isoforms with modified CX8C were isolated. These isoforms produced increased concentrations of H2O2 in the presence of flavin mononucleotide (FMN) and greatly increased the affinity between MtrC and FMN. X-ray crystallography revealed that the molecular structure of MtrC isoforms was largely unchanged, while small-angle X-ray scattering suggested that a change in flexibility was responsible for controlling FMN binding. Together, these results reveal that FMN reduction in S. oneidensis MR-1 is controlled by the redox-active disulfide on the cytochrome surface. In the presence of oxygen, the disulfide forms, lowering the affinity for FMN and decreasing the rate of peroxide formation. This cysteine pair consequently allows the cell to respond to changes in oxygen level and survive in a rapidly transitioning environment. IMPORTANCE Bacteria that live at the oxic/anoxic interface have to rapidly adapt to changes in oxygen levels within their environment. The facultative anaerobe Shewanella oneidensis MR-1 can use EET to respire in the absence of oxygen, but on exposure to oxygen, EET could directly reduce extracellular oxygen and generate harmful reactive oxygen species that damage the bacterium. By modifying an extracellular cytochrome called MtrC, we show how preventing a redox-active disulfide from forming causes the production of cytotoxic concentrations of peroxide. The disulfide affects the affinity of MtrC for the redox-active flavin mononucleotide, which is part of the EET pathway. Our results demonstrate how a cysteine pair exposed on the surface controls the path of electron transfer, allowing facultative anaerobic bacteria to rapidly adapt to changes in oxygen concentration at the oxic/anoxic interface.

Deciphering Molecular Factors That Affect Electron Transfer at the Cell Surface of Electroactive Bacteria: The Case of OmcA from Shewanella oneidensis MR-1

Multiheme cytochromes play a central role in extracellular electron transfer, a process that allows microorganisms to sustain their metabolism with external electron acceptors or donors. In Shewanella oneidensis MR-1, the decaheme cytochromes OmcA and MtrC show functional specificity for interaction with soluble and insoluble redox partners. In this work, the capacity of extracellular electron transfer by mutant variants of S. oneidensis MR-1 OmcA was investigated. The results show that amino acid mutations can affect protein stability and alter the redox properties of the protein, without affecting the ability to perform extracellular electron transfer to methyl orange dye or a poised electrode. The results also show that there is a good correlation between the reduction of the dye and the current generated at the electrode for most but not all mutants. This observation opens the door for investigations of the molecular mechanisms of interaction with different electron acceptors to tailor these surface exposed cytochromes towards specific bio-based applications.

Bioelectrochemical Treatment Technology—The New Practical Approach for Wastewater Management and GHG Emissions Reduction

This study presents BioElectrochemical Treatment Technology (BETT) as a new wastewater management solution toward the Net-Zero future. The results reported herein were collected from a BETT pilot system installed at a large brewery in Los Angeles, CA, United States processing 0.6 m3. day-1 of raw brewery wastewater with a high content of fruit pulp. Removal of Chemical Oxygen Demand (COD), Total Suspended Solids (TSS) and protein in mg.L-1 per day or percentage were evaluated over 2 months of continuous operation of the Demo Unit. The GHG emissions associated with the power consumed, biomass produced, and carbon dioxide emitted were estimated and compared to aerobic and anaerobic solutions. It was demonstrated that BETT can process wastewater with higher organic load than most conventional anaerobic systems. The inflow COD loading varied between 48,550 mg/L to 116,200 mg/L, and BETT achieved up to 33% COD removal in 4-h HRT. The TSS removal reached values as high as 79% with incoming TSS concentrations up to 34,000 mg/L TSS. BETT did not directly generate methane and demonstrated 89 and 49% lower landfill methane emissions than aerobic and anaerobic technologies, respectively. The overall reduction in CO2 emissions, both direct and indirect, was estimated to be 85–90% compared to existing practices.

Single molecule tracking of bacterial cell surface cytochromes reveals dynamics that impact long-distance electron transport

Multiheme cytochromes in Shewanella oneidensis MR-1 transport electrons across the cell wall, in a process called extracellular electron transfer. These electron conduits can also enable electron transport along and between cells. While the underlying mechanism is thought to involve a combination of electron hopping and lateral diffusion of cytochromes along membranes, these diffusive dynamics have never been observed in vivo. Here, we observe the mobility of quantum dot-labeled cytochromes on living cell surfaces and membrane nanowires, quantify their diffusion with single-particle tracking techniques, and simulate the contribution of these dynamics to electron transport. This work reveals the impact of redox molecule dynamics on bacterial electron transport, with implications for understanding and harnessing this process in the environment and bioelectronics.

Using a series of multiheme cytochromes, the metal-reducing bacterium Shewanella oneidensis MR-1 can perform extracellular electron transfer (EET) to respire redox-active surfaces, including minerals and electrodes outside the cell. While the role of multiheme cytochromes in transporting electrons across the cell wall is well established, these cytochromes were also recently found to facilitate long-distance (micrometer-scale) redox conduction along outer membranes and across multiple cells bridging electrodes. Recent studies proposed that long-distance conduction arises from the interplay of electron hopping and cytochrome diffusion, which allows collisions and electron exchange between cytochromes along membranes. However, the diffusive dynamics of the multiheme cytochromes have never been observed or quantified in vivo, making it difficult to assess their hypothesized contribution to the collision-exchange mechanism. Here, we use quantum dot labeling, total internal reflection fluorescence microscopy, and single-particle tracking to quantify the lateral diffusive dynamics of the outer membrane-associated decaheme cytochromes MtrC and OmcA, two key components of EET in S. oneidensis. We observe confined diffusion behavior for both quantum dot-labeled MtrC and OmcA along cell surfaces (diffusion coefficients D_MtrC = 0.0192 ± 0.0018 µm²/s, D_OmcA = 0.0125 ± 0.0024 µm²/s) and the membrane extensions thought to function as bacterial nanowires. We find that these dynamics can trace a path for electron transport via overlap of cytochrome trajectories, consistent with the long-distance conduction mechanism. The measured dynamics inform kinetic Monte Carlo simulations that combine direct electron hopping and redox molecule diffusion, revealing significant electron transport rates along cells and membrane nanowires.

Outer Membrane c-Type Cytochromes OmcA and MtrC Play Distinct Roles in Enhancing the Attachment of Shewanella oneidensis MR-1 Cells to Goethite

The outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC in Shewanella are key terminal reductases that bind and transfer electrons directly to iron (hydr)oxides. Although the amounts of OmcA and MtrC at the cell surface and their molecular structures are largely comparable, MtrC is known to play a more important role in dissimilatory iron reduction. To explore the roles of these outer membrane c-Cyts in the interaction of Shewanella oneidensis MR-1 with iron oxides, the processes of attachment of S. oneidensis MR-1 wild type and c-type cytochrome-deficient mutants (the ΔomcA, ΔmtrC, and ΔomcA ΔmtrC mutants) to goethite are compared via quartz crystal microbalance with dissipation monitoring (QCM-D). Strains with OmcA exhibit a rapid initial attachment. The quantitative model for QCM-D responses reveals that MtrC enhances the contact area and contact elasticity of cells with goethite by more than one and two times, respectively. In situ attenuated total reflectance Fourier transform infrared two-dimensional correlation spectroscopic (ATR-FTIR 2D-CoS) analysis shows that MtrC promotes the initial interfacial reaction via an inner-sphere coordination. Atomic force microscopy (AFM) analysis demonstrates that OmcA enhances the attractive force between cells and goethite by about 60%. As a result, OmcA contributes to a higher attractive force with goethite and induces a rapid short-term attachment, while MtrC is more important in the longer-term interaction through an enhanced contact area, which promotes interfacial reactions. These results reveal that c-Cyts OmcA and MtrC adopt different mechanisms for enhancing the attachment of S. oneidensis MR-1 cells to goethite. It improves our understanding of the function of outer membrane c-Cyts and the influence of cell surface macromolecules in cell-mineral interactions.IMPORTANCE Shewanella species are one group of versatile and widespread dissimilatory iron-reducing bacteria, which are capable of respiring insoluble iron minerals via six multiheme c-type cytochromes. Outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC are the terminal reductases in this pathway and have comparable protein structures. In this study, we elucidate the different roles of OmcA and MtrC in the interaction of S. oneidensis MR-1 with goethite at the whole-cell level. OmcA confers enhanced affinity toward goethite and results in rapid attachment. Meanwhile, MtrC significantly increases the contact area of bacterial cells with goethite and promotes the interfacial reaction, which may explain its central role in extracellular electron transfer. This study provides novel insights into the role of bacterial surface macromolecules in the interfacial interaction of bacteria with minerals, which is critical to the development of a comprehensive understanding of cell-mineral interactions.

Function of c-type cytochromes of Shewanella xiamenensis in enhanced anaerobic bioreduction of Cr(VI) by graphene oxide and graphene oxide/polyvinyl alcohol films

Graphene-based materials have been demonstrated to facilitate electron extracellular transfer (EET) of Shewanella. In this study, compared to group lacking graphene oxide (GO)-based materials, GO films-added group and graphene oxide/polyvinyl alcohol (GO/PVA) film-added group delivered 2.67- and 3.13-fold increases in the Cr(VI) reduction by Shewanella xiamenensis, respectively. The whole reduction process could be divided into three stages, including microbial Cr(VI) reduction and GO reduction stage, microbial GO reduction stage and microbial Cr(VI) reduction mediated by reduced graphene oxide (rGO) stage. Moreover, gene analysis revealed that addition of GO and GO/PVA films stimulated overexpression of several c-type cytochrome (c-Cyts) genes, including mtrA, mtrB, mtrC, mtrD, mtrE, mtrF, omcA, petC and SO-4047. Specifically, appreciable Cr(VI) reduction by the strains that overexpressed mtrA, mtrB, mtrC, mtrD, mtrE, mtrF and omcA further confirmed that overexpression of c-Cyts genes indeed enhanced the efficiency of Cr(VI) reduction. Based on these results, the specific function of every c-Cyt was clearly found in Cr(VI) reduction by the induction of GO-based materials. Our finding has disclosed a synergetic mechanism stimulated by GO-based materials to enhance Cr(VI) bioreduction that was not only mediated through the modification of material but also upregulated the expression of functional genes.

Geobacter cytochrome OmcZs binds riboflavin: implications for extracellular electron transfer

Geobacter sulfurreducens is an important model organism for understanding extracellular electron transfer (EET), i.e. transfer of electrons from the cell's interior (quinone pool) to an extracellular substrate. This exoelectrogenic functionality can be exploited in bioelectrochemical applications. Nonetheless, key questions remain regarding the mechanisms of this functionality. G. sulfurreducens has been hypothesized to employ both multi-heme cytochromes and soluble, small molecule redox shuttles, as the final, redox-active species in EET. However, interactions between flavin redox shuttles and outer membrane, redox proteins in Geobacter have not been demonstrated. Herein, the heterologous expression and purification from E. coli of a soluble form of the multi-heme cytochrome OmcZs from G. sulfurreducens is reported. UV-vis absorption assays show that riboflavin can be reduced by OmcZs with concomitant oxidation of the protein. Fluorescence assays show that oxidized OmcZs and riboflavin interact with a binding constant of 34 μM. Furthermore, expression of OmcZs in E. coli enables EET in the host, and the current produced by these E. coli in a bioelectrochemical cell increases when riboflavin is introduced. These results support the hypothesis that OmcZs functions in EET by transiently binding riboflavin, which shuttles electrons from the outer membrane to the extracellular substrate.

Insight into the short-term effect of fulvic acid on nitrogen removal performance and N-acylated-L-homoserine lactones (AHLs) release in the anammox system

Fulvic acid (FA) can serve as electron shuttles between bacteria and electron acceptors. It explored the short-term effect of FA dose on nitrogen removal performance and N-acylated-_L-homoserine lactones (AHLs) release change in the anaerobic ammonium oxidation (anammox) system. The results demonstrated that the total inorganic nitrogen removal efficiency increased with the FA dosages from 0.5 mM to 1 mM. FA addition improved anammox bacteria activity, together with extracellular polymeric substances production. FA addition from 0.5 mM to 1 mM stimulated AHLs release in both water and biomass phases, which indicated that the quorum sensing could be improved. These findings revealed that the addition of FA could improve quorum sensing and then enhance nitrogen removal performance.

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.

How Thermophilic Gram-Positive Organisms Perform Extracellular Electron Transfer: Characterization of the Cell Surface Terminal Reductase OcwA

Thermophilic Gram-positive organisms were recently shown to be a promising class of organisms to be used in bioelectrochemical systems for the production of electrical energy. These organisms present a thick peptidoglycan layer that was thought to preclude them to perform extracellular electron transfer (i.e., exchange catabolic electrons with solid electron acceptors outside the cell). In this paper, we describe the structure and functional mechanisms of the multiheme cytochrome OcwA, the terminal reductase of the Gram-positive bacterium Thermincola potens JR found at the cell surface of this organism. The results presented here show that this protein can take the role of a respiratory “Swiss Army knife,” allowing this organism to grow in environments with soluble and insoluble substrates. Moreover, it is shown that it is unrelated to terminal reductases found at the cell surface of other electroactive organisms. Instead, OcwA is similar to terminal reductases of soluble electron acceptors. Our data reveal that terminal oxidoreductases of soluble and insoluble substrates are evolutionarily related, providing novel insights into the evolutionary pathway of multiheme cytochromes.

Extracellular electron transfer is the key process underpinning the development of bioelectrochemical systems for the production of energy or added-value compounds. Thermincola potens JR is a promising Gram-positive bacterium to be used in these systems because it is thermophilic. In this paper, we describe the structural and functional properties of the nonaheme cytochrome OcwA, which is the terminal reductase of this organism. The structure of OcwA, determined at 2.2-Å resolution, shows that the overall fold and organization of the hemes are not related to other metal reductases and instead are similar to those of multiheme cytochromes involved in the biogeochemical cycles of nitrogen and sulfur. We show that, in addition to solid electron acceptors, OcwA can also reduce soluble electron shuttles and oxyanions. These data reveal that OcwA can work as a multipurpose respiratory enzyme allowing this organism to grow in environments with rapidly changing availability of terminal electron acceptors without the need for transcriptional regulation and protein synthesis.

Effects of Flavin-Goethite Interaction on Goethite Reduction by Shewanella decolorationis S12

Flavin mononucleotide (FMN) and riboflavin are structurally similar flavins, except for the presence of a phosphate group on the FMN molecule. They are used by a variety of electroactive bacteria as extracellular electron shuttles in microbial Fe reduction and inevitably interact with Fe (hydr)oxides in the extracellular environment. It is currently unknown whether flavin/Fe (hydr)oxide interaction interferes with extracellular electron transfer (EET) to the mineral surface. In this study, we found that the goethite reduction rate was lower when mediated by FMN than by RF, suggesting that FMN was less effective in shuttling electrons between cells and minerals. Nevertheless, the phosphate group did not prevent the FMN molecule from accepting electrons from bacterial cells and transferring electrons to the mineral. Results of adsorption experiment, attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy, and bacterial attachment trend analyses showed that FMN exhibited strong adsorption on goethite surface by forming phosphate inner-sphere complex, which prevented bacterial cells from approaching goethite. Therefore, the interaction between FMN and goethite surface may increase the distance of electron transfer from bacterial cells to goethite and result in lower EET efficiency in comparison to those mediated by riboflavin. To our knowledge, these data reveal for the first time that the interaction between flavin and Fe (hydr)oxide affect flavin-mediated electron transfer to mineral surface and add a new dimension to our understanding of flavin-mediated microbial Fe reduction processes.

Electrodes Donate Electrons for Nitrate Reduction in a Soil Matrix via DNRA and Denitrification

Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO₃^-) reduction. However, few studies have confirmed this in a soil matrix hitherto. This study investigated if, and how, an electric potential affected NO₃^- reduction in a soil matrix. The results showed that, compared to a control treatment, applying an electric potential of -0.5 V versus the standard hydrogen electrode (SHE) significantly increased the relative abundance of NO₃^--reducing microbes (e.g., Alcaligenaceae and Pseudomonadaceae) and the abundances of the nrfA, nirK, nirS, and nosZ genes in soil matrices. Meanwhile, the electric potential treatment doubled the NO₃^- reduction rate and significantly increased the rates of production of ammonium (NH₄⁺), dinitrogen (N₂), and nitrous oxide (N₂O). The amount of NO₃^--N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased N₂O, N₂, NH₄⁺, and nitrite (NO₂^-) pools. An open-air experiment showed that the electric potential treatment promoted soil NO₃^- reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO₃^- reduction via both denitrification and dissimilatory NO₃^- reduction to ammonium (DNRA) in the soil matrix. The mechanisms revealed in this study have implications for the future development of potential techniques for enhancing NO₃^- reduction in the vadose zone and consequently reducing the risk of NO₃^- leaching.

A brief survey of the "cytochromome"

Multihaem cytochromes c are widespread in nature where they perform numerous roles in diverse anaerobic metabolic pathways. This is achieved in two ways: multihaem cytochromes c display a remarkable diversity of ways to organize multiple hemes within the protein frame; and the hemes possess an intrinsic reactive versatility derived from diverse spin, redox and coordination states. Here we provide a brief survey of multihaem cytochromes c that have been characterized in the context of their metabolic role. The contribution of multihaem cytochromes c to dissimilatory pathways handling metallic minerals, nitrogen compounds, sulfur compounds, organic compounds and phototrophism are described. This aims to set the stage for the further exploration of the vast unknown "cytochromome" that can be anticipated from genomic databases.

Self-adjusting antibacterial properties of Ag-incorporated nanotubes on micro-nanostructured Ti surfaces

The bactericidal activity of a silver loading platform in the “release” mode during the early phase and “contact” mode thereafter.

Electrochemical Detection of pH-Responsive Grafted Catechol and Immobilized Cytochrome c onto Lipid Deposit-Modified Glassy Carbon Surface

The electrochemical systems of both grafted catechol as a pH-responsive electrophore and immobilized cytochrome c as a model redox protein are detected by cyclic voltammetry at an optimized lipid deposit-modified glassy carbon electrode. The catechol covalent grafting is successfully performed by the one-pot/three-step electrochemical reduction of 3,4-dihydroxybenzenediazonium salts generated in situ from 4-nitrocatechol. The resulting glassy carbon electrode electrochemically modified by grafted catechol species is evaluated as an efficient electrochemical pH sensor. The optimized molar ratio for the lipid deposit, promoting cytochrome c electrochemical activity in solution onto glassy carbon electrode, is reached for the lipid mixture composed of 75% 1,2-dioleoyl-sn-glycero-3-phosphocholine and 25% cardiolipin. Cytochrome c immobilization into the optimized supported lipid deposit is efficiently achieved by cyclic voltammetry (10 cycles) recorded at the modified glassy carbon electrode in a cytochrome c solution. The pH-dependent redox response of the grafted catechol and that of the immobilized cytochrome c are finally detected at the same lipid-modified glassy carbon electrode without alteration of their structure and electrochemical properties in the pH range 5-9.

Electron transfer process in microbial electrochemical technologies: The role of cell-surface exposed conductive proteins

Electroactive microorganisms have attracted significant interest for the development of novel biotechnological systems of low ecological footprint. These can be used for the sustainable production of energy, bioremediation of metal-contaminated environments and production of added-value products. Currently, almost 100 microorganisms from the Bacterial and Archaeal domains are considered electroactive, given their ability to efficiently interact with electrodes in microbial electrochemical technologies. Cell-surface exposed conductive proteins are key players in the electron transfer between cells and electrodes. Interestingly, it seems that among the electroactive organisms identified so far, these cell-surface proteins fall into one of four groups. In this review, the different types of cell-surface conductive proteins found in electroactive organisms will be overviewed, focusing on their structural and functional properties.

Structural modeling of an outer membrane electron conduit from a metal-reducing bacterium suggests electron transfer via periplasmic redox partners

Many subsurface microorganisms couple their metabolism to the reduction or oxidation of extracellular substrates. For example, anaerobic mineral-respiring bacteria can use external metal oxides as terminal electron acceptors during respiration. Porin-cytochrome complexes facilitate the movement of electrons generated through intracellular catabolic processes across the bacterial outer membrane to these terminal electron acceptors. In the mineral-reducing model bacterium Shewanella oneidensis MR-1, this complex is composed of two decaheme cytochromes (MtrA and MtrC) and an outer-membrane β-barrel (MtrB). However, the structures and mechanisms by which porin-cytochrome complexes transfer electrons are unknown. Here, we used small-angle neutron scattering (SANS) to study the molecular structure of the transmembrane complexes MtrAB and MtrCAB. Ab initio modeling of the scattering data yielded a molecular envelope with dimensions of ∼105 × 60 × 35 Å for MtrAB and ∼170 × 60 × 45 Å for MtrCAB. The shapes of these molecular envelopes suggested that MtrC interacts with the surface of MtrAB, extending ∼70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. The data also reveal that MtrA fully extends through the length of MtrB, with ∼30 Å being exposed into the periplasm. Proteoliposome models containing membrane-associated MtrCAB and internalized small tetraheme cytochrome (STC) indicate that MtrCAB could reduce Fe(III) citrate with STC as an electron donor, disclosing a direct interaction between MtrCAB and STC. Taken together, both structural and proteoliposome experiments support porin-cytochrome-mediated electron transfer via periplasmic cytochromes such as STC.

Ultrastructure of Shewanella oneidensis MR-1 nanowires revealed by electron cryotomography

Bacterial nanowires have garnered recent interest as a proposed extracellular electron transfer (EET) pathway that links the bacterial electron transport chain to solid-phase electron acceptors away from the cell. Recent studies showed that Shewanella oneidensis MR-1 produces outer membrane (OM) and periplasmic extensions that contain EET components and hinted at their possible role as bacterial nanowires. However, their fine structure and distribution of cytochrome electron carriers under native conditions remained unclear, making it difficult to evaluate the potential electron transport (ET) mechanism along OM extensions. Here, we report high-resolution images of S. oneidensis OM extensions, using electron cryotomography (ECT). We developed a robust method for fluorescence light microscopy imaging of OM extension growth on electron microscopy grids and used correlative light and electron microscopy to identify and image the same structures by ECT. Our results reveal that S. oneidensis OM extensions are dynamic chains of interconnected outer membrane vesicles (OMVs) with variable dimensions, curvature, and extent of tubulation. Junction densities that potentially stabilize OMV chains are seen between neighboring vesicles in cryotomograms. By comparing wild type and a cytochrome gene deletion mutant, our ECT results provide the likely positions and packing of periplasmic and outer membrane proteins consistent with cytochromes. Based on the observed cytochrome packing density, we propose a plausible ET path along the OM extensions involving a combination of direct hopping and cytochrome diffusion. A mean-field calculation, informed by the observed ECT cytochrome density, supports this proposal by revealing ET rates on par with a fully packed cytochrome network.

The Functional Mechanisms and Application of Electron Shuttles in Extracellular Electron Transfer

Electron shuttles extensively exist in various environments. Some kinds of organic substances can be applied by microorganisms to produce electrons, and then the electrons can be transferred to other substances or microorganisms through electron shuttles, resulting in coexistence and interaction of diverse species of microbes. In this review, the functional mechanisms of extracellular electron transfer mediated by different electron shuttles are described. And different subtypes as well as the application of electron shuttles in microbial degradation of pollutants, microbial electricity, and the promotion of energy generation are also discussed. Summary results show that extracellular electron transfer is based on the electrogenesis microorganism with the structure of cytochromes or pili. Materials were usually used in long-distance electron transfer because of their widespread presence and abundance. Therefore, the review is beneficial to perceive the pathways of extracellular electron transfer mediated by electron shuttles and explore the contribution of different electron shuttles in extracellular electron transfer.

Outer membrane cytochromes/flavin interactions in Shewanella spp.-A molecular perspective

Extracellular electron transfer (EET) is intrinsically associated with the core phenomena of energy harvesting/energy conversion in natural ecosystems and biotechnology applications. However, the mechanisms associated with EET are complex and involve molecular interactions that take place at the "bionano interface" where biotic/abiotic interactions are usually explored. This work provides molecular perspective on the electron transfer mechanism(s) employed by Shewanella oneidensis MR-1. Molecular docking simulations were used to explain the interfacial relationships between two outer-membrane cytochromes (OMC) OmcA and MtrC and riboflavin (RF) and flavin mononucleotide (FMN), respectively. OMC-flavin interactions were analyzed by studying the electrostatic potential, the hydrophilic/hydrophobic surface properties, and the van der Waals surface of the OMC proteins. As a result, it was proposed that the interactions between flavins and OMCs are based on geometrical recognition event. The possible docking positions of RF and FMN to OmcA and MtrC were also shown.

Bound Flavin-Cytochrome Model of Extracellular Electron Transfer in Shewanella oneidensis: Analysis by Free Energy Molecular Dynamics Simulations

Flavins are known to enhance extracellular electron transfer (EET) in Shewanella oneidensis MR-1 bacteria, which reduce electron acceptors through outer-membrane (OM) cytochromes c. Free-shuttle and bound-redox cofactor mechanisms were proposed to explain this enhancement, but recent electrochemical reports favor a flavin-bound model, proposing two one-electron reductions of flavin, namely, oxidized (Ox) to semiquinone (Sq) and semiquinone to hydroquinone (Hq), at anodic and cathodic conditions, respectively. In this work, to provide a mechanistic understanding of riboflavin (RF) binding at the multiheme OM cytochrome OmcA, we explored binding configurations at hemes 2, 5, 7, and 10. Subsequently, on the basis of molecular dynamics (MD) simulations, binding free energies and redox potential shifts upon RF binding for the Ox/Sq and Sq/Hq reductions were analyzed. Our results demonstrated an upshift in the Ox/Sq and a downshift in the Sq/Hq redox potentials, consistent with a bound RF-OmcA model. Furthermore, binding free energy MD simulations indicated an RF binding preference at heme 7. MD simulations of the OmcA-MtrC complex interfacing at hemes 5 revealed a small interprotein redox potential difference with an electron transfer rate of 10(7)-10(8)/s.

The effects of fulvic acid on microbial denitrification: promotion of NADH generation, electron transfer, and consumption

The heterotrophic denitrification requires the participation of electrons which are derived from direct electron donor (usually nicotinamide adenine dinucleotide (NADH)), and the electrons are transferred via electron transport system in denitrifiers and then consumed by denitrifying enzymes. Despite the reported electron transfer ability of humic substances (HS), the influences of fulvic acid (FA), an ubiquitous major component of HS, on promoting NADH generation, electron transfer, and consumption in denitrification process have never been reported. The presence of FA, compared with the control, was found not only significantly improved the total nitrogen (TN) removal efficiency (99.9 % versus 74.8 %) but remarkably reduced the nitrite accumulation (0.2 against 43.8 mg/L) and N2O emission (0.003 against 0.240 mg nitrogen/mg TN removed). The mechanisms study showed that FA increased the metabolism of carbon source via glycolysis and tricarboxylic acid (TCA) cycle pathways to produce more available NADH. FA also facilitated the electron transfer activities from NADH to denitrifying enzymes via complex I and complex III in electron transport system, which improved the reduction of nitrate and accelerated the transformations of nitrite and N2O, and lower nitrite and N2O accumulations were therefore observed. In addition, the consumption of electrons in denitrification was enhanced due to FA stimulating the synthesis and the catalytic activity of key denitrifying enzymes, especially nitrite reductase and N2O reductase. It will provide an important new insight into the potential effect of FA on microbial denitrification metabolism process and even nitrogen cycle in nature niches.

Effect of fulvic acids with different characteristics on biological denitrification

Fulvic acids with different molecular structures pose different effect on microbial denitrificationviacarbon source utilization and enzyme activity.

Mechanisms of Bacterial Extracellular Electron Exchange

The biochemical mechanisms by which microbes interact with extracellular soluble metal ions and insoluble redox-active minerals have been the focus of intense research over the last three decades. The process presents two challenges to the microorganism. Firstly, electrons have to be transported at the cell surface, which in Gram-negative bacteria presents an additional problem of electron transfer across the ~6nm of the outer membrane. Secondly, the electrons must be transferred to or from the terminal electron acceptors or donors. This review covers the known mechanisms that bacteria use to transport electrons across the cell envelope to external electron donors/acceptors. In Gram-negative bacteria, electron transfer across the outer membrane involves the use of an outer membrane β-barrel and cytochrome. These can be in the form of a porin-cytochrome protein, such as Cyc2 of Acidithiobacillus ferrooxidans, or a multiprotein porin-cytochrome complex like MtrCAB of Shewanella oneidensis MR-1. For mineral-respiring organisms, there is the additional challenge of transferring the electrons from the cell to mineral surface. For the strict anaerobe Geobacter sulfurreducens this requires electron transfer through conductive pili to associated cytochrome OmcS that directly reduces Fe(III)oxides, while the facultative anaerobe S. oneidensis MR-1 accomplishes mineral reduction through direct membrane contact, contact through filamentous extensions and soluble flavin shuttles, all of which require the outer membrane cytochromes MtrC and OmcA in addition to secreted flavin.

In Situ Analysis of a Silver Nanoparticle-Precipitating Shewanella Biofilm by Surface Enhanced Confocal Raman Microscopy

Shewanella oneidensis MR-1 is an electroactive bacterium, capable of reducing extracellular insoluble electron acceptors, making it important for both nutrient cycling in nature and microbial electrochemical technologies, such as microbial fuel cells and microbial electrosynthesis. When allowed to anaerobically colonize an Ag/AgCl solid interface, S. oneidensis has precipitated silver nanoparticles (AgNp), thus providing the means for a surface enhanced confocal Raman microscopy (SECRaM) investigation of its biofilm. The result is the in-situ chemical mapping of the biofilm as it developed over time, where the distribution of cytochromes, reduced and oxidized flavins, polysaccharides and phosphate in the undisturbed biofilm is monitored. Utilizing AgNp bio-produced by the bacteria colonizing the Ag/AgCl interface, we could perform SECRaM while avoiding the use of a patterned or roughened support or the introduction of noble metal salts and reducing agents. This new method will allow a spatially and temporally resolved chemical investigation not only of Shewanella biofilms at an insoluble electron acceptor, but also of other noble metal nanoparticle-precipitating bacteria in laboratory cultures or in complex microbial communities in their natural habitats.

Redox Linked Flavin Sites in Extracellular Decaheme Proteins Involved in Microbe-Mineral Electron Transfer

Extracellular microbe-mineral electron transfer is a major driving force for the oxidation of organic carbon in many subsurface environments. Extracellular multi-heme cytochromes of the Shewenella genus play a major role in this process but the mechanism of electron exchange at the interface between cytochrome and acceptor is widely debated. The 1.8 Å x-ray crystal structure of the decaheme MtrC revealed a highly conserved CX₈C disulfide that, when substituted for AX₈A, severely compromised the ability of S. oneidensis to grow under aerobic conditions. Reductive cleavage of the disulfide in the presence of flavin mononucleotide (FMN) resulted in the reversible formation of a stable flavocytochrome. Similar results were also observed with other decaheme cytochromes, OmcA, MtrF and UndA. The data suggest that these decaheme cytochromes can transition between highly reactive flavocytochromes or less reactive cytochromes, and that this transition is controlled by a redox active disulfide that responds to the presence of oxygen.

Nature of the Surface-Exposed Cytochrome-Electrode Interactions in Electroactive Biofilms of Desulfuromonas acetoxidans

Metal-respiring bacteria are microorganisms capable of oxidizing organic pollutants present in wastewater and transferring the liberated electrons to an electrode. This ability has led to their application as catalysts in bioelectrochemical systems (BESs), a sustainable technology coupling bioremediation to electricity production. Crucial for the functioning of these BESs is a complex protein architecture consisting of several surface-exposed multiheme proteins, called outer membrane cytochromes, wiring the cell metabolism to the electrode. Although the role of these proteins has been increasingly understood, little is known about the protein-electrode interactions and their impact on the performance of BESs. In this study, we used surface-enhanced resonance Raman spectroscopy in combination with electrochemical techniques to unravel the nature of the protein-electrode interaction for the outer membrane cytochrome OmcB from Desulfuromonas acetoxidans (Dace). Comparing the spectroelectrochemical properties of OmcB bound directly to the electrode surface with those of the same protein embedded inside an electroactive biofilm, we have shown that the surface-exposed cytochromes of Dace biofilms are in direct contact with the electrode surface. Even if direct binding causes protein denaturation, the biofilm possesses the ability to minimize the extent of the damage maximizing the amount of cells in direct electrical communication with the electrode.