Cyclic voltammetric analysis of the electron transfer of Shewanella oneidensis MR-1 and nanofilament and cytochrome knock-out mutants

Biosynthesis of silver nanoparticles using Burkholderia contaminans ZCC and mechanistic analysis at the proteome level

The biogenic synthesis of silver nanoparticles (AgNPs) by microorganisms has been a subject of increasing attention. Despite extensive studies on this biosynthetic pathway, the mechanisms underlying the involvement of proteins and enzymes in AgNPs production have not been fully explored. Herein, we reported that Burkholderia contaminans ZCC was able to reduce Ag+ to AgNPs with a diameter of (10±5) nm inside the cell. Exposure of B. contaminans ZCC to Ag+ ions led to significant changes in the functional groups of cellular proteins, with approximately 5.72% of the (C-OH) bonds being converted to (C-C/C-H) (3.61%) and CO (2.11%) bonds, and 4.52% of the CO (carbonyl) bonds being converted to (C-OH) bonds. Furthermore, the presence of Ag+ and AgNPs induced the ability of extracellular electron transfer for ZCC cells via specific membrane proteins, but this did not occur in the absence of Ag+ ions. Proteomic analysis of the proteins and enzymes involved in heavy metal efflux systems, protein secretion system, oxidative phosphorylation, intracellular electron transfer chain, and glutathione metabolism suggests that glutathione S-transferase and ubiquinol-cytochrome c reductase iron-sulfur subunit play importance roles in the biosynthesis of AgNPs. These findings contribute to a deeper understanding of the functions exerted by glutathione S-transferase and ferredoxin-thioredoxin reductase iron-sulfur subunits in the biogenesis of AgNPs, thereby hold immense potential for optimizing biotechnological techniques aimed at enhancing the yield and purity of biosynthetic AgNPs.

Deciphering the role of extracellular polymeric substances in the adsorption and biotransformation of organic micropollutants during anaerobic wastewater treatment

Extracellular polymeric substances (EPS) participate in the removal of organic micropollutants (OMPs), but the primary pathways of removal and detailed mechanisms remain elusive. We evaluated the effect of EPS on removal for 16 distinct chemical classes of OMPs during anaerobic digestion (AD). The results showed that hydrophobic OMPs (HBOMPs) could not be removed by EPS, while hydrophilic OMPs (HLOMPs) were amenable to removal via adsorption and biotransformation of EPS. The adsorption and biotransformation of HLOMPs by EPS accounted up to 19.4 ± 0.9 % and 6.0 ± 0.8 % of total removal, respectively. Further investigations into the adsorption and biotransformation mechanisms of HLOMPs by EPS were conducted utilizing spectral, molecular dynamics simulation, and electrochemical analysis. The results suggested that EPS provided abundant binding sites for the adsorption of HLOMPs. The binding of HLOMPs to tryptophan-like proteins in EPS formed nonfluorescent complexes. Hydrogen bonds, hydrophobic interactions and water bridges were key to the binding processes and helped stabilize the complexes. The biotransformation of HLOMPs by EPS may be attributed to the presence of extracellular redox active components (c-type cytochromes (c-Cyts), c-Cyts-bound flavins). This study enhanced the comprehension for the role of EPS on the OMPs removal in anaerobic wastewater treatment.

Biological Self-Assembled Transmembrane Electron Conduits for High-Efficiency Ammonia Production in Microbial Electrosynthesis

Usually, CymA is irreplaceable as the electron transport hub in Shewanella oneidensis MR-1 bidirectional electron transfer. In this work, biologically self-assembled FeS nanoparticles construct an artificial electron transfer route and implement electron transfer from extracellular into periplasmic space without CymA involvement, which present similar properties to type IV pili. Bacteria are wired up into a network, and more electron transfer conduits are activated by self-assembled transmembrane FeS nanoparticles (electron conduits), thereby substantially enhancing the ammonia production. In this study, we achieved an average NH4+-N production rate of 391.8 μg·h-1·L reactor-1 with the selectivity of 98.0% and cathode efficiency of 65.4%. Additionally, the amide group in the protein-like substances located in the outer membrane was first found to be able to transfer electrons from extracellular into intracellular with c-type cytochromes. Our work provides a new viewpoint that contributes to a better understanding of the interconnections between semiconductor materials and bacteria and inspires the exploration of new electron transfer chain components.

Electroactive biocake layer-driven advanced removal of dissolved organic matter at membrane interface of anaerobic electrochemical membrane bioreactor

The bio-cake layer is one of the most negative effects during water and wastewater filtration, but its potential behoof of biodegradation is poorly understood. In this study, we activated and reconstructed the bio-cake by using the carbon nanotube membrane (25 cm2 area, 17 LMH flux) as the anode in an anaerobic membrane bioreactor (AnMBR), and investigated its positive role in advanced removal of dissolved organic matter from up-flow anaerobic sludge bed unit (3 L/d) when treating synthetic municipal wastewater. At the anodic membrane interface, the enhanced biodegradation was proved to dominate the DOM reduction (contribution >40%), controlling the effluent COD as low as 19.2 ± 2.5 mg/L. Bio-cake characterizations suggested that the positive potential induced electroactive improvement, cell viability boost, and metabolic optimization. Metatranscriptomic analyses revealed that anode respiratory out-compete methanogenesis, forwarding a synergetic metabolism between enriched fermenters like Proteiniphilum sp. and exoelectrogens like Geobacter sp. Thus, electroactive bio-cake not only accelerated the decomposition of inside foulants to maintain the high flux, but also efficiently intercepted flow-through DOM due to reduced mass-transfer limitations and enhanced metabolic activity. An ordered, non-clogging, and potentially functional "cell filter" was established to achieve a win-win situation between fouling control and effluent improvement, which is promising to upgrade the AnMBR technology for maximizing the sustainable regeneration in future wastewater treatment.

The influence of various microplastics on PBDEs contaminated soil remediation by nZVI and sulfide-nZVI: Impedance, electron-accepting/-donating capacity and aging

The microplastics (MPs) existed in the environment widely has resulted in novel thinking about in-situ remediation techniques, such as nano-zero-valent iron (nZVI) and sulfided nZVI (S-nZVI), which were often compromised by various environmental factors. In this study, three common MPs such as polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP) in soil were found to inhibit the degradation rate of decabromodiphenyl ether (BDE209) by nZVI and S-nZVI to different degrees due to MPs inhibiting of electron transfer which is the main way to degrade BDE209. The inhibition strength was related to its impedance (Z) and electron-accepting (EAC)/-donating capacity (EDC). Based on the explanation of the inhibition mechanism, the reason for different aging degrees of nZVI and S-nZVI in different MPs was illustrated, especially in PVC systems. Furthermore, the aging of reacted MPs, functionalization and fragmentation in particular, indicated that they were involved in the degradation process. Moreover, this work provided new insights into the field application of nZVI-based materials for removing persistent organic pollutants (POPs).

Evolution and interaction of microbial communities in mangrove microbial fuel cells and first description of Shewanella fodinae as electroactive bacterium

Understanding exoelectrogenic bacteria mechanisms and their interactions in complex biofilm is critical for the development of microbial fuel cells (MFCs). In this article, assumptions concerning the benefits of the complex sediment microbial community for electricity production were explored with both the complex microbial community and isolates identified as Shewanella. Analysis of the microbial community revealed a strong influence of the sediment community on anodes and electrolytes compared to that of only water. Moreover, while Pelobacteraceae-related genera were dominant in our MFCs instead of Desulfuromonas and Geobacter as usually reported, the electroactive Shewanella algae and Shewanella fodinae were isolated and cultivated from the anodic biofilm. S. fodinae, described for the first time as an electroactive bacterium to the best of our knowledge, led to a maximal current density of 3.6 A/m² set as 0.3 V/SCE in a three-electrode set-up fed with lactate. S. algae, in a complex medium containing several available substrates, showed several preferential oxidative behaviors including a diauxic behavior. In pure culture and under our conditions, S. fodinae and S. algae were not able to use acetate as a sole electron donor. However, their presence in our acetate-fed MFCs and the adaptive behavior of S. algae hint a syntrophic interaction between the bacteria to optimize the use of the substrate in a complex environment.

Periodic step polarization accelerates electron recovery by electroactive biofilms (EABs)

Relatively low rate of electron recovery is one of the factors that limit the advancement of bioelectrochemical systems. Here, new periodic polarizations were investigated with electroactive biofilms (EABs) enriched from activated sludge and Geobacter sulfurreducens biofilms. When representative anode potentials (E_a ) were applied, redox centers with midpoint potentials (E_mid ) higher than E_a were identified by localized cyclic voltammetry. The electrons held by these redox centers were accessible when E_a was raised to 0.4 V (vs. Ag/AgCl). New periodic polarizations that discharge at 0.4 V recovered electrons faster than normal periodic and fixed-potential polarizations. The best-performing periodic step polarization accelerated electron recovery by 23%-24% and 12%-76% with EABs and G. sulfurreducens biofilms, respectively, compared to the fixed-potential polarization. Quantitative reverse transcription polymerase chain reaction showed an increased abundance of omcZ mRNA transcripts from G. sulfurreducens after periodic step polarization. Therefore, both the rate of energy recovery by EABs and the performance of bioelectrochemical systems can be enhanced by improving the polarization schemes.

Biochar facilitates ferrihydrite reduction by Shewanella oneidensis MR-1 through stimulating the secretion of extracellular polymeric substances

Biochar can mediate extracellular electron transfer (EET) of Shewanella oneidensis MR-1 and subsequently facilitate dissimilatory reduction of iron(III) minerals. Previous studies mainly focused on the interaction of biochar and membrane cytochrome complexes to reveal the mediating mechanisms between biochar and S. oneidensis MR-1. However, the influence of biochar on the production and activity of extracellular polymeric substances (EPS) has long been neglected, despite the fact that EPS are commonly exudated by S. oneidensis MR-1 and can participate in a variety of electron transfer processes due to their redox activity. Here, we performed a series of microbial ferrihydrite reduction experiments in combination with electrochemical voltametric and impedance analyses to investigate the role of biochar in the formation and transformation of cell EPS during EET. Results showed that the added biochar not only functioned as an electron shuttle facilitating electron transfer, but also induced the secretion of five times more EPS by S. oneidensis MR-1, leading to a 1.4-fold faster ferrihydrite reduction in comparison with biochar-free setups. We further extracted the secreted EPS and found that the proportion of redox-active exoproteins was significantly (p < 0.05) increased in the EPS and resulted in a higher electron exchange capacity in secreted EPS. Such increased exoprotein content also induced a higher ratio of exoprotein to exopolysaccharide, which largely dropped diffusion and electron transfer impedance of EPS to 1.1 and 18 Ω, respectively, and accelerated the EET and thus the ferrihydrite reduction. Overall, our findings revealed the interactions between biochar and EPS matrices, which could potentially play a critical role in EET processes in both environmental or biotechnological systems.

Electroactive Biofilms of Activated Sludge Microorganisms on a Nanostructured Surface as the Basis for a Highly Sensitive Biochemical Oxygen Demand Biosensor

The possibility of the developing a biochemical oxygen demand (BOD) biosensor based on electroactive biofilms of activated sludge grown on the surface of a graphite-paste electrode modified with carbon nanotubes was studied. A complex of microscopic methods controlled biofilm formation: optical microscopy with phase contrast, scanning electron microscopy, and laser confocal microscopy. The features of charge transfer in the obtained electroactive biofilms were studied using the methods of cyclic voltammetry and electrochemical impedance spectroscopy. The rate constant of the interaction of microorganisms with the extracellular electron carrier (0.79 ± 0.03 dm³(g s)^-1) and the heterogeneous rate constant of electron transfer (0.34 ± 0.02 cm s^-1) were determined using the cyclic voltammetry method. These results revealed that the modification of the carbon nanotubes' (CNT) electrode surface makes it possible to create electroactive biofilms. An analysis of the metrological and analytical characteristics of the created biosensors showed that the lower limit of the biosensor based on an electroactive biofilm of activated sludge is 0.41 mgO₂/dm³, which makes it possible to analyze almost any water sample. Analysis of 12 surface water samples showed a high correlation (R² = 0.99) with the results of the standard method for determining biochemical oxygen demand.

Coupling riboflavin de novo biosynthesis and cytochrome expression for improving extracellular electron transfer efficiency in Shewanella oneidensis

Shewanella oneidensis MR-1, as a model exoelectrogen with divergent extracellular electron transfer (EET) pathways, has been widely used in microbial fuel cells (MFCs). The electron transfer rate is largely determined by riboflavin (RF) and c-type cytochromes (c-Cyts). However, relatively low RF production and inappropriate amount of c-Cyts substantially impedes the capacity of improving the EET rate. In this work, coupling of riboflavin de novo biosynthesis and c-Cyts expression was implemented to enhance the efficiency of EET in S. oneidensis. Firstly, the upstream pathway of RF de novo biosynthesis was divided into four modules, and the expression level of 22 genes in above four modules was fine-tuned by employing promoters with different strength. Among them, genes zwf*, glyA, ybjU which exhibited the optimal RF production were combinatorially overexpressed, leading to enhancement of maximum output power density by 166%. Secondly, the diverse c-Cyts genes were overexpressed to match high RF production, and omcA was selected for further combination. Thirdly, RF de novo biosynthesis and c-Cyts expression were combined, resulting in 2.34-fold higher power output than the parent strain. This modular and combinatorial manipulation strategy provides a generalized reference to advance versatile practical applications of electroactive microorganisms. This article is protected by copyright. All rights reserved.

Biotransformation of 2D Nanomaterials through Stimulated Bacterial Respiration-Produced Extracellular Reactive Oxygen Species: A Common but Overlooked Process

The biotransformation of 2D nanomaterials is still poorly understood, although their environmental fates are becoming an increasing concern with their broad applications. Here, we found that Ti₃C₂T_x nanosheets, a typical 2D nanomaterial, could be oxidized by reactive oxygen species (ROS) produced by both Gram-negative (Escherichia coli and Shewanella oneidensis) and Gram-positive (Bacillus subtilis) bacteria, with the formation of titanium dioxide (TiO₂) on the nanosheet surfaces and impairment of structural integrity. Specifically, Ti₃C₂T_x nanosheets stimulated bacterial respiration Complex I, leading to increased generation of extracellular O₂^•- and the formation of H₂O₂ and ^•OH via Fenton-like reactions, which intensified the oxidation of the nanosheets. Surface modifications with KOH and hydrazine (HMH), especially HMH, could limit bacterial oxidation of the nanosheets. These findings reveal a common but overlooked process in which oxygen-respiring bacteria are capable of oxidizing 2D nanosheets, providing new knowledge for environmental fate evaluation and future design of functional 2D nanomaterials.

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.

Recent advancements in azo dye decolorization in bio-electrochemical systems (BESs): Insights into decolorization mechanism and practical application

Recent advances in bio-electrochemical systems (BESs) for azo dye removal are gaining momentum due to having electrode biocarrier and electro-active bacteria that could stimulate decolorization via extracellular electron transfer. Enhanced decolorization performance is observed in most laboratory studies, indicating the great potential of BESs as an alternative to the traditional biological processes or serving as a pre-/post-processing unit to improve the performance of biological processes. It is proven more competitive in environmental friendly than physicochemical methods. While, the successful application of BESs to azo dye-containing wastewater remediation requires a deeper evaluation of its performance, mechanism and typical attributes, and a comprehensive potential evaluation of BESs practical application in terms of economic analysis and technical optimizations. This review is organized to address BESs as a practical option for azo dye removal by analyzing the decolorization mechanisms and involved functional microorganisms, followed by the comparisons of device configurations, operational conditions, and economic evaluation. It further highlights the current hurdles and prospects for the abatement of azo dyes via BES related techniques.

Wiring Up Along Electrodes for Biofilm Formation

Millimeter-length cables of bacteria were discovered growing along a graphite-rod electrode serving as an anode of a microbial electrolysis cell (MEC). The MEC had been inoculated with a culture of Fe-reducing microorganisms enriched from a polluted river sediment (Reconquista river, Argentina) and was operated at laboratory controlled conditions for 18 days at an anode poised potential of 240 mV (vs. Ag/AgCl), followed by 23 days at 480 mV (vs. Ag/AgCl). Anode samples were collected for scanning electron microscopy, phylogenetic and electrochemical analyses. The cables were composed of a succession of bacteria covered by a membranous sheath and were distinct from the known "cable-bacteria" (family Desulfobulbaceae). Apparently, the formation of the cables began with the interaction of the cells via nanotubes mostly located at the cell poles. The cables seemed to be further widened by the fusion between them. 16S rRNA gene sequence analysis confirmed the presence of a microbial community composed of six genera, including Shewanella, a well-characterized electrogenic bacteria. The formation of the cables might be a way of colonizing a polarized surface, as determined by the observation of electrodes extracted at different times of MEC operation. Since the cables of bacteria were distinct from any previously described, the results suggest that bacteria capable of forming cables are more diverse in nature than already thought. This diversity might render different electrical properties that could be exploited for various applications.

Seafood Processing Chitin Waste for Electricity Generation in a Microbial Fuel Cell Using Halotolerant Catalyst Oceanisphaera arctica YHY1

In this study, a newly isolated halotolerant strain Oceanisphaera arctica YHY1, capable of hydrolyzing seafood processing waste chitin biomass, is reported. Microbial fuel cells fed with 1% chitin and 40 g L−1 as the optimum salt concentration demonstrated stable electricity generation until 216 h (0.228 mA/cm2). N-acetyl-D-glucosamine (GlcNAc) was the main by-product in the chitin degradation, reaching a maximum concentration of 192.01 mg g−1 chitin at 120 h, whereas lactate, acetate, propionate, and butyrate were the major metabolites detected in the chitin degradation. O. arctica YHY1 utilized the produced GlcNAc, lactate, acetate, and propionate as the electron donors to generate the electric current. Cyclic voltammetry (CV) investigation revealed the participation of outer membrane-bound cytochromes, with extracellular redox mediators partly involved in the electron transfer mechanism. Furthermore, the changes in structural and functional groups in chitin after degradation were analyzed using FTIR and XRD. Therefore, the ability of O. arctica YHY1 to utilize waste chitin biomass under high salinities can be explored to treat seafood processing brine or high salt wastewater containing chitin with concurrent electricity generation.

Shewanella oneidensis MR-1 as a bacterial platform for electro-biotechnology

The genus Shewanella comprises over 70 species of heterotrophic bacteria with versatile respiratory capacities. Some of these bacteria are known to be pathogens of fishes and animals, while many are non-pathogens considered to play important roles in the global carbon cycle. A representative strain is Shewanella oneidensis MR-1 that has been intensively studied for its ability to respire diverse electron acceptors, such as oxygen, nitrate, sulfur compounds, metals, and organics. In addition, studies have been focused on its ability as an electrochemically active bacterium that is capable of discharging electrons to and receiving electrons from electrodes in bioelectrochemical systems (BESs) for balancing intracellular redox states. This ability is expected to be applied to electro-fermentation (EF) for producing value-added chemicals that conventional fermentation technologies are difficult to produce efficiently. Researchers are also attempting to utilize its electrochemical ability for controlling gene expression, for which electro-genetics (EG) has been coined. Here we review fundamental knowledge on this bacterium and discuss future directions of studies on its applications to electro-biotechnology (EB).

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

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

Microbial Electrochemical Systems: Principles, Construction and Biosensing Applications

Microbial electrochemical systems are a fast emerging technology that use microorganisms to harvest the chemical energy from bioorganic materials to produce electrical power. Due to their flexibility and the wide variety of materials that can be used as a source, these devices show promise for applications in many fields including energy, environment and sensing. Microbial electrochemical systems rely on the integration of microbial cells, bioelectrochemistry, material science and electrochemical technologies to achieve effective conversion of the chemical energy stored in organic materials into electrical power. Therefore, the interaction between microorganisms and electrodes and their operation at physiological important potentials are critical for their development. This article provides an overview of the principles and applications of microbial electrochemical systems, their development status and potential for implementation in the biosensing field. It also provides a discussion of the recent developments in the selection of electrode materials to improve electron transfer using nanomaterials along with challenges for achieving practical implementation, and examples of applications in the biosensing field.

Application of macroalgal biomass derived biochar and bioelectrochemical system with Shewanella for the adsorptive removal and biodegradation of toxic azo dye

The present study aimed towards adsorptive removal of the toxic azo dye onto biochar derived from Eucheuma spinosum biomass. Characterization of the produced biochar was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET). Eucheuma spinosum biochar (ES-BC) produced at 600 °C revealed a maximum adsorption capacity of 331.97 mg/g towards reactive red 120 dye. The adsorption data fitted best to the pseudo-second order kinetics (R² > 0.99) and Langmuir isotherm (R² > 0.98) models. These adsorption models signified the chemisorption mechanism with monolayer coverage of the adsorbent surface with dye molecules. Furthermore, the adsorption process was mainly governed by electrostatic interaction, ion exchange, metal complexation, and hydrogen bonding as supported by the solution pH, FTIR, XPS, and XRD investigation. Nevertheless, alone adsorption technology could not offer a complete solution for eliminating the noxious dyes. Therefore, the bioelectrochemical system (BES) equipped with previously isolated marine Shewanella marisflavi BBL25 was intended for the complete remediation of azo dye. The BES II demonstrated highest dye decolorization (97.06%) within 48 h at biocathode where the reductive cleavage of the azo bond occurred. Cyclic voltammetry (CV) studies of the BES revealed perfect redox reactions taking place where the redox mediators shuttled the electrons to the dye molecule to accelerate the dye decolorization. Besides, the GC-MS analysis revealed biotransformation of the dye into less toxic metabolites as tested using a phyto and cytogenotoxicity.

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.

Electric Field-Driven Direct Interspecies Electron Transfer for Bioelectrochemical Methane Production from Fermentable and Non-Fermentable Substrates

The bioelectrochemical methane production from acetate as a non-fermentable substrate, glucose as a fermentable substrate, and their mixture were investigated in an anaerobic sequential batch reactor exposed to an electric field. The electric field enriched the bulk solution with exoelectrogenic bacteria (EEB) and electrotrophic methanogenic archaea, and promoted direct interspecies electron transfer (DIET) for methane production. However, bioelectrochemical methane production was dependent on the substrate characteristics. For acetate as the substrate, the main electron transfer pathway for methane production was DIET, which significantly improved methane yield up to 305.1 mL/g chemical oxygen demand removed (CODr), 77.3% higher than that in control without the electric field. For glucose, substrate competition between EEB and fermenting bacteria reduced the contribution of DIET to methane production, resulting in the methane yield of 288.0 mL/g CODr, slightly lower than that of acetate. In the mixture of acetate and glucose, the contribution of DIET to methane production was less than that of the single substrate, acetate or glucose, due to the increase in the electron equivalent for microbial growth. The findings provide a better understanding of electron transfer pathways, biomass growth, and electron transfer losses depending on the properties of substrates in bioelectrochemical methane production.

Microbial electroactive biofilms dominated by Geoalkalibacter spp. from a highly saline-alkaline environment

Understanding of the extreme microorganisms that possess extracellular electron transfer (EET) capabilities is pivotal to advance electromicrobiology discipline and to develop niche-specific microbial electrochemistry-driven biotechnologies. Here, we report on the microbial electroactive biofilms (EABs) possessing the outward EET capabilities from a haloalkaline environment of the Lonar lake. We used the electrochemical cultivation approach to enrich haloalkaliphilic EABs under 9.5 pH and 20 g/L salinity conditions. The electrodes controlled at 0.2 V vs. Ag/AgCl yielded the best-performing biofilms in terms of maximum bioelectrocatalytic current densities of 548 ± 23 and 437 ± 17 µA/cm² with acetate and lactate substrates, respectively. Electrochemical characterization of biofilms revealed the presence of two putative redox-active moieties with the mean formal potentials of 0.183 and 0.333 V vs. Ag/AgCl, which represent the highest values reported to date for the EABs. 16S-rRNA amplicon sequencing of EABs revealed the dominance of unknown Geoalkalibacter sp. at ~80% abundance. Further investigations on the haloalkaliphilic EABs possessing EET components with high formal potentials might offer interesting research prospects in electromicrobiology.

Extracellular Electron Shuttling Mediated by Soluble c-Type Cytochromes Produced by Shewanella oneidensis MR-1

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.

A review on the contribution of electron flow in electroactive wetlands: Electricity generation and enhanced wastewater treatment

In less than a decade, bioelectrochemical systems/microbial fuel cell integrated constructed wetlands (electroactive wetlands) have gained a considerable amount of attention due to enhanced wastewater treatment and electricity generation. The enhancement in treatment has majorly emanated from the electron transfer or flow, particularly in anaerobic regions. However, the chemistry associated with electron transfer is complex to understand in electroactive wetlands. The electroactive wetlands accommodate diverse microbial community in which each microbe set their own potential to further participate in electron transfer. The conductive materials/electrodes in electroactive wetlands also contain some potential, due to which, several conflicts occur between microbes and electrode, and results in inadequate electron transfer or involvement of some other reaction mechanisms. Still, there is a considerable research gap in understanding of electron transfer between electrode-anode and cathode in electroactive wetlands. Additionally, the interaction of microbes with the electrodes and understanding of mass transfer is also essential to further understand the electron recovery. This review mainly deals with the electron transfer mechanism and its role in pollutant removal and electricity generation in electroactive wetlands.

Single cell electron collectors for highly efficient wiring-up electronic abiotic/biotic interfaces

By electronically wiring-up living cells with abiotic conductive surfaces, bioelectrochemical systems (BES) harvest energy and synthesize electric-/solar-chemicals with unmatched thermodynamic efficiency. However, the establishment of an efficient electronic interface between living cells and abiotic surfaces is hindered due to the requirement of extremely close contact and high interfacial area, which is quite challenging for cell and material engineering. Herein, we propose a new concept of a single cell electron collector, which is in-situ built with an interconnected intact conductive layer on and cross the individual cell membrane. The single cell electron collector forms intimate contact with the cellular electron transfer machinery and maximizes the interfacial area, achieving record-high interfacial electron transfer efficiency and BES performance. Thus, this single cell electron collector provides a superior tool to wire living cells with abiotic surfaces at the single-cell level and adds new dimensions for abiotic/biotic interface engineering.

Abatement of the membrane biofouling: Performance of an in-situ integrated bioelectrochemical-ultrafiltration system

The practical applications of membrane-based water treatment techniques are constrained by the problem of membrane fouling. Various studies have revealed that interactions between extracellular polymeric substances (EPS) and the membrane surface determine the extent of irreversible fouling. Herein, we describe a novel bioelectrochemical system (BES) integrated with an ultrafiltration (UF) membrane in order to provide an enhanced antifouling property. It was found that the integrated BES membrane system had a superior performance compared to a conventional (control) UF system, as manifested by a much lower development of transmembrane pressure. The BES significantly reduced microbial viability in the membrane tank and the imposed electrode potential contributed to the degradation of biopolymers, which favored the alleviation of membrane fouling. Notably, the electron transfer between the acclimated microorganisms and the conductive membrane in the BES integrated system exhibited an increasing trend with the operation time, indicating a gradual increase in microbial electrical activity. Correspondingly, the accumulation of extracellular polymeric substances (EPS) on the membrane surface of the BES integrated system showed a substantial decrease compared to the control system, which could be attributed to a series of synergistic effects induced by the BES integration. The differences in the microbial diversity between the control and the BES integrated system revealed the microbial selectivity of the poised potential. Specifically, microbial strains with relatively high EPS production, like the genus of Zoogloea and Methyloversatilis, were reduced significantly in the BES integrated system, while the expression of the electroactive bacteria was promoted, which facilitated extracellular electron transfer (EET) and therefore the bioelectrochemical reactions. Overall, this study has presented a feasible and promising new approach for membrane fouling mitigation during the process of water treatment.

Accelerated Corrosion of 316L Stainless Steel Caused by Shewanella algae Biofilms

In the marine environment, microbiologically influenced corrosion (MIC) is a major problematic issue, which leads to severe damage to metals and alloys. The prerequisite to mitigate this worldwide problem is to investigate the mechanisms of marine-corroding microbes. Therefore, the corrosion behavior of 316L stainless steel in the presence of marine Shewanella algae was investigated by means of electrochemical measurements and surface analysis. The results revealed that S. algae is capable of forming a dense and thick biofilm on the surfaces of 316L SS coupons after 7 days of incubation, which reached about a thickness of 40.4 μm. According to electrochemical results, the S. algae biofilm also induced the corrosion of 316L SS coupons. The accelerated corrosion of 316L SS coupons was in the form of pits, which was formed underneath the biofilms. The largest pit depth after 14 days of incubation time reached 9.8 μm, which was 6.7 times higher than the one immersed in abiotic medium (1.45 μm). This is the first study demonstrating the MIC of 316L SS due to the S. algae biofilm.

Carbon dots-fed Shewanella oneidensis MR-1 for bioelectricity enhancement

Bioelectricity generation, by Shewanella oneidensis (S. oneidensis) MR-1, has become particularly alluring, thanks to its extraordinary prospects for energy production, pollution treatment, and biosynthesis. Attempts to improve its technological output by modification of S. oneidensis MR-1 remains complicated, expensive and inefficient. Herein, we report on the augmentation of S. oneidensis MR-1 with carbon dots (CDs). The CDs-fed cells show accelerated extracellular electron transfer and metabolic rate, with increased intracellular charge, higher adenosine triphosphate level, quicker substrate consumption and more abundant extracellular secretion. Meanwhile, the CDs promote cellular adhesion, electronegativity, and biofilm formation. In bioelectrical systems the CDs-fed cells increase the maximum current value, 7.34 fold, and power output, 6.46 fold. The enhancement efficacy is found to be strongly dependent on the surface charge of the CDs. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of S. oneidensis MR-1, holding promise in all relevant technologies.

Bacterial fuel cells have generated attention with the prospect of green energy production; current research is focused on optimising the system to improve efficiency. Here, the authors report on the feeding of carbon dots to S. oneidensis MR-1 to enhance metabolic activity and bioelectric generation.

Enhanced extracellular electron transfer between Shewanella putrefaciens and carbon felt electrode modified by bio-reduced graphene oxide

Extracellular electron transfer (EET) is a governing factor for the electrochemical performance of a bioelectrochemical system (BES) such as the microbial fuel cell (MFC). Herein, an in situ method to fabricate a bio-reduced graphene oxide (GO) (br-GO) modified carbon felt electrode to increase EET was developed. GO (0.5mgmL^-1) was spiked into the anode chamber in a three-electrode BES and was transformed to br-GO with a self-assembled three-dimensional (3D) structure. The response of the br-GO modified electrode potential to the attached population of Shewanella putrefaciens increased from 0.071V to 0.517V (vs Ag/AgCl). Meanwhile, br-GO modification resulted a significant enhancement in the total amount of extracellular electrons transferred between the modified electrode and microbe. The process of br-GO modification lowered the charge transfer resistance of the electrode and enhanced the EET. The modified electrode was further employed as an anode in the MFC, and consequently, the power density of the MFC was significantly enhanced. The current study not only gives a simple and effective way for improving the EET with br-GO fabrication, but also provides a strategy to enhance the power density of the MFC.

Investigating the role of anodic potential in the biodegradation of carbamazepine in bioelectrochemical systems

Anode potential is a critical factor in the biodegradation of organics in bioelectrochemical systems (BESs), but research on these systems with complex recalcitrant co-substrates at set anode potentials is scarce. In this study, carbamazepine (CBZ) biodegradation in a BES was examined over a wide range of set anode potentials (-200 to +600 mV vs Ag/AgCl). Current generation and current densities were improved with the increase in positive anode potentials. However, at a negative potential (-200 mV), current generation was higher as compared to that for +000 and +200 mV. The highest CBZ degradation (84%) and TOC removal efficiency (70%) were achieved at +400 mV. At +600 mV, a decrease in CBZ degradation was observed, which can be attributed to a low number of active bacteria and a poor ability to adapt to high voltage. This study signified that BESs operated at optimum anode potentials could be used for enhancing the biodegradation of complex and recalcitrant contaminants in the environment.

EPS bound flavins driven mediated electron transfer in thermophilic Geobacillus sp

Through extracellular electron transfer (EET), bacteria are capable of transforming different insoluble materials of geochemical interest into energy-rich molecules for their growth. For this process, bacteria have been depending directly or indirectly on molecules synthesized within the cells or by various synthetics as mediators. Herein, we studied the in-situ change in electrochemistry and supporting components for EET in the extracellular polysaccharide (EPS) producing biofilm of thermophilic Geobacillus sp. The CV and DPV resultsrevealed that the intact biofilm of bacteria was not able to generate any potential at 25 °C /- ≤50 °C. However, at 55 °C (optimal condition), the potential occurred drastically after the EPS production by bacteria. HPLC and MALDI-TOF results revealed that the presence of Flavins, which can able adsorbed to the electrodes from the cell surface. Moreover, the temperature-dependent EPS production and originally conceived ability of flavins to act as electron shuttles suggest that not much complexity in bacteria with minerals. Additionally, the electrochemical potential was severely affected upon removal of EPS/flavin moiety from the intact biofilm, revealed the necessity of EPS bound flavins in transferring the electrons across its thick cell walls. This paradigm shift to electrogenic nature of Geobacillus sp. biofilm will become evident in the adaptation of other microbes during mineral respiration in extreme environments.

Controlled Layer-By-Layer Deposition of Carbon Nanotubes on Electrodes for Microbial Fuel Cells

Carbon nanotubes (CNTs) and polyelectrolyte poly(allylamine hydrochloride) (PAH) composite modified indium tin oxide (ITO) electrodes, by a layer-by-layer (LBL) self-assembly technique, was evaluated as an anode for microbial fuel cells (MFCs). The bioelectrochemistry of Shewanella loihica PV-4 in an electrochemical cell and the electricity generation performance of MFCs with multilayer (CNTs/PAH)n-deposited ITO electrodes as an anode were investigated. Experimental results showed that the current density generated on the multilayer modified electrode increased initially and then decreased as the deposition of the number of layers (n = 12) increased. Chronoamperometric results showed that the highest peak current density of 34.85 ± 2.80 mA/m2 was generated on the multilayer (CNTs/PAH)9-deposited ITO electrode, of which the redox peak current of cyclic voltammetry was also significantly enhanced. Electrochemical impedance spectroscopy analyses showed a well-formed nanostructure porous film on the surface of the multilayer modified electrode. Compared with the plain ITO electrode, the multilayered (CNTs/PAH)9 anodic modification improved the power density of the dual-compartment MFC by 29%, due to the appropriate proportion of CNTs and PAH, as well as the porous nanostructure on the electrodes.

Promoting Shewanella Bidirectional Extracellular Electron Transfer for Bioelectrocatalysis by Electropolymerized Riboflavin Interface on Carbon Electrode

The extracellular electron transfer (EET) that connects the intracellular metabolism of electroactive microorganisms to external electron donors/acceptors, is the foundation to develop diverse microbial electrochemical technologies. For a particular microbial electrochemical device, the surface chemical property of an employed electrode material plays a crucial role in the EET process owing to the direct and intimate biotic-abiotic interaction. The functional modification of an electrode surface with redox mediators has been proposed as an effectual approach to promote EET, but the underlying mechanism remains unclear. In this work, we investigated the enhancement of electrochemically polymerized riboflavin interface on the bidirectional EET of Shewanella putrefaciens CN32 for boosting bioelectrocatalytic ability. An optimal polyriboflavin functionalized carbon cloth electrode achieved about 4.3-fold output power density (∼707 mW/m2) in microbial fuel cells and 3.7-fold cathodic current density (∼0.78 A/m2) for fumarate reduction in three-electrode cells compared to the control, showing great increases in both outward and inward EET rates. Likewise, the improvement was observed for polyriboflavin-functionalized graphene electrodes. Through comparison between wild-type strain and outer-membrane cytochrome (MtrC/UndA) mutant, the significant improvements were suggested to be attributed to the fast interfacial electron exchange between the polyriboflavin interface with flexible electrochemical activity and good biocompatibility and the outer-membrane cytochromes of the Shewanella strain. This work not only provides an effective approach to boost microbial electrocatalysis for energy conversion, but also offers a new demonstration of broadening the applications of riboflavin-functionalized interface since the widespread contribution of riboflavin in various microbial EET pathways together with the facile electropolymerization approach.

Electrode Potential Dependency of Single-Cell Activity Identifies the Energetics of Slow Microbial Electron Uptake Process

Electrochemical measurements have been widely applied to study microbial extracellular electron transport processes. However, because electrochemistry detects not only microbial electron transport but also other reactions, background signals comparable to or larger than microbial ones hamper the identification of microbial electrochemical properties. This problem is crucial especially for the detection of electron uptake processes by slow-growing microbes in low-energy subsurface sediments, as the environmental samples contain electrochemically active humus and mineral particles. In this study, we report a cell-specific stable isotope analysis to quantify the electrode potential dependency of anabolic activity in individual cells for identifying the electron uptake energetics of slow-growing bacteria. Followed by the incubation of Desulfovibrio ferrophilus IS5 cells with isotopic ¹⁵N-ammonium as the sole N source on electrodes poised at potentials of -0.2, -0.3, -0.4, and -0.5 V [vs. standard hydrogen electrode (SHE)], we conducted nanoscale secondary ion mass spectroscopy (NanoSIMS) to quantify ¹⁵N assimilation in more than 100 individual cells on the electrodes. We observed significant ¹⁵N assimilation at potentials of -0.4 and more ¹⁵N assimilation at -0.5 V, which is consistent with the onset potential for electron uptake via outer-membrane cytochromes (OMCs). The activation of cell energy metabolism was further examined by transcriptome analysis. Our results showed a novel methodology to study microbial electron uptake energetics. The results also serve as the first direct evidence that energy acquisition is coupled to the electron uptake process in sulfate-reducing bacteria that are ubiquitous in the subsurface environments, with implications on the electron-fueled subsurface biosphere hypothesis and other microbial processes, such as anaerobic iron corrosion and anaerobic methane oxidation.

Shewanella oneidensis MR-1 Utilizes both Sodium- and Proton-Pumping NADH Dehydrogenases during Aerobic Growth

Shewanella oneidensis MR-1 is a metal-reducing bacterium with the ability to utilize many different terminal electron acceptors, including oxygen and solid-metal oxides. Both metal oxide reduction and aerobic respiration have been studied extensively in this organism. However, electron transport chain processes upstream of the terminal oxidoreductases have been relatively understudied in this organism, especially electron transfer from NADH to respiratory quinones. Genome annotation indicates that S. oneidensis MR-1 encodes four NADH dehydrogenases, a proton-translocating dehydrogenase (Nuo), two sodium ion-translocating dehydrogenases (Nqr1 and Nqr2), and an “uncoupling” dehydrogenase (Ndh), but none of these complexes have been studied. Therefore, we conducted a study specifically focused on the effects of individual NADH dehydrogenase knockouts in S. oneidensis MR-1. We observed that two of the single-mutant strains, the ΔnuoN and ΔnqrF1 mutants, exhibited significant growth defects compared with the wild type. However, the defects were minor and only apparent under certain growth conditions. Further testing of the ΔnuoN ΔnqrF1 double-mutant strain yielded no growth in minimal medium under oxic conditions, indicating that Nuo and Nqr1 have overlapping functions, but at least one is necessary for aerobic growth. Coutilization of proton- and sodium ion-dependent energetics has important implications for the growth of this organism in environments with varied pH and salinity, including microbial electrochemical systems.

IMPORTANCE Bacteria utilize a wide variety of metabolic pathways that allow them to take advantage of different energy sources, and to do so with varied efficiency. The efficiency of a metabolic process determines the growth yield of an organism, or the amount of biomass it produces per amount of substrate consumed. This parameter has important implications in biotechnology and wastewater treatment, where low growth yields are often preferred to minimize the production of microbial biomass. In this study, we investigated respiratory pathways containing NADH dehydrogenases with varied efficiency (i.e., the number of ions translocated per NADH oxidized) in the metal-reducing bacterium Shewanella oneidensis MR-1. We observed that two different respiratory pathways are used concurrently, and at least one pathway must be functional for growth under oxic conditions.

FeS2 Nanoparticles Decorated Graphene as Microbial-Fuel-Cell Anode Achieving High Power Density

Microbial fuel cells (MFCs) have received great attention worldwide due to their potential in recovering electrical energy from waste and inexhaustible biomass. Unfortunately, the difficulty of achieving the high power, especially in real samples, remains a bottleneck for their practical applications. Herein, FeS₂ nanoparticles decorated graphene is fabricated via a simple hydrothermal reaction. The FeS₂ nanoparticles decorated graphene anode not only benefits bacterial adhesion and enrichment of electrochemically active Geobacter species on the electrode surface but also promotes efficient extracellular electron transfer, thus giving rise to a fast start-up time of 2 d, an unprecedented power density of 3220 mW m^-2 and a remarkable current density of 3.06 A m^-2 in the acetate-feeding and mixed bacteria-based MFCs. Most importantly, the FeS₂ nanoparticles decorated graphene anode successfully achieves a power density of 310 mW m^-2 with simultaneous removal of 1319 ± 28 mg L^-1 chemical oxygen demand in effluents from a beer factory wastewater. The characteristics of improved power generation and enhanced pollutant removal efficiency opens the door toward development of high-performance MFCs via rational anode design for practical application.

The catalytic effect of TiO2 nanosheets on extracellular electron transfer of Shewanella loihica PV-4

Electron transfer kinetics of Shewanella loihica PV-4 at the up-growing TiO₂ nanosheet (TiO₂-NS) modified carbon paper (CP) electrode was investigated. The effect of TiO₂-NSs, which speeds up the interfacial electron transfer of outer membrane c-type cytochromes (OMCs), was revealed for the first time. TiO₂-NSs with a polar surface modified hydrophobic CP into super-hydrophilic TiO₂-NS/CP. The favorable interaction between PV-4 and TiO₂-NSs not only enhanced microbial adhesion, but also altered the redox nature of OMCs. The mid-point potential of OMCs at TiO₂-NS/CP was shifted to a more negative potential, indicating a higher thermodynamic driving force for the protein to release electrons. Moreover, electron transfer from OMCs to TiO₂-NSs was also benefited from the positive shift of flat-band potential V_fb owing to reduced pH at the electrode/microorganism interface, as well as good electrical conductivity of TiO₂-NSs. As a result, the electron transfer rate constant k_et of OMCs at the TiO₂-NS/CP anode was about three times faster than that at the CP anode. The accelerated electron transfer kinetics as well as 15% increase of biomass together accounted for a 97% increase of the maximum output power density in the MFC. The result expanded our knowledge about the role of a designed TiO₂ nanostructure in microbial electron transfer that can be applied in other bio-electrochemical systems.

Nanostructured interfaces for probing and facilitating extracellular electron transfer

Probing and facilitating microbial extracellular electron transfer through nanotechnology enabled platforms are transforming bioenergetic, bioelectronic, and other related research areas.

CRISPRi-sRNA: Transcriptional-Translational Regulation of Extracellular Electron Transfer in Shewanella oneidensis

Extracellular electron transfer (EET) in Shewanella oneidensis MR-1, which is one of the most well-studied exoelectrogens, underlies many microbial electrocatalysis processes, including microbial fuel cells, microbial electrolysis cells, and microbial electrosynthesis. However, regulating the efficiency of EET remains challenging due to the lack of efficient genome regulation tools that regulate gene expression levels in S. oneidensis. Here, we systematically established a transcriptional regulation technology, i.e., clustered regularly interspaced short palindromic repeats interference (CRISPRi), in S. oneidensis MR-1 using green fluorescent protein (GFP) as a reporter. We used this CRISPRi technology to repress the expression levels of target genes, individually and in combination, in the EET pathways (e.g., the MtrCAB pathway and genes affecting the formation of electroactive biofilms in S. oneidensis), which in turn enabled the efficient regulation of EET efficiency. We then established a translational regulation technology, i.e., Hfq-dependent small regulatory RNA (sRNA), in S. oneidensis by repressing the GFP reporter and mtrA, which is a critical gene in the EET pathways in S. oneidensis. To achieve coordinated transcriptional and translational regulation at the genomic level, the CRISPRi and Hfq-dependent sRNA systems were incorporated into a single plasmid harbored in a recombinant S. oneidensis strain, which enabled an even higher efficiency of mtrA gene repression in the EET pathways than that achieved by the CRISPRi and Hfq-dependent sRNA system alone, as exhibited by the reduced electricity output. Overall, we developed a combined CRISPRi-sRNA method that enabled the synergistic transcriptional and translational regulation of target genes in S. oneidensis. This technology involving CRISPRi-sRNA transcriptional-translational regulation of gene expression at the genomic level could be applied to other microorganisms.