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Bioelectronic devices can use electron flux to enable communication between biotic components and abiotic electrodes. We have modified Escherichia coli to electrically interact with electrodes by expressing the cytochrome c from Shewanella oneidensis MR-1. However, we observe inefficient electrical performance, which we hypothesize is due to the limited compatibility of the E. coli cytochrome c maturation (Ccm) systems with MR-1 cytochrome c. Here we test whether the bioelectronic performance of E. coli can be improved by constructing hybrid Ccm systems containing protein domains from both E. coli and S. oneidensis MR-1. The hybrid CcmH increased cytochrome c expression by increasing the abundance of CymA 60%, while only slightly changing the abundance of the other cytochromes c. Electrochemical measurements showed that the overall current from the hybrid ccm strain increased 121% relative to the wildtype ccm strain, with an electron flux per cell of 12.3 ± 0.3 fA·cell-1. Additionally, the hybrid ccm strain doubled its electrical response with the addition of exogenous flavin, and quantitative analysis of this demonstrates CymA is the rate-limiting step in this electron conduit. These results demonstrate that this hybrid Ccm system can enhance the bioelectrical performance of the cyt c expressing E. coli, allowing the construction of more efficient bioelectronic devices.

Extracellular electron transfer (EET) is a process that microorganisms use to reduce or oxidize external insoluble electron acceptors or donors. Much of our mechanistic understanding of this process is derived from studies of transmembrane cytochrome complexes and extracellular redox shuttles that mediate outward EET to anodes and external electron acceptors. In contrast, there are knowledge gaps concerning the reverse process of inward EET from external electron donors to cells. Here, we describe a role for soluble iron (exogenous FeCl2) in enhancing EET from cathodes to the model EET bacterium Shewanella oneidensis MR‐1, with fumarate serving as the intracellular electron acceptor. This iron concentration‐dependent electron uptake was eradicated upon addition of an iron chelator and occurred only in the presence of fumarate reductase, confirming an electron pathway from cathodes to this periplasmic enzyme. Moreover, S. oneidensis mutants lacking specific outer membrane and periplasmic cytochromes exhibited significantly decreased current levels relative to wild‐type. These results indicate that soluble iron can function as an electron carrier to the EET machinery of S. oneidensis. [ABSTRACT FROM AUTHOR]

Constructed wetland (CW) is an effective ecological technology to treat water pollution and has the significant advantages of high impact resistance, simple construction process, and low maintenance cost. However, under extreme conditions such as low temperature, high salt concentration, and multiple types of pollutants, some bottlenecks exist, including the difficulty in improving operating efficiency and the low pollutant removal rate. Microbial electrochemical technology is an emerging clean energy technology and has the similar structure and pollutant removal mechanism to CW. Microbial electrochemistry combined with CW can improve the overall removal effect of pollutants in wetlands. This review summarizes characterization methods of microbial electrochemistry-enhanced constructed wetland systems, construction methods of different composite systems, mechanisms of single and composite systems, and removal effects of composite systems on different pollutants in water bodies. Based on the shortcomings of existing studies, the potential breakthroughs in microbial electrochemistry-enhanced constructed wetlands are proposed for developing the optimization solution of constructed wetlands. [ABSTRACT FROM AUTHOR]

[Display omitted] • The correlation between the power generation and pollutant degradation was determined. • Activated carbon (AC) improves the pollutant transfer from liquid phase to biofilm. • Modified ruthenium (Ru) accelerates the electron transfer from biofilm to anode. • The Ru-AC bioanode enhances chlorobenzenes removal and power generation. • Ru and AC together promote the conversion of chlorophenols to organic acids. Microbial fuel cell (MFC) is a promising device for water decontamination and energy generation. However, the correlation between power generation and pollutant degradation has not been clarified. Herein, a ruthenium–activated carbon (Ru–AC) bioanode was constructed for chlorobenzenes (CBs) treatment. The pollutant tolerance was improved by Ru–AC anode, and the minimum removal efficiencies of CB and ortho -dichlorobenzene (o -DCB) reached 75.1 % and 69.3 %, respectively, which were considerably higher than those of other MFCs (16.3 %–39.7 %). Correspondingly, the maximum output voltage reached 360.7 mV for the Ru–AC anode, whereas the values obtained from others reached 45.2–149.6 mV. Interaction models were introduced to quantify the relationship between power generation and pollutant degradation. The conversion of highly toxic chlorophenols to organic acids could be accelerated by boosting the mass and electron transfer, thereby simultaneously enhancing CBs removal and power generation. This work provided important insights into pollutant-powered MFC development. [ABSTRACT FROM AUTHOR]

In microbial electrochemical systems, microorganisms catalyze chemical reactions converting chemical energy present in organic and inorganic molecules into electrical energy. The concept of microbial electrochemistry has been gaining tremendous attention for the past two decades, mainly due to its numerous applications. This technology offers a wide range of applications in areas such as the environment, industries, and sensors. The biocatalysts governing the reactions could be cell secretion, cell component, or a whole cell. The electroactive bacteria can interact with insoluble materials such as electrodes for exchanging electrons through colonization and biofilm formation. Though biofilm formation is one of the major modes for extracellular electron transfer with the electrode, there are other few mechanisms through which the process can occur. Apart from biofilm formation electron exchange can take place through flavins, cytochromes, cell surface appendages, and other metabolites. The present article targets the various mechanisms of electron exchange for microbiome-induced electron transfer activity, proteins, and secretory molecules involved in the electron transfer. This review also focuses on various proteomics and genetics strategies implemented and developed to enhance the exo-electron transfer process in electroactive bacteria. Recent progress and reports on synthetic biology and genetic engineering in exploring the direct and indirect electron transfer phenomenon have also been emphasized. [ABSTRACT FROM AUTHOR]

Recently several non-traditional electroactive microorganisms have been discovered. These can be considered weak electricigens; microorganisms that typically rely on soluble electron acceptors and donors in their lifecycle but are also capable of extracellular electron transfer (EET), resulting in either a low, unreliable, or otherwise unexpected current. These unanticipated electroactive microorganisms represent a new chapter in electromicrobiology and have important medical, environmental, and biotechnological relevance. As such, it is essential to continue the momentum of their discovery. However, their study poses unique challenges due to their low current output. Capturing their signal necessitates novel approaches including unconventional electrode choice, the use of sensitive electrochemical techniques, and modifications of conventional experiments that use bioelectrochemical systems (BES). Recent years have witnessed the discovery of several non-traditional electroactive microorganisms with important medical, environmental, and biotechnological relevance. These microorganisms typically rely on soluble electron acceptors and donors in their lifecycle but are also capable of extracellular electron transfer (EET), resulting in either a low, unreliable, or otherwise unexpected current. As such they can be categorised as 'weak electricigens'. Due to their low current output, there are specific challenges to capturing their signal. This requires a careful analysis of the techniques available in adjacent fields. [ABSTRACT FROM AUTHOR]

The electrical conductivity measured in Shewanella and Geobacter spp. is an intriguing physical property that is the fundamental basis for possible extracellular electron transport (EET) pathways. There is considerable debate regarding the origins of the electrical conductivity reported in these microbial cellular structures, which is essential for deciphering the EET mechanism. Here, we report systematic on-chip nanoelectronic investigations of both Shewanella and Geobacter spp. under physiological conditions to elucidate the complex basis of electrical conductivity of both individual microbial cells and biofilms. Concurrent electrical and electrochemical measurements of living Shewanella at both few-cell and the biofilm levels indicate that the apparent electrical conductivity can be traced to electrochemical-based electron transfer at the cell/electrode interface. We further show that similar results and conclusions apply to the Geobacter spp. Taken together, our study offers important insights into previously proposed physical models regarding microbial conductivities as well as EET pathways for Shewanella and Geobacter spp.

In microbial electrochemical systems, microorganisms catalyze chemical reactions converting chemical energy present in organic and inorganic molecules into electrical energy. The concept of microbial electrochemistry has been gaining tremendous attention for the past two decades, mainly due to its numerous applications. This technology offers a wide range of applications in areas such as the environment, industries, and sensors. The biocatalysts governing the reactions could be cell secretion, cell component, or a whole cell. The electroactive bacteria can interact with insoluble materials such as electrodes for exchanging electrons through colonization and biofilm formation. Though biofilm formation is one of the major modes for extracellular electron transfer with the electrode, there are other few mechanisms through which the process can occur. Apart from biofilm formation electron exchange can take place through flavins, cytochromes, cell surface appendages, and other metabolites. The present article targets the various mechanisms of electron exchange for microbiome-induced electron transfer activity, proteins, and secretory molecules involved in the electron transfer. This review also focuses on various proteomics and genetics strategies implemented and developed to enhance the exo-electron transfer process in electroactive bacteria. Recent progress and reports on synthetic biology and genetic engineering in exploring the direct and indirect electron transfer phenomenon have also been emphasized.

Desulfarculus baarsii and Desulfurivibrio alkaliphilus are strictly anaerobic bacteria existing in marine sediments. D. baarsii gains energy through reducing sulphate and D. alkaliphilus is able to reduce elemental sulphur, thiosulphate and polysulphide in seawater. Both organisms were previously identified as key organisms in sediment derived, bidirectional electroactive biofilms. Here, we investigated the electrochemical performance of these two bacteria in bio‐electrochemical systems and their possible involvement in anodic and cathodic reactions. The results show that D. baarsii was unable to donate or accept electrons to/from an electrode, while D. alkaliphilus was able to catalyse both anodic and cathodic reactions and interact with the electrode through direct or potentially indirect electron transfer. Raman spectra of D. alkaliphilus electrode biofilms showed a high similarity to Geobacter sulfurreducens biofilms, including the specific bands of cytochromes b and c, explaining the electroactivity of D. alkaliphilus in bioelectrochemical reactions. [ABSTRACT FROM AUTHOR]

Microbial fuel cells (MFCs) have shown immense potential as a one-stop solution for three major sustainability issues confronting the world today—energy security, global warming and wastewater management. MFCs represent a cross-disciplinary platform for research at the confluence of the natural and engineering sciences. The diversity of variables influencing performance of MFCs has garnered research interest across varied scientific disciplines since the beginning of this century. The increasing number of research publications has made it necessary to keep track of work being carried out by research groups across the globe and consolidate significant findings on a regular basis. Review articles are often the nodal points for beginners who are required to undertake an exploratory survey of literature to identify a suitable research problem. This 'review of reviews' is a ready-reckoner that directs readers to relevant reviews and research articles reporting significant developments in MFC research in the last two decades. The article also highlights the areas needing research attention which when addressed could take this technology a few more steps closer to practical implementation. [ABSTRACT FROM AUTHOR]

In this study, an aerobic granular sludge electrochemical system (AGES) was established by applying the micro-electric field to an aerobic granular sludge (AGS) reactor for the degradation of sulfamethoxazole (SMZ). Under the stimulation of the micro-electric field, the granulation of sludge was improved and the degradation rate of SMZ was enhanced. The features of granular sludge were characterized by scanning electron microscopy and X-ray diffraction. The optimal degradation rate of SMZ (88%) was obtained at the voltage of 3 V and the effective electrode area of 800 mm2. The results of kinetics analyses revealed that the degradation of SMZ by AGES can be fitted with the second-order kinetic equation, showing a degradation rate constant (k) of 0.001 L mol−1·min−1. The degradation products of SMZ in the AGES system were detected by LC-MS and their possible degradation routes were elucidated. The micro-electric field in the AGES system played a selective role in microbes' enrichment and growth, changing the diversity of the microbial community. Pseudomonas, Tolumonas, and Acidovorax were the dominant bacteria in the AGES system, which is accountable for the abatement of SMZ and nutrients. This work provides a green means for improving AGS and paves the way for applying the AGS process to real-world wastewater treatment. [Display omitted] • An aerobic granular sludge electrochemical system was established. • The voltage and electrode plate area were optimized. • Micro-electric field played a selective role in microbes' enrichment and growth. • The potential degradation products and pathways of SMZ were proposed. • The main function microbes in AGES system for SMZ degradation was disclosed. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonization (HTC) is a promising technology for chemical and material synthesis. However, HTC produces not only valuable solid coal-materials but also yields process water (PW) with high chemical oxygen demand (COD) that requires extensive treatment. Anaerobic digestion (AD) has been used for initial treatment of HTC-PW, but the AD effluent is still high in COD and particles. Here, we show that microbial electrochemical technologies (MET) can be applied for COD removal from AD effluent of HTC-PW. Bioelectrochemical systems (BES) treating different shares of AD effluent from HTC-PW exhibited similar trends for current production. Thereby, maximum current densities of 0.24 mA cm−2 and COD removal of 65.4 ± 4.4% were reached (n = 3). Microbial community analysis showed that the genus Geobacter dominated anode biofilm and liquid phase of all reactors indicating its central role for COD oxidation and current generation.

Cyanobacteria form dense blooms that impair water quality. Although the impact of blooms on water quality largely differs depending on the species of cyanobacteria, it is challenging to distinguish cyanobacterial species with conventional sensors due to their similar physicochemical properties. Here, we conducted cyclic voltammetry to three major bloom‐forming cyanobacteria, Synechococcus sp. Microcystis aeruginosa, and Anabaena circinalis without addition of redox active compounds nor chemical modifications of the working electrode. These cyanobacteria exhibited redox peaks with distinct potentials from each other, in the range of +0.60 – +0.75 V vs SHE. Because these redox potentials hardly overlap with those of other environmental electroactive bacteria, those redox peaks may be used as indicators of the onset of a cyanobacterial bloom. Given that the photo‐responsive current correlated with the cell density of Synechococcus sp. electrochemical measurement would be a promising technique for a novel cyanobacterial quantification sensor, with in‐built ability to identify cyanobacterial species. [ABSTRACT FROM AUTHOR]

Microbial Electrochemical Fluidized Bed Reactors (ME-FBR) represent a new concept for promoting proper bacteria-electrode interaction and eventually efficient biocatalysis in Microbial Electrochemical Technologies (METs). In the current work we demonstrate how a fluidized cathode, a dynamic and discontinuous design of electrode, can be an effective electron donor for electroactive hydrogen-generating and nitrate reducing bacteria. Furthermore, the oxygen produced in the anodic reaction promoted ammonium oxidation to nitrate by nitrifying bacteria thus expanding the environmental applications of the system. By coupling both anodic and cathodic reactions, it was possible to simultaneously achieve nitrification–denitrification within one chamber and without external oxygen addition. Our proof-of-concept revealed the removal of 98% ammonium and ca. 29% of total nitrogen (31 g-N m−3reactor d−1) from an effluent with low organic matter under continuous mode. This study reveals for first time how fluidized beds can be integrated in METs not only as anodes but also as cathodes, broadening the opportunities and applications to bioremediation and bioelectrosynthesis processes.

In this study, the performance of electroactive bacteria (EAB), cultivated inside tubular electrode ducts, is systematically investigated to derive predictions on the behavior of EAB under conditions limited by electrochemical losses. A modeling approach is applied to assess the influence of the electrochemical losses on the electrochemical performance and scaling characteristics of complex 3D structures, such as sponges and foams. A modular flow reactor is designed that provides laminar and reproducible flow conditions as a platform for the systematic electrochemical and bioelectrochemical characterization of 3D electrodes in bioelectrochemical systems (BES). The bioelectrochemical experiments are carried out in a set of reactors incorporating cylindrical electrodes exhibiting ducts of 1 cm length and different diameters ranging from 0.1 cm up to 1 cm. Single duct calculations are extrapolated to three dimensions through geometrical considerations; trends in 3D bioanode performance are demonstrated using the resulting simplified 3D structure. The combined experimental and modeling approach constitutes a framework for future studies on systematic electrode design. [ABSTRACT FROM AUTHOR]

Bacterial cell appendix formation supports cell‐cell interaction, cell adhesion and cell movement. Additionally, in bioelectrochemical systems (BES), cell appendages have been shown to participate in extracellular electron transfer. In this work, the cell appendix formation of Clostridium acetobutylicum in biofilms of a BES are imaged and compared with conventional biofilms. Under all observed conditions, the cells possess filamentous appendages with a higher number and density in the BES. Differences in the amount of extracellular polymeric substance in the biofilms of the electrodes lead to the conclusion that the cathode can be used as electron donor and the anode as electron acceptor by C. acetobutylicum. When using conductive atomic force microscopy, a current response of about 15 nA is found for the cell appendages from the BES. This is the first report of conductivity for clostridial cell appendices and represents the basis for further studies on their role for biofilm formation and electron transfer. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoFe-LDH@Ni-MOF was prepared by simple hydrothermal method. • The maximum power density in CoFe-LDH@Ni-MOF MFC was 211 mW/m2. • The lamellar stacked structure formed by CoFe-LDH and Ni-MOF promoted ORR performance. Cobalt Iron -layered double hydroxide (CoFe-LDH) nano sheets were attached to Nickel-metal organic frameworks (Ni-MOF) by utilizing hydrothermal reaction method, and CoFe-LDH@Ni-MOF was synthesized and worked as the cathode catalyst in microbial fuel cell. The surface of this composite material provided generous electrochemical active sites, consisting of wrinkled strips of CoFe-LDH adhering to a lamellar structure of Ni-MOF. In terms of the maximum output power density, CoFe-LDH@Ni-MOF as the catalyst was 211 mW/m2, 2.54 times higher than that of Ni-MOF (83 mW/m2), and it was stable at about 225 mV for 150 h. CoFe-LDH@Ni-MOF showed high oxygen reduction reaction capability and high specific surface area, and the electron transfer rate was accelerated. This work might set the stage for the development and utilization of fuel cell cathode catalysts. [ABSTRACT FROM AUTHOR]

Bioelectrochemical techniques are quick, efficient, and sustainable alternatives for treating heavy metal soils. The use of carbon nanomaterials in combination with electroactive microorganisms can create a conductive network that mediates long-distance electron transfer in an electrode system, thereby resolving the issue of low electron transfer efficiency in soil remediation. As a multifunctional soil heavy metal remediation technology, its application in organic remediation has matured, and numerous studies have demonstrated its potential for soil heavy metal remediation. This is a ground-breaking method for remediating soils polluted with high concentrations of heavy metals using soil microbial electrochemistry. This review summarizes the use of bioelectrochemical systems with modified anode materials for the remediation of soils with high heavy metal concentrations by discussing the mass-transfer mechanism of electrochemically active microorganisms in bioelectrochemical systems, focusing on the suitability of carbon nanomaterials and acidophilic bacteria. Finally, we discuss the emerging limitations of bioelectrochemical systems, and future research efforts to improve their performance and facilitate practical applications. The mass-transfer mechanism of electrochemically active microorganisms in bioelectrochemical systems emphasizes the suitability of carbon nanomaterials and acidophilic bacteria for remediating soils polluted with high concentrations of heavy metals. We conclude by discussing present and future research initiatives for bioelectrochemical systems to enhance their performance and facilitate practical applications. As a result, this study can close any gaps in the development of bioelectrochemical systems and guide their practical application in remediating heavy-metal-contaminated soils. [Display omitted] • Carbon nanomaterials improve microorganism extracellular electron transfer and increase soil remediation efficiency. • Carbon nanomaterial anodes overcome the bottleneck in electrochemical microbial soil remediation. • Long-distance electron transfer realizes the double effect of soil remediation. • Electroactive bacteria form a denser biofilm structure with a conductive network. [ABSTRACT FROM AUTHOR]

The marine heterotroph, Marinobacter atlanticus strain CP1, was recently isolated from the electroautotrophic Biocathode MCL community, named for the three most abundant members: Marinobacter, an uncharacterized member of the Chromatiaceae, and Labrenzia. Biocathode MCL catalyzes the production of cathodic current coupled to carbon fixation through the activity of the uncharacterized Chromatiaceae, renamed as “Candidatus Tenderia electrophaga,” but the contribution of M. atlanticus is currently unknown. Here, we report on the electrochemical characterization of pure culture M. atlanticus biofilms grown under aerobic conditions and supplemented with succinate as a carbon source at applied potentials ranging from 160 to 510 mV vs. SHE, and on three different electrode materials (graphite, carbon cloth, and indium tin oxide). M. atlanticus was found to produce either cathodic or anodic current that was an order of magnitude lower than that of the Biocathode MCL community depending on the oxygen concentration, applied potential, and electrode material. Cyclic voltammetry, differential pulse voltammetry (DPV), and square wave voltammetry (SWV) were performed to characterize putative redox mediators at the electrode surface; however no definitive redox peaks were observed. No effect on current was observed when genes encoding a putative rubredoxin (ACP86_RS07295), as well as a putative NADH:flavorubredoxin oxidoreductase (ACP86_RS07290), were deleted to evaluate their role in EET. The addition of either riboflavin or excess trace mineral solution increased anodic current by ca. an order of magnitude under the conditions in which Biocathode MCL is typically grown. These results indicate that M. atlanticus has a non-negligible ability to utilize electrodes as an electron acceptor, which can be enhanced by the presence of excess trace minerals already available in the growth medium. The ability of M. atlanticus to utilize trace minerals as electron shuttles with extracellular electron acceptors may have broader implications for its natural role in biogeochemical cycling.

This study elucidates the role of micrometer-scale electrode surface structures on the growth and the electrochemical performance of mixed culture electrochemically active biofilms (EAB). For this purpose, copper electrodes were machined to generate micro-scale surface structures (roughness and waviness) ranging from a few μm to over 100 μm, which were characterized using confocal laser scanning microscopy (CLSM). The structured electrodes were used to cultivate acetate based, mixed culture, anodic EAB in order to establish relationships between the surface properties and (i) the growth behavior and (ii) the stationary electrocatalytic properties of the resulting EAB. On short time scale, the initial growth phase is shown to be significantly influenced behavior by the surface topology. The long term electrocatalytic biofilm performance, however, does not show any dependence on the surface structures and does thus not profit from the increased specific surface area and micro-scale surface area due to the increasing 3-dimensionality. The results of this study are of great importance for a more systematic development of tailored electrodes for microbial electrochemical technologies.

The presence of inorganic pollutants such as Cadmium(II) and Chromium(VI) could destroy our environment and ecosystem. To overcome this problem, much attention was directed to microbial technology, whereas some microorganisms could resist the toxic effects and decrease pollutants concentration while the microbial viability is sustained. Therefore, we built up a complementary strategy to study the biofilm formation of isolated strains under the stress of heavy metals. As target resistive organisms, Rhizobium-MAP7 and Rhodotorula ALT72 were identified. However, Pontoea agglumerans strains were exploited as the susceptible organism to the heavy metal exposure. Among the methods of sensing and analysis, bioelectrochemical measurements showed the most effective tools to study the susceptibility and resistivity to the heavy metals. The tested Rhizobium strain showed higher ability of removal of heavy metals and more resistive to metals ions since its cell viability was not strongly inhibited by the toxic metal ions over various concentrations. On the other hand, electrochemically active biofilm exhibited higher bioelectrochemical signals in presence of heavy metals ions. So by using the two strains, especially Rhizobium-MAP7, the detection and removal of heavy metals Cr(VI) and Cd(II) is highly supported and recommended.

Halophilic bacteria are remarkable organisms that have evolved strategies to survive in high saline concentrations. These bacteria offer many advances for microbial-based biotechnologies and are commonly used for industrial processes such as compatible solute synthesis, biofuel production, and other microbial processes that occur in high saline environments. Using halophilic bacteria in electrochemical systems offers enhanced stability and applications in extreme environments where common electroactive microorganisms would not survive. Incorporating halophilic bacteria into microbial fuel cells has become of particular interest for renewable energy generation and self-powered biosensing since many wastewaters can contain fluctuating and high saline concentrations. In this perspective, we highlight the evolutionary mechanisms of halophilic microorganisms, review their application in microbial electrochemical sensing, and offer future perspectives and directions in using halophilic electroactive microorganisms for high saline biosensing.

Abstract The direct monitoring of biofilm formation enables valuable insights into the industrial processes, microbiology, and biomedical applications. Therefore, in the present study, nano-structured bioelectrochemical platforms were designed for sensing the formation of biofilm of P. aeruginosa along with monitoring its electrochemical/morphological changes under different stresses. Through the assay optimizations, the performances of different electrode modifiers such as reduced graphene oxide (rGO) nanosheets, hyperbranched chitosan nanoparticles (HBCs NPs), and rGO-HBCs nano-composite were tested to assess the influence of the electrode materials on biofilm progression. As a need for the anodic respiration, the bioelectrochemical responses of the adhered bacterial cells changed from a non-electrochemically active (planktonic state) to an electrochemically active (biofilm matrix) state. Our results demonstrated that electrode modifications with conductive nanostructured elements is highly sensitive and enable direct assay for the biofilm formation without any preachments. Consequently, the morphological changes in bacterial cell wall, upon switching from the planktonic state to the biofilm matrix were imaged using scanning electron microscopy (SEM), and the changes in cell wall chemical composition were monitored by the Energy Dispersive X-ray analysis (EDX). Thus, the designed microbial electrochemical system (MES) was successfully used to monitor changes in the biofilm matrix under different stresses through direct measurements of electron exchanges. Highlights • New nano-engineered microbial electrochemical systems (MESs) were designed. • Electrode modification with new conductive nanostructures promoted biofilm formation. • The MESs were used for exploring the mechanism of biofilm formation in P. aeruginosa. • The designed MES were used successfully to monitor changes in the biofilm matrix. [ABSTRACT FROM AUTHOR]

Microbial electrochemical technologies (METs) have a number of potential technological applications. In this work, we report the use of screen-printed electrodes (SPEs) as a tool to analyze the microbial electroactivity by using Geobacter sulfurreducens as a model microorganism. We took advantage of the small volume required for the assays (75 μL) and the disposable nature of the manufactured strips to explore short-term responses of microbial extracellular electron transfer to conductive materials under different scenarios. The system proved to be robust for identifying the bioelectrochemical response, while avoiding complex electrochemical setups, not available in standard biotechnology laboratories. We successfully validated the system for characterizing the response of Geobacter sulfurreducens in different physiological states (exponential phase, stationary phase, and steady state under continuous culture conditions) revealing different electron transfer responses. Moreover, a combination of SPE and G. sulfurreducens resulted to be a promising biosensor for quantifying the levels of acetate, as well as for performing studies in real wastewater. In addition, the potential of the technology for identifying electroactive consortia was tested, as an example, with a mixed population with nitrate-reducing capacity. We therefore present SPEs as a novel low-cost platform for assessing microbial electrochemical activity at the microscale level.

Reducing the time required for the formation of microbial anodes from environmental inocula is a great challenge. The possibility of reaching this objective by increasing the temperature during the bioanode preparation was investigated here. Microbial anodes were formed at 25 °C and 40 °C under controlled potential with successive acetate additions. At 25 °C, around 40 days were required to perform three acetate batches, which led to current density of 9.4 ± 2 A.m −2 , while at 40 °C, 20 days were sufficient to complete three similar batches, leading to 22.9 ± 4.2 A.m −2 . The bioanodes formed at 40 °C revealed three redox systems and those formed at 25 °C only one. The temperature also impacted the biofilm structure, which was less compact at 40 °C. When the bioanodes formed at 40 °C were switched to 25 °C, they produced current densities similar to those of bioanodes formed at 25 °C; they recovered the single redox system that was developed by the bioanodes formed at 25 °C and the difference in biofilm structures was mitigated. It is consequently fully appropriate to accelerate the formation of microbial anodes by increasing the temperatures to 40 °C even if they are finally intended to operate at room temperature. [ABSTRACT FROM AUTHOR]

Microbial electrochemistry is a cross-disciplinary field that has become a hot topic in environmental engineering and science. The microbial electrochemical cell (MxC) is a technology platform for removing biochemical oxygen demand (BOD) from organic wastes while generating valuable outputs. The unique feature of MxCs is the biofilm of anode-respiring bacteria (ARB) that live on the anode, oxidize organic molecules, and efficiently respire the electron to the anode by extracellular electron transport (EET). Valuable outputs depend on the reduction reaction at the cathode: They include electrical power, hydrogen gas, hydrogen peroxide, and acetate. Those wanting to create viable MxC technologies need to recognize four important ironies about what actually controls the performance of a MxC: (1) although the unique features of a MxC are the ARB and EETof electrons to the anode, the anode's current density is determined by the diffusion of protons out of the biofilm, which depends on a weak-base buffer; (2) the main energy loss in a MxC is at the cathode, and minimizing cathode overpotential depends on the transport of hydroxide away from the cathode with a weak-acid buffer; (3) selection and accumulation of the most efficient ARB in the biofilm anode is brought about by using a stressful negative anode potential, even though a high anode potential gives the fastest EET once the biofilm is established; and (4) efficient ARB have adaptable EET that takes advantage of changes of potential and pH within the biofilm. [ABSTRACT FROM AUTHOR]

Background: Microbial electrolysis cells (MECs) use bioelectrochemical reactions to remove organic contaminants at the bioanode and produce hydrogen gas at the cathode. High local pH conditions near the cathode can also be utilized to produce struvite from nutrient-rich wastewater. This beneficial aspect was investigated using lab-scale MECs fed with dewatering centrate collected at a local wastewater treatment plant. The main objective was to improve phosphorus recovery by examining various cathode configurations and electric current conditions. Results: The stainless steel mesh (SSM) cathode was relatively inefficient to achieve complete phosphorus recovery because struvite crystals were smaller (a few to tens of micrometers) than the open space between mesh wires (80 μm). As a result, the use of multiple pieces of SSM also showed a limited improvement in the phosphorus recovery up to only 68% with 5 SSM pieces. Readily available organic substrates were not sufficient in the dewatering centrate, resulting in relatively low electric current density (mostly below 0.2 A/m²). The slow electrode reaction did not provide sufficiently high pH conditions near the cathode for complete recovery of phosphorus as struvite. Based on these findings, additional experiments were conducted using stainless steel foil (SSF) as the cathode and acetate (12 mM) as an additional organic substrate for exoelectrogens at the bioanode. With the high electric current (>2 A/mBackground: Microbial electrolysis cells (MECs) use bioelectrochemical reactions to remove organic contaminants at the bioanode and produce hydrogen gas at the cathode. High local pH conditions near the cathode can also be utilized to produce struvite from nutrient-rich wastewater. This beneficial aspect was investigated using lab-scale MECs fed with dewatering centrate collected at a local wastewater treatment plant. The main objective was to improve phosphorus recovery by examining various cathode configurations and electric current conditions. Results: The stainless steel mesh (SSM) cathode was relatively inefficient to achieve complete phosphorus recovery because struvite crystals were smaller (a few to tens of micrometers) than the open space between mesh wires (80 μm). As a result, the use of multiple pieces of SSM also showed a limited improvement in the phosphorus recovery up to only 68% with 5 SSM pieces. Readily available organic substrates were not sufficient in the dewatering centrate, resulting in relatively low electric current density (mostly below 0.2 A/m2). The slow electrode reaction did not provide sufficiently high pH conditions near the cathode for complete recovery of phosphorus as struvite. Based on these findings, additional experiments were conducted using stainless steel foil (SSF) as the cathode and acetate (12 mM) as an additional organic substrate for exoelectrogens at the bioanode. With the high electric current (>2 A/m2), a thick layer of struvite crystals was formed on the SSF cathode. The phosphorus recovery increased to 96% with the increasing MEC operation time from 1 to 7 days. With the high phosphorus recovery, estimated energy requirement was relatively low at 13.8 kWh (with acetate) and 0.30 kWh (without acetate) to produce 1 kg struvite from dewatering centrate. Conclusions: For efficient phosphorus recovery from real wastewater, a foil-type cathode is recommended to avoid potential losses of small struvite crystals. Also, presence of readily available organic substrates is important to maintain high electric current and establish high local pH conditions near the cathode. Struvite precipitation was relatively slow, requiring 7 days for nearly complete removal (92%) and recovery (96%). Future studies need to focus on shortening the time requirement. Keywords: Phosphorus recovery, Municipal wastewater treatment, Struvite, Dewatering downstream, Microbial electrochemistry, Cathode structure ), a thick layer of struvite crystals was formed on the SSF cathode. The phosphorus recovery increased to 96% with the increasing MEC operation time from 1 to 7 days. With the high phosphorus recovery, estimated energy requirement was relatively low at 13.8 kWh (with acetate) and 0.30 kWh (without acetate) to produce 1 kg struvite from dewatering centrate. Conclusions: For efficient phosphorus recovery from real wastewater, a foil-type cathode is recommended to avoid potential losses of small struvite crystals. Also, presence of readily available organic substrates is important to maintain high electric current and establish high local pH conditions near the cathode. Struvite precipitation was relatively slow, requiring 7 days for nearly complete removal (92%) and recovery (96%). Future studies need to focus on shortening the time requirement. [ABSTRACT FROM AUTHOR]

We have explored a new concept in bacteria-electrode interaction based on the use of fluid-like electrodes and planktonic living cells. We show for the first time that living in a biofilm is not a strict requirement for Geobacter sulfurreducens to exchange electrons with an electrode. The growth of planktonic electroactive G. sulfurreducens could be supported by a fluid-like anode as soluble electron acceptors and with electron transfer rates similar to those reported for electroactive biofilms. This growth was maintained by uncoupling the charge (catabolism) and discharge (extracellular respiration) processes of the cells. Our results reveal a novel method to culture electroactive bacteria in which every single cell in the medium could be instantaneously wired to a fluid-like electrode. Direct extracellular electron transfer is occurring but with a new paradigm behind the bacteria-electrode interaction. [ABSTRACT FROM AUTHOR]

MFC research has been conducted enormously in the last decade, and their electrochemical data has been accumulated enormously. Although reproducible measurement method and fair evaluation are essential elements in research, these elements are missing in MFC research. Herein, using single-chamber cubic MFCs, diverse polarization methods were conducted along with other electrochemical tests. 3-hour optimum resistance operation followed by 2-h open circuit time prevented power overshoots. In a faster polarization method, data reproducibility increased drastically, power and current were larger, and polarization resistance was smaller. In ERC-30 min, LSV-0.1, LSV-1, LSV-10 mV/s and, maximum power density was 1242, 1518, 2287 and 3417 mW/m2, and internal resistance 76, 63, 48 and 35 Ω, respectively. For fair evaluation of different polarization data, the interconversion factors were developed and applied. During the optimum resistance operation, average power and current densities were 994 mW/m2 and 3767 mA/m2, respectively. As similarly in the polarization tests, cathodic impedances (24.1 Ω) were 2.4 times larger than the anodic impedances (10.2 Ω). Charge transfer impedance (32.4 Ω) was the major component (>95%) in the electrodes and the whole cell (34.3 Ω). Ohmic impedance was detected only in the anode (0.2 Ω), and diffusion impedance was detected only in the cathodes (1.7 Ω). Cathodic capacitance (1.62 F) was 2.5 times larger than the anodic capacitance (0.64 F). Cyclic voltammogram cathodic capacitance (7.78 F) was 1.6 times larger than the anodic capacitance (4.76 F). Longer operation, lower internal resistance and higher anode performance increased coulombic and energy efficiencies. This study contains very important information on experimental accuracy and reproducibility, essential elements in research, and will greatly contribute to the firm foundation and the meaningful growth of the MFC research. [Display omitted] • Reproducible data production in MFCs is explained. • Faster polarization tests produce more reproducible results. • Interconversion factors are developed for fair evaluation of MFC data. • Cathodic impedances are 2.5 time larger than anodic impedances. • Charge transfer impedance was the major component in the cell impedance. [ABSTRACT FROM AUTHOR]

Keywords: electron uptake; electrosynthesis; extracellular electron transfer; microbial electrochemistry; Shewanella EN electron uptake electrosynthesis extracellular electron transfer microbial electrochemistry Shewanella 1 1 1 02/17/23 20230213 NES 230213 B The Cover Feature b illustrates soluble iron acting as an electron mediator for enhanced extracellular electron uptake by I Shewanella oneidensis i MR-1. Cover Feature: Soluble Iron Enhances Extracellular Electron Uptake by Shewanella oneidensis MR-1 (ChemElectroChem 4/2023) Electron uptake, electrosynthesis, extracellular electron transfer, microbial electrochemistry, Shewanella. [Extracted from the article]

Rhodopseudomonas palustris has emerged as a model microbe for the anaerobic metabolism of p-coumarate, which is an aromatic compound and a primary component of lignin. However, under anaerobic conditions, R. palustris must actively eliminate excess reducing equivalents through a number of known strategies (e.g., CO2 fixation, H2 evolution) to avoid lethal redox imbalance. Others had hypothesized that to ease the burden of this redox imbalance, a clonal population of R. palustris could functionally differentiate into a pseudo-consortium. Within this pseudo-consortium, one sub-population would perform the aromatic moiety degradation into acetate, while the other sub-population would oxidize acetate, resulting in a single-genotype syntrophy through acetate sharing. Here, the objective was to test this hypothesis by utilizing microbial electrochemistry as a research tool with the extracellular-electron-transferring bacterium Geobacter sulfurreducens as a reporter strain replacing the hypothesized acetate-oxidizing sub-population. We used a 2x4 experimental design with pure cultures of R. palustris in serum bottles and co-cultures of R. palustris and G. sulfurreducens in bioelectrochemical systems. This experimental design included growth medium with and without bicarbonate to induce non-lethal and lethal redox imbalance conditions, respectively, in R. palustris. Finally, the design also included a mutant strain (NifA*) of R. palustris, which constitutively produces H2, to serve both as a positive control for metabolite secretion (H2) to G. sulfurreducens, and as a non-lethal redox control for without bicarbonate conditions. Our results demonstrate that acetate sharing between different sub-populations of R. palustris does not occur while degrading p-coumarate under either non-lethal or lethal redox imbalance conditions. This work highlights the strength of microbial electrochemistry as a tool for studying microbial syntrophy.

Dissimilatory metal-reducing bacteria are widespread in terrestrial ecosystems, especially in anaerobic soils and sediments. Thermodynamically, dissimilatory metal reduction is more favorable than sulfate reduction and methanogenesis but less favorable than denitrification and aerobic respiration. It is critical to understand the complex relationships, including the absence or presence of terminal electron acceptors, that govern microbial competition and coexistence in anaerobic soils and sediments, because subsurface microbial processes can effect greenhouse gas emissions from soils, possibly resulting in impacts at the global scale. Here, we elucidated the effect of an inexhaustible, ferrous-iron and humic-substance mimicking terminal electron acceptor by deploying potentiostatically poised electrodes in the sediment of a very specific stream riparian zone in Upstate New York state. At two sites within the same stream riparian zone during the course of six weeks in the spring of 2013, we measured CH4 and N2/N2O emissions from soil chambers containing either poised or unpoised electrodes, and we harvested biofilms from the electrodes to quantify microbial community dynamics. At the upstream site, which had a lower vegetation cover and highest soil temperatures, the poised electrodes inhibited CH4 emissions by ~45% (when normalized to remove temporal effects). CH4 emissions were not significantly impacted at the downstream site. N2/N2O emissions were generally low at both sites and were not impacted by poised electrodes. We did not find a direct link between bioelectrochemical treatment and microbial community membership; however, we did find a correspondence between environment/function and microbial community dynamics.

Rhodopseudomonas palustris has emerged as a model microbe for the anaerobic metabolism of p-coumarate, which is an aromatic compound and a primary component of lignin. However, under anaerobic conditions, R. palustris must actively eliminate excess reducing equivalents through a number of known strategies (e.g., CO2 fixation, H2 evolution) to avoid lethal redox imbalance. Others had hypothesized that to ease the burden of this redox imbalance, a clonal population of R. palustris could functionally differentiate into a pseudo-consortium. Within this pseudo-consortium, one sub-population would perform the aromatic moiety degradation into acetate, while the other sub-population would oxidize acetate, resulting in a single-genotype syntrophy through acetate sharing. Here, the objective was to test this hypothesis by utilizing microbial electrochemistry as a research tool with the extracellular-electron-transferring bacterium Geobacter sulfurreducens as a reporter strain replacing the hypothesized acetate-oxidizing sub-population. We used a 2 #215; 4 experimental design with pure cultures of R. palustris in serum bottles and co-cultures of R. palustris and G. sulfurreducens in bioelectrochemical systems. This experimental design included growth medium with and without bicarbonate to induce non-lethal and lethal redox imbalance conditions, respectively, in R. palustris. Finally, the design also included a mutant strain (NifA*) of R. palustris, which constitutively produces H2, to serve both as a positive control for metabolite secretion (H2) to G. sulfurreducens, and as a non-lethal redox control for without bicarbonate conditions. Our results demonstrate that acetate sharing between different sub-populations of R. palustris does not occur while degrading p-coumarate under either non-lethal or lethal redox imbalance conditions. This work highlights the strength of microbial electrochemistry as a tool for studying microbial syntrophy. [ABSTRACT FROM AUTHOR]

A denitrifying biocathode in a microbial fuel cell was developed to investigate the replacement of the costly Pt-coated abiotic cathodes for electricity generation. The denitrifying biocathode was sandwiched between the dual-anode systems. The study investigated the performance for simultaneous treatment of wastewater on the anode, biological denitrification on the cathode and the potential recovery of electrical energy. Autotrophic biofilms performed denitrification on the cathode using supplied electrons by the biodegradation of organics on the anode. Graphite granules were used as electrodes for biofilm attachment, and nafion membranes were used as separators between electrodes. The system achieved a volumetric power of 7 ± 0.4 W m net cathodic compartment (NCC) with the simultaneous removal of 229.5 ± 18 mg L COD on anode and 88.9 g m NCC day nitrogen on cathode, respectively. The columbic efficiency for cathodic and anodic reactions was 98.9 ± 0.57 and 23.54 ± 0.87 %, respectively. This is a combined study for domestic wastewater treatment and biological denitrification in a compact MFC reactor. Further optimization of the system is desired to improve its performance and applicability. [ABSTRACT FROM AUTHOR]

ABSTRACT Extracellular electron transfer pathways allow certain bacteria to transfer energy between intracellular chemical energy stores and extracellular solids through redox reactions. Microorganisms containing these pathways, exoelectrogens, are a critical part of microbial electrochemical technologies that aim to impact applications in bioenergy, biosensing, and biocomputing. However, there are not yet any examples of economically viable microbial electrochemical technologies due to the limitations of naturally occurring exoelectrogens. Here we first briefly summarize recent discoveries in understanding extracellular electron transfer pathways, then review in-depth the creation of customized and novel exoelectrogens for biotechnological applications. We analyze engineering efforts to increase current production in native exoelectrogens, which reveals that modulating certain processes within extracellular electron transfer are more effective than others. We also review efforts to create new exoelectrogens and highlight common challenges in this work. Lastly, we summarize work utilizing engineered exoelectrogens for biotechnological applications and the key obstacles to their future development. Fueled by the development of genetic tools, these approaches will continue to expand and genetically modified organisms will continue to improve the outlook for microbial electrochemical technologies. Biotechnol. Bioeng. 2016;113: 687-697. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Staphylococcus aureus is one of the most dangerous human pathogens and is the cause of numerous illnesses ranging from moderate skin infections to life-threatening diseases. Despite advances made in identifying microorganisms, rapid detection methods for the viability of bacteria are still missing. Here, we report a rapid electrochemical assay for cell viability combining the use of double redox mediators and multiwall carbon nanotubes-screen printed electrodes (MWCNTs-SPE), ferricyanide (FCN) and 2,6-dichlorophenolindophenol (DCIP), which served as electron shuttle to enable the bacterial-electrode communications. The current originating from the metabolically active cells was recorded for probing the activity of the intracellular redox centers. Blocking of the respiratory chain pathways with electron transfer inhibitors demonstrated the involvement of the electron transport chain in the reaction. A good correlation between the number of the metabolically active cells and the current was obtained. The proposed assay has been exploited for monitoring cell proliferation of S. aureus during the growth. The sensitivity of the detection method reached 0.1 OD. Therefore, the technique described is promising for estimating the cell number, measuring the cell viability, and probing intracellular redox center(s). [ABSTRACT FROM AUTHOR]

Dissimilatory metal-reducing bacteria are widespread in terrestrial ecosystems, especially in anaerobic soils and sediments. Thermodynamically, dissimilatory metal reduction is more favorable than sulfate reduction and methanogenesis but less favorable than denitrification and aerobic respiration. It is critical to understand the complex relationships, including the absence or presence of terminal electron acceptors, that govern microbial competition and coexistence in anaerobic soils and sediments, because subsurface microbial processes can effect greenhouse gas emissions from soils, possibly resulting in impacts at the global scale. Here, we elucidated the effect of an inexhaustible, ferrous-iron and humic-substance mimicking terminal electron acceptor by deploying potentiostatically poised electrodes in the sediment of a very specific stream riparian zone in Upstate New York state. At two sites within the same stream riparian zone during the course of 6 weeks in the spring of 2013, we measured CH4 and N2/N2O emissions from soil chambers containing either poised or unpoised electrodes, and we harvested biofilms from the electrodes to quantify microbial community dynamics. At the upstream site, which had a lower vegetation cover and highest soil temperatures, the poised electrodes inhibited CH4 emissions by ~45% (when normalized to remove temporal effects). CH4 emissions were not significantly impacted at the downstream site. N2/N2O emissions were generally low at both sites and were not impacted by poised electrodes. We did not find a direct link between bioelectrochemical treatment and microbial community membership; however, we did find a correspondence between environment/function and microbial community dynamics. [ABSTRACT FROM AUTHOR]

Microbial electrochemical technologies (METs) have a number of potential technological applications. In this work, we report the use of screen-printed electrodes (SPEs) as a tool to analyze the microbial electroactivity by using Geobacter sulfurreducens as a model microorganism. We took advantage of the small volume required for the assays (75 μL) and the disposable nature of the manufactured strips to explore short-term responses of microbial extracellular electron transfer to conductive materials under different scenarios. The system proved to be robust for identifying the bioelectrochemical response, while avoiding complex electrochemical setups, not available in standard biotechnology laboratories. We successfully validated the system for characterizing the response of Geobacter sulfurreducens in different physiological states (exponential phase, stationary phase, and steady state under continuous culture conditions) revealing different electron transfer responses. Moreover, a combination of SPE and G. sulfurreducens resulted to be a promising biosensor for quantifying the levels of acetate, as well as for performing studies in real wastewater. In addition, the potential of the technology for identifying electroactive consortia was tested, as an example, with a mixed population with nitrate-reducing capacity. We therefore present SPEs as a novel low-cost platform for assessing microbial electrochemical activity at the microscale level. [ABSTRACT FROM AUTHOR]

AFM and confocal resonance Raman microscopy (CRRM) of single-cells were used to study the transition of anode-grown Geobacter sulfurreducens biofilms from lag phase (initial period of low current) to exponential phase (subsequent period of rapidly increasing current). Results reveal that lag phase biofilms consist of lone cells and tightly packed single-cell thick clusters crisscrossed with extracellular linear structures that appear to be comprised of nodules approximately 20 nm in diameter aligned end to end. By early exponential phase cell clusters expand laterally and a second layer of closely packed cells begins to form on top of the first. Abundance of c-type cytochromes (c-Cyt) is > 3-fold greater in 2-cell thick regions than in 1-cell thick regions. The results indicate that early biofilm growth involves two transformations. The first is from lone cells to 2-dimensionally associated cells during lag phase when current remains low. This is accompanied by formation of extracellular linear structures. The second is from 2- to 3-dimensionally associated cells during early exponential phase when current begins to increases rapidly. This is accompanied by a dramatic increase in c-Cyt abundance.