Bidirectional extracellular electron transfers of electrode-biofilm: Mechanism and application

Enhancing water toxicity determination sensitivity by using TMAO as electron acceptor of inward extracellular electron transfer in electrochemically active bacteria

Toxicity determination based on electrochemically active bacteria (EAB) shows great prospects for early warning of sudden water pollution. However, the main bottleneck for practical application is the low sensitivity. Extracellular electron transfer (EET) is a key parameter influencing sensitivity. Our previous research has demonstrated that EAB exhibit higher sensitivity when performing inward EET compared with outward EET. Inward EET relies on electron acceptors, but the effects of electron acceptors on sensitivity remain unclear. In this study, the sensitivity of toxicity determination with different electron acceptors was compared. Results indicated that the choice of electron acceptors significantly changed the sensitivity. When Trimethylamine N-oxide (TMAO) was chosen as the electron acceptor, EAB exhibited the highest sensitivity, with a lower response limit of 0.05 mg/L Cd2+. The main reason was that the utilization of TMAO for inward EET increases the membrane permeability of EAB cells, facilitates toxic pollutant penetration, and results in high mortality after toxicity exposure.

Unlocking interfacial electron transfer in biophotoelectrochemical processes: Role of extracellular polymeric substances in aquatic environments

The biophotoelectrochemical process (BPECs) integrates the light-absorbing capabilities of nano-semiconductors with the catalytic efficiency of microorganisms, demonstrating significant potential for the development, utilization, transformation, and ecological restoration of water resources. In aquatic environments, extracellular polymeric substances (EPS) serve as a critical interfacial barrier between microorganisms and semiconductor materials, with the underlying electron transfer mechanisms playing a pivotal role in determining the efficiency of bio-photochemical reactions. Despite their importance, the rapidity and complexity of the electron transfer process within EPS pose significant challenges to a comprehensive understanding of BPECs. In this study, an in-situ characterization strategy was employed to rapidly and accurately analyze the components and pathways of photogenerated electron transfer involving EPS at interfaces. The findings indicate that EPS significantly accelerates the transfer of photogenerated electrons within BPECs. Specifically, proteins and redox-active substances within EPS act as efficient conduits for electron transfer, accounting for up to 84.2% of the increased speed in electron transfer rates at bio-abiotic interfaces. Conversely, polysaccharides within EPS impede the electron transfer process but serve as substrates that facilitate methane (CH4) production. The in-situ characterization approach used in this research provides valuable insights into the interfacial electron transfer mechanisms of EPS in BPECs, emphasizing their relevance in aquatic environments. This study establishes a theoretical framework for designing high-performance BPECs, with significant implications for the energy utilization of water resources and the transformation of water pollutants.

Sulfate reducing bacteria corrosion of a 90/10 Cu-Ni alloy coupled to an Al sacrificial anode

This study investigates the corrosion of 90/10 copper-nickel (Cu-Ni) alloy caused by sulfate-reducing bacteria (SRB) in the presence of aluminum anodes, with particular emphasis on the role of electron supply in microbial corrosion and the resulting local corrosion failures. The study reveals that the electron supply from the anode supports SRB growth on the Cu-Ni alloy through an "Electrons-siphoning" mechanism. However, the supply is insufficient to sustain the SRB population, resulting in ineffective cathodic protection (icorr = 2.34 × 10-6 A cm-2). The addition of 20 ppm riboflavin (RF) to the SRB biofilm enhances electrical activity and increases the electron donor density, thereby restoring the anode's protective effect. As a result, the icorr of the 90/10 Cu-Ni alloy decreases by an order of magnitude (to 3.5 × 10-7 A cm-2). These findings provide valuable new insights into the mechanisms of microbial corrosion.

In situ bioelectrochemical remediation of contaminated soil and groundwater: A review

Contamination of the subsurface environment poses a serious hazard to the environment and human health. Recently, the bioelectrochemical system (BES) has drawn great attention in soil and groundwater remediation as it does not necessitate the addition of chemicals and exhibits minimal energy consumption to facilitate microbial degradation of pollutants. However, the complexity of the subsurface environment and the design parameters of the BES significantly affect the remediation performance and the current literature on BES primarily concentrates on its application in wastewater treatment, with a lack of summary of that in the subsurface environment. Therefore, the purpose of this review was to provide the current status, challenges, and outlooks of BES in situ treatment of pollutants from soil and groundwater. Firstly, the principles and efficacies of BES in treating the typical pollutants from the subsurface environment were discussed. Secondly, the factors that impact the BES treatment efficiencies, especially soil properties, the distinctive and pivotal factors for BES in situ application, were discussed specifically. Finally, the challenges and outlooks of BES for the in situ remediation of the contaminated soil and groundwater were addressed. BES is a green and sustainable in situ remediation technology and future advancements may necessitate the integration with complementary technologies and innovative system configurations to advance the practical implementation of BES.

Fast preparing bioelectrode with conductive bioink for nitrite detection in high sensitivity and stability

Electrochemically active biofilms (EABs) for nitrite detection have high specificity, rapid response, operational simplicity, and extended lifespan advantages. However, their scale production remains challenging due to time-consuming and uniform preparation. In this study, a novel approach was proposed to fast fabricate an EAB biosensor with a synthetic biofilm electrode for nitrite detection. The biofilm electrode was prepared by coating bioinks with varying conductive materials onto the surface of the graphite sheets, showing short incubation time and good reproducibility. Incorporating conductive materials into the bioinks remarkably enhanced the maximum voltage of the first cycle of bioelectrode incubation, with an increase of up to 633% for carbon nanofibers. The nitrite reduction current was amplified by a factor of 2.97, due to the enhancement of extracellular electron transfer (EET). The developed nitrite biosensor exhibited a detection range of 0.1-15 mg NO2--N L-1, with a high sensitivity of 610.8 μA mM-1 cm-2, and a stabilization operation time of at least 280 cycles. This study not only provided valuable insights into conductive materials for synthetic biofilms but also presented a practical approach for the rapid preparation, scale production, and optimization of highly sensitive and stable EAB sensors.

Harnessing microbes to pioneer environmental biophotoelectrochemistry

In seeking sustainable environmental strategies, microbial biophotoelectrochemistry (BPEC) systems represent a significant advancement. In this review, we underscore the shift from conventional bioenergy systems to sophisticated BPEC applications, emphasizing their utility in leveraging solar energy for essential biochemical conversions. Recent progress in BPEC technology has facilitated improved photoelectron transfer and system stability, resulting in substantial advancements in carbon and nitrogen fixation, degradation of pollutants, and energy recovery from wastewater. Advances in system design and synthetic biology have expanded the potential of BPEC for environmental clean-up and sustainable energy generation. We also highlight the challenges of environmental BPEC systems, ranging from performance improvement to future applications.

Carbon chain elongation characterizations of electrode-biofilm microbes in electro-fermentation

The higher efficiency of electro-fermentation in synthesizing medium-chain fatty acids (MCFAs) compared to traditional fermentation has been acknowledged. However, the functional mechanisms of electrode-biofilm enhancing MCFAs synthesis remain research gaps. To address this, this study proposed a continuous flow electrode-biofilm reactor for chain elongation (CE). After 225 days of operation, stable electrode-biofilms formed and notably improved caproate yield by more than 38 %. The electrode-biofilm was enriched with more CE microorganisms and electroactive bacteria compared to the suspended sludge microorganisms, including Caproicibacterium, Oscillibacter and Pseudoramibacter. Besides, the upregulated CE pathways were evaluated by metagenomic analysis, and the results indicated that the pathways such as acetyl-CoA and malonyl-[acp] formation, reverse beta-oxidation, and fatty acid biosynthesis pathway were all markedly enhanced in cathodic biofilm, more than anodic biofilm and suspended microorganisms. Moreover, microbial community regulated processes like bacterial chemotaxis, flagellar assembly and quorum sensing, crucial for electrode-biofilm formation. Electron transfer, energy metabolism, and microbial interactions were found to be prominently upregulated in the cathodic biofilm, surpassing levels observed in anodic biofilm and suspended sludge microorganisms, which further enhanced CE efficiency. In addition, the statistical analyses further highlighted key microbial functions and interactions within the cathodic biofilm. Oscillospiraceae_bacterium was identified to be the most active microbe, alongside pivotal roles played by Caproiciproducens_sp._NJN-50, Clostridiales_bacterium, Prevotella_sp. and Pseudoclavibacter_caeni. Eventually, the proposed microbial collaboration mechanisms of cathodic biofilm were ascertained. Overall, this study uncovered the biological effects of the electrode-biofilm on MCFAs electrosynthesis, thereby advancing biochemicals production and filling the knowledge gaps in CE electroactive biofilm reactors.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Research on the conversion of biowaste to MCCAs: A review of recent advances in the electrochemical synergistic anaerobic pathway

Medium-chain carboxylic acids (MCCAs) show great promise as commercial chemicals due to their high energy density, significant product value, and wide range of applications. The production of MCCAs from waste biomass through coupling chain extension with anaerobic fermentation represents a new and innovative approach to biomass utilization. This review provides an overview of the principles of MCCAs production through coupled chain extension and anaerobic fermentation, as well as the extracellular electron transfer pathways and microbiological effects involved. Emphasis is placed on the mechanisms, limitations, and microbial interactions in MCCAs production, elucidating metabolic pathways, potential influencing factors, and the cooperative and competitive relationships among various microorganisms. Additionally, this paper delves into a novel technology for the bio-electrocatalytic generation of MCCAs, which promotes electron transfer through the use of different three-dimensional electrodes, various electrical stimulation methods, and hydrogen-assisted approaches. The insights and conclusions from previous studies, as well as the identification of existing challenges, will be valuable for the further development of high-product-selectivity strategies and environmentally friendly treatments.

Hydrogen production in microbial electrolysis cells with biocathodes

Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe-electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production.

Metabolic engineering strategies to enable microbial electrosynthesis utilization of CO2: recent progress and challenges

Microbial electrosynthesis (MES) is a promising technology that mainly utilizes microbial cells to convert CO2 into value-added chemicals using electrons provided by the cathode. However, the low electron transfer rate is a solid bottleneck hindering the further application of MES. Thus, as an effective strategy, genetic tools play a key role in MES for enhancing the electron transfer rate and diversity of production. We describe a set of genetic strategies based on fundamental characteristics and current successes and discuss their functional mechanisms in driving microbial electrocatalytic reactions to fully comprehend the roles and uses of genetic tools in MES. This paper also analyzes the process of nanomaterial application in extracellular electron transfer (EET). It provides a technique that combines nanomaterials and genetic tools to increase MES efficiency, because nanoparticles have a role in the production of functional genes in EET although genetic tools can subvert MES, it still has issues with difficult transformation and low expression levels. Genetic tools remain one of the most promising future strategies for advancing the MES process despite these challenges.

Bidirectional extracellular electron transfer pathways of Geobacter sulfurreducens biofilms: Molecular insights into extracellular polymeric substances

The basis for bioelectrochemical technology is the capability of electroactive bacteria (EAB) to perform bidirectional extracellular electron transfer (EET) with electrodes, i.e. outward- and inward-EET. Extracellular polymeric substances (EPS) surrounding EAB are the necessary media for EET, but the biochemical and molecular analysis of EPS of Geobacter biofilms on electrode surface is largely lacked. This study constructed Geobacter sulfurreducens-biofilms performing bidirectional EET to explore the bidirectional EET mechanisms through EPS characterization using electrochemical, spectroscopic fingerprinting and proteomic techniques. Results showed that the inward-EET required extracellular redox proteins with lower formal potentials relative to outward-EET. Comparing to the EPS extracted from anodic biofilm (A-EPS), the EPS extracted from cathodic biofilm (C-EPS) exhibited a lower redox activity, mainly due to a decrease of protein/polysaccharide ratio and α-helix content of proteins. Furthermore, less cytochromes and more tyrosine- and tryptophan-protein like substances were detected in C-EPS than in A-EPS, indicating a diminished role of cytochromes and a possible role of other redox proteins in inward-EET. Proteomic analysis identified a variety of redox proteins including cytochrome, iron-sulfur clusters-containing protein, flavoprotein and hydrogenase in EPS, which might serve as an extracellular redox network for bidirectional EET. Those redox proteins that were significantly stimulated in A-EPS and C-EPS might be essential for outward- and inward-EET and warranted further research. This work sheds light on the mechanism of bidirectional EET of G. sulfurreducens biofilms and has implications in improving the performance of bioelectrochemical technology.

Housing of electrosynthetic biofilms using a roll-up carbon veil electrode increases CO2 conversion and faradaic efficiency in microbial electrosynthesis cells

Electrode-driven microbial electron transfer enables the conversion of CO2 into multi-carbon compounds. The electrosynthetic biofilms grow slowly on the surface and are highly susceptible to operational influences, such as hydrodynamic shear stress. In this study, a cylindrical roll-up carbon felt electrode was developed as a novel strategy to protect biofilms from shear stress within the reactor. The fabricated electrode allowed hydrogen bubble formation inside the structure, which enabled microbes to uptake hydrogen and convert CO2 to multi-carbon organic compounds. The roll-up electrode exhibited faster start-up and biofilm formation than the conventional linear shape carbon felt. The acetate yield and cathodic faradaic efficiency increased by 80% and 34%, respectively, and the bioelectrochemical stability was improved significantly. The roll-up structure increased biofilm development per unit electrode surface by three to five-fold. The roll-up configuration improved biofilm formation on the electrode, which enhanced the performance of microbial electrosynthesis-based CO2 valorization.

Autoinducer-2 quorum sensing regulates biofilm formation and chain elongation metabolic pathways to enhance caproate synthesis in microbial electrochemical system

Quorum sensing (QS) have been explored extensively. However, most studies focused on N-acyl homoserine lactones (AHLs) participating in intraspecies QS. In this study, autoinducer-2 (AI-2, participating in interspecies QS) with different concentration was investigated for chain elongation in microbial electrosynthesis (MES). The results demonstrated that the R3 treatment, which involved adding 10 μM of 4,5-dihydroxy-2,3-pentanedione (DPD) in the reactor, exhibited the best performance. The concentration of caproate was increased by 66.88% and the redox activity of cathodic electroactive biofilms (EABs) was enhanced. Meanwhile, microbial community data indicated that Negativicutes relative abundance was increased obviously in R3 treatment. In this study, the transcriptome Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases were used to analyze the metabolic pathway of chain elongation involving fatty acid biosynthesis (FAB) pathway and reverse β-oxidization (RBO) pathway. KEGG analysis revealed that fatty acid elongation metabolism (p < 0.001), tryptophan metabolism (p < 0.01), arginine and proline metabolism (p < 0.05) were significantly improved in R3 treatment. GO analysis suggested that R3 treatment mainly upregulated significantly transmembrane signaling receptor activity (p < 0.01), oxidoreductase activity (p < 0.05), and phosphorelay signal transduction (p < 0.05). Moreover, metatranscriptomic analyses also showed that R3 treatment could upregulate the LuxP extracellular receptor, LuxO transcriptional activator, LsrB periplasmic protein, and were beneficial to both FAB and RBO pathways. These findings provided a new insight into chain elongation in MES system.

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

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

Retracted: The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Enhanced bidirectional extracellular electron transfer based on biointerface interaction of conjugated polymers-bacteria biohybrid system

The low bacteria loading capacity and low extracellular electron transfer (EET) efficiency are two major bottlenecks restricting the performance of the bioelectrochemical systems from practical applications. Herein, we demonstrated that conjugated polymers (CPs) could enhance the bidirectional EET efficiency through the intimate biointerface interactions of CPs-bacteria biohybrid system. Upon the formation of CPs/bacteria biohybrid, thick and intact CPs-biofilm formed which ensured close biointerface interactions between bacteria-to-bacteria and bacteria-to-electrode. CPs could promote the transmembrane electron transfer through intercalating into the cell membrane of bacteria. Utilizing the CPs-biofilm biohybrid electrode as anode in microbial fuel cell (MFC), the power generation and lifetime of MFC had greatly improved based on accelerated outward EET. Moreover, using the CPs-biofilm biohybrid electrode as cathode in electrochemical cell, the current density was increased due to the enhanced inward EET. Therefore, the intimate biointerface interaction between CPs and bacteria greatly enhanced the bidirectional EET, indicating that CPs exhibit promising applications in both MFC and microbial electrosynthesis.

Microbe-Anode Interactions: Comparing the impact of genetic and material engineering approaches to improve the performance of microbial electrochemical systems (MES)

Microbial electrochemical systems (MESs) are a highly versatile platform technology with a particular focus on power or energy production. Often, they are used in combination with substrate conversion (e.g., wastewater treatment) and production of value-added compounds via electrode-assisted fermentation. This rapidly evolving field has seen great improvements both technically and biologically, but this interdisciplinarity sometimes hampers overseeing strategies to increase process efficiency. In this review, we first briefly summarize the terminology of the technology and outline the biological background that is essential for understanding and thus improving MES technology. Thereafter, recent research on improvements at the biofilm-electrode interface will be summarized and discussed, distinguishing between biotic and abiotic approaches. The two approaches are then compared, and resulting future directions are discussed. This mini-review therefore provides basic knowledge of MES technology and the underlying microbiology in general and reviews recent improvements at the bacteria-electrode interface.

Enhanced nitrogen removal via biochar-mediated nitrification, denitrification, and electron transfer in constructed wetland microcosms

This study investigated the effect of biochar on real domestic wastewater treatment by constructed wetlands (CWs). To evaluate the role of biochar as a substrate and electron transfer medium on nitrogen transformation, three treatments of CW microcosms were established: conventional substrate (T1), biochar substrate (T2), and biochar-mediated electron transfer (T3). Nitrogen removal increased from 74% in T1 to 77.4% in T2 and 82.1% in T3. Nitrate generation increased in T2 (up to 2 mg/L) but decreased in T3 (lower than 0.8 mg/L), and the nitrification genes (amoA, Hao, and nxrA) in T2 and T3 increased by 132-164% and 129-217%, respectively, compared with T1 (1.56 × 10⁴- 2.34 × 10⁷ copies/g). The nitrifying Nitrosomonas, denitrifying Dechloromonas, and denitrification genes (narL, nirK, norC, and nosZ) in the anode and cathode of T3 were significantly higher than those of the other treatments (increased by 60-fold, 35-fold, and 19-38%). The genus Geobacter, related to electron transfer, increased in T3 (by 48-fold), and stable voltage (~150 mV) and power density (~9 uW/m²) were achieved. These results highlight the biochar-mediated enhancement of nitrogen removal in constructed wetlands via nitrification, denitrification, and electron transfer, and provide a promising approach for enhanced nitrogen removal by constructed wetland technology.

Identification of Emerging Industrial Biotechnology Chassis Vibrio natriegens as a Novel High Salt-Tolerant and Feedstock Flexibility Electroactive Microorganism for Microbial Fuel Cell

The development of MFC using electroactive industrial microorganisms has seen a surge of interest because of the co-generation for bioproduct and electricity production. Vibrio natriegens as a promising next-generation industrial microorganism chassis and its application for microbial fuel cells (MFC) was first studied. Mediated electron transfer was found in V. natriegens MFC (VMFC), but V. natriegens cannot secrete sufficient electron mediators to transfer electrons to the anode. All seven electron mediators supplemented are capable of improving the electronic transfer efficiency of VMFC. The media and carbon sources switching study reveals that VMFCs have excellent bioelectricity generation performance with feedstock flexibility and high salt-tolerance. Among them, 1% glycerol as the sole carbon source produced the highest power density of 111.9 ± 6.7 mW/cm². The insight of the endogenous electronic mediators found that phenazine-1-carboxamide, phenazine-1-carboxylic acid, and 1-hydroxyphenazine are synthesized by V. natriegens via the shikimate pathway and the phenazine synthesis and modification pathways. This work provides the first proof for emerging industrial biotechnology chassis V. natriegens as a novel high salt-tolerant and feedstock flexibility electroactive microorganism for MFC, and giving insight into the endogenous electron mediator biosynthesis of VMFC, paving the way for the application of V. natriegens in MFC and even microbial electrofermentation (EF).

Clostridium kluyveri enhances caproate production by synergistically cooperating with acetogens in mixed microbial community of electro-fermentation system

As a chain elongation (CE) model strain, Clostridium kluyveri has been used in the studies of bioaugmentation of caproate production. However, its application in the novel electro-fermentation CE system for bioaugmentation is still unclear. In this study, the CE performances, with or without bioaugmentation and in conventional or electro-fermentation systems were compared. And the mechanism of electrochemical-bioaugmentation by constructing a co-culture of Acetobacterium woodii and Clostridium kluyveri were further verified. Results demonstrated that the bioaugmentation treatments have better CE performance, especially in electro-fermentation system, with a highest caproate concentration of 4.68 g·L^-1. Mechanism analysis revealed that C. kluyveri responded to the electric field and emerged synergy with the acetogens, which was proved by the increases of C. kluyveri colonization and the acetogens abundance in biofilm and supported by the co-culture experiment. This study provides a novel insight of microbial synergy mechanism of C. kluyveri during CE bioaugmentation in electro-fermentation system.

Enhancing the extracellular electron transfer ability via Polydopamine@S. oneidensis MR-1 for Cr(VI) reduction

Microbial reduction of hexavalent chromium (Cr (VI)) shows better efficiency and cost-effectiveness. However, immobilization of Cr (III) remains a challenge as there is a limited supply of electron donors. A greener and cleaner option for donating external electrons was using bioelectrochemical systems to perform the microbial reduction of Cr(VI). In this system, we constructed a polydopamine (PDA) decorated Shewanella oneidensis MR-1 (S. oneidensis MR-1) bioelectrode with bidirectional electron transport, abbreviated as PDA@S. oneidensis MR-1. The conjugated PDA distributed on the intracellular and extracellular of individual S. oneidensis MR-1 has been shown to accelerate electron transfer by outer membrane C-type cytochromes and flavin-bound MtrC/OmcA pathway by various electrochemical analyses. As expected, the PDA@S. oneidensis MR-1 biofilm achieved 88.1% Cr (VI) removal efficiency (RE) and 58.1% Cr (III) immobilization efficiency (IE) within 24 h under the autotrophic conditions at the optimal voltage (-150 mV) compared with the control potential (0 mV). The PDA@S. oneidensis MR-1 biofilm showed increased RE activity was attributed to the shortening of the distance between individual bacteria by PDA. This research provides a viable strategy for in situ bioremediation of Cr(VI) polluted aquatic environment.

Two polarity reversal modes lead to different nitrate reduction pathways in bioelectrochemical systems

Polarity reversal is one of the effective strategies to rapidly start up denitrifying BESs，but the long-term performances of the denitrifying BESs operated under polarity reversal receive little attention. This study investigated the effects of periodic polarity reversal (PPR) and polarity reversal once only (PRO) on the long-term performances of denitrifying BESs. Repeatable oxidative and reductive currents were observed in the BESs obtained by PPR (PPR-BESs). The peak reductive currents of the PPR-BESs reached 0.95 A/m², and nitrate was mainly removed by dissimilatory nitrate reduction to ammonium pathway with removal rates higher than 95 %. In contrast, the peak reductive currents of the BESs obtained by PRO (PRO-BESs) progressively decreased from 1.01 A/m² to 0.12 A/m². The nitrate removal rates of the PRO-BESs were <50 %, and the product of nitrate reduction turned to N₂ instead of ammonium. 16S rDNA sequencing and metatranscriptomic analysis revealed that Geobacter capable of bidirectional extracellular electron transfer (EET) and Afipia capable of autotrophic growth were the dominant genera in the two types of BESs. Outer membrane cytochrome c and formate dehydrogenase were potentially involved in the cathodic electron uptake. These findings contribute to a better understanding of the EET mechanisms of electroautotrophic denitrifiers.

Cysteine-Mediated Extracellular Electron Transfer of Lysinibacillus varians GY32

Microbial extracellular electron transfer (EET) is essential in many natural and engineering processes. Compared with the versatile EET pathways of Gram-negative bacteria, the EET of Gram-positive bacteria has been studied much less and is mainly limited to the flavin-mediated pathway. Here, we investigate the EET pathway of a Gram-positive filamentous bacterium Lysinibacillus varians GY32. Strain GY32 has a wide electron donor spectrum (including lactate, acetate, formate, and some amino acids) in electrode respiration. Transcriptomic, proteomic, and electrochemical analyses show that the electrode respiration of GY32 mainly depends on electron mediators, and c-type cytochromes may be involved in its respiration. Fluorescent sensor and electrochemical analyses demonstrate that strain GY32 can secrete cysteine and flavins. Cysteine added shortly after inoculation into microbial fuel cells accelerated EET, showing cysteine is a new endogenous electron mediator of Gram-positive bacteria, which provides novel information to understand the EET networks in natural environments. IMPORTANCE Extracellular electron transport (EET) is a key driving force in biogeochemical element cycles and microbial chemical-electrical-optical energy conversion on the Earth. Gram-positive bacteria are ubiquitous and even dominant in EET-enriched environments. However, attention and knowledge of their EET pathways are largely lacking. Gram-positive bacterium Lysinibacillus varians GY32 has extremely long cells (>1 mm) and conductive nanowires, promising a unique and enormous role in the microenvironments where it lives. Its capability to secrete cysteine renders it not only an EET pathway to respire and survive, but also an electrochemical strategy to connect and shape the ambient microbial community at a millimeter scale. Moreover, its incapability of using flavins as an electron mediator suggests that the common electron mediator is species-dependent. Therefore, our results are important to understanding the EET networks in natural and engineering processes.

Biocathode prepared at low anodic potentials achieved a higher response for water biotoxicity monitoring after polarity reversal

Microbial electrochemical sensors equipped with biocathode sensing elements have attracted a growing interest, but their startup and recovery properties remain unclear. In this study, the approach of polarity reversal was applied for the biocathode sensing element fabrication and biosensor recovery. The stimulating/suppressing effect of formaldehyde was determined by the anode potential before polarity reversal as well as the increased trials of toxic exposure. Increasing anode potential from -0.3 V to +0.3 V before polarity reversal, the baseline electric signal was changed from -0.028 ± 0.001 mA to -0.005 ± 0.003 mA, while the maximum normalized electrical signal (NES) was increased from 1.1 ± 0.1 to 4.1 ± 1.9, and thus a general downtrend was observed for Response as a newly induced indicator. Polarity reversal failed to recover the electroactivity of these poisoned bioelectrodes. This study demonstrated that electrode potential was critical when using the approach of polarity reversal to construct the biocathode sensing element, and revealed an urgent need for strategies toward high recoverability of such biosensors.

Ferrous iron oxidation microflora from rust deposits improve the performance of bioelectrochemical system

Ferric iron (Fe(III)) ions are efficient electron acceptor in bioelectrochemical systems (BESs). For the first time, this study applied the enriched Fe(II)-oxidizing microflora individually from rust deposits, aerobic sludge, or topsoil to catholyte to regenerate Fe(III) ions to boost BES operation. Among three microflora, the rust-microflora had the highest Fe2+ oxidation rate and the lowest Fe ion loss rate since Acidithiobacillus sp., Ferrovum sp., Rhodobacter sp., Sphingomonas sp., and others enriched it. The rust-seeded BES generated the maximum power density of 77.15 ± 1.62 Wm-3 at 15 ℃, 38.9 %, and 31.4 % higher than those in sludge and topsoil-seeded BES, respectively. The rust-microflora with enriched Fe(II)-oxidizing bacteria could enhance the performance of BES, reaching coulombic efficiencies of 98.2 ± 2.6 at reduced internal resistance (5.14 Ω), with 1.59 Ω by activation resistance and 0.77 Ω by diffusion resistance.

Interrelation between macrophytes roots and cathode in constructed wetland-microbial fuel cells: Further evidence

As an essential component in constructed wetland-microbial fuel cells (CW-MFC) system, the macrophytes play multiple roles in bioelectricity generation and decontaminants performance. However, the interrelation between macrophytes roots and cathode has not been fully investigated despite the fact that plant cultivation strategy is a critical issue in practice. For the first time, this study was designed to explore the interaction between macrophytes and cathode in CW-MFC by planting Cyperus altrnlifolius at relatively different positions from the cathode. The results showed that plants exhibited higher bioelectricity generation and dramatically improved pollution removal, as well as the improved richness and diversity of cathode microbes. More significantly, the relative locations between the plant roots and the cathode could lead to different cathode working patterns, while the optimal cathode pattern "plant root-assisted bio- & air-cathode" was formed when the plant roots are directly placed on the air-cathode layer in CW-MFC. The insight into the plant root and cathode relationship lies in whether the "multi-function cathode" can be established. This study contributes to increase the knowledge regarding the presence and behavior of plant roots and cathode throughout a CW-MFC system.

The role of microbiome in carbon sequestration and environment security during wastewater treatment

Wastewater treatment is an essential aspect of the earth's sustainable future. However, different wastewater treatment methods are responsible for carbon discharge into the environment, raising environmental risks. Hence, such wastewater treatment methods are required that can minimize carbon release without compromising the treatment quality. Microbiome-based carbon sequestration is a potential method for achieving this goal. Limited studies have been carried out to investigate how microbes can capture and utilize CO₂. This review summarizes the approaches including microbial electrolytic carbon capture, microbial electrosynthesis, microbial fuel cell, microalgae cultivation, and constructed wetlands that employ microbes to capture and utilize CO₂. Electroactive Bacteria (EAB) convert carbon dioxide to carbonates and bicarbonates in subsequent steps after organic matter decomposition. Similarly, microbial electrosynthesis (MES) not only helps capture carbon but also produces secondary products (production of polyhydroxyalkanoates by Gram-negative rod Aeromonas hydrophila bacteria) of commercial importance during wastewater treatment. In addition to this, microbial carbon capture cells (MCCs) have been now utilized for energy generation and carbon sequestration at the same time during wastewater treatment. Moreover, microalgae cultivation has also been found to capture CO₂ at a rapid pace while releasing O₂ as a consequence of photosynthesis. Hence, microbe-based wastewater treatment has quite a potential due to two-fold benefits like carbon sequestration and by-product formation.

Autotrophic nitrate reduction to ammonium via reverse electron transfer in Geobacter dominated biofilm

Geobacter dominated electroactive biofilms (EABs) have been demonstrated to perform bidirectional extracellular electron transfer (EET) in bioelectrochemical systems, but it is largely unknown when nitrate is the electron acceptor at the cathode. If reverse EET occurs on biocathode, this EAB has to perform dissimilatory nitrate reduction to ammonia (DNRA) rather than denitrification according to genomes. Here, we have proven the feasibility of reverse bioelectron transfer in EAB, achieving a DNRA efficiency up to 93 ± 3% and high Faraday efficiency of 74 ± 1%. Constant current was found to be more effective than constant potential to maintain Geobacter on the cathode, which highly determines this electrotrophic respiration. The prevalent DNRA at constant current surpassed denitrification, demonstrated by the reverse tendencies of DNRA (nrfA) and denitrification (nirS and nirK) gene transcription. Metatranscriptomics further revealed the possible electron uptake mechanisms by which the outer membrane (OmcZ and OmcB) and periplasmic cytochromes (PpcB and PpcD) may be involved. These findings extend our understanding of the bidirectional electron transfer and advance the applications of EABs.

Microbial electrosynthesis of methane and acetate—comparison of pure and mixed cultures

Abstract

The electrochemical process of microbial electrosynthesis (MES) is used to drive the metabolism of electroactive microorganisms for the production of valuable chemicals and fuels. MES combines the advantages of electrochemistry, engineering, and microbiology and offers alternative production processes based on renewable raw materials and regenerative energies. In addition to the reactor concept and electrode design, the biocatalysts used have a significant influence on the performance of MES. Thus, pure and mixed cultures can be used as biocatalysts. By using mixed cultures, interactions between organisms, such as the direct interspecies electron transfer (DIET) or syntrophic interactions, influence the performance in terms of productivity and the product range of MES. This review focuses on the comparison of pure and mixed cultures in microbial electrosynthesis. The performance indicators, such as productivities and coulombic efficiencies (CEs), for both procedural methods are discussed. Typical products in MES are methane and acetate, therefore these processes are the focus of this review. In general, most studies used mixed cultures as biocatalyst, as more advanced performance of mixed cultures has been seen for both products. When comparing pure and mixed cultures in equivalent experimental setups a 3-fold higher methane and a nearly 2-fold higher acetate production rate can be achieved in mixed cultures. However, studies of pure culture MES for methane production have shown some improvement through reactor optimization and operational mode reaching similar performance indicators as mixed culture MES. Overall, the review gives an overview of the advantages and disadvantages of using pure or mixed cultures in MES.

Key points

• Undefined mixed cultures dominate as inoculums for the MES of methane and acetate, which comprise a high potential of improvement

• Under similar conditions, mixed cultures outperform pure cultures in MES

• Understanding the role of single species in mixed culture MES is essential for future industrial applications

A novel real-time TMAO detection method based on microbial electrochemical technology

Trimethylamine N-oxide (TMAO) is considered to be a novel biomarker of cardiovascular diseases. However, the traditional TMAO detection method has failed to meet the requirements of real-time and point-of-care tests. Herein, a novel TMAO detection method based on microbial electrochemical technology is established, which realizes the direct conversion of TMAO concentration into electrical signals. Attached Shewanella loihica PV-4 was first proven to be capable of simultaneous inward extracellular electron transfer and TMAO reduction. The TMAO detection method showed a wide linear range of 0 to 250 μM, a high sensitivity of 23.92 μA/mM, and a low limit of detection of 5.96 μM. In addition, the TMAO detection process was accomplished within 600 s, with an acceptable accuracy of 90% in the real serum, showing high feasibility in clinical applications.

Physiological metabolism of electrochemically active bacteria directed by combined acetate and Cd(II) in single-chamber microbial electrolysis cells

It is of great interest to explore physiological metabolism of electrochemically active bacteria (EAB) for combined organics and heavy metals in single-chamber microbial electrolysis cells (MECs). Four pure culture EAB varying degrees responded to the combined acetate (1.0-5.0 g/L) and Cd(II) (20-150 mg/L) at different initial concentrations in the single-chamber MECs, shown as significant relevance of Cd(II) removal (2.57-7.35 mg/L/h) and H₂ production (0-0.0011 m³/m³/h) instead of acetate removal (73-130 mg/L/h), to these EAB species at initial Cd(II) below 120 mg/L and initial acetate below 3.0 g/L. A high initial acetate (5.0 g/L) compensated the Cd(II) inhibition and broadened the removal of Cd(II) to 150 mg/L. These EAB physiologically released variable amounts of extracellular polymeric substances with a compositional diversity in response to the changes of initial Cd(II) and circuital current whereas the activities of typical intracellular enzymes were more apparently altered by the initial Cd(II) than the circuital current. These results provide experimental validation of the presence, the metabolic plasticity and the physiological response of these EAB directed by the changes of initial Cd(II) and acetate concentrations in the single-chamber MECs, deepening our understanding of EAB physiological coping strategies in metallurgical microbial electro-ecological cycles.

Bioelectrochemical system for dehalogenation: A review

Halogenated organic compounds are persistent pollutants, whose persistent contamination and rapid spread seriously threaten human health and the safety of ecosystems. It is difficult to remove them completely by traditional physicochemical techniques. In-situ remediation utilizing bioelectrochemical technology represents a promising strategy for degradation of halogenated organic compounds, which can be achieved through potential modulation. In this review, we summarize the reactor configuration of microbial electrochemical dehalogenation systems and relevant organohalide-respiring bacteria. We also highlight the mechanisms of electrode potential regulation of microbial dehalogenation and the role of extracellular electron transfer in dehalogenation process, and further discuss the application of bioelectrochemical technology in bioremediation of halogenated organic compounds. Therefore, this review summarizes the status of research on microbial electrochemical dehalogenation systems from macroscopic to microscopic levels, providing theoretical support for the development of rapid and efficient in situ bioremediation technologies for halogenated organic compounds contaminated sites, as well as insights for the removal of refractory fluorides.

Biofabrication and characterization of multispecies electroactive biofilms in stratified paper-based scaffolds

This work develops novel biofabrication and analysis platforms by creating innovative, paper-based 3-D systems that accurately recapitulate the structure, function, and physiology of living multispecies biofilms.

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

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

Redox potential-induced regulation of extracellular polymeric substances in an electroactive mixed community biofilm

Electroactive biofilms are promising in achieving efficient wastewater treatment and energy conversion in bioelectrochemical systems (BESs). Extracellular polymeric substances (EPS) are important for physical contact with electrode surface and extracellular electron transfer (EET) within biofilm. Redox potential is an important trigger for the regulation of EPS in microbial aggregates, but this yet is lacking for electroactive mixed community biofilms. This study first explored how redox potential affected EPS of electroactive mixed community biofilms, which were cultured in BES reactors with different anode potentials (-0.3 V, 0 V, +0.3 V, +0.6 V vs. SCE) using artificial brewery wastewater as substrate. The anode potential regulated biocurrent generation, overall EPS production, EPS composition and EPS redox properties. The biofilms poised at 0 V exhibited the highest current production (7.2 mA) and EPS redox capacity, while the +0.6 V biofilms had the lowest current production (1.2 mA) with lowest EPS redox capacity. The steady-state current exhibited a significant positive correlation with EPS redox capability, suggesting an important role of EPS in anode potential-dependent current production. Significant positive correlations between proteins or humic substances in EPS and EPS redox properties further verified that EPS redox feature raised from proteins and humic substances. This study provided a potential mechanism that anode potential determined the electroactivity of anode biofilm via regulating EPS composition and redox properties, and will facilitate the use of electroactive biofilms in bioelectrochemical applications.

Aggressive corrosion of carbon steel by Desulfovibrio ferrophilus IS5 biofilm was further accelerated by riboflavin

EET (extracellular electron transfer) is behind MIC (microbiologically influenced corrosion) of carbon steel by SRB (sulfate reducing bacteria). This work evaluated 20 ppm (w/w) riboflavin (an electron mediator) acceleration of C1018 carbon steel MIC by Desulfovibrio ferrophilus IS5 in enriched artificial seawater (EASW) after 7-d incubation in anaerobic vials at 28 °C. Twenty ppm riboflavin did not significantly change cell growth or alter the corrosion product varieties, but it led to 52% increase in weight loss and 105% increase in pit depth, compared to the control without 20 ppm riboflavin. With 20 ppm riboflavin supplement in EASW, D. ferrophilus yielded weight loss-based corrosion rate of 1.57 mm/y (61.8 mpy), and pit depth growth rate of 2.88 mm/y (113 mpy), highest reported for short-term pure-strain SRB MIC of carbon steel. Electrochemical tests in 450 mL glass cells indicated that the biofilm responded rather quickly to the riboflavin injection (20 ppm in broth) to the culture medium. Polarization resistance (R_p) began to decrease within minutes after injection. Within 2 h, the riboflavin injection led to 31% decrease in R_p and 35% decrease in R_ct + R_f from electrochemical impedance spectroscopy (EIS). The Tafel corrosion current density increased 63% 2 h after the injection.

Bidirectional electroactive microbial biofilms and the role of biogenic sulfur in charge storage and release

The formation of combined electrogenic/electrotrophic biofilms from marine sediments for the development of microbial energy storage systems was studied. Sediment samples from the German coasts of the Baltic and the North Sea were used as inocula for biofilm formation. Anodic biofilm cultivation was applied for a fast and reproducible biofilm formation. North-Sea- and Baltic-Sea-derived biofilms yielded comparable anodic current densities of about 7.2 A m⁻². The anodic cultivation was followed by a potential reversal regime, transitioning the electrode potential from 0.2 V to −0.8 V every 2 h to switch between anodic and cathodic conditions. The charge-discharge behavior was studied, revealing an electrochemical conversion of biogenic elemental sulfur as major charge-discharge mechanism. The microbial sequencing revealed strong differences between North- and Baltic-Sea-derived biofilms; however with a large number of known sulfur-converting and electrochemically active bacteria in both biofilms.

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Bidirectional electroactive biofilms are cultivated from marine sediments

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Cultivation is based on anodic growth followed by periodic potential reversal

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Combined electrogenic and electrotrophic activity is shown

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Biogenic, elemental sulfur plays a key role in charge storage and release

Rechargeable microbial fuel cell based on bidirectional extracellular electron transfer

Rechargeable microbial electrochemical systems can be used as renewable energy storage systems or as potable bioelectronics devices. In this study, a bioelectrode capable of bidirectional extracellular electron transfer was firstly introduced to construct the rechargeable microbial fuel cell (MFC). The performance of rechargeable MFC was enhanced with the increase of charge/discharge cycles, and a maximum energy efficiency of 4.5 ± 0.2% and Coulombic efficiency of 29.4 ± 4.1% were obtained. H₂ was the main charge carrier, while the accumulated acetate was only about 10 mg L^-1. The charge time under constant current mode largely affected the energy recovery. A decreased abundance of Mycobacteria, Geobacter, and Azospirillum, accompanied by an increase of Azonexus and Rhodococcus was observed in the rechargeable MFC, compared to control tests fueled with acetate. This study demonstrates the potential of bioelectrode for energy storage and recovery.

Enhancing bio-cathodic nitrate removal through anode-cathode polarity inversion together with regulating the anode electroactivity

Bio-cathodic nitrate removal uses autotrophic nitrate-reducing bacteria as catalysts to realize the nitrate removal process and has been considered as a cost-effective way to remove nitrate contamination. However, the present bio-cathodic nitrate removal process has problems with long start-up time and low performance, which are urgently required to improve for its application. In this study, we investigated an anode-cathode polarity inversion method for rapidly cultivating high-performance nitrate-reducing bio-cathode by regulating bio-anodic bio-oxidation electroactivities under different external resistances and explored at the first time the correlation between the oxidation performance and the reduction performance of one mixed-bacteria bioelectrode. A high bio-electrochemical nitrate removal rate of 2.74 ± 0.03 gNO₃^--N m^-2 d^-1 was obtained at the bioelectrode with high bio-anodic bio-oxidation electroactivity, which was 4.0 times that of 0.69 ± 0.03 gNO₃^--N m^-2 d^-1 at the bioelectrode with low bio-oxidation electroactivity, and which was 1.3-7.9 times that of reported (0.35-2.04 gNO₃^--N m^-2 d^-1). 16S rRNA gene sequences and bacterial biomass analysis showed higher bio-cathodic nitrate removal came from higher bacterial biomass of electrogenic bacteria and nitrate-reducing bacteria. A good linear correlation between the bio-cathodic nitrate removal performance and the reversed bio-anodic bio-oxidation electroactivity was presented and likely implied that electrogenic biofilm had either action as autotrophic nitrate reduction or promotion to the development of autotrophic nitrate removal system. This study provided a novel strategy not only to rapidly cultivate high-performance bio-cathode but also to possibly develop the bio-cathode with specific functions for substance synthesis and pollutant detection.

Micro-microbial electrochemical sensor equipped with combined bioanode and biocathode for water biotoxicity monitoring

The development of low-cost biosensors for water monitoring is expected to reduce potential risks from contamination accidents. This study reported a novel micro-microbial electrochemical sensor using combined bioanode and biocathode as the sensing element, characterized by a sequential flowing membrane-free channel and a bilateral passive oxygen supply. A decrease in the ratio of number of bioanode to biocathode resulted in a lower power generation, whereas, achieving a similar or even higher toxic response. The voltage was affected by both the flow rate and the acetate concentration. With the increased acetate concentration, a clear trade-off was observed between the electroactivity stimulation of bioanode vs. the electroactivity maintenance of biocathode. Biosensors made good response to the injection of formaldehyde (10 µL of 0.25%, and 100 µL of 0.025%) into the inlet. A high microbial diversity was observed. This work can lead to a revolutionizing way of water monitoring using self-powered micro-biosensors.

Oxygen-reducing bidirectional microbial electrodes designed in real domestic wastewater

Microbial electrodes were designed in domestic wastewaters to catalyse the oxidation of organic matter (anode) and the reduction of oxygen (cathode) alternately. The successive aeration phases (cathode) enhanced the anodic efficiency, resulting in current densities of up to 6.4 Am^-2 without the addition of any substrate. Using nitrogen during the anodic phases affected the microbial populations and the electrodes showed a lower ability to subsequently turn to O₂ reduction than the microbial anodes formed in open-to-air conditions did. No strong difference was observed between internal and external biofilm, both of which showed a very large variety of taxa in terms of abundance as well as variance. They comprised a mix of aerobic and anaerobic species, many of which have already been identified separately in bioelectrochemical systems. Such a large diversity, which had not been observed in aerobic bidirectional bioelectrodes so far, can explain the efficiency and robustness observed here.

Enhanced bio-production from CO2 by microbial electrosynthesis (MES) with continuous operational mode

Technologies able to convert CO2 to various feedstocks for fuels and chemicals are emerging due to the urge of reducing greenhouse gas emissions and de-fossilizing chemical production. Microbial electrosynthesis (MES) has been shown a promising technique to synthesize organic products particularly acetate using microorganisms and electrons. However, the efficiency of the system is low. In this study, we demonstrated the simple yet efficient strategy in enhancing the efficiency of MES by applying continuous feeding regime. Compared to the fed-batch system, continuous operational mode provided better control of pH and constant medium refreshment, resulting in higher acetate production rate and more diverse bio-products, when the cathodic potential of -1.0 V Ag/AgCl and dissolved CO2 were provided. It was observed that hydraulic retention time (HRT) had a direct effect on the pattern of production, acetate production rate and coulombic efficiency. At HRT of 3 days, pH was around 5.2 and acetate was the dominant product with the highest production rate of 651.8 ± 214.2 ppm per day and a significant coulombic efficiency of 90%. However at the HRT of 7 days, pH was lower at around 4.5, and lower but stable acetate production rate of 280 ppm per day and a maximum coulombic efficiency of 80% was obtained. In addition, more diverse and longer chain products, such as butyrate, isovalerate and caproate, were detected with low concentrations only at the HRT of 7 days. Although microbial community analysis showed the change in the planktonic cells communities after switching the fed-batch mode to continuous feeding regime, Acetobacterium still remained as the responsible bacteria for CO2 reduction to acetate, dominating the cathodic biofilm.