Extracellular Electron Transfer Mechanisms Research Articles

Advances and insights into modeling extracellular electron transfer in anaerobic bioprocesses.

Extracellular electron transfer (EET) plays an important role in maintaining redox balance in both natural and engineered anaerobic microbial systems, driving key biochemical processes such as energy generation, bioremediation, and waste degradation. While EET has been characterized in a limited number of microbes and applied in anaerobic digestion and bioelectrochemical systems, further research is needed to explore its mechanism across a broader range of microbial species and anaerobic processes. This review highlights advanced modeling frameworks that provide deeper insights into EET mechanisms and dynamics, aiming to optimize research efforts and minimize time and resource expenditure. Mechanistic models, encompassing thermodynamics and kinetics, are discussed for their utility in calculating conduction rates of electroactive microbes and assessing the energetics of medium chain carboxylic acids production. Genome-scale metabolic models are highlighted for elucidating the roles of cytochromes and conductive pili in the EET pathway. Machine learning is presented as a tool to improve model accuracy and predict EET mechanisms. Furthermore, the integration of quantum mechanics/molecular mechanics methods offers molecular-level insights into electron transfer, while quantum computing addresses limitations of classical computers by simulating complex electron transfer processes in multi-heme cytochromes. Developing advanced modeling techniques will complement experimental techniques, enabling precise predictions and optimization strategies for developing innovative and sustainable anaerobic biotechnologies.

Co-occurrence of direct and indirect extracellular electron transfer mechanisms during electroactive respiration in a dissimilatory sulfate reducing bacterium.

Understanding the extracellular electron transfer mechanisms of electroactive bacteria could help determine their potential in microbial fuel cells (MFCs) and their microbial syntrophy with redox-active minerals in natural environments. However, the mechanisms of extracellular electron transfer to electrodes by sulfate-reducing bacteria (SRB) remain underexplored. Here, we utilized double-chamber MFCs with carbon cloth electrodes to investigate the extracellular electron transfer mechanisms of Desulfovibrio vulgaris Hildenborough (DvH), a model SRB, under varying lactate and sulfate concentrations using different DvH mutants. Our MFC setup indicated that DvH can harvest electrons from lactate at the anode and transfer them to cathode, where DvH could further utilize these electrons. Patterns in current production compared with variations of electron donor/acceptor ratios in the anode and cathode suggested that attachment of DvH to the electrode and biofilm density were critical for effective electricity generation. Electron microscopy analysis of DvH biofilms indicated DvH utilized filaments that resemble pili to attach to electrodes and facilitate extracellular electron transfer from cell to cell and to the electrode. Proteomics profiling indicated that DvH adapted to electroactive respiration by presenting more pili- and flagellar-related proteins. The mutant with a deletion of the major pilus-producing gene yielded less voltage and far less attachment to both anodic and catholic electrodes, suggesting the importance of pili in extracellular electron transfer. The mutant with a deficiency in biofilm formation, however, did not eliminate current production indicating the existence of indirect extracellular electron transfer. Untargeted metabolomics profiling showed flavin-based metabolites, potential electron shuttles.IMPORTANCEWe explored the application of Desulfovibrio vulgaris Hildenborough in microbial fuel cells (MFCs) and investigated its potential extracellular electron transfer (EET) mechanism. We also conducted untargeted proteomics and metabolomics profiling, offering insights into how DvH adapts metabolically to different electron donors and acceptors. An understanding of the EET mechanism and metabolic flexibility of DvH holds promise for future uses including bioremediation or enhancing efficacy in MFCs for wastewater treatment applications.

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins.

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

Sulfamethoxazole degradation in tri-electrode microbial electrochemical systems: Metabolomic and Metagenomic insights into organic pollution effects

Organic pollutants can alter the physicochemical properties and microbial communities of water bodies. In water contaminated with organic pollutants, the unique extracellular electron transfer mechanisms that promote sulfamethoxazole (SMX) degradation in tri-electrode microbial electrochemical systems (TE-MES) may be impacted. To simulate biodegradable organic matter contamination, glucose (GLU) was added. Metagenomics and metabolomics were used to analyze changes in microbial community structure, metabolism, and function on the electrodes. GLU addition accelerated water quality deterioration, and enhanced SMX degradation. Microbial taxa on the electrodes experienced selective enrichment. Notably, methanogens and SMX-degrading bacteria were enriched, while denitrifying bacteria and antibiotic-resistant bacteria were suppressed. Enriched metabolites were linked to 15 metabolic pathways and other functions like microbial signaling and genetics. Non-redundant genes also clustered in metabolic pathways, aligning with metabolite enrichment results. Additional pathways involved life cycle processes and protein interactions. Enzymes related to carbon metabolism, particularly glycoside hydrolases, increased significantly, indicating a shift in carbon metabolism on microbial electrodes after GLU addition. The abundance of intracellular electron transfer enzymes rose, while outer membrane proteins decreased. This contrasts with the typical TE-MES mechanism where outer membrane proteins facilitate SMX degradation. The presence of organic pollution may shift SMX degradation from an extracellular electrochemical process to an intracellular metabolic process, possibly involving co-metabolism with simple organic compounds. This study provides mechanistic insights and theoretical guidance for using TE-MES with embedded microbial electrodes to treat antibiotic-contaminated water affected by organic pollution.

Regulatory roles of extracellular polymeric substances in uranium reduction via extracellular electron transfer by Desulfovibrio vulgaris UR1

The pathway of reducing U(VI) to insoluble U(IV) using electroactive bacteria has become an effective and promising approach to address uranium-contaminated water caused by human activities. However, knowledge regarding the roles of extracellular polymeric substances (EPS) in the uranium reduction process involving in extracellular electron transfer (EET) mechanisms is limited. Here, this study isolated a novel U(VI)-reducing strain, Desulfovibrio vulgaris UR1, with a high uranium removal capacity of 2.75 mM/(g dry cell). Based on a reliable EPS extraction method (45 °C heating), manipulation of EPS in D. vulgaris UR1 suspensions (removal or addition of EPS) highlighted its critical role in facilitating uranium reduction efficiency. On the second day, U(VI) removal rates varied significantly across systems with different EPS contents: 60.8% in the EPS-added system, 48.5% in the pristine system, and 22.2% in the EPS-removed system. Characterization of biogenic solids confirmed the reduction of U(VI) by D. vulgaris UR1, and the main products were uraninite and UO2 (2.88–4.32 nm in diameter). As EPS formed a permeable barrier, these nanoparticles were primarily immobilized within the EPS in EPS-retained/EPS-added cells, and within the periplasm in EPS-removed cells. Multiple electroactive substances, such as tyrosine/tryptophan aromatic compounds, flavins, and quinone-like substances, were identified in EPS, which might be the reason for enhancement of uranium reduction via providing more electron shuttles. Furthermore, proteomics revealed that a large number of proteins in EPS were enriched in the subcategories of catalytic activity and electron transfer activity. Among these, iron-sulfur proteins, such as hydroxylamine reductase (P31101), pyruvate: ferredoxin oxidoreductase (A0A0H3A501), and sulfite reductase (P45574), played the most critical role in regulating EET in D. vulgaris UR1. This work highlighted the importance of EPS in the uranium reduction by D. vulgaris UR1, indicating that EPS functioned as both a reducing agent and a permeation barrier for access to heavy metal uranium.

Enhanced denitrification performance and extracellular electron transfer for eletrotrophic bio-cathode mediated by anthraquinone-2, 6-disulfonate (AQDS) at low temperature

The slow electron transfer rate and low temperature are the bottleneck in biological denitrification process, and quinone electron shuttle (e.g. AQDS) has been confirmed as a viable approach to improve the efficiency of biological denitrification by accelerating electron transport. In this study, we employed the eletrotrophic bio-cathode with cold-tolerant to explore the mechanisms of extracellular electron transfer (EET) mediated by AQDS in simultaneous nitrification and denitrification (SND) for the first time. The results indicated that adding 0.5 mmol/L AQDS increased NH4+-N removal rate and SND efficiency by 18.1% and 17.9%, respectively, and the optimal hydraulic retention time (HRT) was shortened from 5 d to 3 d. The potential mechanism was that AQDS resulted in a 1.47-fold increase in the relative abundance of cathodic electroactive microorganisms, and the relative abundance of functional genes such as cytochrome c, flavin, cold acclimation and translocator increased by about 1.15–300.42%, which accelerated the transfer of extracellular electron and significantly improved the energy metabolism efficiency. In addition, AQDS may act as an electron shuttle to accelerate the electronic transition in long-distance electron transfer. In summary, this study expands our understanding of the role of AQDS in the SND process by bio-cathode under low temperature and provides an insight into the EET of eletrotrophic microorganisms with cold-tolerant.

Nanowire Electrode Structures Enhanced Direct Extracellular Electron Transport via Cell-Surface Multi-Heme Cytochromes in Desulfovibrio ferrophilus IS5

Extracellular electron transfer (EET) by sulfate-reducing bacteria (SRB), such as Desulfovibrio ferrophilus IS5, enables bacterial interactions with minerals, which are vital for biogeochemical cycling and environmental chemistry. Here, we explore the direct EET mechanisms through outer-membrane cytochromes (OMCs) using IS5 as a model SRB. We employed nanostructured electrodes arrayed with 0, 50, 200, and 500 nm long nanowires (NWs) coated with indium–tin–doped oxide to examine the impact of electrode morphology on the direct EET efficacy. Compared to flat electrodes, NW electrodes significantly enhanced current production in IS5 with OMCs. However, this enhancement was diminished when OMC expression was reduced. Differential pulse voltammetry revealed that NW electrodes specifically augmented redox peaks associated with OMCs without affecting those related to redox mediators, suggesting that NWs foster direct EET through OMCs. Scanning electron microscopy observations following electrochemical analyses revealed a novel vertical cell attachment and aggregation on NW electrodes, contrasting with the horizontal monolayer cell attachment on flat electrodes. This study presents the first evidence of the critical role of electrode nanoscale topography in modulating SRB cell orientation and aggregation behavior. The findings underscore the significant influence of electrode morphology on the direct EET kinetics, highlighting the potential impact of mineral morphology on mineral reduction and biogeochemical processes.

Modulating anodic performance of halophilic electroactive microbial consortia with selective enrichment using metal-based terminal electron acceptors

Long start-up times and lack of robust halophilic electroactive microbes (EAM) challenge the implementation of microbial fuel cells (MFC) for treating saline wastewater. Here a pragmatic strategy is presented to selectively enrich halophilic EAM consortium capitalising on their cellular respiration process using graphene oxide (GO) and metal ions (Fe3+, Mn4+ and Cu2+) as the primary and secondary terminal electron acceptors (TEA), respectively. Enrichment of GO-respiring EAM consortium with Fe3+ ions enabled growth of an efficient anodic biofilm within a short start-up time of 50 h and generated up to 1.1 W/m3 while treating highly saline electrolyte (2 % NaCl). In conjunction with predictive genetic profiling, in-depth electrochemical analysis of the biofilm unravelled key mechanisms of direct extracellular electron transfer for generating high anodic currents. Additionally, the inclusion of Fe3+ ions as secondary TEA upregulated the metabolic activity of GO-respiring EAM, which subsequently influenced their anodic performance rather than the relative abundance of exoelectrogens in the biofilm. Thus, this study stimulates the discovery of novel EAM communities using an advanced and sensible design of selective media by varying the electron donor and TEA couplings.

Lack of physiological evidence for cytochrome filaments functioning as conduits for extracellular electron transfer.

Unraveling microbial extracellular electron transfer mechanisms has profound implications for environmental processes and advancing biological applications. This study on Geobacter sulfurreducens challenges prevailing beliefs on cytochrome filaments as crucial components thought to facilitate long-range electron transport. The discovery of an OmcS-deficient strain's unexpected effectiveness in Fe(III) oxide reduction prompted a reevaluation of the key conduits for extracellular electron transfer. By exploring the impact of genetic modifications on G. sulfurreducens' performance, this research sheds light on the importance of 3-nm diameter electrically conductive pili in Fe(III) oxide reduction. Reassessing these mechanisms is essential for uncovering the true drivers of extracellular electron transfer in microbial systems, offering insights that could revolutionize applications across diverse fields.

Mechanisms of extracellular electron transfer in anaerobic methanotrophic archaea

Anaerobic methanotrophic (ANME) archaea are environmentally important, uncultivated microorganisms that oxidize the potent greenhouse gas methane. During methane oxidation, ANME archaea engage in extracellular electron transfer (EET) with other microbes, metal oxides, and electrodes through unclear mechanisms. Here, we cultivate ANME-2d archaea (‘Ca. Methanoperedens’) in bioelectrochemical systems and observe strong methane-dependent current (91–93% of total current) associated with high enrichment of ‘Ca. Methanoperedens’ on the anode (up to 82% of the community), as determined by metagenomics and transmission electron microscopy. Electrochemical and metatranscriptomic analyses suggest that the EET mechanism is similar at various electrode potentials, with the possible involvement of an uncharacterized short-range electron transport protein complex and OmcZ nanowires.

Electrochemical Characterization of Two Gut Microbial Strains Cooperatively Promoting Multiple Sclerosis Pathogenesis.

In this study, we explored the extracellular electron transfer (EET) capabilities of two bacterial strains, OTU0001 and OTU0002, which are demonstrated in biofilm formation in mouse gut and the induction of autoimmune diseases like multiple sclerosis. OTU0002 displayed significant electrogenic behaviour, producing microbial current on an indium tin-doped oxide electrode surface, particularly in the presence of glucose, with a current density of 60 nA/cm2. The presence of cell-surface redox substrate potentially mediating EET was revealed by the redox-based staining method and electrochemical voltammetry assay. However, medium swapping analyses and the addition of flavins, a model redox mediator, suggest that the current production is dominated by soluble endogenous redox substrates in OTU0002. Given redox substrates were detected at the cell surface, the secreted redox molecule may interact with the cellular surface of OTU0002. In contrast to OTU0002, OTU0001 did not exhibit notable electrochemical activity, lacking cell-surface redox molecules. Further, the mixture of the two strains did not increase the current production from OTU0001, suggesting that OTU0001 does not support the EET mechanism of OTU0002. The present work revealed the coexistence of EET and non-EET capable pathogens in multi-species biofilm.

Nonelectroactive clostridium obtains extracellular electron transfer-capability after forming chimera with Geobacter.

Extracellular electron transfer (EET) of microorganisms is a major driver of the microbial growth and metabolism, including reactions involved in the cycling of C, N, and Fe in anaerobic environments such as soils and sediments. Understanding the mechanisms of EET, as well as knowing which organisms are EET-capable (or can become so) is fundamental to electromicrobiology and geomicrobiology. In general, Gram-positive bacteria very seldomly perform EET due to their thick non-conductive cell wall. Here, we report that a Gram-positive Clostridium intestinale (C.i) attained EET-capability for ethanol metabolism only after forming chimera with electroactive Geobacter sulfurreducens (G.s). Mechanism analyses demonstrated that the EET was possible after the cell fusion of the two species was achieved. Under these conditions, the ethanol metabolism pathway of C.i was integrated by the EET pathway of G.s, by which achieved the oxidation of ethanol for the subsequent reduction of extracellular electron acceptors in the coculture. Our study displays a new approach to perform EET for Gram-positive bacteria via recruiting the EET pathway of an electroactive bacterium, which suggests a previously unanticipated prevalence of EET in the microbial world. These findings also provide new perspectives to understand the energetic coupling between bacterial species and the ecology of interspecies mutualisms.

Magnetite-equipped algal-rich sediments for microbial fuel cells: Remediation of sediment organic matter pollution and mechanisms of remote electron transfer

Using the bio-electrochemical methods for the restoration of high algae sediments is full of potential and challenges. How to promote extracellular electron transfer (EET) process in microbial fuel cells (MFC) is the key bottleneck. The study had explored the potential application of magnetite on accelerating electron transfer for improving the output of MFC and sediment pollution remediation. The results indicated that the organic matter degradation rate showed a remarkable increase of 27.45 %, and the voltage output was approximately 1.68 times higher compared to the MFC configured with regular sediment. Abundant electroactive bacteria (EABs), such as Geobacter and Burkholderiaceae, and fermentative bacteria were responsible for these results, accompanied by the enhanced fluorescence of humic substances (HS), increased concentration and activity of cytochrome C (25.05 % and 21.12 %), as well as elevated extracellular polymeric substance content. Moreover, the intrinsic EET mechanisms among Fe-oxides, HS, and EABs were explored. According to the electrochemical analysis and substance transformation, the EET process involved four stages: magnetite-enhanced direct electron transfer via strong conductivity, iron respiration mediating electron transfer to the electrode, the model quinone substance acting as an electron shuttle facilitating EET and iron reduction, and iron cycling mediating electron transfer. This study provides an effective strategy for pollution remediation in algal-rich sediment, which was beneficial for the harmless treatment and resource utilization of both algae and sediment, simultaneously.

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.

Extracellular electron transfer for aerobic denitrification mediated by the bioelectric catalytic system with zero-carbon source

The slow rate of electron transfer and the large consumption of carbon sources are technical bottlenecks in the biological treatment of wastewater. Here, we first proposed to domesticate aerobic denitrifying bacteria (ADB) from heterotrophic to autotrophic by electricity (0.6 V) under zero organic carbon source conditions, to accelerate electron transfer and shorten hydraulic retention time (HRT) while increasing the biodegradation rate. Then we investigated the extracellular electron transfer (EET) mechanism mediated by this process, and additionally examined the integrated nitrogen removal efficiency of this system with composite pollution. It was demonstrated that compared with the traditional membrane bioreactor (MBR), the BEC displayed higher nitrogen removal efficiency. Especially at C/N = 0, the BEC exhibited a NO3--N removal rate of 95.42 ± 2.71 % for 4 h, which was about 6.5 times higher than that of the MBR. Under the compound pollution condition, the BEC still maintained high NO3--N and tetracycline removal (94.52 ± 2.01 % and 91.50 ± 0.001 %), greatly superior to the MBR (10.64 ± 2.01 % and 12.00 ± 0.019 %). In addition, in-situ electrochemical tests showed that the nitrate in the BEC could be directly converted to N2 by reduction using electrons from the cathode, which was successfully demonstrated as a terminal electron acceptor.

Influence of shear stress on electroactive biofilm characteristics and performance in microbial fuel cells

This study has provided comprehensive insights into the intricate relationship between shear stress and the development, structure, and functionality of electroactive biofilms in Microbial Fuel Cells (MFCs). A multichannel microfluidic MFC reactors that created specific shear stress on the anode, were designed for the simultaneous study of multiple flow conditions using the same medium. Then, the evolution of the biofilm growth under different shear stress conditions (1, 5 and 10 mPa) were compared. The taxonomic and functional structure was studied by 16S rRNA gene and metagenomic sequencing and the physical biofilm characteristics were measured via fluorescence microscopy. The results demonstrate the pivotal role of shear stress in influencing the growth kinetics, electrical performance, and physical structure of anodic biofilms. Notably, the selection of specific EAB was observed to be shear stress-dependent, with a marked increase in specific EAB abundance as shear stress increased. The power density, while not directly correlated with the relative abundance of specific or nonspecific EAB, exhibited a strong linear relationship with biofilm coverage. This suggests that factors beyond the microbial composition, potentially including mass transport or electrochemical conditions, might be instrumental in determining electricity production. The functional metagenomic analysis further highlighted the complexities of extracellular electron transfer (EET) mechanisms in electroactive biofilm. While certain genes associated with EET in known species such as Geobacter and Shewanella were identified, the study also examined the limitations of solely relying on genetic markers to infer EET capabilities, emphasizing the need for complementary metaproteomic analyses. This study demonstrates the multifaceted impact of shear stress on electroactive biofilm and paves the way for future investigations aimed at harnessing the potential of electroactive biofilms in microbial fuel cell applications.

Synergistic effect between sulfate-reducing bacteria and Shewanella algae on corrosion behavior of 321 stainless steel

Microbiologically influenced corrosion (MIC) caused by mixed microorganisms limits the application of 321 stainless steel (SS) in the field of ocean engineering. Therefore, in this paper, the synergistic effect of sulfate-reducing bacteria (SRB) and Shewanella algae (S. algae) on the corrosion behavior of 321 SS was studied by surface analysis and electrochemical measurements. The results revealed that riboflavin secreted by S. algae promoted the electron transfer between SRB and 321 SS by obtaining the electrons generated by the dissolution of metallic iron on the surface of SS and then transferring them to SRB, thus accelerating the MIC of 321 SS. Meanwhile, SRB gained electrons and reacted with SO42− to form HS−, which reacted with iron oxides in the Cr-rich passive film on the surface of SS to generate iron sulfides, providing thermodynamic and kinetic conditions for the formation of Cr6+. The formation of thermodynamically unstable Cr6+ destructed the passive film on the SS surface and promoted the development of pitting corrosion. The outcome provided a new perspective for studying the corrosion mechanism of extracellular electron transfer (EET)-MIC.

Bioelectrochemical system accelerates reductive dechlorination through extracellular electron transfer networks

Bioelectrochemical system is considered as a promising approach for enhanced bio-dechlorination. However, the mechanism of extracellular electron transfer in the dechlorinating consortium is still a controversial issue. In this study, bioelectrochemical systems were established with cathode potential settings at −0.30 V (vs. SHE) for trichloroethylene reduction. The average dechlorination rate (102.0 μM Cl·d−1) of biocathode was 1.36 times higher than that of open circuit (74.7 μM Cl·d−1). Electrochemical characterization via cyclic voltammetry illustrated that electrostimulation promoted electrochemical activity for redox reactions. Moreover, bacterial community structure analyses indicated electrical stimulation facilitated the enrichment of electroactive and dechlorinating populations on cathode. Metagenomic and quantitative polymerase chain reaction (qPCR) analyses revealed that direct electron transfer (via electrically conductive pili, multi-heme c-type cytochromes) between Axonexus and Desulfovibrio/cathode and indirect electron transfer (via riboflavin) for Dehalococcoides enhanced dechlorination process in BES. Overall, this study verifies the effectiveness of electrostimulated bio-dechlorination and provides novel insights into the mechanisms of dechlorination process enhancement in bioelectrochemical systems through electron transfer networks.

Advances in the Application of Quorum Sensing to Regulate Electrode Biofilms in Bioelectrochemical Systems

Bioelectrochemical systems (BESs) are an emerging technology for wastewater treatment and resource recovery. These systems facilitate electron transfer between microorganisms and electrodes, enabling their application in various fields, such as electricity production, bioremediation, biosensors, and biocatalysis. However, electrode biofilms, which play a critical role in BESs, face several challenges (e.g., a long acclimation period, low attached biomass, high electron transfer resistance, and poor tolerance and stability) that limit the development of this technology. Quorum sensing (QS) is a communication method among microorganisms that can enhance the performance of BESs by regulating electrode biofilms. QS regulation can positively impact electrode biofilms by enhancing extracellular electron transfer (EET), biofilm formation, cellular activity, the secretion of extracellular polymeric substances (EPS), and the construction of microbial community. In this paper, the characteristics of anode electrogenic biofilms and cathode electrotrophic biofilms in BESs, EET mechanisms, and the main factors affecting biofilm formation were summarized. Additionally, QS regulation mechanisms for biofilm formation, strategies for enhancing and inhibiting QS, and the application of QS regulation for electrode biofilms in BESs were systematically reviewed and discussed. This paper provides valuable background information and insights for future research and development of BES platforms based on QS regulation of electrode biofilms.

Comparing theoretical and practical biomass yields calls for revisiting thermodynamic growth models for electroactive microorganisms

Research on electroactive microorganisms (EAM) often focuses either on their physiology and the underlying mechanisms of extracellular electron transfer or on their application in microbial electrochemical technologies (MET). Thermodynamic understanding of energy conversions related to growth and activity of EAM has received only a little attention. In this study, we aimed to prove the hypothesized restricted energy harvest of EAM by determining biomass yields by monitoring growth of acetate-fed biofilms presumably enriched in Geobacter, using optical coherence tomography, at three anode potentials and four acetate concentrations. Experiments were concurrently simulated using a refined thermodynamic model for EAM. Neither clear correlations were observed between biomass yield and anode potential nor acetate concentration, albeit the statistical significances are limited, mainly due to the observed experimental variances. The experimental biomass yield based on acetate consumption (YX/ac = 37 ± 9 mgCODbiomass gCODac−1) was higher than estimated by modeling, indicating limitations of existing growth models to predict yields of EAM. In contrast, the modeled biomass yield based on catabolic energy harvest was higher than the biomass yield from experimental data (YX/cat = 25.9 ± 6.8 mgCODbiomass kJ−1), supporting restricted energy harvest of EAM and indicating a role of not considered energy sinks. This calls for an adjusted growth model for EAM, including, e.g., the microbial electrochemical Peltier heat to improve the understanding and modeling of their energy metabolism. Furthermore, the reported biomass yields are important parameters to design strategies for influencing the interactions between EAM and other microorganisms and allowing more realistic feasibility assessments of MET.