Acetate limitation selects Geobacter from mixed inoculum and reduces polysaccharide in electroactive biofilm

Deciphering of electron transfer and microbial community of electrogenic oxygen reducing biofilms to sulfamethoxazole stress

This study first evaluated the sulfamethoxazole (SMX) effects on oxygen-reducing biocathodes in microbial fuel cells (MFCs). Low SMX (0.5 mg L-1) enhanced current density by 20 % via increased direct electron transfer and lower charge transfer resistance. High SMX (10-30 mg L-1) suppressed electrochemical performance. SMX preferentially bound protein-like EPS components over fulvic-like fractions, inducing sequential structural changes (1054 > 970 > 3464 > 2921 > 1643 > 1350 cm-1). SMX exposure reshaped microbial communities, enriching antibiotic-resistant genera (Truepera, Nitrospira, Brevundimonas, etc.). Network analysis revealed low SMX enhanced community complexity/stability, while high doses simplified biofilm structure. Functional genes for electron transfer, carbon metabolism and oxidative phosphorylation increased at 0.5 mg L-1 SMX but decreased under high concentrations. Overall, this study elucidates the dual role of SMX in modulating oxygen-reducing biofilm composition, function, and capability, laying the groundwork for optimized application of MFC in treating SMX-contaminated wastewater.

Enhanced dissimilatory nitrate reduction to ammonium and electron transfer mechanisms in bidirectional electron transfer biofilm constructed by iron phthalocyanine

Bidirectional electron transfer biofilms (BETB) could efficiently reduce nitrate without accumulating nitrite, representing a promising biological electrochemical denitrification technology. This study utilized iron phthalocyanine modified carbon felt (FePc-CF) to enrich electroactive bacteria, constructing a long-term stable FePc-BETB. Its nitrate removal rate reached 91%, far exceeding the traditional nitrate-reducing biocathode (45%) and Con-BETB (46%). The dissimilatory nitrate reduction to ammonium (DNRA) dominated nitrate reduction in FePc-BETB, consuming 35% of the total electrons. Additionally, FePc-BETB effectively reduced the accumulation of NO2--N and N2O. Electrochemical analysis demonstrated FePc-BETB exhibited stronger electrochemical activity and electron transfer capability. Mediated electron transfer (MET) enhanced by increased extracellular humic acid in FePc-BETB favored the electron supplement for nitrate removal. The relative abundance of nrfA, marker of the DNRA, increased significantly. This study provided new insights into regulating denitrification and DNRA pathways and treating nitrate wastewater lacking electron donors.

Metabolic allocation strategies of Geobacter in electroactive biofilms to adapt to varying acetate supply concentrations

Geobacter species play a key role in acetate-fed electroactive biofilms (EABs), but their competitiveness varies with acetate supply concentration for unclear reasons. By continuously supplying different concentrations of acetate, we discovered an adaptive metabolic strategy of Geobacter biofilms, centered on regulating carbon allocation to protein synthesis, polysaccharide production, and the TCA cycle. Growth and reproduction were prioritized in response to acetate limitation under low supply concentrations, whereas catabolic efficiency was enhanced when acetate was sufficient. Excess acetate also induced the toxic effects of intracellular acetyl-CoA accumulation, triggering metabolic processes including stress responses, acetyl-CoA hydrolase synthesis, and carbon source storage. These metabolic adaptations ultimately determined the competitive niche of Geobacter in wastewater EABs, allowing for population dominance under acetate limitation and enhancing current production when acetate was abundant. Our findings give new insights into Geobacter's survival strategies in various environments with different acetate availability, and provide a theoretical basis for targeted regulation of the performance and stability of EABs to achieve environmental functions.

Electrode functional microorganisms in bioelectrochemical systems and its regulation: A review

Bioelectrochemical systems (BES) as environmental remediation biotechnologies have boomed in the last two decades. Although BESs combined technologies with electro-chemistry, -biology, and -physics, microorganisms and biofilms remain at their core. In this review, various functional microorganisms in BESs for CO2 reduction, dehalogenation, nitrate, phosphate, and sulfate reduction, metal removal, and volatile organic compound oxidation are summarized and compared in detail. Moreover, interrelationship regulation approaches for functional microorganisms and methods for electroactive biofilm development, such as targeted electrode surface modification, chemical treatment, physical revealing, biological optimization, and genetic programming are pointed out. This review provides promising guidance and suggestions for the selection of microbial inoculants and provides an analysis of the role of individual microorganisms in mixed microbial communities and its metabolisms.

Evaluating advancements and opportunities in electro-assisted biodehalogenation of emerging halogenated contaminants

Electro-assisted biodehalogenation (EASB) as a biostimulation strategy can accelerate the slow attenuation of emerging halogenated contaminants (EHCs) in anaerobic aqueous environments. A timely review is urgent to evaluate the knowledge gaps and potential opportunities, further facilitating its design and application. Till now, EASB achieves promising progress in accelerating biohalogenation rates, promoting the detoxification of EHCs to cope with unfavourable environments and mitigating greenhouse gas emissions. However, EASB of EHCs still faces several knowledge gaps. Exploring crucial microbes and deciphering insights into dehalogenase characteristics and extracellular electron transfer (EET) pathways remain the prominent task for EASB of EHCs. Moreover, microbial ecological relationships and intricate environmental factors affecting performances and applications are largely underexplored. The emergence of emerging tools holds promises for sorting the intricate changes and addressing these knowledge gaps. Judicious use of emerging tools will rejuvenate EASB strategy, from EET to scale-up, to purposefully and effectively address cascading EHCs.

Selenium treatment via integrating flow electrode capacitive deionization (FCDI) and bio-electrochemical systems (BES)

Selenium pollution in aquatic environments poses a major global challenge, with a significant gap in effective treatment technologies. In this study, we explored a novel approach integrating flow-electrode capacitive deionization (FCDI) with bio-electrochemical systems (BES) for the removal and reduction of selenate and selenite ions in one compact reactor. Our integrated system was electricity-driven, eliminating chemical usage. Up to 76 % selenium removal from the waste streams was achieved, followed by up to 66 % and 54 % reduction of selenate and selenite to elemental selenium respectively. The addition of acetate, a carbon source, enhanced selenate reduction by 14 % but lowered selenite reduction by 21 %, suggesting the substrate-dependent and bio-electrochemical-driven nature of selenate and selenite reduction respectively. Metagenomic sequencing revealed that Geobacter sulfurreducens and Pseudomonas stutzeri two known Se-reducing species, likely contributed to both selenite and selenate reduction through up-regulating functional genes related to sulfide reductase, fumarate reductase, and multi-heme c-type cytochromes. Thauera spp. and Alishewanella spp., two species not previously associated with selenium reduction, were likely involved in selenite reduction via the up-regulation of genes related to sulfite reductase and selenium reductase.

Fabricating an advanced electrogenic chassis by activating microbial metabolism and fine-tuning extracellular electron transfer

Exploiting electrogenic microorganisms as unconventional chassis hosts offers potential solutions to global energy and environmental challenges. However, their limited electrogenic efficiency and metabolic versatility, due to genetic and metabolic constraints, hinder broader applications. Herein, we developed a multifaceted approach to fabricate an enhanced electrogenic chassis, starting with streamlining the genome by removing extrachromosomal genetic material. This reduction led to faster lactate consumption, higher intracellular NADH/NAD+ and ATP/ADP levels, and increased growth and biomass accumulation, as well as promoted electrogenic activity. Transcriptome profiling showed an overall activation of cellular metabolism. We further established a molecular toolkit with a vector vehicle incorporating native replication block and refined promoter components for precise gene expression control. This enabled engineered primary metabolism for greater environmental robustness and fine-tuned extracellular electron transfer (EET) for improved efficiency. The enhanced chassis demonstrated substantially improved pollutant biodegradation and radionuclide removal, establishing a new paradigm for utilizing electrogenic organisms as novel biotechnology chassis.

Insights to the cooperation of double-working potential electroactive biofilm for performance of sulfamethoxazole removal: ARG fate and microorganism communities

Bioelectrochemical systems (BESs) have shown great potential in enhancing sulfamethoxazole (SMX) removal. However, electroactive biofilms (EBs) constructed with single potentials struggle due to limited biocatalytic activity, hindering deep SMX degradation. Here, we constructed a double-working potential BES (BES-D) to investigate its ability to eliminate SMX and reduce the levels of corresponding antibiotic resistance genes (ARGs). The preferable electrochemical activity of EB in BES-D was confirmed by electrochemical characterization, EPS analysis, physical structure, viability of the biofilm, and cytochrome content. BES-D exhibited a notably greater SMX removal efficiency (94.2 %) than did the single-working potential BES (BES-S) and the open-circuit group (OC). Degradation pathway analysis revealed that the cooperative EB could accelerate the in-depth removal of SMX. Moreover, EB interaction in BES-D decreased the relative abundance of ARGs in biofilms compared to that in BES-S, although the absolute number of ARG copies increased in BES-D effluents. Compared to those in BES-S and OC, more complex cross-niche microbial associations in the EB of BES-D were observed by network analysis of the bacterial community and ARG hosts, enhancing the degradation efficiency of SMX. In conclusion, BES-D has significant potential for SMX removal and the enhancement of EB activity. Nonetheless, the risk of ARG dissemination in effluent remains a concern.

Natural defence mechanisms of electrochemically active biofilms: From the perspective of microbial adaptation, survival strategies and antibiotic resistance

Electrochemically active biofilms (EABs) play an ever-growingly critical role in the biological treatment of wastewater due to its low carbon footprint and sustainability. However, how the multispecies biofilms adapt, survive and become tolerant under acute and chronic toxicity such as antibiotic stress still remains well un-recognized. Here, the stress responses of EABs to tetracycline concentrations (CTC) and different operation schemes were comprehensively investigated. Results show that EABs can quickly adapt (start-up time is barely affected) to low CTC (≤ 5 μM) exposure while the adaptation time of EABs increases and the bioelectrocatalytic activity decreases at CTC ≥ 10 μM. EABs exhibit a good resilience and high anti-shocking capacity under chronic and acute TC stress, respectively. But chronic effects negatively affect the metabolic activity and extracellular electron transfer, and simultaneously change the spatial morphology and microbial community structure of EABs. Particularly, the typical exoelectrogens Geobacter anodireducens can be selectively enriched under chronic TC stress with relative abundance increasing from 45.11% to 85.96%, showing stronger TC tolerance than methanogens. This may be attributed to the effective survival strategies of EABs in response to TC stress, including antibiotic efflux regulated by tet(C) at the molecular level and the secretion of more extracellular proteins in the macro scale, as the C=O bond in amide I of aromatic amino acids plays a critical role in alleviating the damage of TC to cells. Overall, this study highlights the versatile defences of EABs in terms of microbial adaptation, survival strategies, and antibiotic resistance, and deepens the understanding of microbial communities' evolution of EABs in response to acute and chronic TC stress.

Fabrication and optimization of bioelectrochemical system using tetracycline-degrading bacterial strains for antibiotic wastewater treatment

In this study, a microbial fuel cell was constructed using Raoultella sp. XY-1 to efficiently degrade tetracycline (TC) and assess the effectiveness of the electrochemical system. The degradation rate reached 83.2 ± 1.8 % during the 7-day period, in which the system contained 30 mg/L TC, and the degradation pathway and intermediates were identified. Low concentrations of TC enhanced anodic biofilm power production, while high concentrations of TC decreased the electrochemical activity of the biofilm, extracellular polymeric substances, and enzymatic activities associated with electron transfer. Introducing electrogenic bacteria improved power generation efficiency. A three-strain hybrid system was fabricated using Castellaniella sp. A3, Castellaniella sp. A5 and Raoultella sp. XY-1, leading to the enhanced TC degradation rate of 90.4 % and the increased maximum output voltage from 200 to 265 mV. This study presents a strategy utilizing tetracycline-degrading bacteria as bioanodes for TC removal, while incorporating electrogenic bacteria to enhance electricity generation.

Recent advances in microbial fuel cell-based self-powered biosensors: a comprehensive exploration of sensing strategies in both anode and cathode modes

With the rapid development of society, it is of paramount importance to expeditiously assess environmental pollution and provide early warning of toxicity risks. Microbial fuel cell-based self-powered biosensors (MFC-SPBs) have emerged as a pivotal technology, obviating the necessity for external power sources and aligning with the prevailing trends toward miniaturization and simplification in biosensor development. In this case, vigorous advancements in MFC-SPBs have been acquired in past years, irrespective of whether the target identification event transpires at the anode or cathode. The present article undertakes a comprehensive review of developed MFC-SPBs, categorizing them into substrate effect and microbial activity effect based on the nature of the target identification event. Furthermore, various enhancement strategies to improve the analytical performance like accuracy and sensitivity are also outlined, along with a discussion of future research trends and application prospects of MFC-SPBs for their better developments.

Highly sensitive standardized toxicity biosensors for rapid water quality warning

Microbial electrochemical sensor (MES) using electroactive biofilm (EAB) as the sensing element represents a broad-spectrum technology for early warning of biotoxicity of water samples. However, its commercial application is impeded by limited sensitivity and repeatability. Here, we proposed a layered standardized EAB (SEAB) with enriched Geobacter anodireducens SD-1 in the inner layer and self-matched outer layer. The SEAB sensors showed a 2.3 times higher sensitivity than conventional EAB acclimated directly from wastewater (WEAB). A highly repeatable response sensitivity was concentrated at 0.011 ± 0.0006 A/m2/ppm in 4 replicated batches of SEAB sensors (R2 > 0.95), highlighting their potential for reliable toxicity monitoring in practical applications. In contrast, the sensing performance of all WEAB sensors was unpredictable. SEAB also exhibited a better tolerance towards low concentration of formaldehyde, with only a 4 % loss in viability. Our findings improved the sensitivity and reproducibility of standardized MES for toxicity early warning.

Propionate outperforms conventional acetate as electron donors for highly-sensitive electrochemical active biofilm sensors in water biotoxicity early-warning

With the ability to generate in situ real-time electric signals, electrochemically active biofilm (EAB) sensors have attracted wide attention as a promising water biotoxicity early-warning device. Organic matters serving as the electron donors potentially affect the electric signal's output and the sensitivity of the EAB sensor. To explore the influence of organic matters on EAB sensor's performance, this study tested six different organic matters during the sensor's inoculation. Besides the acetate, a conventional and widely used organic matter, propionate and lactate were also found capable of starting up the sensor. Moreover, the propionate-fed (PF) sensor delivered the highest sensitivity, which are respectively 1.4 times and 2.8 times of acetate-fed (AF) sensor and lactate-fed (LF) sensor. Further analysis revealed that EAB of PF sensor had more vulnerable intracellular metabolism than the others, which manifested as the most severe energy metabolic suppression and reactive oxygen species attack. Regarding the microbial function, a two-component system that was deemed as an environment awareness system was found in the EAB of PF, which also contributed to its high sensitivity. Finally, PF sensor was tested in real water environment to deliver early-warning signals.

Dynamic membrane filtration accelerates electroactive biofilms in bioelectrochemical systems

Bioelectrochemical systems (BES) have emerged as a dual-function technology for treating wastewater and recovering energy. A vital element of BES is the rapid formation and maintenance of electroactive biofilms (EABs). Previous attempts to accelerate EAB formation and improve electroactivities focused on enhancing the bacterial adhesion process while neglecting the rate-limiting step of the bacterial transport process. Here, we introduce membrane filtration into BES, establishing a dynamic membrane filtration system that enhances overall performance. We observed that optimal membrane flux considerably reduced the startup time for EAB formation. Specifically, EABs established under a 25 L m-2 h-1 flux (EAB25 LMH) had a formation time of 43.8 ± 1.3 h, notably faster than the 51.4 ± 1.6 h in the static state (EAB0 LMH). Additionally, EAB25 LMH exhibited a significant increase in maximum current density, approximately 2.2 times higher than EAB0 LMH. Pearson correlation analysis indicated a positive relationship between current densities and biomass quantities and an inverse correlation with startup time. Microbial analysis revealed two critical findings: (i) variations in maximum current densities across different filtration conditions were associated with redox-active substances and biomass accumulation, and (ii) the incorporation of a filtration process in EAB formation enhanced the proportion of viable cells and encouraged a more diverse range of electroactive bacteria. Moreover, the novel electroactive membrane demonstrated sustained current production and effective solid-liquid separation during prolonged operation, indicating its potential as a viable alternative in membrane-based systems. This approach not only provides a new operational model for BES but also holds promise for expanding its application in future wastewater treatment solutions.

Refining microbial potentiometric sensor performance with unique cathodic catalytic properties for targeted application scenarios

Traditional microbial electrochemical sensors encounter challenges due to their inherent complexity. In response to these challenges, the microbial potentiometric sensor (MPS) technology was introduced, featuring a straightforward high-impedance measurement circuit tailored for environmental monitoring. Nonetheless, the practical implementation of conventional MPS is constrained by issues such as the exposure of the reference electrode to the monitored water and the absence of methodologies to stimulate microbial metabolism. In this study, our objective was to enhance MPS performance by imbuing it with unique cathodic catalytic properties, specifically tailored for distinct application scenarios. Notably, the anodic region served as the sensing element, with both the cathodic region and reference electrode physically isolated from the analyzed water sample. In the realm of organic monitoring, the sensor without Pt/C coated in the cathodic region exhibited a faster response time (1 h) and lower detection limits (1 mg L-1 BOD, 1 mM acetic acid). Conversely, when monitoring toxic substances, the sensor with Pt/C showcased a lower detection limit (0.004% formaldehyde), while the Pt/C-free sensor demonstrated superior reusability. The sensor with Pt/C displayed a heightened anode biofilm thickness and coverage, predominantly composed of Rhodococcus. In conclusion, this study introduces simple, cost-effective, and tailorable biosensors holding substantial promise for water quality monitoring.

Electroactive properties of EABs in response to long-term exposure to polystyrene microplastics/nanoplastics and the underlying adaptive mechanisms

Given widespread presence of polystyrene (PS) microplastics/nanoplastics (MPs/NPs), the electroactive responses and adaptation mechanisms of electroactive biofilms (EABs) exposed long-term to PS-containing aquatic environments remain unclear. Therefore, this study investigated the impacts of PS MPs/NPs on electroactivity of EABs. Results found that EABs exhibited delayed formation upon initially exposure but displayed an increased maximum current density (Imax) after subsequent exposure for up to 55 days. Notably, EABs exposure to NH2PS NPs (EAB-NH2PSNPs) demonstrated a 50% higher Imax than the control, along with a 17.84% increase in viability and a 58.10% increase in biomass. The cytochrome c (c-Cyts) content in EAB-NH2PSNPs rose by 178.35%, benefiting the extracellular electron transfer (EET) of EABs. Moreover, bacterial community assembly indicated the relative abundance of electroactive bacteria increased to 87.56% in EAB-NH2PSNPs. The adaptability mechanisms of EABs under prolonged exposure to PS MPs/NPs predominantly operate by adjusting viability, EET, and bacterial community assembly, which were further confirmed a positive correlation with Imax through structural equation model. These findings provide deeper insights into long-term effects and mechanisms of MPs/NPs on the electroactive properties of EABs and even functional microorganisms in aquatic ecosystems.

Scaling up of dual-chamber microbial electrochemical systems - An appraisal using systems design approach

Impetus to minimise the energy and carbon footprints of evolving wastewater resource recovery facilities has promoted the development of microbial electrochemical systems (MES) as an emerging energy-neutral and sustainable platform technology. Using separators in dual-chamber MES to isolate anodic and cathodic environments creates endless opportunities for its myriad applications. Nevertheless, the high internal resistance and the complex interdependencies among various system factors have challenged its scale-up. This critical review employed a systems approach to examine the complex interdependencies and practical issues surrounding the implementation and scalability of dual-chamber MES, where the anodic and cathodic reactions are mutually appraised to improve the overall system efficiency. The robustness and stability of anodic biofilms in large-volume MES is dependent on its inoculum source, antecedent history and enrichment strategies. The composition and anode-respiring activity of these biofilms are modulated by the anolyte composition, while their performance demands a delicate balance between the electrode size, macrostructure and the availability of substrates, buffers and nutrients when using real wastewater as anolyte. Additionally, the catholyte governed the reduction environment and associated energy consumption of MES with scalable electrocatalysts needed to enhance the sluggish reaction kinetics for energy-efficient resource recovery. A comprehensive assessment of the dual-chamber reactor configuration revealed that the tubular, spiral-wound, or plug-in modular MES configurations are suitable for pilot-scale, where it could be designed more effectively using efficient electrode macrostructure, suitable membranes and bespoke strategies for continuous operation to maximise their performance. It is anticipated that the critical and analytical understanding gained through this review will support the continuous development and scaling-up of dual-chamber MES for prospective energy-neutral treatment of wastewater and simultaneous circular management of highly relevant environmental resources.

The influencing mechanism of AD-MEC domesticated sludge to alleviates propionate accumulation and enhances methanogenesis

Anaerobic digestion combined with microbial electrolysis cell (AD-MEC) could maintain stable reactor operation and alleviating the anaerobic digestion (AD) propionate accumulation. In this study, the addition of sludge to AD-MEC was examined as a way to enhance system performance and explore the microbial interaction mechanism after electric field domestication. The results showed that under 1000 and 4000 mg/L propionate, the methane production of the sludge from AD-MEC increased by 34.29 % and 9.70 %, respectively, as compared to the AD sludge. Gompertz fitting analysis showed that sludge after electric field domestication enhancing its continuous methanogenic capacity. Further analysis showed that sludge extracellular electron transfer capacity was enhanced in AD-MEC and that its domesticated granular sludge formed a microbial community function with acid-degrading synergistic methanogenesis. The results of the study may provide theoretical support and optimization strategies for the application of AD-MEC system.

Instant water toxicity detection based on magnetically-constructed electrochemically active biofilm

Water toxicity determination with electrochemically active bacteria (EAB) is promising in the early warning of water pollution. However, limited by tedious biofilm formation, natural EAB biofilms are uncapable of the instant detection of water toxicity, resulting in the failure for the emergency monitoring of water pollution. To solve this problem, a novel method for the rapid construction of EAB biofilms using magnetic adsorption was established, and the performance of instant water toxicity detection with magnetically-constructed EAB biofilm was investigated. The results demonstrate that EAB biofilms were magnetically constructed in less than 30 min, and magnetically-constructed EAB biofilm generated stable currents even under continuous flow conditions. Magnetically-constructed EAB biofilms realized instant water toxicity detection, and the sensitivity increased with the decrease of magnetic field intensity. Low magnetic field intensity resulted in a loose biofilm structure, which is conducive to toxic pollutant penetration. The detection limit for Cu2+, phenol, and Cd2+ achieved 0.07 mg/L with optimal magnetic field intensity, and the detection time was less than 30 min. This study broadens the application of water toxicity determination with EAB, and establishes a foundation for the instant and continuous detection of water toxicity with EAB.

Polydopamine and calcium functionalized fiber carrier for enhancing microbial attachment and Cr(VI) resistance

The formation of biofilm determines the performance and stability of biofilm system. Increasing the hydrophilicity of the carrier surface could efficiently accelerate the attachment and growth of microorganisms. Here, the surface of polypropylene (PP) fiber carrier was modified with polydopamine (PDA) and calcium (Ca(II)) to enhance microbial attachment and toxicity resistance. The results of surface characteristic confirmed the self-polymerization of PDA and the chelation mechanism of Ca(II). Subsequently, the biofilm formation experiments were conducted in sequencing batch biofilm reactors using both normal and chromium-containing wastewater. The biofilm on the surface of the modified carrier exhibited better nitrogen removal and Cr(VI) reduction ability. The biomass of the modified carrier was significantly increased, and the maximum microbial attachment amounts in normal wastewater and chrome-containing wastewater were 1153.34 and 511.78 mg/g carrier, respectively. Furthermore, the confocal laser scanning microscope (CLSM) indicated that the modified carrier coated with PDA and Ca(II) were both biocompatible, and the cell activity was significantly increased. 16S rRNA sequencing results showed that the modified carrier efficiently enriched both denitrification bacteria (Thauera and Flavobacterium) and chrome-reducing bacteria (Simplicispira and Arenimonas) to improve system stability and Cr(VI) resistance. Microbial phenotype prediction based on BugBase analysis further verified the enrichment effect of modified carriers on microorganisms responsible for biofilm formation and oxidative stress resistance. Overall, this work proposed a novel functional carrier that could provide references for advancing the application of biofilm systems in wastewater treatment.

Acclimation of electroactive biofilms under different operating conditions: comprehensive analysis from architecture, composition, and metabolic activity

Electroactive biofilms (EABs) have aroused wide concern in waste treatment due to their unique capability of extracellular electron transfer with solid materials. The combined effect of different operating conditions on the formation, microbial architecture, composition, and metabolic activity of EABs is still unknown. In this study, the impact of three different factors (anode electrode, substrate concentration, and resistance) on the acclimation and performance of EABs was investigated. The results showed that the shortest start-up time of 127.3 h and highest power density of 0.84 W m-2 were obtained with carbon brush as electrode, low concentration of substrate (1.0 g L-1), and 1000 Ω external resistance (denoted as N1). The EABs under N1 condition also represented strongest redox capacity, lowest internal resistance, and close arrangement of bacteria. Moreover, the EABs cultured under different conditions both showed similar results, with direct electron transfer (DET) dominated from EABs to anode. Microbial community compositions indicated that EABs under N1 condition have lowest diversity and highest abundance of electroactive bacteria (46.68%). Higher substrate concentration (3.0 g L-1) promoted the proliferation of some other bacteria without electroactivity, which was adverse to EABs. The metabolic analysis showed the difference of genes related to electron transfer (cytochrome C and pili) and biofilm formation (xap) of EABs under different conditions, which further demonstrated the higher electroactivity of EABs under N1. These results provided a comprehensive understanding of the effect of different operating conditions on EABs including biofilm formation and electrochemical activity.

Revealing the roles of chemical communication in restoring the formation and electroactivity of electrogenic biofilm under electrical signaling disruption

Electrogenic biofilms in microbial electrochemical systems have played significant roles in simultaneous wastewater treatment and energy recovery owing to their unique extracellular electron transfer. Their formation has been shown to be regulated by electrical and chemical communication, but the interaction between these signal communication pathways has not been studied. This research investigated the coordination between intracellular c-di-GMP signaling and reinforced quorum sensing with or without exogenous HSL (a common quorum sensing molecule), on the formation of mixed-cultured electrogenic biofilm under electrical signaling disruption by tetraethylammonium (TEA, a broad-range potassium channel blocker). Intracellular c-di-GMP was spontaneously reinforced in response to TEA stress, and metagenomic analysis revealed that the dominant DGC (the genes for producing c-di-GMP) induced the eventual biofilm formation by mediating exopolysaccharide synthesis. Meanwhile, reinforced quorum sensing by exogenous HSL could also benefit the biofilm restoration, however, it alleviated the TEA-induced communication stress, resulting in the weakening of c-di-GMP dominance. Interestingly, suppressing electrical communication with or without HSL addition both induced selective enrichment of Geobacter of 85.5% or 30.1% respectively. Functional contribution analysis revealed the significant roles of Geobacter and Thauera in c-di-GMP signaling, especially Thauera in resistance to TEA stress. This study proposed a potential strategy for electrogenic biofilm regulation from the perspectives of cell-to-cell communication.

Fe(III) oxide microparticles modulate extracellular electron transfer in anodic biofilms dominated by bacteria of the Pelobacter genus

Exo-electrogenic microorganisms have been extensively studied for their ability to transfer electrons with solid surfaces using a large variety of metabolic pathways. Most of the studies on these microorganisms consist in the replacement of solid electron acceptors such as Fe(III) oxides found in nature by electrodes with the objective of generating harvestable current in devices such as microbial fuel cells. In this study we show how the presence of solid ferric oxide (Fe₂O₃) particles in the inoculum during bio-anode development influences extracellular electron transfer to the electrode. Amplification and sequencing of the 16S rRNA (V4-V5 region) show bacteria and archaea communities with a large predominance of the Pelobacter genus, which is known to be phylogenetically close to the Geobacter genus, regardless of the presence or absence of ferric oxide in the inoculum. Data indicate that the bacteria at the bio-anode surface can preferentially utilize solid ferric oxide as terminal electron acceptors instead of the anode, though extracellular electron transfer to the anode can be restored by removing the particles. Mixed inoculum commonly used to develop bioanodes may produce similar bacterial communities with divergent electrochemical responses due to the presence of alternate electron acceptors, with direct implications for microbial fuel cell performance.

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

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

Recent advances in enrichment, isolation, and bio-electrochemical activity evaluation of exoelectrogenic microorganisms

Exoelectrogenic microorganisms (EEMs) catalyzed the conversion of chemical energy to electrical energy via extracellular electron transfer (EET) mechanisms, which underlay diverse bio-electrochemical systems (BES) applications in clean energy development, environment and health monitoring, wearable/implantable devices powering, and sustainable chemicals production, thereby attracting increasing attentions from academic and industrial communities in the recent decades. However, knowledge of EEMs is still in its infancy as only ~100 EEMs of bacteria, archaea, and eukaryotes have been identified, motivating the screening and capture of new EEMs. This review presents a systematic summarization on EEM screening technologies in terms of enrichment, isolation, and bio-electrochemical activity evaluation. We first generalize the distribution characteristics of known EEMs, which provide a basis for EEM screening. Then, we summarize EET mechanisms and the principles underlying various technological approaches to the enrichment, isolation, and bio-electrochemical activity of EEMs, in which a comprehensive analysis of the applicability, accuracy, and efficiency of each technology is reviewed. Finally, we provide a future perspective on EEM screening and bio-electrochemical activity evaluation by focusing on (i) novel EET mechanisms for developing the next-generation EEM screening technologies, and (ii) integration of meta-omics approaches and bioinformatics analyses to explore nonculturable EEMs. This review promotes the development of advanced technologies to capture new EEMs.

Self-forming electroactive dynamic membrane for enhancing the decolorization of methyl orange by weak electrical stimulation

An electroactive dynamic membrane (EADM), which enabled simultaneous solid-liquid separation and contaminants removal, has been developed by electrostimulation using domestic wastewater as inoculum. Results showed that both the control dynamic membrane (CDM), without electrical stimulation, and the EADM systems exhibited stable removal performance with chemical oxygen demand (COD), and a robustness in responding to a fluctuating organic load. With the introduction of a weak electrical field, the EADM transmembrane pressure (TMP) was significantly reduced (0.02 kPa/d) compared with the control (0.20 kPa/d). In the treatment of methyl orange (MO), the EADM system achieved a decolorization efficiency of 85.87 %, much higher than the control dynamic membrane (CDM) system (58.84 %), which can be attributed to electrical stimulation and H₂ production on cathode. Microbial analysis has established that electrostimulation enriched the electroactive bacteria in the dynamic biofilm, and shaped the microbial structure, with improved contaminant removal. The results of this study highlight the potential of regulating the microbial community and creating a beneficial biofilm as a dynamic layer to facilitate contaminant removal.

Heterotrophic anodic denitrification coupled with cathodic metals recovery from on-site smelting wastewater with a bioelectrochemical system inoculated with mixed Castellaniella species

Although Castellaniella species are crucial for denitrification, there is no report on their capacity to carry out denitrification and anode respiration simultaneously in a bioelectrochemical system (BES). Herein, the ability of a mixed inoculum of electricigenic Castellaniella species to perform simultaneous denitrification and anode respiration coupled with cathodic metals recovery was investigated in a BES. Results showed that 500 mg/L NO₃^--N significantly decreased power generation, whereas 100 and 250 mg/L NO₃^--N had a lesser impact. The single-chamber MFCs (SCMFCs) fed with 100 and 250 mg/L NO₃^--N concentrations achieved a removal efficiency higher than 90% in all cycles. In contrast, the removal efficiency in the SCMFCs declined dramatically at 500 mg/L NO₃^--N, which might be attributable to decreased microbial viability as revealed by SEM and CLSM. EPS protein content and enzymatic activities of the biofilms decreased significantly at this concentration. Cyclic voltammetry results revealed that the 500 mg/L NO₃^--N concentration decreased the redox activities of anodic biofilms, while electrochemical impedance spectroscopy showed that the internal resistance of the SCMFCs at this concentration increased significantly. In addition, BES inoculated with the Castellaniella species was able to simultaneously perform heterotrophic anodic denitrification and cathodic metals recovery from real wastewater. The BES attained Cu²⁺, Hg²⁺, Pb²⁺, and Zn²⁺ removal efficiencies of 99.86 ± 0.10%, 99.98 ± 0.014%, 99.98 ± 0.01%, and 99.17 ± 0.30%, respectively, from the real wastewater. Cu²⁺ was bio-electrochemically reduced to Cu⁰ and Cu₂O, whereas Hg⁰ and HgO constituted the Hg species recovered via bioelectrochemical reduction and chemical deposition, respectively. Furthermore, Pb²⁺ and Zn²⁺ were bio-electrochemically reduced to Pb⁰ and Zn⁰, respectively. Over 89% of NO₃^--N was removed from the BES anolyte during the recovery of the metals. This research reveals promising denitrifying exoelectrogens for enhanced power generation, NO₃^--N removal, and heavy metals recovery in BES.

Extracellular electron transfer and the conductivity in microbial aggregates during biochemical wastewater treatment: A bottom-up analysis of existing knowledge

Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process.

Combining Geobacter spp. Dominated Biofilms and Anaerobic Digestion Effluents─The Effect of Effluent Composition and Electrode Potential on Biofilm Activity and Stability

The combination of anaerobic digestion (AD) and microbial electrochemical technologies (METs) offers different opportunities to increase the efficiency and sustainability of AD processes. However, methanogenic archaea and/or particles may partially hinder combining MET and AD processes. Furthermore, it is unclear if the applied anode potential affects the activity and efficiency of electroactive microorganisms in AD-MET combinations as it is described for more controlled experimental conditions. In this study, we confirm that 6-week-old Geobacter spp. dominated biofilms are by far more active and stable in AD-effluents than 3-week-old Geobacter spp. dominated biofilms. Furthermore, we show that the biofilms are twice as active at -0.2 V compared to 0.4 V, even under challenging conditions occurring in AD-MET systems. Paired-end amplicon sequencing at the DNA level using 16S-rRNA and mcrA gene shows that hydrogenotrophic methanogens incorporate into biofilms immersed in AD-effluent without any negative effect on biofilm stability and electrochemical activity.

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

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

Endogenous electric field accelerates phenol degradation in bioelectrochemical systems with reduced electrode spacing

Reducing the electrode spacing in bioelectrochemical systems (BESs) are widely reported to improve power output, which was mainly attributed to the decrease of ohmic resistance (R_ohm) for a long time. Here we found the change of endogenous electric field (EF) intensity was the key to improve electroactivity in response to a reduced electrode spacing, which also accelerated phenol biodegradation. Correlation and principal components analysis revealed that the microbial community of electroactive biofilm (EAB) was independent of R_ohm, while the EF intensity was found closely related to most of predominant genera. A strong EF selectively enriched phenol-degrading bacteria Comamonas in suspension and Geobacter in EAB, contributed to the improvement of degradation efficiency. EF also induced the secretion of extracellular polymeric substances, protected EAB from being inactivated by phenol. Our findings highlighted the importance of EF intensity on BESs performance, providing new insights into the design and application of BESs in wastewater treatment.

Response of electroactive biofilms from real wastewater to metal ion shock in bioelectrochemical systems

The electrochemical activity of bioelectrochemical systems (BESs) was proven to be dependent on the stability of electroactive biofilms (EABs), but the response of EABs based on real wastewater to external disturbances is not fully known. Herein, we used real wastewater (beer brewery wastewater) as a substrate for culturing EABs and found that current generation, biomass, redox activity and extracellular polymeric substances (EPS) content in those EABs were lower as compared to EABs cultured with synthetic wastewaters (acetate and glucose). However, the EABs from the beer brewery wastewater showed moderate anti-shock resistance capability. The proteins and humic acid in loosely bound EPS (LB-EPS) exhibited a positive linear relationship with current recovery after Ag⁺ shock, indicating the importance of LB-EPS for protecting the EABs. Fluorescence and Fourier transform infrared spectroscopy integrated with two-dimensional correlation spectroscopy verified that the spectra of the protein-like region of LB-EPS changed considerably under the interference of Ag⁺ concentration and the CO group of humic acid or proteins was mainly responsible for binding with Ag⁺ to attenuate its toxicity to the EABs. This is the first study revealing the underlying molecular mechanism of EABs cultured with real wastewater against external heavy metal shock and provides useful insights into enhancing the application of BESs in future water treatment.

Deciphering the role of extracellular polymeric substances in the regulation of microbial extracellular electron transfer under low concentrations of tetracycline exposure: Insights from transcriptomic analysis

Low concentrations of antibiotics can regulate the formation of electroactive biofilms, however, the underlying mechanisms, especially the composition and spatial distribution of extracellular polymeric substances (EPS) and their effects on extracellular electron transfer (EET) process, have not been fully deciphered. Here, the response of EPS of Geobacter sulfurreducens biofilm to low concentrations of tetracycline (μg L^-1 to mg L^-1) was explored, and the impact of such EPS variations on EET efficiency was further elucidated by transcriptomic analysis. Results showed that 0.05 mg L^-1 of tetracycline achieved both beneficial quantitative and spatial regulation of redox-active proteins and non-conducting exopolysaccharides in EPS, while higher concentrations induced negative effects. Moreover, 1 mg L^-1 of tetracycline upregulated multiple exopolysaccharide biosynthesis-related genes, indicating a stress response for cell-protection, while 0.05 mg L^-1 of tetracycline upregulated most direct EET-related gene expressions, resulting in the promoted EET efficiency. Furthermore, 0.05 mg L^-1 of tetracycline selectively enriched Geobacter (45.55% vs 19.55% in control, respectively) from mixed inoculum. This research provides a new insight of how antibiotics at low concentrations regulated EET process through modulation of EPS.

High current density with spatial distribution of Geobacter in anodic biofilm of the microbial electrolysis desalination and chemical-production cell with enlarged volumetric anode

The aim of this study was to establish the relationship between spatial distribution of Geobacter and electric intensity in the microbial electrolysis desalination and chemical-production cell (MEDCC) and to investigate the effect of enlarged volumetric anode on the performance of MEDCC. The MEDCC was constructed with nine carbon brush anodes (length × diameter = 11 cm × 3 cm) as enlarged volumetric anode, and operated by feeding with 1 g/L acetate as substrate and 35 g/L NaCl as artificial seawater under the applied voltages of 1.2-4.5 V. Spatial distribution of Geobacter in the anodic biofilm was determined according to the bacterial community analysis on 27 biofilm samples from the top, middle and bottom layers of anodes (i.e., with distance of 4.5, 10, and 15.5 cm to the cathode, respectively). Results showed that the enlarged volumetric anode significantly improved the performance of MEDCC. The maximum desalination rate and current density reached 338.5 ± 21.8 mg/L∙h and 55.7 ± 3.7 A/m² in the MEDCC, respectively. The electric intensity values decreased with the distance from the anode to the cathode and formed an uneven distribution in the anode chamber. The samples in the top layer of anodes had the highest average 16S rRNA gene copy number of Geobacter of 1.55 × 10⁷ copies/μL, which was 18 times higher than that in the bottom layer of anodes. A linear relation was established between the spatial distribution of Geobacter and electric intensity (R² = 0.994-0.999). The electric intensity gradient created the uneven spatial distribution of Geobacter in the biofilms of volumetric anode. Results from this study could be useful to enrich Geobacter in the anodic biofilm thus to improve the performance of MEDCC.

Spatial-type skeleton induced Geobacter enrichment and tailored bio-capacitance of electroactive bioanode for efficient electron transfer in microbial fuel cells

Microbial fuel cell (MFC) is a promising alternative to energy-intensive conventional wastewater technology. However, poor electron transfer efficiency, low coulombic recovery (CR), and high capital cost highly restricted its practical application. In this work, spatial electroactive biofilm is successfully developed on the carbonaceous skeleton derived from phenolic foam, which highly improved the bio-capacitance and Geobacter abundance of bioanode. Compared with carbon cloth (CC) anode, the optimal spatial electroactive biofilm (3DP_900) enriched the Geobacter abundance up to 56.8% from 17.2%, and obtained an extraordinary electroactive biomass loading of about 339 ± 63 μg cm^-2 and a remarkable bio-capacitance of about 3.4 F. In general, spatial biofilm highly reduces the barriers to electron transfer (R_ct) and mass transfer (R_d) in anodic substrate oxidation reaction and obtains the lowest R_ct of 2.0 ± 0.2 Ω and R_d of 35 ± 3.3 Ω in 3DP_900, which also supports the highest power density at 0.347 ± 0.027 W m^-2 and the highest CR at 69.2%. More importantly, due to its mature preparation technology, carbonized phenolic foam (2 cm thick pieces) reduces the capital cost of electrode preparation by three orders of magnitude from 1157.3 USD m^-2 of CC to 5.2 USD m^-2. Overall, this work offers an effective and scalable electrode to achieve high substrate utilization rate and energy recovery efficiency, and considers the economic cost of electrode fabrication for the further construction of pilot-scale MFCs equipment.

Enhanced photodegradation of antibiotics based on anoxygenic photosynthetic bacteria and bacterial metabolites: A sustainably green strategy for the removal of high-risk organics from secondary effluent

Antibiotic residues in effluents discharged from wastewater treatment plants (WWTPs) have been considered high-risk organics due to biorefractory property and potential toxicity. Secondary pollution and unsustainability existed in advanced treatment of secondary effluent are currently in urgent need of improvement. In this study, a sustainably green strategy based on Rhodopseudomonas palustris (R.palustris) by regulating the structure of extracellular polymeric substances (EPS) was proposed for the first time to achieve efficiently removal of sulfadiazine (SDZ). Results showed that 0.2 V was the optimal external potential for R.palustris to efficiently remove SDZ, where the biodegradation rate constant obtained at this potential was 4.87-folds higher than that in open-circuit mode and a complete removal was achieved within 58 h in the presence of EPS extracted at this potential. Three-dimensional excitation-emission matrix (3D-EEM) spectra analysis suggested that tryptophan protein-like, tyrosine protein-like, humic acid-like and fulvic acid-like substances present in EPS were the main effective components which was responsible for the indirect photodegradation of SDZ. The quenching experiments showed that ³EPS* was the dominant reactive species which accounted for 90% of SDZ removal. This study provides new implications for the advanced treatment of secondary effluent organic matters by developing eco-friendly bioaugmentation technology and biomaterials.

A high-sensitive and durable electrochemical sensor based on Geobacter-dominated biofilms for heavy metal toxicity detection

A highly sensitive electrochemical sensor for detecting low concentrations of heavy metals (Cd²⁺, Ni²⁺, Pb²⁺ and Cu²⁺) based on Geobacter-dominated biofilms was developed. The biosensor showed a high sensitivity for the determination of Cd²⁺ (109.7 μAμM^-1cm^-2) and the determination of Pb²⁺ (161.7 μAμM^-1cm^-2). The performance of three fitting models for biosensor response to heavy metal toxicity was investigated based on the relationship between total coulomb yield and heavy metal concentration. The full-area model (Equation a) provided the best fit, and the response times tended to be the fastest based on the peak current model (Equation c). Recovery methods were proposed to ensure the electrical activity of the biofilm for long-term monitoring. 16S rRNA gene sequence analysis showed that the most dominant genus in the anodic biofilm was Geobacter (44.1%-45.8%), indicating a stable community structure after continuous toxicity shock for 22 days. The confocal laser scanning microscope (CLSM) further proved the restorable and reusability of the biosensor. Thanks to the thin and electrically active Geobacter-dominated biofilms, it could be a good alternative biosensor for groundwater analysis etc. The results of this study contribute to the development of a highly sensitive and accurate biosensor with long-term usage towards on-site monitoring of heavy metals at low concentrations, improving the test performance of the biosensor for practical application.

Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH₄⁺ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.

Enrichment of Clostridia enhances Geobacter population and electron harvesting in a complex electroactive biofilm

Current research on microbial fuel cell or microbial electrolysis cell dealt with finding new electroactive bacteria and understanding the mechanisms of electronic exchange. Complex consortia allowed to obtain better performances than pure cultures in part thanks to inter-species cooperation. However, the role of each bacterium in a complex biofilm in the electron harvest on an electrode remains unclear. Thus, we combined electrochemical monitoring of electron exchange and high throughput sequencing analysis in order to describe the bacterial composition and the electroactive performance of mangrove mud biofilms. In this study, secondary electroactive biofilms were formed on carbon electrodes from Desulfuromonas-dominated inoculum of pre-formed bioanodes. The performances and the Desulfuromonas-dominated profile were the same as those of primary bioanodes when the planktonic community was conserved. However, a Clostridium enrichment allowed to restore the performance in maximal current densities promoting an increase of Geobacter population, becoming the most dominant group among the Deltaproteobacteria, replacing Desulfuromonas. These results highlight a positive collaboration between Clostridium and Geobacter spp. helping a bacterial population to achieve with the depletion of their environment. Our study provides new insight into relationships between dominant electroactive bacteria and other bacteria species living in an organic matter-rich environment as mangrove sediments.

Enhanced electricity generation and tetracycline removal of bioelectro-Fenton with electroactive biofilm induced by multi external resistance

A simple multi electric resistance mode is used to regulate electroactive anode film, which improves the electricity generation, H₂O₂ production and pollutants removal. This external electron transport path (double cathode with different resistance) exhibits higher H₂O₂ production (571.9 ± 0.1 mg m^-2 h^-1), tetracycline removal (71.4 ± 0.4% to 50 mg L^-1), and power (615.3 ± 9.9 mW m^-2 plus 680.6 ± 10.3 mW m^-2), which is 75.4%, 23.1% and 1.25 times higher than that of single cathode mode. The double cathode improves the relative abundance of Geobacter (exoelectrogens), which is 9.45 times higher than that of single cathode mode. The anodic capacitance of double cathode mode is more than 10 times higher than that of single cathode mode. Electrons (generate by exoelectrogens) participate in two- (cathodic chamber) and four- (anodic chamber) electron reaction at cathode surface, and facilitates electricity generation of bioelectro-Fenton. The removal rate of double cathode mode is 342.7 mg L^-1 d^-1 (50 mg L^-1 tetracycline) and 170.1 mg L^-1 d^-1 (20 mg L^-1 tetracycline), which is much higher than that of reported. These results indicate that external electron transport path enhances the electrochemical activity of anode film and performance of bioelectro-Fenton. This paper provides a new power supply method for the future practical application and field experiment of bioelectrio-Fenton.

Heterogeneous Structure Regulated by Selection Pressure on Bacterial Adhesion Optimized the Viability Stratification Structure of Electroactive Biofilms

As the core of microbial fuel cells (MFCs), the components and structure of electroactive biofilms (EABs) are essential for MFC performance. Bacterial adhesion plays a vital role in shaping the structure of EABs, but the effect of bacterial adhesion under selection pressure on EABs has not been systematically studied. Here, the response of the composition, structure, and electrochemical performance of EABs to the selective adhesion pressure due to the selective coordination of Fe(III) and Co(II) with thiol and the different affinities for bacteria on hybrid electrodes (Fe₁Co, Fe₄Co, and Fe₁₀Co) were comprehensively investigated. Compared with carbon cloth (CC), the appropriate selective adhesion pressure of Fe₄Co activated the dead inner core of EABs and optimized their viability stratification structure. Both the total viability and the viability of the inner core layer in the Fe₄Co EAB (0.67, 0.70 ± 0.01) were higher than those of the CC (0.46, 0.54 ± 0.01), Fe₁Co (0.50, 0.48 ± 0.03), and Fe₁₀Co (0.51, 0.51 ± 0.03). Moreover, a higher proportion of proteins was detected in the Fe₄Co EAB, enhancing the redox activity of extracellular polymeric substances. Fe₄Co enriched Geobacter and stimulated microbial metabolism. Electrochemical analysis revealed that the Fe₄Co EAB was the most electroactive EAB, with a maximum power density of 2032.4 mW m^-2, which was 1.7, 1.3, and 1.1 times that of the CC (1202.6 mW m^-2), Fe₁Co (1610.3 mW m^-2), and Fe₁₀Co (1824.4 mW m^-2) EABs, respectively. Our findings confirmed that highly active EABs could be formed by imposing selection pressure on bacterial adhesion.

Efficient decolorization of azo dye wastewater with polyaniline/graphene modified anode in microbial electrochemical systems

Azo dye pollution has become a worldwide issue, and the current treatment methods can hardly meet the expected emission standards. Microbial electrochemical systems (MESs) show promising applications for decolorization, but their performance critically depends on the microorganisms. Electrode modification is an interesting method of improving decolorization performance. However, the mechanisms of how the modification can affect microbial communities and the decolorization process remain unclear. Here, a modified anode with polyaniline (PANI) and graphene was fabricated via electro-deposition. Consequently, the highest decolorization efficiency was obtained. The Congo red (CR) decolorization rate of the MESs with the PANI/graphene-modified electrode (PG) reached 90% at 54 h. By contrast, the CR decolorization rates of the MESs with the PANI-modified electrode (P) and those of the MESs with the unmodified electrode (C) only reached 68% and 79%, respectively. Results of the microbial community analysis showed abundant Methanobrevibacter arboriphilus in PG (11%), which was 5.5 times that in C (2%) at 18 h. This phenomenon may be related to the rapid decolorization. The upregulated metabolism pathways, including arginine and proline metabolism, purine metabolism, arginine biosynthesis, and riboflavin metabolism, provided more electron shuttles and redox mediators that facilitated the extracellular electron transfer. Therefore, the PG-modified electrode facilitated the decolorization by altering certain metabolic pathways. This study can help to improve the guideline on the potential application of MESs for wastewater treatment.

Long-Term Succession Shows Interspecies Competition of Geobacter in Exoelectrogenic Biofilms

Geobacter spp. are well-known exoelectrogenic microorganisms that often predominate acetate-fed biofilms in microbial fuel cells (MFCs) and other bioelectrochemical systems (BESs). By using an amplicon sequence variance analysis (at one nucleotide resolution), we observed a succession between two closely related species (98% similarity in 16S RNA), Geobacter sulfurreducens and Geobacter anodireducens, in the long-term studies (20 months) of MFC biofilms. Geobacter spp. predominated in the near-electrode portion of the biofilm, while the outer layer contained an abundance of aerobes, which may have helped to consume oxygen but reduced the relative abundance of Geobacter. Removal of the outer aerobes by norspermidine washing of biofilms revealed a transition from G. sulfurreducens to G. anodireducens. This succession was also found to occur rapidly in co-cultures in BES tests even in the absence of oxygen, suggesting that oxygen was not a critical factor. G. sulfurreducens likely dominated in early biofilms by its relatively larger cell size and production of extracellular polymeric substances (individual advantages), while G. anodireducens later predominated due to greater cell numbers (quantitative advantage). Our findings revealed the interspecies competition in the long-term evolution of Geobacter genus, providing microscopic insights into Geobacter's niche and competitiveness in complex electroactive microbial consortia.

External resistance acclimation regulates bio-anode: new perspective from biofilm structure and its correlation with anode performance

External resistance is important for the anode and cell performance. However, little attentions were paid on the effect of external resistance on the variation of biofilm structure. Here, we used external resistance ranged from 4000 to 500 Ω for anodic acclimation to investigate the correlation between anode performance and biofilm structure. With the reduce of external resistance, the maximum current density of anode increased from 1.0 to 3.4 A/m², which was resulted from a comprehensive effect of reduced charge transfer resistance and increased diffusion resistance. Biological analysis showed that with the reduce of external resistance, biomass and extracellular polymeric substances content increased by 109 and 286%, cell viability increased by 22.7%, which contributed to the reduced charge transfer resistance. But the porosity of anodic biofilm decreased by 27.8%, which led to an increased diffusion resistance of H⁺. This work provided a clear correlation between the electrochemical performance and biofilm structure.

Synthesis of silver nanoparticles using living electroactive biofilm protected by polydopamine

The biosynthesis of metal nanoparticles from precious metals has been of wide concern. Their antibacterial activity is a main bottleneck restricting the bacterial activity and reduction performance. Here, bio-electrochemical systems were used to harvest electroactive biofilms (EABs), where bacteria were naturally protected by extracellular polymeric substances to keep activity. The biofilm was further encapsulated with polydopamine (PDA) as additional shield. Silver nanoparticles (AgNPs) were biosynthesized on EABs, whose electroactivity could be fully recovered after Ag⁺ reduction. The PDA increased bacterial viability by 90%–105%, confirmed as an effective protection against antibacterial activity of Ag⁺/AgNPs. The biosynthetic process changed the component and function of the microbial community, shifting from bacterial Fe reduction to archaeal methanogenesis. These results demonstrated that the electrochemical acclimation of EABs and encapsulation with PDA were effective protective measures during the biosynthesis of AgNPs. These approaches have a bright future in the green synthesis of nanomaterials, biotoxic wastewater treatment, and sustainable bio-catalysis.

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The EABs formed using BESs had an efficient ability to recover Ag⁺ to AgNPs

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The bio-reduction efficiencies of AgNPs reached more than 94%

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The PDA increased by 90%–105% of the bacterial viability

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The biosynthesis process changed the microbial community

Novel insights into the anaerobic digestion of propionate via Syntrophobacter fumaroxidans and Geobacter sulfurreducens: Process and mechanism

The accumulation of volatile fatty acids, particularly propionic acid, significantly inhibits the efficiency of the anaerobic digestion system. In propionate degradation metabolism, the unfavorable thermodynamics of syntrophic reactions, strict ecological niche of syntrophic priopionate oxidizing bacteria, and slow metabolic rate of methanogens are regarded as major limitations. In this study, Geobacter sulfurreducens was co-cultured with Syntrophobacter fumaroxidans in bioelelectrochemical cells to analyze the propionate degradation process, impact factor, mechanism metabolic pathways, and electron transfer comprehensively. The results revealed that the syntroph S. fumaroxidans and syntrophic partner G. sulfurreducens achieved more efficient propionate degradation than the control group, comprising S. fumaroxidans and methanogens. Moreover, the carbon resource concentration and pH were both significantly correlated with propionate degradation (P < 0.01). The results further confirmed that G. sulfurreducen strengthened the consumption of H₂ and acetate via direct interspecific electron transfer in propionate degradation. These findings indicate that G. sulfurreducens plays an unidentified functional role in propionate degradation.