Electrochemical activities of Geobacter biofilms growing on electrodes with various potentials

The role of electroactive biofilms in enhanced para-chlorophenol transformation collaborated with biosynthetic palladium nanoparticles

Bioremediation is a cost-effective strategy for decomposition of chlorinated organic contaminants, but its application is often hindered by the generation of toxic chlorinated byproducts. Though the design of functional biofilms, incorporating microbially-inspired catalytic materials, has emerged as a promising solution for tackling the byproducts issues, the microbial mechanisms driving these processes remain inadequately understood. This study demonstrates a hybrid electroactive biofilm (EAB)-palladium nanoparticles (Pd NPs) system that effectively separates the dechlorination and mineralization of para-chlorophenol (4-CP), and most importantly, it provides new insights into the microbial and genetic roles of EABs in this process. Under an applied potential of -0.6 V, Pd NPs via palladate reduction were biogenically synthesized and deposited on the cytomembranes within the biofilm, achieving an 82 % decrease in 4-CP concentration within 48 h. The ultra-performance liquid chromatogram and mass spectrum confirmed that 4-CP was initially dechlorinated to phenol by the biogenic Pd NPs before undergoing further degradation by the biofilm, effectively preventing toxic chlorinated byproducts. The Dechloromonas, Pseudomonas, and Geobacter were identified as predominant genera in the system and the metagenomics analysis noted increased relative abundance of ring-cleavage genes like pcaG, dmpB/xylE, and catA. Importantly, the abundance of dmpB/xylE was primarily associated with Dechloromonas and Pseudomonas, further highlighted that the dmpB/xylE-pathway was important for rapid 4-CP decomposition in the system. This study advances the understanding of EAB-Pd NPs synergy, showcasing an innovative and sustainable approach for the efficient removal of halogenated pollutants.

Electrogenic dynamics of biofilm formation: Correlation between genetic expression and electrochemical activity in Bacillus subtilis

Biofilms are structured microbial communities, known for their electron transfer properties, which are essential for metabolic processes and microbial survival. Here, we investigated the electrogenic properties of Bacillus subtilis, a bacterial producer of electron-donating biofilms. Interdigitated gold electrodes were utilized to continuously measure the electrochemical activity of biofilm-forming B. subtilis cells and genetic mutants unable to create them (biofilm-deficient). The formation of extracellular polymeric substances (EPS) and filamentous appendages was monitored via scanning electron microscopy (SEM). Chronoamperometry was used to assess electrochemical activity, which showed fluctuations in electrical current at specific time points in biofilm-forming cells. Cyclic voltammetry (CV) revealed significant differences between the voltammograms of biofilm-forming and biofilm-deficient cells, hypothesized to be a result of the reduction of secreted flavodoxin. Electrochemical impedance spectroscopy (EIS) was also performed at various intervals and analyzed using an equivalent circuit. We identified the presence of a charge transfer resistance (Rct) exclusively in biofilm which correlated to the time of increased electrochemical activity measured using chronoamperometry. Finally, through confocal microscopy, we found that the expression of a gene involved in biofilm matrix formation, tasA, was correlated with the time when charge transfer was measured. These results indicate that electrochemical activity is primarily present in biofilm-forming cells rather than in biofilm-deficient mutants. By combining electrochemical and microscopic methods, we developed a methodology to continuously monitor the stages of biofilm formation and showed that electrochemical activities within biofilms vary over time and there is a temporal relationship between these processes and the expression of genes responsible for biofilm development.

Effect of single and hybrid microplastic exposures on anaerobic sludge in microbial electrochemical technology (MET)

Microplastics (MPs) in wastewater treatment pose significant environmental risks. While microbial electrochemical technology (MET) is effective in removing refractory pollutants, most studies focus on single MP types, despite real-world wastewater often containing mixtures. This study examines the effects of single and hybrid MPs (HD-MPs) on wastewater treatment performance and microbial communities in MET systems. Results show that MPs significantly impair methanogenesis, reducing methane production by 25.27-36.46 %, decreasing COD removal efficiency by 26.82-33.33 %, and increasing volatile fatty acid accumulation by 17.23-26.79 %, with PVC exhibiting the strongest inhibition. Electrical stimulation (0.7 V) exacerbates oxidative stress, elevating reactive oxygen species (ROS) and lactate dehydrogenase (LDH) release in biofilm sludge (BFS) compared to anaerobic granular sludge (AGS). Extracellular polymeric substances (EPS) shift from protein-dominant (65-77 % under PVC-MPs and HD-MPs) to polysaccharide-rich (52-55 % under PE-MPs), reflecting type-specific microbial responses. High-throughput 16S rRNA sequencing reveals that applied voltage selectively enriches fermentative bacteria (Firmicutes and Bacteroidota), creating distinct microbial profiles between AGS and BFS. Metatranscriptomic analyses show that HD-MPs under electrical stimulation induce structural reorganization of key functional microorganisms, including a 144 % increase in fermentation bacteria, 61.7 % rise in acetogens, and 3.89-fold enhancement of electrogenic bacteria at the BFS anode compared to AGS. Transcriptional upregulation of genes in glycolysis, TCA cycle, and methane metabolism confirms the dominance of the acetoclastic methanogenic pathway (79.18-86.97 % of total methanogenesis). This study enhances understanding of electrostimulated microbial consortia in complex pollutant environments and proposes practical MET configurations for real-world applications.

Redox potential shapes spatial heterogeneity of mixed-cultured electroactive biofilm treating wastewater

The core of bioelectrochemical systems (BESs) is electrochemically active microorganisms (EAMs), which exert spatial heterogeneity on electrode surface and influences BESs performance. Setting an optimal potential is an effective strategy for improving and optimizing BESs performance, however, how the electrode potential affects spatial structure of microbial community within anode biofilm is not known. Using a complex substrate-fed BES with a wastewater inoculum, this study investigated the community structure and composition of the stratified biofilm developed under the potential of -0.3 V, 0 V, +0.3 V and +0.6 V (vs. saturated calomel electrode) by freezing microtome method and high-throughput sequencing analysis. The spatial heterogeneity of biofilm community was found to be dependent on the electrode potential and a less stratified community structure was observed for +0.6 V than other potentials. Within the biofilms, the inner layers selected more Geobacter and the outer layers enriched more Acinetobacter and Serratia, potentially suggested a stratification of electron transfer pathway and metabolite-based interspecies communications. The results demonstrated the response of spatial heterogeneity of anode biofilm community to the change of electrode potential, which helps to understand the selectivity and enrichment of kinetically efficient anodic microbiome by electron potential.

An in-depth analysis of microbial response to exposure to high concentrations of microplastics in anaerobic wastewater fermentation

The impact of microplastics (MPs) in anaerobic wastewater treatment on microbial metabolism is significant. Anaerobic granular sludge (AS) and biofilm (BF) are two common ways, and their responses to microplastics will have a direct impact on their application potential. This study investigated the microbial reactions of AS and BF to three types of MPs: polyethylene (PE), polyvinyl chloride (PVC), and a mixture of both (MIX). Results exhibited that MPs reduced methane output by 44.65 %, 55.89 %, and 53.18 %, elevated short-chain fatty acid (SCFA) levels by 95.93 %, 124.49 %, and 110.78 %, and lowered chemical oxygen demand (COD) removal by 28.77 %, 36.78 %, and 33.99 % for PE-MP, PVC-MP, and MIX-MP, respectively, with PVC-MP showing the greatest inhibition. Meanwhile, microplastics also facilitated the relative production of reactive oxygen species (ROS, 40.29 %-96.99 %), lactate dehydrogenase (LDH, 20.01 %-75.02 %), and adenosine triphosphate (ATP, 26.64 %-43.80 %), while reducing cytochrome c (cyt c, 23.60 %-49.02 %) and extracellular polymeric substances (EPS, 17.44 %-26.58 %). AS and BF displayed distinct enzymatic activities under MPs exposure. Correspondingly, 16S-rRNA sequencing indicated that AS was mainly involved in acetate generation by Firmicutes, while BF performed polysaccharide degradation by Bacteroidota. Metatranscriptomic analysis showed AS to be rich in acetogens (Bacillus, Syntrophobacter) and methanogens (Methanothrix, Methanobacterium), while BF contained more fermentation bacteria (Mesotoga, Lentimicrobium) and electroactive microorganisms (Clostridium, Desulfuromonas) under MIX-MP. Moreover, BF exhibited higher glycolysis gene expression, whereas AS was more active in methane metabolism, primarily through the acetoclastic methanogenic pathway's direct acetate conversion. This study provides new insights into understanding the microbial response produced by microplastics during anaerobic wastewater digestion.

Effects of marine eutrophication environment on microbial corrosion: A review

Metal materials undergo severe corrosion in eutrophic environments. The effect of DO decay stimulated by high concentrations of nitrogen and phosphorus pollutants on microorganisms leads to the coupling of electrochemical and microbial corrosion processes. However, there are few studies on microbial corrosion mechanisms in eutrophic environments. This article discusses the corrosive factors of marine eutrophication, summarizes the impact of marine eutrophication on microbial corrosion and the potential mechanisms, including aerobic biofilm corrosion, aerobic & anaerobic mixed biofilm corrosion, and anaerobic microbial electron transfer corrosion, and expounds on the research methods for microbial corrosion of materials serving in estuarine areas prone to pollution. Microbial prevention and control, such as nutrient restriction and microbial interspecies competition, are of research value in the field of green protection. Microbial corrosion mechanisms studies in marine eutrophication environments are significant for environment monitor development, water intake and algae control technologies, and corrosion protection in polluted environments.

Exploration of the mechanism of 2-CP degradation by Acinetobacter sp. stimulated by Lactobacillus plantarum fermentation waste: A bio-waste reuse

Green and economical pollution management methods which reusing bio-waste as biostimulant to effectively improve the removal of target pollutants are receiving more and more attention. In this study, Lactobacillus plantarum fermentation waste solution (LPS) was used to investigate its facilitative effect and the stimulation mechanisms on the degradation of 2-chlorophenol (2-CP) by strain Acinetobacter sp. Strain ZY1 in terms of both cell physiology and transcriptomics. The degradation efficiency of 2-CP was improved from 60% to >80% under LPS treatment. The biostimulant maintained the morphology of strain, reduced the level of reactive oxygen species, and recovered the cell membrane permeability from 39% to 22%. It also significantly increased the level of electron transfer activity and extracellular polymeric substances secretion and improved the metabolic activity of the strain. The transcriptome results revealed the stimulation of LPS to promote biological processes such as bacterial proliferation, metabolism, membrane structure composition, and energy conversion. This study provided new insights and references for the reuse of fermentation waste streams in biostimulation methods.

Complete pentachlorophenol biodegradation in a dual-working electrode bioelectrochemical system: Performance and functional microorganism identification

Bioelectrochemical system (BES) can effectively promote the reductive dechlorination of chlorophenols (CPs). However, the complete degradation of CPs with sequential dechlorination and mineralization processes has rarely achieved from the BES. Here, a dual-working electrode BES was constructed and applied for the complete degradation of pentachlorophenol (PCP). Combined with DNA-stable isotope probing (DNA-SIP), the biofilms attached on the anodic and cathodic electrode in the BES were analyzed to explore the dechlorinating and mineralizing microorganisms. Results showed that PCP removal efficiency in the dual-working BES (84% for 21 days) was 4.1 and 4.7 times higher than those of conventional BESs with a single anodic or cathodic working electrode, respectively. Based on DNA-SIP and high-throughput sequencing analysis, the cathodic working electrode harbored the potential dechlorinators (Comamonas, Pseudomonas, Methylobacillus, and Dechlorosoma), and the anodic working enriched the potential intermediate mineralizing bacteria (Comamonas, Stenotrophomonas, and Geobacter), indicating that PCP could be completely degraded under the synergetic effect of these functional microorganisms. Besides, the potential autotrophic functional bacteria that might be involved in the PCP dechlorination were also identified by SIP labeled with ¹³C-NaHCO₃. Our results proved that the dual-working BES could accelerate the complete degradation of PCP and enrich separately the functional microbial consortium for the PCP dechlorination and mineralization, which has broad potential for bioelectrochemical techniques in the treatment of wastewater contaminated with CPs or other halogenated organic compounds.

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.

Methylmercury formation in biofilms of Geobacter sulfurreducens

Introduction

Mercury (Hg) is a major environmental pollutant that accumulates in biota predominantly in the form of methylmercury (MeHg). Surface-associated microbial communities (biofilms) represent an important source of MeHg in natural aquatic systems. In this work, we report MeHg formation in biofilms of the iron-reducing bacterium Geobacter sulfurreducens.

Methods

Biofilms were prepared in media with varied nutrient load for 3, 5, or 7 days, and their structural properties were characterized using confocal laser scanning microscopy, cryo-scanning electron microscopy and Fourier-transform infrared spectroscopy.

Results

Biofilms cultivated for 3 days with vitamins in the medium had the highest surface coverage, and they also contained abundant extracellular matrix. Using 3 and 7-days-old biofilms, we demonstrate that G. sulfurreducens biofilms prepared in media with various nutrient load produce MeHg, of which a significant portion is released to the surrounding medium. The Hg methylation rate constant determined in 6-h assays in a low-nutrient assay medium with 3-days-old biofilms was 3.9 ± 2.0 ∙ 10^-14  L ∙ cell^-1 ∙ h^-1, which is three to five times lower than the rates found in assays with planktonic cultures of G. sulfurreducens in this and previous studies. The fraction of MeHg of total Hg within the biofilms was, however, remarkably high (close to 50%), and medium/biofilm partitioning of inorganic Hg (Hg(II)) indicated low accumulation of Hg(II) in biofilms.

Discussion

These findings suggest a high Hg(II) methylation capacity of G. sulfurreducens biofilms and that Hg(II) transfer to the biofilm is the rate-limiting step for MeHg formation in this systems.

Towards BioMnOx-mediated intra/extracellular electron shuttling for doxycycline hydrochloride metabolism in Bacillus thuringiensis

Doxycycline hydrochloride (DCH) could be continuously removed by Bacillus thuringiensis S622 with the in-situ biogenic manganese oxide (BioMnOx) via oxidizing/regenerating. The DCH removal rate was significantly increased by 3.01-fold/1.47-fold at high/low Mn loaded via the integration of biological (intracellular/extracellular electron transfer (IET/EET)) and abiotic process (BioMnOx, Mn(III) and •OH). BioMnOx accelerated IET via activating coenzyme Q to enhance electrons transfer (ET) from complex I to complex III, and as an alternative electron acceptor for respiration and provide another electron transfer transmission channel. Additionally, EET was also accelerated by stimulating to secrete flavins, cytochrome c (c-Cyt) and flavin bounded with c-Cyt (Flavins & Cyts). To our best knowledge, this is the first report about the role of BioMnOx on IET/EET during antibiotic biodegradation. These results suggested that Bacillus thuringiensis S622 incorporated with BioMnOx could adopt an alternative strategy to enhance DCH degradation, which may be of biogeochemical and technological significance.

Self-produced biophotosensitizers enhance the degradation of organic pollutants in photo-bioelectrochemical systems

Bioelectrochemical systems (BESs) with integrated photoactive components have been shown to be a promising strategy for enhancing the performance for bioenergy generation and pollutant removal. This study revealed an efficient photo-BES with an enhanced pollutant degradation rate by utilizing self-produced biomolecules as photosensitizers in situ. Results showed that the BES could increase the coulombic efficiency from 60.8% to 73.0% and the degradation rate of bisphenol A (BPA) from 0.030 to 0.189 h^-1 when the suspension in the reactor was illuminated with simulated sunlight in the absence of any external photosensitizers. We identified that the regular BES released many organic substances into the reactor during operation. These substances, including dissolved biomolecules and solid cell residues, were photoactive for producing hydroxyl radicals during light illumination. Quenching experiments verified that the •OH generated from the self-produced biophotosensitizers contributed to the enhanced degradation of BPA. Additionally, the phototransformation of biophotosensitizers was also observed in photo-BES. The quantity of tyrosine protein-like components decreased, but that of the humic components remained relatively stable. Our findings imply that BESs with integrated self-produced biophotosensitizers may be promising for constructing advanced electrochemical and biological systems for synchronous bioelectricity production and degradation of organic pollutants in wastewater treatments.

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.

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

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

Bimetallic oxide MnFe2O4 modified carbon felt anode by drip coating: an effective approach enhancing power generation performance of microbial fuel cell

The anode electrode of microbial fuel cell (MFC) is the key component to determine its power generation performance because it is the habitat and electron transfer center of the electricity-producing microorganisms. Carbon-based anodes have been confirmed to improve MFC performance. Its large surface area, excellent conductivity and low cost make it very suitable for electrode materials used in MFC. However, the low biocompatibility and instability of common carbon-based materials restrict their practical application in MFC. In this work, a bimetal oxide MnFe₂O₄ was prepared and used to modify carbon felt anode by a simple drop coating method. The influence of the amount of MnFe₂O₄ material on the performance of MFC was systematically studied. The results showed that the power density of the carbon felt anode with a MnFe₂O₄ modified amount of 1 mg/cm² increased by 66.9% compared with the unmodified anode. Meanwhile, the MFC cycle using MnFe₂O₄ modified anode was more stable. After 6 months of long-term operation, the power density reached 3836 mW/m². The anode modified by MnFe₂O₄ has capacitance characteristics, good biocompatibility and fast electron transmission rate, which significantly improves the power generation performance of MFC. In addition, the use of a simple drop coating method to prepare electrodes can reduce the difficulty of electrode fabrication and the cost of MFC, laying a certain foundation for the industrialization of MFC.

Bioelectrochemically enhanced degradation of bisphenol S: mechanistic insights from stable isotope-assisted investigations

Electroactive microbes is the driving force for the bioelectrochemical degradation of organic pollutants, but the underlying microbial interactions between electrogenesis and pollutant degradation have not been clearly identified. Here, we combined stable isotope-assisted metabolomics (SIAM) and 13C-DNA stable isotope probing (DNA-SIP) to investigate bisphenol S (BPS) enhanced degradation by electroactive mixed-culture biofilms (EABs). Using SIAM, six 13C fully labeled transformation products were detected originating via hydrolysis, oxidation, alkylation, or aromatic ring-cleavage reactions from 13C-BPS, suggesting hydrolysis and oxidation as the initial and key degradation pathways for the electrochemical degradation process. The DNA-SIP results further displayed high 13C-DNA accumulation in the genera Bacteroides and Cetobacterium from the EABs and indicated their ability in the assimilation of BPS or its metabolites. Collectively, network analysis showed that the collaboration between electroactive microbes and BPS assimilators played pivotal roles the improvement in bioelectrochemically enhanced BPS degradation.

Insight into the electrocatalytic performance of in-situ fabricated electroactive biofilm-Pd: The role of biofilm thickness, initial Pd(II) concentration and the exposure time to Pd precursor

Biogenic palladium (bio-Pd) nanoparticles have been considered as promising biocatalyst for energy generation and contaminants remediation in water and sediment. Recently, an electroactive biofilm-Pd (EAB-Pd) network, which can be used directly as electrocatalyst and show enhanced electrocatalytic performance, has exhibited tremendous application potential. However, the information regarding to the controllable biosynthetic process and corresponding catalytic properties is scarce. This study demonstrated that the catalytic performance of EAB-Pd could be influenced by Pd loading on bacteria cells (Pd/cells), which was crucial to determine the final distribution characteristic of Pd nanocrystal on EAB skeleton. For instance, the high Pd/cells (over 0.18 pg cell^-1) exhibited almost 6-fold and 1.5-fold enhancement over EAB-Pds with Pd/cells below 0.03 in catalytic current toward hydrogen evolution reaction and nitrobenzene reduction, respectively. In addition, the Pd/cells was found to be affected by the synthesis factors, such as the ratio of biomass to initial Pd(II) concentration (cells/Pd_II) and the exposure time of EAB to Pd(II) precursor solution. The Pd/cells increased significantly as the cell/Pd_II ratio decreased from ~5.5 × 10⁷ to ~1.3 × 10⁷ cells L mg^-1 or the prolongation of exposure time from 3 h to 24 h. The findings developed in this work extensively expand our knowledge for the in-situ designing biogenic electrocatalyst and provide important information for the development of its catalytic property.

Ammonia oxidation and denitrification in a bio-anode single-chambered microbial electrolysis cell

In this study, anodic ammonia oxidation and denitrification were performed in single-chamber bioelectrochemical systems at a wide range of anodic potentials (-400 to +400 mV) versus Ag/AgCl. The low coulombic efficiencies (~30.84%) in reactors were mainly due to electrons being transferred to atmospheric oxygen through the electrode and reversal of the electrode. The removal efficiencies of acetate, ammonia, and total nitrogen were 100%, 100%, and 40.44% at +200 mV and 100%, 100%, and 50.24% at -200 mV, respectively. The nitrogen-removal mechanisms were nitrogen respiration/nitrate reduction at +200 mV and denitrification at -200 mV, and ammonia oxidation occurred by coupling with sulfate-reducing at -300 and -400 mV. Thauera, Comamonas, Alicycliphilus, Nitrosomonas, Desulforhabdus, Dethiosulfatibacter, and Desulfomicrobium were the dominant genera at the anode which participated in the nitrification/denitrification or sulfate-reducing processes. In summary, ammonia oxidation and denitrification could be coupled with carbon-removal or sulfur-reduction using a bio-anode with a suitable anodic potential.

Anode potential-dependent protection of electroactive biofilms against metal ion shock via regulating extracellular polymeric substances

Extracellular polymeric substances (EPS) have been considered as a barrier for toxic species penetration into the cells, but their function in protecting electroactive biofilms (EABs) had been rarely revealed. In this study, the anode potential was used to regulate the EPS quantity and components in mixed-culture EABs, where their resistance to Ag⁺ shock was assessed. The results showed that the EAB grown at 0 V showed the highest anti-shock capability by the Ag⁺ exposure compared to those grown at -0.2, 0.2, and 0.4 V. The EAB produced at 0 V had both of the highest amounts of loosely bound EPS (LB-EPS; 61.9 mg-EPS/g-VSS) and tightly bound EPS (TB-EPS; 74.8 mg-EPS/g-VSS) than those grown under other potentials, where proteins and humic acid were the predominated components. The abundance of genes associated with EPS biosynthesis were also confirmed to be related with the applied anode potentials, based on the metagenomic analysis. Considering proteins and humic acid in LB-EPS showed positive linearity with the current recovery and viability of the EABs, these two main components might play important roles in reducing the Ag⁺ toxicity. Synchronous fourier transform infrared (FTIR) spectroscopy integrated two-dimensional correlation spectroscopy (2D-COS) analyses further confirmed that the oxygen and nitrogen moieties (i.e. amide, carbonyl CO, phenolic, and C-O-C) in proteins and humic acid of the LB-EPS were response for the binding with the Ag⁺ to prevent the penetration into the cells. The underlying molecular mechanisms of EPS in protecting EABs from the Ag⁺ shock explored in this study can provide implications for developing new methods to construct highly stable EABs.

Enhanced degradation of triphenyl phosphate (TPHP) in bioelectrochemical systems: Kinetics, pathway and degradation mechanisms

Triphenyl phosphate (TPHP) is one of the major organophosphate esters (OPEs) with increasing consumption. Considering its largely distribution and high toxicity in aquatic environment, it is important to explore an efficient treatment for TPHP. This study aimed to investigate the accelerated degradation of TPHP in a three-electrode single chamber bioelectrochemical system (BES). Significant increase of degradation efficiency of TPHP in the BES was observed compared with open circuit and abiotic controls. The one-order degradation rates of TPHP (1.5 mg L^-1) were increased with elevating sodium acetate concentrations and showed the highest value (0.054 ± 0.010 h^-1) in 1.0 g L^-1 of sodium acetate. This result indicated bacterial metabolism of TPHP was enhanced by the application of micro-electrical field and addition acetate as co-substrates. TPHP could be degraded into diphenyl phosphate (DPHP), hydroxyl triphenyl phosphate (OH-TPHP) and three byproducts. DPHP was the most accumulated degradation product in BES, which accounted more than 35.5% of the initial TPHP. The composition of bacterial community in BES electrode was affected by the acclimation by TPHP, with the most dominant bacteria of Azospirillum, Petrimonas, Pseudomonas and Geobacter at the genera level. Moreover, it was found that the acute toxic effect of TPHP to Vibrio fischeri was largely removed after the treatment, which revealed that BES is a promising technology to remove TPHP threaten in aquatic environment.

Anode potentials regulate Geobacter biofilms: New insights from the composition and spatial structure of extracellular polymeric substances

The extracellular electron transfer (EET) efficiency in bioelectrochemical systems has been proven to be dependent on anode potentials. To explore the underlying mechanism, previous studies have mainly focused on EET conduit and bacterial biomass but rarely concerned with the role of extracellular polymeric substances (EPS) surrounding electroactive cells. In this study, the response of Geobacter biofilms to anode potentials was investigated with a special emphasis on the mechanistic role of EPS. The electrochemical activities and cell viabilities of Geobacter soli biofilms were simultaneously attenuated at 0.4 and 0.6 V compared to -0.2 and 0 V. It was found that the biofilms (especially the biofilm region closer to electrode surface) grown at -0.2 and 0 V produced relatively more extracellular redox-active proteins and less extracellular polysaccharides, which conferred higher electron accepting/donating capacities to EPS and consequently facilitated EET. Meanwhile, electrically nonconductive extracellular polysaccharide-dominated interior layers were formed in the biofilms grown at 0.4 and 0.6 V, which limited direct EET but might serve as physical barriers for protecting cells in these biofilms from the increasing stress by poised electrodes. These results demonstrated that the production of EPS under different anode potentials might be finely regulated by cells to keep balance between EET efficiency and cell-protection. This study provides a new insight to investigate the Geobacter biofilms coping with various environments, and is useful for optimizing electrochemical activity of anode biofilms.

Sustainable synthesis of novel carbon microwires for the modification of a Ti mesh anode in bioelectrochemical systems

Herein, an effective method was developed to integrate carbon microwires on Ti mesh (denoted as CM/TiM) to fabricate high-performance anodes with long-time stability in microbial fuel cell. CM/TiM was synthesized by colonizing filamentous fungi on the bread modified Ti mesh followed by carbonization, which could convert the attached mycelium into carbon microwires (denoted as CM). Benefiting from the biocompatibility and 3D interlaced structure of carbon microwires, the biomass accumulation (1027 ± 83 μg cm^-2) of CM/TiM have been significantly improved nearly 3 folds, thus the fabricated CM/TiM demonstrated 2-fold higher current density (12.19 ± 0.07 A m^-2) with significantly increased stability compared with TiM. Therefore, the present high power output, chemical stability and hydrophilic carbon microwires make CM/TiM stable, scalable and environmentally sustainable anodes in bioelectrochemical systems.

Monitoring microbial communities' dynamics during the start-up of microbial fuel cells by high-throughput screening techniques

Microbial Electrochemical Technologies are based on the use of electrochemically active microorganisms that can carry out extracellular electron transfer to an electrode while they are oxidizing the organic compounds. The dynamics and changes of the bacterial community in the anode biofilm and planktonic broth of an acetate fed batch single chamber air cathode MFC have been studied by combing flow-cytometry and Illumina sequencing techniques. At the beginning of the test, from 0 h to 70 h, microbial planktonic communities changed from four groups to two groups, as revealed by DNA content, and from three groups to one group based on the cell membrane polarization revealed by a DiOC₆(3) probe. Between 4^th day and 13^th day, microbial communities changed from one group to a maximum of three groups, monitoring DNA content, and from one group to two based on the cell membrane polarization. The 16S rDNA gene profiling confirmed the shift in microbial communities, with Acinetobacter (39.34%), Azospirillum (27.66%), Arcobacter (4.17%) and Comamonas (2.62%) being the most abundant genera at the beginning of MFC activation. After 70 h the main genera detected were Azospirillum (46.42%), Acinetobacter (34.66%), Enterococcus (2.32%), Dysgonomonas (2.14%). Data obtained have shown that flow cytometry and illumina sequencing are useful tools to monitor "online" the changes in microbial communities during the MFCs start-up and the increase of Azospirillum and Acinetobacter genera is in good agreement with the MFC voltage generation. Moreover, monitoring planktonic populations, instead of the less accessible anode biofilm, was in good agreement with the evolution of MFC voltage.