Geobacter sulfurreducens biofilms developed under different growth conditions on glassy carbon electrodes: insights using cyclic voltammetry

Geobacter sulfurreducens promoted the biosynthesis of reduced graphene oxide and coupled it for nitrobenzene reduction

In order to explore an efficient and green method to deal with nitrobenzene (NB) pollutant, reduced graphene oxide (rGO) as an electron shuttle was applied to enhance the extracellular electron transfer (EET) process of Geobacter sulfurreducens, which was a typical electrochemically active bacteria (EAB). In this study, rGO biosynthesis was achieved via the reduction of graphene oxide (GO) by G. sulfurreducens PCA within 3 days. Also, the rGO-PCA combining system completely reduced 50-200 µmol/L of NB to aniline as end product within one day. SEM characterization revealed that PCA cells were partly wrapped by rGO, and therefore the distance of electron transfer between strain PCA and rGO material was reduced. Beside, the ID/IG of GO, rGO, and rGO-PCA combining system were 0.990, 1.293 and 1.31, respectively. Moreover, highest currents were observed in rGO-PCA-NB as 12.950 µA/-12.560 µA at -408 mV/156 mV, attributing to the faster electron transfer efficiency in EET process. Therefore, the NB reduction was mainly due to: (I) direct EET process from G. sulfurreducens PCA to NB; (II) rGO served as electron shuttle and accelerated electron transfer to NB, which was the main degradation pathway. Overall, the biosynthesis of rGO via GO reduction by Geobacter promoted the NB removal process, which provided a facile strategy to alleviate the problematic nitroaromatic pollution in the environment.

Comparative study of exoelectrogenic utilization preferences and hydrogen conversion among major fermentation products in microbial electrolysis cells

This study comparatively investigated the exoelectrogenic utilization and hydrogen conversion of major dark fermentation products (acetate, propionate, butyrate, lactate, and ethanol) from organic wastes in dual-chamber microbial electrolysis cells (MECs) alongside their mixture as a simulated dark fermentation effluent (DFE). Acetate-fed MECs showed the highest hydrogen yield (1,465 mL/g chemical oxygen demand), near the theoretical maximum yield, with the highest coulombic efficiency (105%) and maximum current density (7.9 A/m2), followed by lactate-fed, propionate-fed, butyrate-fed, mixture-fed, and ethanol-fed MECs. Meanwhile, the highest hydrogen production rate (514 mL/L anolyte∙d) was observed in ethanol-fed MECs despite their lower coulombic efficiency. Butyrate was the least favored substrate, followed by propionate, leading to significantly delayed startup and reaction. The active anodic microbial community structure varied considerably among the MECs utilizing different substrates, particularly between Geobacter and Acetobacterium dominance. The results highlight the substantial effect of the DFE composition on its utilization and current-producing bioanode development.

Self-assembled electrochemically active biofilms doped with carbon nanotubes: Electron exchange efficiency and cytotoxicity evaluation

Thick electrochemically active biofilms (EABs) will lead to insufficient extracellular electron transfer (EET) rate because of the limitation of both substrate diffusion and electron exchange. Herein, carbon nanotubes (CNTs)-doped EABs are developed through self-assembly. The highly conductive biofilms (internal resistance of ∼211 Ω) are efficiently enriched at CNTs dosage of 1 g L-1, with the stable power output of 0.568 W m-2 over three months. The embedded CNTs can act as electron tunnel to accelerate the EET rate in thick biofilm. Self-charging/discharging experiments and Nernst-Monod model stimulation demonstrate a higher net charge storage capacity (0.15 C m-2) and more negative half-saturation potential (-0.401 V) for the hybrid biofilms than that of the control (0.09 C m-2, and -0.378 V). Enzyme activity tests and the observation of confocal laser scanning microscopy by live/dead staining show a nearly negligible cytotoxicity of CNTs, and non-targeted metabonomics analysis reveals fourteen differential metabolites that do not play key roles in microbial central metabolic pathways according to KEGG compound database. The abundance of typical exoelectrogens Geobacter sp. is 2-fold of the control, resulting in a better bioelectrocatalytic activity. These finding provide a possible approach to prolong electron exchange and power output by developing a hybrid EABs doped with conductive material.

Mechanisms of magnetic sensing and regulating extracellular electron transfer of electroactive bacteria under magnetic fields

Electroactive bacteria can display notable plasticity in their response to magnetic field (MF), which prompted bioelectrochemical system as promising candidates for magnetic sensor applications. In this study, we explored the sensing and stimulatory effect of MF on current generation by Geobacter sulfurreducens, and elucidated the related molecular mechanism at the transcriptomic level. MF treatment significantly enhanced electricity generation and overall energy efficiency of G. sulfurreducens by 50 % and 22 %, respectively. The response of current to MFs was instantaneous and reversible. Cyclic voltammetry analysis of the anode biofilm revealed that the redox couples changed from -0.31 to -0.39 V (vs. Ag/AgCl), suggesting that MFs could alter electron transfer related components. Differential gene expression analysis further verified this hypothesis, genes associated with electron transfer were upregulated in G. sulfurreducens under MF treatment relative to the control group, specifically, genes encoding periplasmic c-type cytochromes (ppcA and ppcD), outer membrane cytochrome (omcF, omcZ, omcB), pili (pilA-C, pilM, and pilV2), and ribosome. The enhanced bacterial extracellular electron transfer process was also linked to the overexpression of the NADH dehydrogenase I subunit, the ABC transporter, transcriptional regulation, and ATP synthase. Overall, our findings shed light on the molecular mechanism underlying the effects of magnetic field stimuli on EAB and provide a theoretical basis for its further application in magnetic sensors and other biological system.

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.

Contribution analysis of different electron transfer pathways to methane production in anaerobic digestion coupled with bioelectrochemical system

The contribution analysis of different electron transfer pathways to CH₄ production was investigated in bioelectrochemical anaerobic digestion (BEAD). It demonstrates that the indirect interspecies electron transfer (IIET) pathway and the direct interspecies electron transfer (DIET) pathways contributed to 41.7 % and 58.3 % of the CH₄ production in the BEAD reactor, respectively. The DIET pathway was further divided into DIET via electrode (eDIET) and biological DIET (bDIET) in the bulk solution, and contributed 11.1 % and 47.2 % of CH₄ production, respectively. This indicates that the dominant electron transfer pathway for CH₄ production is from the bulk solution, rather than on the polarized electrode. The electroactive microorganisms were well enriched in the bulk solution by the electric field generated between anode and cathode. The enriched electroactive microorganisms significantly improved the CH₄ production in the bulk solution through the bDIET pathway.

Enrichment of sulfur-oxidizing bacteria using S-doped NiFe2O4 nanosheets as the anode in microbial fuel cell enhances power production and sulfur recovery

Microbial fuel cells (MFCs) have great promise for power generation by oxidizing organic wastewater, yet the challenge to realize high efficiency in simultaneous energy production and resource recovery remains. In this study, we designed a novel MFC anode by synthesizing S-doped NiFe₂O₄ nanosheet arrays on carbon cloth (S10-NiFe₂O₄@CC) to build a three-dimensional (3D) hierarchically porous structure, with the aim to regulate the microbial community of sulfur-cycling microbes in order to enhance power production and elemental sulfur (S⁰) recovery. The S10-NiFe₂O₄@CC anode obtained a faster start-up time of 2 d and the highest power density of 4.5 W/m² in acetate-fed and mixed bacteria-based MFCs. More importantly, sulfide removal efficiency (98.3 %) (initial concentration of 50 mg/L S^2-) could be achieved within 3 d and sulfur (S₈) could be produced. Microbial community analysis revealed that the S10-NiFe₂O₄@CC anode markedly enriched sulfur-oxidizing bacteria (SOB) and promoted enrichment of SOB and sulfate-reducing bacteria (SRB) in the bulk solution as well, leading to the enhancement of power generation and S⁰ recovery. This study shows how carefully designing and optimizing the composition and structure of the anode can lead to the enrichment of a multifunctional microbiota with excellent potential for sulfide removal and resource recovery.

3D Hierarchical Co8FeS8-FeCo2O4/N-CNTs@CF with an Enhanced Microorganisms-Anode Interface for Improving Microbial Fuel Cell Performance

Microbial fuel cells (MFCs) are promising ecofriendly techniques for harvesting bioenergy from organic and inorganic matter. Currently, it is challenging to design MFC anodes with favorable microorganism attachment and fast extracellular electron transfer (EET) rate for high MFC performance. Here we prepared N-doped carbon nanotubes (NCNTs) on carbon felt (CF) and used it as a support for growing hierarchical Co₈FeS₈-FeCo₂O₄/NCNTs core-shell nanostructures (FeCo/NCNTs@CF). We observed improved wettability, specific areal capacitance, and diffusion coefficient, as well as small charge transfer resistance compared with bare CF. MFCs equipped with FeCo/NCNTs@CF displayed a power density of 3.04 W/m² and COD removal amount of 221.0 mg/L/d, about 47.6 and 290.1% improvements compared with that of CF. Biofilm morphology and 16s rRNA gene sequence analysis proved that our anode facilitated the enrichment growth of exoelectrogens. Flavin secretion was also promoted on our hierarchical elelctrode, effectively driving the EET process. This work disclosed that hierarchical nanomaterials modified electrode with tailored physicochemical properties is a promising platform to simultaneously enhance exoelectrogen attachment and EET efficiency for MFCs.

GSU1771 regulates extracellular electron transfer and electroactive biofilm formation in Geobacter sulfurreducens: Genetic and electrochemical characterization

Type IV pili and the >100c-type cytochromes in Geobacter sulfurreducens are essential for extracellular electron transfer (EET) towards metal oxides and electrodes. A previous report about a mutation in the gsu1771 gene indicated an enhanced reduction of insoluble Fe(III) oxides coupled with increased pilA expression. Herein, a marker-free gsu1771-deficient mutant was constructed and characterized to assess the role of this regulator in EET and the formation of electroactive biofilms. Deleting this gene delayed microbial growth in the acetate/fumarate media (electron donor and acceptor, respectively). However, this mutant reduced soluble and insoluble Fe(III) oxides more efficiently. Heme staining, western blot, and RT-qPCR analyses demonstrated that GSU1771 regulates the transcription of several genes (including pilA) and many c-type cytochromes involved in EET, suggesting the broad regulatory role of this protein. DNA-protein binding assays indicated that GSU1771 directly regulates the transcription of pilA, omcE, omcS, and omcZ. Additionally, gsu1771-deficient mutant biofilms are thicker than wild-type strains. Electrochemical studies revealed that the current produced by this biofilm was markedly higher than the wild-type strains (approximately 100-fold). Thus, demonstrating the role of GSU1771 in the EET pathway and establishing a methodology to develop highly electroactive G. sulfurreducens mutants.

Effects of ferroferric oxide on azo dye degradation in a sulfate-containing anaerobic reactor: From electron transfer capacity and microbial community

Anaerobic decolorization of azo dye in sulfate-containing wastewater has been regarded as an economical and effective method, but it is generally limited by the high concentration of azo dye and accumulation of toxic intermediates. To address this problem, Fe₃O₄ was added to one of the anaerobic reactors to investigate the effects on system performances. Results showed that AO7 removal rate, COD removal rate, and sulfate reduction were enhanced with the addition of Fe₃O₄ under various influent AO7 concentrations (153 mgCOD/L - 1787 mgCOD/L). According to the proposed pathway for the degradation of AO7, more intermediates (2-hydroxy-1,4-naphthoquinone, phthalide, 4-methylphenol) were produced in the presence of Fe₃O₄. The electron transfer capacity of sludge was also increased since Fe₃O₄ could stimulate to secrete humic acid-like organics in EPS. Microbial analysis showed that iron-reducing bacteria like Clostridium and Geobacter were also enriched, which were capable of azo dye and aromatic compounds degradation.

In Situ Spectroelectrochemical Characterization Reveals Cytochrome-Mediated Electric Syntrophy in Geobacter Coculture

Direct interspecies electron transfer (DIET) between microbial species prevails in some key microbial consortia. However, the electron transfer mechanism(s) in these consortia is controversial due to lack of efficient characterization methods. Here, we provide an in situ anaerobic spectroelectrochemical coculture cell (in situ ASCC) to induce the formation of DIET coculture biofilm on the interdigitated microelectrode arrays and characterize the electron transfer directly. Two typical Geobacter DIET cocultures, Geobacter metallireducens and wild-type Geobacter sulfurreducens (G.m&G.s) and G. metallireducens and a G. sulfurreducens strain deficient in citrate synthase (G.m&G.s-ΔgltA), were selected. In situ Raman and electrochemical Fourier transform infrared (FTIR) spectroscopy indicated that cytochromes are abundant in the electric syntrophic coculture. Cyclic voltammetry and potential step experiment revealed a diffusion-controlled electron transfer process and the electrochemical gating measurements further demonstrated a cytochrome-mediated electron transfer in the DIET coculture. Furthermore, the G.m&G.s-ΔgltA coculture displayed a higher redox conductivity than the G.m&G.s coculture, consistent with the existence of an intimate and efficient electrical connection between these two species. Our findings provide the first report of a redox-gradient-driven electron transport facilitated by c-type cytochromes in DIET coculture, supporting the model that DIET is mediated by cytochromes and suggest a platform to explore the other DIET consortia.

Competition of two highly specialized and efficient acetoclastic electroactive bacteria for acetate in biofilm anode of microbial electrolysis cell

Maintaining functional stability of microbial electrolysis cell (MEC) treating wastewater depends on maintaining functional redundancy of efficient electroactive bacteria (EAB) on the anode biofilm. Therefore, investigating whether efficient EAB competing for the same resources (electron donor and acceptor) co-exist at the anode biofilm is key for the successful application of MEC for wastewater treatment. Here, we compare the electrochemical and kinetic properties of two efficient acetoclastic EAB, Geobacter sulfurreducens (GS) and Desulfuromonas acetexigens (DA), grown as monoculture in MECs fed with acetate. Additionally, we monitor the evolution of DA and GS in co-culture MECs fed with acetate or domestic wastewater using fluorescent in situ hybridization. The apparent Monod kinetic parameters reveal that DA possesses higher jmax (10.7 ± 0.4 A/m2) and lower KS, app (2 ± 0.15 mM) compared to GS biofilms (jmax: 9.6 ± 0.2 A/m2 and KS, app: 2.9 ± 0.2 mM). Further, more donor electrons are diverted to the anode for respiration in DA compared to GS. In acetate-fed co-culture MECs, DA (98% abundance) outcompete GS for anode-dependent growth. In contrast, both EAB co-exist (DA: 55 ± 2%; GS: 24 ± 1.1%) in wastewater-fed co-culture MECs despite the advantage of DA over GS based on kinetic parameters alone. The co-existence of efficient acetoclastic EAB with high current density in MECs fed with wastewater is significant in the context of functional redundancy to maintain stable performance. Our findings also provide insight to future studies on bioaugmentation of wastewater-fed MECs with efficient EAB to enhance performance.

Customizable design strategies for high-performance bioanodes in bioelectrochemical systems

Bioelectrochemical systems (BESs) can fulfill the demand for renewable energy and wastewater treatment but still face significant challenges to improve their overall performance. Core efforts have been made to enhance the bioelectrode performance, yet, previous approaches are fragmented and have limited applicability, unable to flexibly adjust physicochemical and structural properties of electrodes for specific requirements in various applications. Here, we propose a facile electrode design strategy that integrates three-dimensional printing technology and functionalized modular electrode materials. A customized graphene-based electrode with hierarchical pores and functionalized components (i.e., ferric ions and magnetite nanoparticles) was fabricated. Owing to efficient mass and electron transfer, a high volumetric current density of 10,608 ± 1,036 A/m³ was achieved, the highest volumetric current density with pure Geobacter sulfurreducens to date. This strategy can be readily applied to existing BESs (e.g., microbial fuel cells and microbial electrosynthesis) and provide a feasibility for practical application.

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A 3D-printed graphene aerogel electrode was proposed for BESs

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The optimized electrode mass transfer resistance was less than 5% of carbon felt

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A high volumetric current density of 10,608 ± 1,036 A/m³ was achieved

The reduction of nitrobenzene by extracellular electron transfer facilitated by Fe-bearing biochar derived from sewage sludge

In this work, the incorporation of Fe-bearing sludge-derived biochar greatly enhanced both biotic and abiotic reduction of nitrobenzene (NB) to aniline, which was attributed to the concomitant microbial dissimilatory iron reduction. Biogenic Fe(II) produced by Geobacter sulfurreducens dominated the anaerobic reduction of NB following the pseudo-first-order kinetic. Besides, the increase of pyrolysis temperature from 600 to 900 ℃ to generate biochar resulted in an accelerated removal rate of NB in Geobacter-biochar combined system. The morphology and structural characterization of biochar with G. sulfurreducens confirmed the formation of conductive bacteria-biochar aggregates. Electrochemical measurements suggested the presence of graphitized domains and quinone-like moieties in biochar as redox-active centers, which might play an important role in accelerating electron transfer for microbial dissimilatory iron reduction and NB degradation. This study provides a feasible way of using Fe-bearing sludge as a valuable feedstock for biochar generation and its application with electrochemically active bacteria for the bioremediation of nitroaromatic compounds-polluted wastewater.

Magnetite nanoparticle anchored graphene cathode enhances microbial electrosynthesis of polyhydroxybutyrate by Rhodopseudomonas palustris TIE-1

Microbial electrosynthesis (MES) is an emerging technology that can convert carbon dioxide (CO₂) into value-added organic carbon compounds using electrons supplied from a cathode. However, MES is affected by low product formation due to limited extracellular electron uptake by microbes. Herein, a novel cathode was developed from chemically synthesized magnetite nanoparticles and reduced graphene oxide nanocomposite (rGO-MNPs). This nanocomposite was electrochemically deposited on carbon felt (CF/rGO-MNPs), and the modified material was used as a cathode for MES production. The bioplastic, polyhydroxybutyrate (PHB) produced by Rhodopseudomonas palustris TIE-1 (TIE-1), was measured from reactors with modified and unmodified cathodes. Results demonstrate that the magnetite nanoparticle anchored graphene cathode (CF/rGO-MNPs) exhibited higher PHB production (91.31 ± 0.9 mg l^-1). This is ∼4.2 times higher than unmodified carbon felt (CF), and 20 times higher than previously reported using graphite. This modified cathode enhanced electron uptake to -11.7 ± 0.1 μA cm^-2, ∼5 times higher than CF cathode (-2.3 ± 0.08 μA cm^-2). The faradaic efficiency of the modified cathode was ∼2 times higher than the unmodified cathode. Electrochemical analysis and scanning electron microscopy suggest that rGO-MNPs facilitated electron uptake and improved PHB production by TIE-1. Overall, the nanocomposite (rGO-MNPs) cathode modification enhances MES efficiency.

Electrochemical and phylogenetic comparisons of oxygen-reducing electroautotrophic communities

The mechanisms of extracellular electron transfer and the microbial taxa associated with the observed electroactivity are fundamental to oxygen-reducing microbial cathodes. Here we confirmed the apparent 'electroautotrophic' behavior of electroactive biofilms (EABs) grown on carbon electrodes at + 0.20V vs. Ag/AgCl under air. The EABs catalyzed O₂ electroreduction into water ─ as demonstrated by a rotating ring disc experiment ─ and performed quasi-reversible heterogeneous electron transfer (HET). By using electrodes of low surface capacitance, we report for the first time nonturnover redox peaks that are very likely intrinsic to the redox protein(s) performing the HET. Because the formal potential of redox proteins is pH-dependent, we investigated the evolution of characteristic potentials of the EABs with the solution pH: (i) open circuit potential, (ii) half-wave potential, and (iii) averaged peak potential of nonturnover cyclic voltammograms, which is presumably the formal potential of the primary electron acceptor(s) for the community. In addition to describing the redox thermodynamics behind HET, we suggest that the corresponding data provides an electrochemical fingerprint that could help in comparing the electroactivity of diverse microbial communities. The taxon with the highest relative abundance in our EABs was an unclassified member of the Gammaproteobacteria that was phylogenetically closely related to most other abundant unclassified Gammaproteobacteria commonly reported in EABs reducing O₂ at high potentials, further suggesting that those taxa are responsible for the bioelectroactivity. Phylogenetic and electrochemical similarities between reported EABs jointly support the hypothesis that similar biomolecular mechanisms may be responsible for this highly probable electroautotrophic metabolism.

The synergistic effect of biophoto anode for the enhancement of current generation and degradation

The demand for removal of refractory organic pollutants limits the application of microbial fuel cells. In this study, the synergistic effects of bioelectrochemical and photocatalysis methods were captured by constructing a biophoto anode from a combination of WO₃/TiO₂ and carbon felt. This biophoto electrode was able to decrease the aniline concentration from 63.3 ± 6.2 to 9.3 ± 5.5 mg/L. The structure of the benzene ring was broken through strong oxidation by photocatalysis. Electrochemical analysis showed that photocatalysis also enhanced the extracellular electron transfer of microorganisms and reduced the resistance of the anode from 136.9 Ω to 69.9 Ω. In addition, the maximum current output increased by 28.5% under the composite biophoto electrode. Further analysis of the microbial community indicated that the biophoto electrode promoted the enrichment of Geobacter in the anode. This biophoto electrode provided a method for overcoming the disadvantages of anaerobic degradation of refractory organics.

Electroactive biofilms on surface functionalized anodes: The anode respiring behavior of a novel electroactive bacterium, Desulfuromonas acetexigens

Surface chemistry is known to influence the formation, composition, and electroactivity of electron-conducting biofilms. However, understanding of the evolution of microbial composition during biofilm development and its impact on the electrochemical response is limited. Here we present voltammetric, microscopic and microbial community analysis of biofilms formed under fixed applied potential for modified graphite electrodes during early (90 h) and mature (340 h) growth phases. Electrodes modified to introduce hydrophilic groups (-NH₂, -COOH and -OH) enhance early-stage biofilm formation compared to unmodified or electrodes modified with hydrophobic groups (-C₂H₅). In addition, early-stage films formed on hydrophilic electrodes are dominated by the gram-negative sulfur-reducing bacterium Desulfuromonas acetexigens while Geobacter sp. dominates on -C₂H₅ and unmodified electrodes. As biofilms mature, current generation becomes similar, and D. acetexigens dominates in all biofilms irrespective of surface chemistry. Electrochemistry of pure culture D. acetexigens biofilms reveal that this microbe is capable of forming electroactive biofilms producing considerable current density of > 9 A/m² in a short period of potential-induced growth (~19 h following inoculation) using acetate as an electron donor. The inability of D. acetexigens biofilms to use H₂ as a sole source electron donor for current generation shows promise for maximizing H₂ recovery in single-chambered microbial electrolysis cell systems treating wastewaters.

Effect of heterotrophic anodic denitrification on anolyte pH control and bioelectricity generation enhancement of bufferless microbial fuel cells

Heterotrophic anodic denitrification (HAD) in the single-chamber microbial fuel cell (MFC) is a promising nitrogen removal technology. In this paper, the benefit (anolyte pH increase) and challenge (substrate consumption) brought by the heterotrophic anodic denitrification process for the electricity generation of bufferless MFCs were studied for the first time. Substrate anaerobic hydrolysis dramatically decreased the anolyte pH to 5.1, which seriously restricted the electric power output of the Control. The anolyte pH of the heterotrophic anodic denitrification MFCs (HADMFCs) with 60 mg/L (HADMFC-60), 90 mg/L (HADMFC-90), and 120 mg/L (HADMFC-120) nitrate nitrogen (NO₃^--N), retained above 6.0, 6.5, and 6.8 in every running cycles, due to the protons (H⁺) consumption by nitrate reduction. In the HADMFC-60 and HADMFC-90, 17.6% and 26.1% of the total organic carbons (TOC) were used for the nitrate reduction, but their electric power output significantly increased. The maximum power densities of the HADMFC-60 and HADMFC-90 were 3.3 and 5.4 times higher than that of the Control. However, when the proportion of TOC consumption for nitrate reduction increased to 35.8%, substrate insufficiency became a serious limitation for the electricity generation. The P_max of the HADMFC-120 dramatically decreased to 17.3 mW/m². Dysgonomonas was the dominant electro-active genus, and Petrimonas, Acidovorax and Devosia appeared as the denitrifying bacteria genera.

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

Bioelectrochemical systems (BESs) are widely investigated as a promising technology to recover bioenergy or synthesize value-added products from wastewaters. The performance of BES depends on the activity of electroactive biofilm (EAB). As the core of BES, it is still unclear how the EAB is formed from mixed inoculum, and how exoelectrogens compete with non-exoelectrogens. Here we confirmed that microbial community composition and the morphology of EAB on the electrode including the thickness and porosity of the biofilm are critical for the performance of BES, and these properties can be simply controlled by the substrate concentration during EAB formation. The EAB formed with 0.1 g/L of acetate (EAB-0.1) exhibited a 90% higher current density than that formed with 1.0 g/L acetate (EAB-1.0). EAB-0.1 had a 50% higher electroactivity per biomass and a 20% thinner thickness than EAB-1.0, which was partly due to the 54% decrease of insulative polysaccharide in biofilm. Limited acetate also imposed a selective pressure to enrich Geobacter up to 88% compared to 72% when acetate was abundant. Our findings demonstrate that a highly active EAB can be formed by limiting substrate concentration, providing a broader understanding of the EAB formation process, the ecology of interspecies competitions and potential applications for bioenergy recovery and trace toxicant detection in the future.

Conductive magnetite nanoparticles trigger syntrophic methane production in single chamber microbial electrochemical systems

Performance of methane-producing microbial electrochemical systems (MESs) is highly reliant on electron transfer efficiency from electrode to microorganisms and vice versa. In this study, magnetite nanoparticles were used as electron carriers to enhance extracellular electron transfer in single chamber MESs. The MES with magnetite exhibited the highest methane yield and current generation of 0.37 ± 0.009 L_CH4/g_COD and 9.6 mA, respectively among the tested reactors. The experimental data was observed to be highly consistent with modified Gompertz model results (R² > 0.99), which also showed 74.2% and 22.1% enhanced methane production rate in MES with magnetite as compared to control AD and MES without magnetite, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy analysis confirmed that magnetite enhanced catalytic activity of biofilm and lowered both solution and charge transfer resistance. Therefore, supplementing magnetite in MESs could be a strategy to develop an efficient syntrophic biomethanation in field scale applications.

Thermophiles; or, the Modern Prometheus: The Importance of Extreme Microorganisms for Understanding and Applying Extracellular Electron Transfer

Approximately four billion years ago, the first microorganisms to thrive on earth were anaerobic chemoautotrophic thermophiles, a specific group of extremophiles that survive and operate at temperatures ∼50 - 125°C and do not use molecular oxygen (O2) for respiration. Instead, these microorganisms performed respiration via dissimilatory metal reduction by transferring their electrons extracellularly to insoluble electron acceptors. Genetic evidence suggests that Gram-positive thermophilic bacteria capable of extracellular electron transfer (EET) are positioned close to the root of the Bacteria kingdom on the tree of life. On the contrary, EET in Gram-negative mesophilic bacteria is a relatively new phenomenon that is evolutionarily distinct from Gram-positive bacteria. This suggests that EET evolved separately in Gram-positive thermophiles and Gram-negative mesophiles, and that EET in these bacterial types is a result of a convergent evolutionary process leading to homoplasy. Thus, the study of dissimilatory metal reducing thermophiles provides a glimpse into some of Earth's earliest forms of respiration. This will provide new insights for understanding biogeochemistry and the development of early Earth in addition to providing unique avenues for exploration and discovery in astrobiology. Lastly, the physiological composition of Gram-positive thermophiles, coupled with the kinetic and thermodynamic consequences of surviving at elevated temperatures, makes them ideal candidates for developing new mathematical models and designing innovative next-generation biotechnologies. KEY CONCEPTS Anaerobe: organism that does not require oxygen for growth. Chemoautotroph: organism that obtains energy by oxidizing inorganic electron donors. Convergent Evolution: process in which organisms which are not closely related independently evolve similar traits due to adapting to similar ecological niches and/or environments. Dissimilatory Metal Reduction: reduction of a metal or metalloid that uses electrons from oxidized organic or inorganic electron donors. Exoelectrogen: microorganism that performs dissimilatory metal reduction via extracellular electron transfer. Extremophiles: organisms that thrive in physical or geochemical conditions that are considered detrimental to most life on Earth. Homoplasy: a character shared by a set of species that is not shared by a common ancestor Non-synonymous Substitutions (K a ): a substitution of a nucleotide that changes a codon sequence resulting in a change in the amino acid sequence of a protein. Synonymous Substitutions (K s ): a substitution of a nucleotide that may change a codon sequence, but results in no change in the amino acid sequence of a protein. Thermophiles: a specific group of extremophiles that survive and operate at temperatures ∼50-125°C.

Influence of the major pilA transcriptional regulator in electrochemical responses of Geobacter sulfureducens PilR-deficient mutant biofilm formed on FTO electrodes

Geobacter sulfurreducens is a model organism for understanding the role of bacterial structures in extracellular electron transfer mechanism (EET). This kind of bacteria relies on different structures such as type IV pili and over 100 c-type cytochromes to perform EET towards soluble and insoluble electron acceptors, including electrodes. To our knowledge, this work is the first electrochemical study comparing a G. sulfurreducens PilR-deficient mutant and wild type biofilms developed on fluorine-doped tin oxide (FTO) electrodes. Open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), were used to evaluate the electroactive properties of biofilms grown without externally imposed potential. Parallel studies of Confocal Laser Scanning Microscopy (CLSM) correlated with the electrochemical results. PilR is a transcriptional regulator involved in the expression of a wide variety of genes, including pilA (pilus structural protein) relevant c-type cytochromes and some other genes involved in biofilm formation and EET processes. Our findings suggest that PilR-deficient mutant forms a thinner (CLSM analysis) and less conductive biofilm (EIS analysis) than wild type, exhibiting different and irreversible redox processes at the interface (CV analysis). Additionally, this work reinforces some of the remarkable features described in previous reports about this G. sulfurreducens mutant.

Inhibitory effect of cadmium(II) ion on anodic electrochemically active biofilms performance in bioelectrochemical systems

Cadmium(II) ion can affect the anode performance of bioelectrochemical systems (BES); however, how the presence of Cd²⁺ affect the extracellular electron transfer of anodic electrochemically active biofilms (EABs), the microbial viability and species composition of microorganism on the anode remain poorly understood. Here, we investigated the inhibitory effect of Cd²⁺ at different concentrations on the electrochemical performance and the biofilm community in mixed-culture enriched BES. The electrochemical performance of the BES was not inhibited at 2 mg L^-1 Cd²⁺, while higher concentrations of 5-20 mg L^-1 resulted in the decrease in the maximum power density, with 0.34 ± 0.01 W m^-2 at 5 mg L^-1, 0.28 ± 0.01 W m^-2 at 10 mg L^-1, and 0.17 ± 0 W m^-2 at 20 mg L^-1, respectively. When adding 30 mg/L Cd²⁺, there was almost no power output. The decline of the power output was possibly ascribed to the suppressed viability and the change of species richness as evident from confocal laser scanning microscopy and microbial community analysis. Cyclic voltammogram and electrochemical impedance spectroscopy revealed that high concentration of Cd²⁺ exceeding 5 mg L^-1 can inhibit the secretion of outer membrane cytochromes, thus reducing the electron transfer between the EABs and the anode surface. Analysis of bacterial structures showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas and Azospira in response to Cd²⁺ at 10 and 20 mg L^-1. This study added to the in-depth analysis of the inhibition of Cd²⁺ on EABs, and provided new insights into the removing Cd²⁺ and organics simultaneously in BES.

Alkaline treatment of used carbon-brush anodes for restoring power generation of microbial fuel cells

Long-term operation of microbial fuel cells (MFCs) results in an electrochemical activity decline by the degradation of the anodic biofilm. In this work, an alkaline soaking treatment is proposed as an efficient and simple method for anode regeneration. The alkaline treatment was employed in a used carbon-brush anode, and its performance was compared with those of two other traditional treatment methods, i.e. air drying and carbonization. Among all the treated MFC anodes, the one treated by alkaline soaking exhibited the highest recovery rate. A series of tests including a start-up process, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and MFC performance were performed. The results show that alkaline soaking can modify the carbon fiber by introducing carboxyl groups onto the carbon surface and completely remove the aged biofilm, demonstrating that the alkaline treatment of used anodes is a practically effective method for the performance recovery of MFCs.

An alkaline soaking treatment is proposed as an efficient and simple method for anode regeneration.

Framework of Cytochrome/Vitamin B2 Linker/Graphene for Robust Microbial Electricity Generation

A bioelectrochemical system (BES) allows direct electricity production from wastes, but its low-power density, which is mainly associated with its poor anodic performance, limits its practical applications. Here, the anodic performance of a BES can be significantly improved by electrodepositing vitamin B₂ (VB₂) onto a graphene [reduced graphene oxide (rGO)]-modified glassy carbon electrode (VB₂/rGO/GC) with Geobacter sulfurreducens as the model microorganisms. The VB₂/rGO/GC electrode results in 200% higher electrochemical activity than a bare GC anode. Additionally, in microbial electrolysis cells, the current density of this composite electrode peaks at ∼210 μA cm^-2 after 118 h and is maintained for 113 h. An electrochemical analysis coupled with molecular simulations reveals that using VB₂ as a linker between the electrochemically active protein of this model strain and the rGO surface accelerates the electron transfer, which further improves the bioelectricity generation and favors the long-term stability of the BES. The VB₂ bound with a flexible ribityl group as the organic molecular bridge efficiently mediates energy conversion in microbial metabolism and artificial electronics. This work provides a straightforward and effective route to significantly enhance the bioenergy generation in a BES.

Positive effects of bio-nano Pd (0) toward direct electron transfer in Pseudomona putida and phenol biodegradation

This study constructed a biological-inorganic hybrid system including Pseudomonas putida (P. putida) and bioreduced Pd (0) nanoparticles (NPs), and inspected the influence of bio-nano Pd (0) on the direct electron transfer and phenol biodegradation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX) showed that bio-nano Pd (0) (~10 nm) were evenly dispersed on the surface and in the periplasm of P. putida. With the incorporation of bio-nano Pd (0), the redox currents of bacteria in the cyclic voltammetry (CV) became higher and the oxidation current increased as the addition of lactate, while the highest increase rates of two electron transfer system (ETS) rates were 63.97% and 33.79%, respectively. These results indicated that bio-nano Pd (0) could directly promote the electron transfer of P. putida. In phenol biodegradation process, P. putida-Pd (0)- 2 showed the highest k (0.2992 h^-1), μ_m (0.035 h^-1) and K_i (714.29 mg/L) and the lowest apparent K_s (76.39 mg/L). The results of kinetic analysis indicated that bio-nano Pd (0) markedly enhanced the biocatalytic efficiency, substrate affinity and the growth of cells compared to native P. putida. The positive effects of bio-nano Pd (0) to the electron transfer of P. putida would promote the biodegradation of phenol.

Recent progress of metal-graphene nanostructures in photocatalysis

Metal-graphene nanostructures (NSs) as photocatalysts, prepared using simple and scalable synthesis methods, are gaining heightened attention as novel materials for water treatment and environmental remediation applications. Graphene, the unique few layers sheet-like arrangement of sp2 hybridized carbon atoms, has an inimitable two-dimensional (2D) structure. The material is highly conductive, has high electron mobility and an extremely high surface area, and can be produced on a large scale at low cost. Accordingly, it has been considered as an essential base component for producing various metal-based NSs. In particular, metal-graphene NSs as photocatalysts have attracted considerable attention because of their special surface plasmon resonance (SPR) effect that can improve their performance for the removal of toxic dyes and other pollutants. This review summarizes the recent and advanced progress for the easy fabrication and design of graphene-based NSs as photocatalysts, as a novel tool, using a range of approaches, including green and biogenic approaches.

FeS2 Nanoparticles Decorated Graphene as Microbial-Fuel-Cell Anode Achieving High Power Density

Microbial fuel cells (MFCs) have received great attention worldwide due to their potential in recovering electrical energy from waste and inexhaustible biomass. Unfortunately, the difficulty of achieving the high power, especially in real samples, remains a bottleneck for their practical applications. Herein, FeS₂ nanoparticles decorated graphene is fabricated via a simple hydrothermal reaction. The FeS₂ nanoparticles decorated graphene anode not only benefits bacterial adhesion and enrichment of electrochemically active Geobacter species on the electrode surface but also promotes efficient extracellular electron transfer, thus giving rise to a fast start-up time of 2 d, an unprecedented power density of 3220 mW m^-2 and a remarkable current density of 3.06 A m^-2 in the acetate-feeding and mixed bacteria-based MFCs. Most importantly, the FeS₂ nanoparticles decorated graphene anode successfully achieves a power density of 310 mW m^-2 with simultaneous removal of 1319 ± 28 mg L^-1 chemical oxygen demand in effluents from a beer factory wastewater. The characteristics of improved power generation and enhanced pollutant removal efficiency opens the door toward development of high-performance MFCs via rational anode design for practical application.

Electrochemical and microbial community responses of electrochemically active biofilms to copper ions in bioelectrochemical systems

Heavy metals play an important role in the conductivity of solution, power generation and activity of microorganisms in bioelectrochemical systems (BESs). However, effect of heavy metal on the process of exoelectrogenesis metabolism and extracellular electron transfer of electrochemically active biofilms (EABs) was poorly understood. Herein, we investigated the impact of Cu²⁺ at gradually increasing concentration on the morphological and electrochemical performance and bacterial communities of anodic biofilms in mixed-culture BESs. The voltage output decreased continuously and dropped to zero at 10 mg L^-1, which was attributed to the toxic inhibition that cased anodic biofilm damage and decreased secretion of outer membrane cytochromes. When stopping the introduction of Cu²⁺ to anodic chamber, the maximum voltage production recovered 75.1% of the voltage produced from BES and coulombic efficiency was higher but acetate removal rate was lower than that before Cu²⁺ addition, demonstrating the recovery capability of EABs was higher compared to nonelectroactive bacteria. Moreover, SEM-EDS and XPS suggested that most of Cu²⁺ was adsorbed by the anode electrode and reduced by EABs on anode. Compared to the open-circuit BES, the flow of electrons through a circuit could improve the reduction of copper. Community analysis showed a decrease in Geobacter accompanied by an increase in Stenotrophomonas in response to Cu²⁺ shock in anodic chamber.

Silica immobilization of Geobacter sulfurreducens for constructing ready-to-use artificial bioelectrodes

Microbial electrochemical technologies (METs) rely on the control of interactions between microorganisms and electronic devices, enabling to transform chemical energy into electricity. We report a new approach to construct ready-to-use artificial bioelectrodes by immobilizing Geobacter sulfurreducens cells in composite materials associating silica gel and carbon felt fibres. Viability test confirmed that the majority of bacteria (ca. 70 ± 5%) survived the encapsulation process in silica and that cell density did not increase in 96 h. The double entrapment within the silica-carbon composite prevented bacterial release from the electrode but allowed a suitable mass transport (ca. 5 min after electron donor pulse), making the electrochemical characterization of the system possible. The artificial bioelectrodes were evaluated in three-electrode reactors and the maximum current displayed was ca. 220 and 150 μA cm-3 using acetate and lactate as electron donors respectively. Cyclic voltammetry of acetate-fed bioelectrodes revealed a sigmoidal catalytic oxidation wave, typical of more advanced-stage biofilms. The presence of G. sulfurreducens within composites was ascertained by SEM analysis, suggesting that only part of the bacterial population was in direct contact with the carbon fibres. Preliminary analyses of the transcriptomic response of immobilized G. sulfurreducens enlightened that encapsulation mainly induces an osmotic stress to the cells. Therefore, ready-to-use artificial bioelectrodes represent a versatile time- and cost-saving strategy for microbial electrochemical systems.

Development and characterisation of self-assembled graphene hydrogel-based anodes for bioelectrochemical systems

In this work, we report a simple and scalable method to produce high efficiency 3D graphene-based electrodes (GH) for bioelectrochemical systems. GH were obtained by self-assembly of graphene oxide, through slow reduction with ascorbic acid over conductive mesh-works (carbon cloth and stainless-steel). The GH structure and composition were characterised by electron microscopy (SEM) and spectroscopy (FTIR and Raman), whereas the electrodes' performance was tested by chronoamperometry and cyclic voltammetry in a microbial electrolysis cell (MEC) inoculated with a pure culture of G. sulfurreducens. The hydrogel had a broad pore size distribution (>1 μm), which allowed bacterial colonisation within the framework. The macro-porous structure and chemical properties of the hydrogel rendered a higher bacterial loading capacity and substrate oxidation rate than other carbonaceous materials, including different reported graphene electrodes, which significantly increased MEC performance.

A cheap, robust and versatile hydrogel-electrode is easily obtained by reduction of graphene-oxide; its colonisation by Geobacter resulted in high current densities.

A comprehensive overview on electro-active biofilms, role of exo-electrogens and their microbial niches in microbial fuel cells (MFCs)

Microbial fuel cells (MFCs) are biocatalyzed systems which can drive electrical energy by directly converting chemical energy using microbial biocatalyst and are considered as one of the important propitious technologies for sustainable energy production. Much research on MFCs experiments is under way with great potential to become an alternative to produce clean energy from renewable waste. MFCs have been one of the most promising technologies for generating clean energy industry in the future. This article summarizes the important findings in electro-active biofilm formation and the role of exo-electrogens in electron transfer in MFCs. This study provides and brings special attention on the effects of various operating and biological parameters on the biofilm formation in MFCs. In addition, it also highlights the significance of different molecular techniques used in the microbial community analysis of electro-active biofilm. It reviews the challenges as well as the emerging opportunities required to develop MFCs at commercial level, electro-active biofilms and to understand potential application of microbiological niches are also depicted. Thus, this review is believed to widen the efforts towards the development of electro-active biofilm and will provide the research directions to overcome energy and environmental challenges.

Polarity inversion of bioanode for biocathodic reduction of aromatic pollutants

The enrichment of specific pollutant-reducing consortium is usually required prior to the startup of biocathode bioelectrochemical system (BES) and the whole process is time consuming. To rapidly establish a non-specific functional biocathode, direct polar inversion from bioanode to biocathode is proposed in this study. Based on the diverse reductases and electron transfer related proteins of anode-respiring bacteria (ARB), the acclimated electrochemically active biofilm (EAB) may catalyze reduction of different aromatic pollutants. Within approximately 12 d, the acclimated bioanodes were directly employed as biocathodes for nitroaromatic nitrobenzene (NB) and azo dye acid orange 7 (AO7) reduction. Our results indicated that the established biocathode significantly accelerated the reduction of NB to aniline (AN) and AO7 to discolored products compared with the abiotic cathode and open circuit controls. Several microbes possessing capabilities of nitroaromatic/azo dye reduction and bidirectional electron transfer were maintained or enriched in the biocathode communities. Cyclic voltammetry highlighted the decreased over-potentials and enhanced electron transfer of biocathode as well as demonstrated the ARB Geobacter containing cytochrome c involved in the backward electron transfer from electrode to NB. This study offers new insights into the rapid establishment and modularization of functional biocathodes for the potential treatment of complicated electron acceptors-coexisting wastewaters.

The Planktonic Relationship Between Fluid-Like Electrodes and Bacteria: Wiring in Motion

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

Electrochemical techniques reveal that total ammonium stress increases electron flow to anode respiration in mixed-species bacterial anode biofilms

When anode-respiring bacteria (ARB) respire electrons to an anode in microbial electrochemical cells (MXCs), they harvest only a small amount of free energy. This means that ARB must have a high substrate-oxidation rate coupled with a high ratio of electrons used for respiration compared to total electrons removed by substrate utilization. It also means that they are especially susceptible to inhibition that slows anode respiration or lowers their biomass yield. Using several electrochemical techniques, we show that a relatively high total ammonium-nitrogen (TAN) concentration (2.2 g TAN/L) induced significant stress on the ARB biofilms, lowering their true yield and forcing the ARB to boost the ratio of electrons respired per electrons consumed from the substrate. In particular, a higher respiration rate, measured as current density (j), was associated with slower growth and a lower net yield, compared to an ARB biofilm grown with a lower ammonium concentration (0.2 g TAN/L). Further increases in influent TAN (to 3 and then to 4.4 g TAN/L) caused nearly complete inhibition of anode respiration. However, the ARB could recover from high-TAN inhibition after a shift of the MXC's feed to 0.2 g TAN/L. In summary, ARB biofilms were inhibited by a high TAN concentration, but could divert more electron flow toward anode respiration with modest inhibition and recover when severe inhibition was relieved. Biotechnol. Bioeng. 2017;114: 1151-1159. © 2017 Wiley Periodicals, Inc.