Extracellular Polymeric Substances from Geobacter sulfurreducens Biofilms in Microbial Fuel Cells

Potential mechanisms of propionate degradation and methanogenesis in anaerobic digestion coupled with microbial electrolysis cell system: Importance of biocathode

Microbial electrolysis cells (MEC) have the potential for enhancing the efficiency of anaerobic digestion (AD). In this study, microbiological and metabolic pathways in the biocathode of anaerobic digestion coupled with microbial electrolysis cells system (AD-MEC) were revealed to separate bioanode. The biocathode efficiently degraded 90 % propionate within 48 h, leading to a methane production rate of 3222 mL·m-2·d-1. The protein and heme-rich cathodic biofilm enhanced redox capacity and facilitated interspecies electron transfer. Key acid-degrading bacteria, including Dechloromonas agitata, Ignavibacteriales bacterium UTCHB2, and Syntrophobacter fumaroxidans, along with functional proteins such as cytochrome c and e-pili, established mutualistic relationships with Methanothrix soehngenii. This synergy facilitated a multi-pathway metabolic process that converted acetate and CO2 into methane. The study sheds light on the intricate microbial dynamics within the biocathode, suggesting promising prospects for the scalable integration of AD-MEC and its potential in sustainable energy production.

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

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

Global transcriptional analysis of Geobacter sulfurreducens gsu1771 mutant biofilm grown on two different support structures

Electroactive biofilms formation by the metal-reducing bacterium Geobacter sulfurreducens is a step crucial for bioelectricity generation and bioremediation. The transcriptional regulator GSU1771 controls the expression of essential genes involved in electron transfer and biofilm formation in G. sulfurreducens, with GSU1771-deficient producing thicker and more electroactive biofilms. Here, RNA-seq analyses were conducted to compare the global gene expression patterns of wild-type and Δgsu1771 mutant biofilms grown on non-conductive (glass) and conductive (graphite electrode) materials. The Δgsu1771 biofilm grown on the glass surface exhibited 467 differentially expressed (DE) genes (167 upregulated and 300 downregulated) versus the wild-type biofilm. In contrast, the Δgsu1771 biofilm grown on the graphite electrode exhibited 119 DE genes (79 upregulated and 40 downregulated) versus the wild-type biofilm. Among these DE genes, 67 were also differentially expressed in the Δgsu1771 biofilm grown on glass (56 with the same regulation and 11 exhibiting counter-regulation). Among the upregulated genes in the Δgsu1771 biofilms, we identified potential target genes involved in exopolysaccharide synthesis (gsu1961-63, gsu1959, gsu1972-73, gsu1976-77). RT-qPCR analyses were then conducted to confirm the differential expression of a selection of genes of interest. DNA-protein binding assays demonstrated the direct binding of the GSU1771 regulator to the promoter region of pgcA, pulF, relA, and gsu3356. Furthermore, heme-staining and western blotting revealed an increase in c-type cytochromes including OmcS and OmcZ in Δgsu1771 biofilms. Collectively, our findings demonstrated that GSU1771 is a global regulator that controls extracellular electron transfer and exopolysaccharide synthesis in G. sulfurreducens, which is crucial for electroconductive biofilm development.

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.

Geobacter sulfurreducens' Unique Metabolism Results in Cells with a High Iron and Lipid Content

Geobacter sulfurreducens is a ubiquitous iron-reducing bacterium in soils, and in engineered systems, it can respire an electrode to produce measurable electric current. Its unique metabolism, heavily dependent on an extensive network of cytochromes, requires a unique cell composition. In this work, we used metallomics, cell fraction and elemental analyses, and transcriptomics to study and analyze the cell composition of G. sulfurreducens. Elemental composition studies (C, H, O, N, and ash content) showed high C:O and H:O ratios of approximately 1.7:1 and 0.25:1, indicative of more reduced cell composition that is consistent with high lipid content. Our study shows that G. sulfurreducens cells have a large amount of iron (2 ± 0.2 μg/g dry weight) and lipids (32 ± 0.5% dry weight/dry weight) and that this composition does not change whether the cells are grown with a soluble or an insoluble electron acceptor. The high iron concentration, higher than similar microorganisms, is attributed to the production of cytochromes that are abundant in transcriptomic analyses in both solid and soluble electron acceptor growth. The unique cell composition of G. sulfurreducens must be considered when growing this microorganism for lab studies and commercial applications. IMPORTANCE Geobacter sulfurreducens is an electroactive microorganism. In nature, it grows on metallic minerals by transferring electrons to them, effectively "breathing" metals. In a manmade system, it respires an electrode to produce an electric current. It has become a model organism for the study of electroactive organisms. There are potential biotechnological applications of an organism that can bridge the gap between biology and electrical signal and, as a ubiquitous iron reducer in soils around the world, G. sulfurreducens has an impact on the global iron cycle. We measured the concentrations of metals, macromolecules, and basic elements in G. sulfurreducens to define this organism's composition. We also used gene expression data to discuss which proteins those metals could be associated with. We found that G. sulfurreducens has a large amount of lipid and iron compared to other bacteria-these observations are important for future microbiologists and biotechnologists working with the organism.

Improved energy efficiency in microbial fuel cells by bioethanol and electricity co-generation

Background

Microbial electricity production has received considerable attention from researchers due to its environmental friendliness and low price. The increase in the number of intracellular electrons in a microbial fuel cell (MFC) helps to improve the MFC performance.

Results

In this study, we accumulated excess electrons intracellularly by knocking out the gene related to intracellular electron consumption in Saccharomyces cerevisiae, and the elevated intracellular electron pool positively influenced the performances of MFCs in terms of electricity production, while helping to increase ethanol production and achieve ethanol and electricity co-production, which in turn improved the utilization of substrates. The final knockout strain reached a maximum ethanol yield of 7.71 g/L and a maximum power density of 240 mW/m2 in the MFC, which was 12 times higher than that of the control bacteria, with a 17.3% increase in energy utilization.

Conclusions

The knockdown of intracellular electron-consuming genes reported here allowed the accumulation of excess electrons in cells, and the elevated intracellular electron pool positively influenced the electrical production performance of the MFC. Furthermore, by knocking out the intracellular metabolic pathway, the yield of ethanol could be increased, and co-production of ethanol and electricity could be achieved. Thus, the MFC improved the utilization of the substrate.

Microbial biofilms for electricity generation from water evaporation and power to wearables

Employing renewable materials for fabricating clean energy harvesting devices can further improve sustainability. Microorganisms can be mass produced with renewable feedstocks. Here, we demonstrate that it is possible to engineer microbial biofilms as a cohesive, flexible material for long-term continuous electricity production from evaporating water. Single biofilm sheet (~40 µm thick) serving as the functional component in an electronic device continuously produces power density (~1 μW/cm²) higher than that achieved with thicker engineered materials. The energy output is comparable to that achieved with similar sized biofilms catalyzing current production in microbial fuel cells, without the need for an organic feedstock or maintaining cell viability. The biofilm can be sandwiched between a pair of mesh electrodes for scalable device integration and current production. The devices maintain the energy production in ionic solutions and can be used as skin-patch devices to harvest electricity from sweat and moisture on skin to continuously power wearable devices. Biofilms made from different microbial species show generic current production from water evaporation. These results suggest that we can harness the ubiquity of biofilms in nature as additional sources of biomaterial for evaporation-based electricity generation in diverse aqueous environments.

Though water evaporation-driven electricity generation is an attractive sustainable energy production strategy, existing electronic devices suffer from poor performance or is costly. Here, the authors report sustainable biofilms for efficient, low-cost evaporation-based electricity production

R-based method for quantitative analysis of biofilm thickness by using confocal laser scanning microscopy

Microscopy is mostly the method of choice to analyse biofilms. Due to the high local heterogeneity of biofilms, single and punctual analyses only give an incomplete insight into the local distribution of biofilms. In order to retrieve statistically significant results a quantitative method for biofilm thickness measurements was developed based on confocal laser scanning microscopy and the programming language R. The R-script allows the analysis of large image volumes with little hands-on work and outputs statistical information on homogeneity of surface coverage and overall biofilm thickness. The applicability of the script was shown in microbial fuel cell experiments. It was found that Geobacter sulfurreducens responds differently to poised anodes of different material so that the optimum potential for MFC on poised ITO anodes had to be identified with respect to maximum current density, biofilm thickness and MFC start-up time. Thereby, a positive correlation between current density and biofilm thickness was found, but with no direct link to the applied potential. The optimum potential turned out to be +0.1 V versus SHE. The script proved to be a valuable stand-alone tool to quantify biofilm thickness in a statistically valid manner, which is required in many studies.

Lysozyme regulates the extracellular polymer of activated sludge and promotes the formation of electroactive biofilm

The formation of electroactive biofilm from activated sludge on electrode surface is a key step to construct a bio-electrochemical system, yet it is greatly limited by the poor affinity between the bacteria and the electrode interface. Herein, we report a new method to promote the formation of electroactive biofilm by regulating the extracellular polymeric substance (EPS) content in activated sludge with lysozyme. The investigation of the effect of lysozyme treatment on the content of extracellular polymers and the biofilm formation of electroactive bacteria suggests that lysozyme can improve the permeability of the positive bacterial cell membrane and thus increase the EPS content in the activated sludge. The characterizations of electrochemical activity, surface morphology and community structure of the anode biofilm indicate that increasing EPS content promotes the adhesion of the mixed bacteria in the activated sludge on the electrode and results in denser biofilms with better conductivities. The microbial fuel cell (MFC) inoculated with the sludge of high EPS content exhibits the power density up to 2.195 W/m², much higher than that inoculated with the untreated sludge (1.545 W/m²). The strategy of adjusting EPS content in activated sludge with a biological enzyme can effectively enhance the ability of the bacterial community to form biofilms and exhibits great application potentials in the construction of high efficiency bio-electrochemical systems.

Facet-engineered hematite boosts microbial electrogenesis by synergy of promoting electroactive biofilm formation and extracellular electron transfer

Hematite has been proven to be an excellent material for enhancing extracellular electron transfer (EET) in microbial bioelectrochemical systems (BESs). However, the effect of hematite with different exposed facets on microbial EET remains unclear. Here, we synthesized two types of hematite nanoparticles with high {100} and {001} facet exposure (Hem_{100} and Hem_{001}), respectively, which were coated on ITO electrode to stimulate the microbial EET in the BESs. The results showed that the maximum biocurrent density of commercial hematite nanoparticles (Hem_NPs), Hem_{100} and Hem_{001} electrodes reached 73.33 ± 5.68, 129.33 ± 9.12 and 287.00 ± 19.89 μA cm^-2 from three replicates of each treatment, respectively. The current generation achieved from the Hem_{001} electrode was nearly 199-times higher than that of the blank ITO electrode (1.44 ± 0.10 μA cm^-2). The electrochemical measurements showed that the lowest charge transfer resistance (R_ct) was observed for the Hem_{001}, and the promoted biofilm formation and EPS secretion on the Hem_{001} electrode were also revealed, which could contribute the high performance of this electrode. Moreover, metagenomic analysis revealed that Hem_{001} might facilitate the microbial EET by stimulating the expression of genes related to cytochrome c and conductive nanowires. This study not only provides a new strategy to enhance microbial electrogenesis but also expands the knowledge of the effect of facet on microbial EET, helping to develop more efficient electrode materials in the future.

Microbial fuel cells for bioelectricity production from waste as sustainable prospect of future energy sector

Microbial fuel cell (MFC) is lauded for its potentials to solve both energy crisis and environmental pollution. Technologically, it offers the capability to harness electricity from the chemical energy stored in the organic substrate with no intermediate steps, thereby minimizes the entropic loss due to the inter-conversion of energy. The sciences underneath such MFCs include the electron and proton generation from the metabolic decomposition of the substrate by microbes at the anode, followed by the shuttling of these charges to cathode for electricity generation. While its promising prospects were mutually evinced in the past investigations, the upscaling of MFC in sustaining global energy demands and waste treatments is yet to be put into practice. In this context, the current review summarizes the important knowledge and applications of MFCs, concurrently identifies the technological bottlenecks that restricted its vast implementation. In addition, economic analysis was also performed to provide multiangle perspectives to readers. Succinctly, MFCs are mainly hindered by the slow metabolic kinetics, sluggish transfer of charged particles, and low economic competitiveness when compared to conventional technologies. From these hindering factors, insightful strategies for improved practicality of MFCs were formulated, with potential future research direction being identified too. With proper planning, we are delighted to see the industrialization of MFCs in the near future, which would benefit the entire human race with cleaner energy and the environment.

Effect of hydraulic retention time on the denitrification performance and metabolic mechanism of a multi-chambered bio-electrochemical system

The effects of hydraulic retention time (HRT) on the denitrification performance of the multi-chambered bio-electrochemistry system and the metabolic mechanism of the microbial community were investigated. Results indicated that the NO₃^--N and NO₂^--N removal efficiency was up to 99.5% and 99.9%, respectively. The electricity generation performance of the system was optimum at 24 h HRT, with the maximum power density and output voltage of the fourth chamber to be 471.2 mW/m³ and 602.4 mV, respectively. With the decrease of HRT from 24 h to 8 h, the protein-like substance in extracellular polymeric substance (EPS) of granular sludge was reduced and the fluorescence intensities were weakened. Besides, the abundance of metabolism pathway was the highest at 50.0% and 49.9%, respectively, and the methane metabolism (1.8% and 2.0%, respectively) and the nitrogen metabolism (0.8% and 0.9%, respectively) in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway played important roles in providing guaranteed stability and efficient removal of organic matter and nitrogen from the system.

Conductive Geopolymers as Low-Cost Electrode Materials for Microbial Fuel Cells

Geopolymer (GP) inorganic binders have a superior acid resistance compared to conventional cement (e.g., Portland cement, PC) binders, have better microbial compatibility, and are suitable for introducing electrically conductive additives to improve electron and ion transfer properties. In this study, GP-graphite (GPG) composites and PC-graphite (PCG) composites with a graphite content of 1-10 vol % were prepared and characterized. The electrical conductivity percolation threshold of the GPG and PCG composites was around 7 and 8 vol %, respectively. GPG and PCG composites with a graphite content of 8 to 10 vol % were selected as anode electrodes for the electrochemical analysis in two-chamber polarized microbial fuel cells (MFCs). Graphite electrodes were used as the positive control reference material. Geobacter sulfurreducens was used as a biofilm-forming and electroactive model organism for MFC experiments. Compared to the conventional graphite anodes, the anode-respiring biofilms resulted in equal current production on GPG composite anodes, whereas the PCG composites showed a very poor performance. The largest mean value of the measured current densities of a GPG composite used as anodes in MFCs was 380.4 μA cm^-2 with a standard deviation of 129.5 μA cm^-2. Overall, the best results were obtained with electrodes having a relatively low Ohmic resistance, that is, GPG composites and graphite. The very first approach employing sustainable GPs as a low-cost electrode binder material in an MFC showed promising results with the potential to greatly reduce the production costs of MFCs, which would also increase the feasibility of MFC large-scale applications.

Empower C1: Combination of Electrochemistry and Biology to Convert C1 Compounds

The idea to somehow combine electrical current and biological systems is not new. It was subject of research as well as of science fiction literature for decades. Nowadays, in times of limited resources and the need to capture greenhouse gases like CO₂, this combination gains increasing interest, since it might allow to use C1 compounds and highly oxidized compounds as substrate for microbial production by "activating" them with additional electrons. In this chapter, different possibilities to combine electrochemistry and biology to convert C1 compounds into useful products will be discussed. The chapter first shows electrochemical conversion of C1 compounds, allowing the use of the product as substrate for a subsequent biosynthesis in uncoupled systems, further leads to coupled systems of biology and electrochemical conversion, and finally reaches the discipline of bioelectrosynthesis, where electrical current and C1 compounds are directly converted by microorganisms or enzymes. This overview will give an idea about the potentials and challenges of combining electrochemistry and biology to convert C1 molecules.

Anodic and cathodic biofilms coupled with electricity generation in single-chamber microbial fuel cell using activated sludge

Microbial fuel cell (MFC) is used to remove organic pollutants while generating electricity. Biocathode plays as an efficient electrocatalyst for accelerating the Oxidation Reduction Reaction (ORR) of oxygen in MFC. This study integrated biocathode into a single-chamber microbial fuel cell (BSCMFC) to produce electricity from an organic substrate using aerobic activated sludge to gain more insights into anodic and cathodic biofilms. The maximum power density, current density, chemical oxygen demand (COD) removal, and coulombic efficiency were 0.593 W m^-3, 2.6 A m^-3, 83 ± 8.4%, and 22 ± 2.5%, respectively. Extracellular polymeric substances (EPS) produced by biofilm from the biocathode were higher than the bioanode. Infrared spectroscopy and Scanning Electron Microscope (SEM) examined confirmed the presence of biofilm by the adhesion on electrodes. The dominant phyla in bioanode were Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria, while the dominant phylum in the biocathode was Proteobacteria. Therefore, this study demonstrates the applicable use of BSCMFC for bioelectricity generation and pollution control.

Anode biofilm influence on the toxic response of microbial fuel cells under different operating conditions

The response of microorganisms in microbial fuel cells (MFCs) to toxic compounds under different operating conditions, such as flow rate and culture time, was investigated herein. While it has been reported that MFCs can detect some toxic substances, it is unclear if operating conditions affect MFCs toxicity response. In this study, the toxic response time of MFCs decreased when the flow rate increased from 0.5 mL/min to 2 mL/min and then increased with 5 mL/min. The inhibition rates at 0.5 mL/min, 2 mL/min, and 5 mL/min were 8.4% ± 1.6%, 45.1% ± 5.3%, and 4.9% ± 0.3%, respectively. With the increase of culture time from 7 days to 90 days, the toxic response time of MFCs gradually increased. The inhibition rates at culture times of 7 days, 45 days, and 90 days were 45.1% ± 5.3%, 32.6% ± 6.6%, and 23.2% ± 1.3%, respectively. Increasing the culture time will reduce the sensitivity of MFC. The results showed that MFCs can respond quickly at a flow rate of 2 mL/min after cultivation for 7 days. Under these conditions, the power density can reach 1137.0 ± 65.5 mW/m², the relative content of Geobacter sp. is 57%, and the ORP of the multilayers changed from -159.2 ± 1.6 mV to -269.9 ± 1.7 mV within 200 μm biofilm thickness. These findings show that increasing the flow rate and shortening the culture time are conducive for the toxicity response of MFCs, which will increase the sensitivity of MFCs in practical applications.

Artificial electrochemically active biofilm for improved sensing performance and quickly devising of water quality early warning biosensors

A major challenge for devising an electrochemically active biofilm (EAB)-based biosensor for real-time water quality early-warning is the formation of EAB that requires several days to weeks. Besides the onerous and time-consuming preparation process, the naturally formed EABs are intensively concerned as they can hardly deliver repeatable electrical signals even at identical experimental conditions. To address these concerns, this study employed sodium alginate as immobilization agent to encapsulate Shewanella oneidensis MR-1 and prepared EAB for devising a biosensor in a short period of less than 1 h. The artificial EAB were found capable of delivering highly consistent electrical signals with each other when fed with the same samples. Morphology and bioelectrochemical properties of the artificial EAB were investigated to provide interpretations for these findings. Different concentrations of bacteria and alginate in forming the EAB were investigated for their effects on the biosensor's sensitivity. Results suggested that lower concentration of bacteria would be beneficial until it increased to 0.06 (OD₆₆₀). Concentration of sodium alginate affected the sensitivity as well and 1% was found an optimum amount to serve in the formation of EAB. A long-term operation of the biosensor with artificial EAB for 110 h was performed. Clear warning signals for incoming toxicants were observed over random signal fluctuations. All results suggested that the artificial EAB electrode would support a rapid devised and highly sensitivity biosensor.

Investigation of Polymer Biofilm Formation on Titanium-Based Anode Surface in Microbial Fuel Cells with Poplar Substrate

Microbial fuel cells (MFCs) have attracted attention by directly converting the bioelectrochemical energy possessed by the organic materials that make up the biomass into electrical energy. In this study, the relationship between the biofilm formed on the titanium-based anode electrode surface, and the chemical composition of the substrate, the energy source of MFC, was investigated. For this, MFCs were made by using poplar wood shavings rich in organic material as the substrate, titanium-based material as the anode electrode, and natural soil as bacterial habitat. Three types of MFCs containing 1%, 10%, and 20% poplar wood shavings by weight were made and named P1-MFC, P2-MFC, and P3-MFC, respectively. According to electrochemical analysis, P3-MFC provided the highest open circuit voltage with 490 mV value, and the highest power density with 5.11 mW/m² value compared to other MFCs. According to optical microscopy examinations, there were Bacillus and Coccus species of bacteria in the soil structure, and these bacteria also existed around the fiber of poplar wood shavings in MFCs. Scanning electron microscopy (SEM), energy-dispersive spectrum (EDS), and Fourier transform infrared spectroscopy (FTIR) analysis showed that MFCs formed biofilm in the titanium-based anode, and the chemical composition of this biofilm with poplar tree was similar. As a result, due to the catalysis reactions of bacteria, the titanium-based anode electrode surface was coated with polymer biofilm released from poplar wood shavings.

Impact of Carbon Felt Electrode Pretreatment on Anodic Biofilm Composition in Microbial Electrolysis Cells

Sustainable technologies for energy production and storage are currently in great demand. Bioelectrochemical systems (BESs) offer promising solutions for both. Several attempts have been made to improve carbon felt electrode characteristics with various pretreatments in order to enhance performance. This study was motivated by gaps in current knowledge of the impact of pretreatments on the enrichment and microbial composition of bioelectrochemical systems. Therefore, electrodes were treated with poly(neutral red), chitosan, or isopropanol in a first step and then fixed in microbial electrolysis cells (MECs). Four MECs consisting of organic substance-degrading bioanodes and methane-producing biocathodes were set up and operated in batch mode by controlling the bioanode at 400 mV vs. Ag/AgCl (3M NaCl). After 1 month of operation, Enterococcus species were dominant microorganisms attached to all bioanodes and independent of electrode pretreatment. However, electrode pretreatments led to a decrease in microbial diversity and the enrichment of specific electroactive genera, according to the type of modification used. The MEC containing isopropanol-treated electrodes achieved the highest performance due to presence of both Enterococcus and Geobacter. The obtained results might help to select suitable electrode pretreatments and support growth conditions for desired electroactive microorganisms, whereby performance of BESs and related applications, such as BES-based biosensors, could be enhanced.

Identification of a Diguanylate Cyclase That Facilitates Biofilm Formation on Electrodes by Shewanella oneidensis MR-1

In many bacteria, cyclic diguanosine monophosphate (c-di-GMP), synthesized by diguanylate cyclase (DGC), serves as a second messenger involved in the regulation of biofilm formation. Although studies have suggested that c-di-GMP also regulates the formation of electrochemically active biofilms (EABFs) by Shewanella oneidensis MR-1, DGCs involved in this process remained to be identified. Here, we report that the SO_1646 gene, hereafter named dgcS, is upregulated under medium flow conditions in electrochemical flow cells (EFCs), and its product (DgcS) functions as a major DGC in MR-1. In vitro assays demonstrated that purified DgcS catalyzed the synthesis of c-di-GMP from GTP. Comparisons of intracellular c-di-GMP levels in the wild-type strain and a dgcS deletion mutant (ΔdgcS mutant) showed that production of c-di-GMP was markedly reduced in the ΔdgcS mutant when cells were grown in batch cultures and on electrodes in EFCs. Cultivation of the ΔdgcS mutant in EFCs also revealed that the loss of DgcS resulted in impaired biofilm formation and decreased current generation. These findings demonstrate that MR-1 uses DgcS to synthesize c-di-GMP under medium flow conditions, thereby activating biofilm formation on electrodes.IMPORTANCE Bioelectrochemical systems (BESs) have attracted wide attention owing to their utility in sustainable biotechnology processes, such as microbial fuel cells and electrofermentation systems. In BESs, electrochemically active bacteria (EAB) form biofilms on electrode surfaces, thereby serving as effective catalysts for the interconversion between chemical and electric energy. It is therefore important to understand mechanisms for the formation of biofilm by EAB grown on electrodes. Here, we show that a model EAB, S. oneidensis MR-1, expresses DgcS as a major DGC, thereby activating the formation of biofilms on electrodes via c-di-GMP-dependent signal transduction cascades. The findings presented herein provide the molecular basis for improving electrochemical interactions between EAB and electrodes in BESs. The results also offer molecular insights into how Shewanella regulates biofilm formation on solid surfaces in the natural environment.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.

The electrode potential determines the yield coefficients of early-stage Geobacter sulfurreducens biofilm anodes

Geobacter sulfurreducens is the model for electroactive microorganisms (EAM). EAM can use solid state terminal electron acceptors (TEA) including anodes via extracellular electron transfer (EET). Yield coefficients relate the produced cell number or biomass to the oxidized substrate or the reduced TEA. These data are not yet sufficiently available for EAM growing at anodes. Thus, this study provides information about kinetics as well as yield coefficients of early-stage G. sulfurreducens biofilms using anodes as TEA at the potentials of -200 mV, 0 mV and +200 mV (vs. Ag/AgCl sat. KCl). The selected microorganism was therefore cultivated in single and double chamber batch reactors on graphite or AuPd anodes. Interestingly, whereas the lag time and maximum current density within 12 days of growth differed, the anode potential does not influence the coulombic efficiency and the formal potential of the EET, which remains constant for all the experiments at ~ -300 to -350 mV. We demonstrated for the first time that the anode potential has a strong influence on single cell yield coefficients which ranged from 2.69 × 10¹² cells mol_e-^-1 at -200 mV and 1.48 × 10¹² cells mol_e-^-1 at 0 mV to 2.58 × 10¹¹ cells mol_e-^-1 at +200 mV in single chamber reactors and from 1.15 × 10¹² cells mol_e-^-1 at -200 mV to 8.98× 10¹¹ cells mol_e-^-1 at 0 mV in double chamber reactors. This data can be useful for optimization and scaling-up of primary microbial electrochemical technologies.

Sorting the main bottlenecks to use paper-based microbial fuel cells as convenient and practical analytical devices for environmental toxicity testing

Three of the primary bottlenecks, which should be consider for practical, point-of-need use of microbial fuel cell (MFC) analytical devices were surpassed in this work: i) the use of a diffusive barrier, hence, an electrogenic biofilm; ii) longer enrichment/stabilization times to produce a biofilm, made in a laboratory environment, over the electrode; and iii) difficulty comparing results obtained from MFCs based on electrogenic biofilms with standardized bioassays, a setback to be adopted as a new method. Here we show an easy way to determine water toxicity employing planktonic bacteria as biorecognition agents. The paper-based MFC contain an electron carrier (or mediator) to facilitate charge transfer from bacteria to the anode. In this way, there is no need to use biofilms. As far as we know this is the first paper-based MFC containing P. putida KT2440, a well characterized non-pathogenic bacteria previously used in standardized water toxicity bioassays. Results were obtained in 80 min and an effective concentration 50 of 9.02 mg L^-1, calculated for Zn²⁺ (a reference toxic agent), was successfully compared with previously published and ISO standardized bioassays, showing a promising future for this technology. The practical design and cost (less than one U.S. dollar) of the paper-based MFC toxicity test presented will open new market possibilities for rapid and easy-to-use MFC analytical devices.

Hydrogen production in two-chamber MEC using a low-cost and biodegradable poly(vinyl) alcohol/chitosan membrane

Hydrogen production was evaluated in two-chamber microbial electrolysis cells (MEC), where the chambers of the cell were separated using a new economical and environmentally friendly membrane made of poly (vinyl) alcohol/chitosan (PVA/CS). The MEC performance was compared to that of Nafion. The obtained results indicated that the MEC performance for hydrogen production did not show significant differences between the PVA/CS and Nafion membranes. MEC with PVA/CS showed the hydrogen production rate and hydrogen yield of 1277 ± 46 mL H₂L_cat^-1d^-1 and 974 ± 116 mL H₂ g_acetate^-1, respectively. The PVA/CS membrane allowed acetate removal that was 7% higher than that of Nafion due to the lower pH gradient and a lower voltage drop that increased the ion transfer across the membrane.

Complexation by cysteine and iron mineral adsorption limit cadmium mobility during metabolic activity of Geobacter sulfurreducens

Cadmium (Cd) adversely affects human health by entering the food chain via anthropogenic activity. In order to mitigate risk, a better understanding of the biogeochemical mechanisms limiting Cd mobility in the environment is needed. While Cd is not redox-active, Cd speciation varies (i.e., aqueous, complexed, adsorbed), and influences mobility. Here, the cycling of Cd in relation to initial speciation during the growth of Geobacter sulfurreducens was studied. Either fumarate or ferrihydrite (Fh) was provided as an electron acceptor and Cd was present as: (1) an aqueous cation, (2) an aqueous complex with cysteine, which is often present in metal stressed soil environments, or (3) adsorbed to Fh. During microbial Fe(iii) reduction, the removal of Cd was substantial (∼80% removal), despite extensive Fe(ii) production (ratio Fe(ii)total : Fetotal = 0.8). When fumarate was the electron acceptor, there was higher removal from solution when Cd was complexed with cysteine (97-100% removal) compared to aqueous Cd (34-50%) removal. Confocal laser scanning microscopy (CLSM) demonstrated the formation of exopolymeric substances (EPS) in all conditions and that Cd was correlated with EPS in the absence of Fe minerals (r = 0.51-0.56). Most notable is that aqueous Cd was more strongly correlated with Geobacter cells (r = 0.72) compared to Cd-cysteine complexes (r = 0.51). This work demonstrates that Cd interactions with cell surfaces and EPS, and Cd solubility during metabolic activity are dependent upon initial speciation. These processes may be especially important in soil environments where sulfur is limited and Fe and organic carbon are abundant.

Potential-dependent extracellular electron transfer pathways of exoelectrogens

Exoelectrogens are distinct from other bacteria owing to their unique extracellular electron transfer (EET) abilities that allow for anaerobic respiration with various external redox-active surfaces, including electrode and metal oxides. Although the EET process is known to trigger diverse extracellular redox reactions, the reverse impact has been long overlooked. Recent evidences show that exoelectrogens can sense the potential changes of external surfaces and alter their EET strategies accordingly, which imparts them remarkable abilities in adapting to diverse and redox-variable environment. This mini-review provides a condensed overview and critical analysis about the recent discoveries on redox-dependent EET pathways of exoelectrogens, with focus on Geobacter sulfurreducens and Shewanella oneidensis. We summarize the detailed responses of various EET components, analyze the drives and mechanisms of such responses, highlight the diversity of EET dynamics among different bacterial species and under integrated effects of redox potential and surface chemistry, and discusses the future research needs.

Redox-Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

Microbial electrochemical systems in which metabolic electrons in living microbes have been extracted to or injected from an extracellular electrical circuit have attracted considerable attention as environmentally-friendly energy conversion systems. Since general microbes cannot exchange electrons with extracellular solids, electron mediators are needed to connect living cells to an extracellular electrode. Although hydrophobic small molecules that can penetrate cell membranes are commonly used as electron mediators, they cannot be dissolved at high concentrations in aqueous media. The use of hydrophobic mediators in combination with small hydrophilic redox molecules can substantially increase the efficiency of the extracellular electron transfer process, but this method has side effects, in some cases, such as cytotoxicity and environmental pollution. In this Review, recently-developed redox-active polymers are highlighted as a new type of electron mediator that has less cytotoxicity than many conventional electron mediators. Owing to the design flexibility of polymer structures, important parameters that affect electron transport properties, such as redox potential, the balance of hydrophobicity and hydrophilicity, and electron conductivity, can be systematically regulated.

Abatement of the membrane biofouling: Performance of an in-situ integrated bioelectrochemical-ultrafiltration system

The practical applications of membrane-based water treatment techniques are constrained by the problem of membrane fouling. Various studies have revealed that interactions between extracellular polymeric substances (EPS) and the membrane surface determine the extent of irreversible fouling. Herein, we describe a novel bioelectrochemical system (BES) integrated with an ultrafiltration (UF) membrane in order to provide an enhanced antifouling property. It was found that the integrated BES membrane system had a superior performance compared to a conventional (control) UF system, as manifested by a much lower development of transmembrane pressure. The BES significantly reduced microbial viability in the membrane tank and the imposed electrode potential contributed to the degradation of biopolymers, which favored the alleviation of membrane fouling. Notably, the electron transfer between the acclimated microorganisms and the conductive membrane in the BES integrated system exhibited an increasing trend with the operation time, indicating a gradual increase in microbial electrical activity. Correspondingly, the accumulation of extracellular polymeric substances (EPS) on the membrane surface of the BES integrated system showed a substantial decrease compared to the control system, which could be attributed to a series of synergistic effects induced by the BES integration. The differences in the microbial diversity between the control and the BES integrated system revealed the microbial selectivity of the poised potential. Specifically, microbial strains with relatively high EPS production, like the genus of Zoogloea and Methyloversatilis, were reduced significantly in the BES integrated system, while the expression of the electroactive bacteria was promoted, which facilitated extracellular electron transfer (EET) and therefore the bioelectrochemical reactions. Overall, this study has presented a feasible and promising new approach for membrane fouling mitigation during the process of water treatment.

Effects of Extracellular Polymeric Substances on the Formation and Methylation of Mercury Sulfide Nanoparticles

Growing evidence has suggested that microbial biofilms are potential environmental "hotspots" for the production and accumulation of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate that extracellular polymeric substances (EPS), the main components of biofilm matrices, significantly interfere with mercury sulfide precipitation and lead to the formation of nanoparticulate metacinnabar available for microbial methylation, a natural process predominantly responsible for the environmental occurrence of methylmercury. EPS derived from mercury methylating bacteria, particularly Desulfovibrio desulfuricans ND132, substantially increase the methylation potential of nanoparticulate mercury. This is likely due to the abundant aromatic biomolecules in EPS that strongly interact with mercury sulfide via inner-sphere complexation and consequently enhance the short-range structural disorder while mitigating the aggregation of nanoparticulate mercury. The EPS-elevated bioavailability of nanoparticulate mercury to D. desulfuricans ND132 is not induced by dissolution of these nanoparticles in aqueous phase, and may be dictated by cell-nanoparticle interfacial reactions. Our discovery is the first step of mechanistically understanding methylmercury production in biofilms. These new mechanistic insights will help incorporate microbial EPS and particulate-phase mercury into mercury methylation models, and may facilitate the assessment of biogeochemical cycling of other nutrient or toxic elements driven by EPS-producing microorganisms that are prevalent in nature.

Molecular Insights into Extracellular Polymeric Substances in Activated Sludge

Extracellular polymeric substances (EPS) are known to crucially affect the properties and performance of activated sludge, but the detailed influential mechanisms and the pertinence to specific compositional, structural properties of EPS are still elusive. Such knowledge gaps have severely limited our ability in optimizing biological wastewater treatment processes, for which long-term robust and efficient sludge performance remains one of the main bottlenecks. Here, we overview the new knowledge on the molecular structure of sludge EPS gained over the past few years and discuss the future challenges and opportunities for further advancing EPS study and engineering. The structural and functional features of several macromolecules in sludge EPS and their important structural roles in granular sludge are analyzed in detail. The EPS-pollutant interactions and environment-dependent regulation machinery on EPS production are deciphered. Lastly, the remaining knowledge gaps are identified, and the future research needs that may lead to molecular-level understanding and precise engineering of sludge EPS are highlighted.

The shuttling effects and associated mechanisms of different types of iron oxide nanoparticles for Cu(II) reduction by Geobacter sulfurreducens

Iron oxide nanoparticles (IONPs), commonly occurring in soils, aquifers and subsurface sediments, may serve as important electron shuttles for the biotransformation of coexisting toxic metals. Here, we explored the impact of different IONPs (low-crystallinity goethite and ferrihydrite, high-crystallinity magnetite and hematite) on the reduction of Cu(II) by Geobacter sulfurreducens and the associated electron shuttle mechanisms. All four IONPs tested can function as electron shuttles to enhance long distance electron transfer from bacteria to Cu(II). Upon IONPs addition, the rate of Cu(II) reduction increased from 14.9 to 65.0-83.8 % in solution after 7 days of incubation. Formation of both Cu(I) and Cu(0) on the iron oxide nanoparticles was revealed by the X-ray absorption near-edge spectroscopy. The IONPs can be utilized as conduits by bacteria to directly transfer electrons and they can also reversibly accept and donate electrons as batteries through a charging-discharging cycle to transfer electron. The latter mechanism (geo-battery) played an important role in all four types of IONPs while the former one (geo-conductor) can only be found in the magnetite and hematite treatments due to the higher crystallinity. Our results shed new light on the biogeochemically mediated electron flux in microbe-IONPs-metal networks under anaerobic iron-reduction conditions.

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.

Effective Treatment of Acid Mine Drainage with Microbial Fuel Cells: An Emphasis on Typical Energy Substrates

Acid mine drainage (AMD), characterized by a high concentration of heavy metals, poses a threat to the ecosystem and human health. Bioelectrochemical system (BES) is a promising technology for the simultaneous treatment of organic wastewater and recovery of metal ions from AMD. Different kinds of organic wastewater usually contain different predominant organic chemicals. However, the effect of different energy substrates on AMD treatment and microbial communities of BES remains largely unknown. Here, results showed that different energy substrates (such as glucose, acetate, ethanol, or lactate) affected the startup, maximum voltage output, power density, coulombic efficiency, and microbial communities of the microbial fuel cell (MFC). Compared with the maximum voltage output (55 mV) obtained by glucose-fed-MFC, much higher maximum voltage output (187 to 212 mV) was achieved by MFCs fed individually with other energy substrates. Acetate-fed-MFC showed the highest power density (195.07 mW/m2), followed by lactate (98.63 mW/m2), ethanol (52.02 mW/m2), and glucose (3.23 mW/m2). Microbial community analysis indicated that the microbial communities of anodic electroactive biofilms changed with different energy substrates. The unclassified_f_Enterobacteriaceae (87.48%) was predominant in glucose-fed-MFC, while Geobacter species only accounted for 0.63%. The genera of Methanobrevibacter (23.70%), Burkholderia-Paraburkholderia (23.47%), and Geobacter (11.90%) were the major genera enriched in the ethanol-fed-MFC. Geobacter was most predominant in MFC enriched by lactate (45.28%) or acetate (49.72%). Results showed that the abundance of exoelectrogens Geobacter species correlated to electricity-generation capacities of electroactive biofilms. Electroactive biofilms enriched with acetate, lactate, or ethanol effectively recovered all Cu2+ ion (349 mg/L) of simulated AMD in a cathodic chamber within 53 h by reduction as Cu0 on the cathode. However, only 34.65% of the total Cu2+ ion was removed in glucose-fed-MFC by precipitation with anions and cations rather than Cu0 on the cathode.

Recovery of Metals from Acid Mine Drainage by Bioelectrochemical System Inoculated with a Novel Exoelectrogen, Pseudomonas sp. E8

Acid mine drainage (AMD) is a typical source of environmental pollution ascribing to its characteristics of high acidity and heavy metal content. Currently, most strategies for AMD treatment merely focus on metal removal rather than metal recovery. However, bioelectrochemical system (BES) is a promising technology to simultaneously remove and recover metal ions from AMD. In this study, both cupric ion and cadmium ion in simulated AMD were effectively recovered by BES inoculated with a novel exoelectrogen, Pseudomonas sp. E8, that was first isolated from the anodic electroactive biofilm of a microbial fuel cell (MFC) in this study. Pseudomonas sp. E8 is a facultative anaerobic bacterium with a rod shape, 0.43–0.47 μm wide, and 1.10–1.30 μm long. Pseudomonas sp. E8 can agglomerate on the anode surface to form a biofilm in the single-chamber MFC using diluted Luria-Bertani (LB) medium as an energy substrate. A single-chamber MFC containing the electroactive Pseudomonas sp. E8 biofilms has a maximum output voltage of 191 mV and a maximum power density of 70.40 mW/m², which is much higher than those obtained by most other exoelectrogenic strains in the genus of Pseudomonas. Almost all the Cu²⁺ (99.95% ± 0.09%) and Cd²⁺ (99.86% ± 0.04%) in simulated AMD were selectively recovered by a microbial fuel cell (MFC) and a microbial electrolysis cell (MEC). After the treatment with BES, the high concentrations of Cu²⁺(184.78 mg/L), Cd²⁺(132.25 mg/L), and total iron (49.87 mg/L) in simulated AMD were decreased to 0.02, 0.19, and 0 mg/L, respectively. Scanning electron micrograph (SEM), energy dispersive X-ray spectrometry (EDXS) and X-ray diffraction (XRD) analysis indicate that the Cu²⁺ and Cd²⁺ in simulated AMD were selectively recovered by microbial electrochemical reduction as Cu⁰ (together with trace amounts of Cu₂O) or Cd⁰ on the cathode surface. Collectively, data suggest that Pseudomonas sp. E8 has great potential for AMD treatment and metal recovery.

H2 O2 Production at Low Overpotentials for Electroenzymatic Halogenation Reactions

Various enzymes utilize hydrogen peroxide as an oxidant. Such "peroxizymes" are potentially very attractive catalysts for a broad range of oxidation reactions. Most peroxizymes, however, are inactivated by an excess of H₂ O₂ . The electrochemical reduction of oxygen can be used as an in situ generation method for hydrogen peroxide to drive the peroxizymes at high operational stabilities. Using conventional electrode materials, however, also necessitates significant overpotentials, thereby reducing the energy efficiency of these systems. This study concerns a method to coat a gas-diffusion electrode with oxidized carbon nanotubes (oCNTs), thereby greatly reducing the overpotential needed to perform an electroenzymatic halogenation reaction. In comparison to the unmodified electrode, with the oCNTs-modified electrode the overpotential can be reduced by approximately 100 mV at comparable product formation rates.