Deciphering the differentiated performance on electricity generation and COD degradation by Rhodopseudomonas-dominated bioanode in light or dark

Anoxygenic phototrophic bacteria is a promising catalyst for constructing bioanode, but the mixed culture with non-photosynthetic bacteria is inevitable in an open environment application. In this study, a Rhodopseudomonas-dominated mixed culture with other electrogenic bacteria was investigated for deciphering the differentiated performance on electricity generation in light or dark conditions. The kinetic study showed that reaction rate of OM degradation was 9 times higher than that under dark condition, demonstrating that OM degradation was enhanced by photosynthesis. However, CE under light condition was lower. It indicated that part of OM was used to provide hydrogen donors for the fixation of CO2 or hydrogen production in photosynthesis, decreasing the OM used for electron transfer. In addition, higher COD concentration was not conducive to electricity generation. EIS analysis demonstrated that higher OM concentration would increase Rct to hinder the transfer of electrons from bacteria to the electrode. Indirect and direct electron transfer were revealed by CV analysis for light and dark biofilm, respectively, and nanowires were also observed by SEM graphs, further revealing the differentiate performance. Microbial community analysis demonstrated Rhodopseudomonas was dominated in light and decreased in dark, but Geobacter increased apparently from light to dark, resulting in different power generation performance. The findings revealed the differentiated performance on electricity generation and pollutant removal by mixed culture of phototrophic bacteria in light or dark, which will improve the power generation from photo-microbial fuel cells.

In Situ Biosynthesis of FeS Nanoparticles Boosts Current Generation in Bioelectrochemical Systems Through Efficient Electron Transfer

The utility of electrochemical active biofilm in bioelectrochemical systems has received considerable attention for harvesting energy and chemical products. However, the slow electron transfer between biofilms and electrodes hinders the enhancement of performance and still remains challenging. Here, using Fe3O4 /L-Cys nanoparticles as precursors to induce biomineralization, a facile strategy for the construction of an effective electron transfer pathway through biofilm and biological/inorganic interface is proposed, and the underlying mechanisms are elucidated. Taking advantage of an on-chip interdigitated microelectrode array (IDA), the conductive current of biofilm that is related to the electron transfer process within biofilm is characterized, and a 2.10-fold increase in current output is detected. The modification of Fe3O4/L-Cys on the electrode surface facilitates the electron transfer between the biofilm and the electrode, as the bio/inorganic interface electron transfer resistance is only 16% compared to the control. The in-situ biosynthetic Fe-containing nanoparticles (e.g., FeS) enhance the transmembrane EET and the EET within biofilm, and the peak conductivity increases 3.4-fold compared to the control. The in-situ biosynthesis method upregulates the genes involved in energy metabolism and electron transfer from the transcriptome analysis. This study enriches the insights of biosynthetic nanoparticles on electron transfer process, holding promise in bioenergy conversion.

Influence of support materials on the electroactive behavior, structure and gene expression of wild type and GSU1771-deficient mutant of Geobacter sulfurreducens biofilms

Geobacter sulfurreducens DL1 is a metal-reducing dissimilatory bacterium frequently used to produce electricity in bioelectrochemical systems (BES). The biofilm formed on electrodes is one of the most important factors for efficient electron transfer; this is possible due to the production of type IV pili and c-type cytochromes that allow it to carry out extracellular electron transfer (EET) to final acceptors. In this study, we analyzed the biofilm formed on different support materials (glass, hematite (Fe2O3) on glass, fluorine-doped tin oxide (FTO) semiconductor glass, Fe2O3 on FTO, graphite, and stainless steel) by G. sulfurreducens DL1 (WT) and GSU1771-deficient strain mutant (Δgsu1771). GSU1771 is a transcriptional regulator that controls the expression of several genes involved in electron transfer. Different approaches and experimental tests were carried out with the biofilms grown on the different support materials including structure analysis by confocal laser scanning microscopy (CLSM), characterization of electrochemical activity, and quantification of relative gene expression by RT-qPCR. The gene expression of selected genes involved in EET was analyzed, observing an overexpression of pgcA, omcS, omcM, and omcF from Δgsu1771 biofilms compared to those from WT, also the overexpression of the epsH gene, which is involved in exopolysaccharide synthesis. Although we observed that for the Δgsu1771 mutant strain, the associated redox processes are similar to the WT strain, and more current is produced, we think that this could be associated with a higher relative expression of certain genes involved in EET and in the production of exopolysaccharides despite the chemical environment where the biofilm develops. This study supports that G. sulfurreducens is capable of adapting to the electrochemical environment where it grows.

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.

Bifunctional lighting/supporting substrate for microalgal photosynthetic biofilm to bio-remove ammonia nitrogen from high turbidity wastewater

Treatment technologies based on microalgal biofilms have an enormous potential for dealing with water pollution because they can efficiently redirect nutrients from wastewater to renewable biomass feedstock. However, poor light transmittance is caused by the high turbidity of wastewater, which hinders the commercial application of microalgal biofilm-based wastewater treatment. Here, a bifunctional substrate with lighting and biofilm support functions was constructed using a light guide plate. In a biofilm photobioreactor (bPBR) with a bifunctional lighting/supporting substrate (BL/S substrate), light can directly irradiate the biofilm to avoid attenuation by the turbid wastewater. Direct irradiation of light onto the biofilm led to a 93.0% enhancement of microalgal photoconversion efficiency when compared to that of a supporting substrate without lighting (SO substrate). Meanwhile, the maximum growth rate of the microalgal biofilm on the BL/S substrate was 8.7 g m^-2 d^-1, which was increased by 60.3%. The removal rate of ammonia nitrogen (NH₄⁺-N) from the digested wastewater contributed by the microalgal biofilm reached 22.6 mg L^-1 d^-1, which was higher than the previously reported that of NH₄⁺-N from turbid digested wastewater by the biofilms. Furthermore, the BL/S substrate can facilitate the secretion of abundant extracellular polymeric substrates, which results in the stable adhesion of the biofilm onto the BL/S substrate. The optical density of the microalgae cells at the outlet of the bPBR with BL/S substrate was below 0.1, which was 94% lower than that of the bPBR with the SO substrate. The results indicated the BL/S substrate may avoid the loss of microalgal biomass, and almost all biomass could be easily harvested from the biofilm for algae-based biomass resources. Consequently, this study can offer a promising alternative with efficient treatment technologies for wastewater with high turbidity.

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.

Inhibition of AHL-mediated quorum sensing to control biofilm thickness in microbial fuel cell by using Rhodococcus sp. BH4

Anode biofilm thickness is a key point for high and sustainable power generation in microbial fuel cells (MFCs). Over time, the formation of a thicker biofilm on anode electrode hinders the power generation performance of MFC by causing a longer electron transfer path and the accumulation of undesirable components in anode biofilm. To overcome these limitations, we used a novel strategy named quorum quenching (QQ) for the first time in order to control the biofilm thickness on the anode surface by inactivation of signal molecules among microorganisms. For this purpose, the isolated QQ bacteria (Rhodococcus sp. BH4) were immobilized into alginate beads (20, 40, and 80 mg/10 ml sodium alginate) and added to the anode chamber of MFCs. The MFC exhibited the best electrochemical activity (1924 mW m^-2) with a biofilm thickness of 26 μm at 40 mg Rhodococcus sp. BH4/10 ml sodium alginate. The inhibition of signal molecules in anode chamber reduced the production of extracellular polymeric substance (EPS) by preventing microbial communication amonganode microorganisms. Microscopic observations revealed that anode biofilm thickness and the abundance of dead bacteria significantly decreased with an increase in Rhodococcus sp. BH4 concentration in MFCs. Microbiome diversity showed an apparent difference among the microbial community structures of anode biofilms in MFCs containing vacant and Rhodococcus sp. BH4 beads. The data revealed that the QQ strategy is an efficient application for improving MFC performance and may shed light on future studies.

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.

A chip-based 128-channel potentiostat for high-throughput studies of bioelectrochemical systems: Optimal electrode potentials for anodic biofilms

The presence of microorganisms performing extracellular electron transfer has been established in many environments. Research to determine their role is moving slowly due to the high cost of potentiostats and the variance of data with small number of replicates. Here, we present a 128-channel potentiostat, connected to a 128 gold electrode array. Whereas the system is able to perform simultaneously 128 (bio)electrochemical measurements with an independent electrical signal input, the present manufacturing of the array limited the number of effective channels for this study to 77. We assessed the impact of 11 electrode potentials ranging from -0.45V to +0.2V vs. Ag/AgCl (7 replicates per potential) on the growth and electrochemical characteristics of anodic electroactive biofilms (EABs) formed by acetate-fed microbial communities. After 7 days of growth, maximum current was reached for electrodes poised at -0.3V, closely followed by -0.25V and -0.1V to +0.1V, a range well-fitting the midpoint potential of minerals naturally reduced by electroactive bacteria such as Geobacter Sulfurreducens. There was no significant difference in apparent midpoint potential of the EABs (-0.35V), suggesting that the mechanism of heterogeneous electron transfer was not affected by the electrode potential. The EABs poised below current plateau potential (≤-0.3V) exhibited slower growth but higher charge transfer parameters. The high-throughput and high reproducibility provided by the array may have a major facilitating impact on the field of electromicrobiology. Key aspects to improve are data processing algorithms to deal with the vast amount of generated data, and manufacturing of the electrode array itself.

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.

3D biofilm visualization and quantification on granular bioanodes with magnetic resonance imaging

The use of microbial fuel cells (MFCs) for wastewater treatment fits in a circular economy context, as they can produce electricity by the removal of organic matter in the wastewater. Activated carbon (AC) granules are an attractive electrode material for bioanodes in MFCs, as they are cheap and provide electroactive bacteria with a large surface area for attachment. The characterization of biofilm growth on AC granules, however, is challenging due to their high roughness and three-dimensional structure. In this research, we show that 3D magnetic resonance imaging (MRI) can be used to visualize biofilm distribution and determine its volume on irregular-shaped single AC granules in a non-destructive way, while being combined with electrochemical and biomass analyses. Ten AC granules with electroactive biofilm (i.e. granular bioanodes) were collected at different growth stages (3 to 21 days after microbial inoculation) from a multi-anode MFC and T₁-weighted 3D-MRI experiments were performed for three-dimensional biofilm visualization. With time, a more homogeneous biofilm distribution and an increased biofilm thickness could be observed in the 3D-MRI images. Biofilm volumes varied from 0.4 μL (day 4) to 2 μL (day 21) and were linearly correlated (R² = 0.9) to the total produced electric charge and total nitrogen content of the granular bioanodes, with values of 66.4 C μL^-1 and 17 μg N μL^-1, respectively. In future, in situ MRI measurements could be used to monitor biofilm growth and distribution on AC granules.

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.

Continuous shear stress alters metabolism, mass-transport, and growth in electroactive biofilms independent of surface substrate transport

Electroactive bacteria such as Geobacter sulfurreducens and Shewanella onedensis produce electrical current during their respiration; this has been exploited in bioelectrochemical systems. These bacteria form thicker biofilms and stay more active than soluble-respiring bacteria biofilms because their electron acceptor is always accessible. In bioelectrochemical systems such as microbial fuel cells, corrosion-resistant metals uptake current from the bacteria, producing power. While beneficial for engineering applications, collecting current using corrosion resistant metals induces pH stress in the biofilm, unlike the naturally occurring process where a reduced metal combines with protons released during respiration. To reduce pH stress, some bioelectrochemical systems use forced convection to enhance mass transport of both nutrients and byproducts; however, biofilms’ small pore size limits convective transport, thus, reducing pH stress in these systems remains a challenge. Understanding how convection is necessary but not sufficient for maintaining biofilm health requires decoupling mass transport from momentum transport (i.e. fluidic shear stress). In this study we use a rotating disc electrode to emulate a practical bioelectrochemical system, while decoupling mass transport from shear stress. This is the first study to isolate the metabolic and structural changes in electroactive biofilms due to shear stress. We find that increased shear stress reduces biofilm development time while increasing its metabolic rate. Furthermore, we find biofilm health is negatively affected by higher metabolic rates over long-term growth due to the biofilm’s memory of the fluid flow conditions during the initial biofilm development phases. These results not only provide guidelines for improving performance of bioelectrochemical systems, but also reveal features of biofilm behavior. Results of this study suggest that optimized reactors may initiate operation at high shear to decrease development time before decreasing shear for steady-state operation. Furthermore, this biofilm memory discovered will help explain the presence of channels within biofilms observed in other studies.

Geobacter Dominates the Inner Layers of a Stratified Biofilm on a Fluidized Anode During Brewery Wastewater Treatment

In this study, we designed a microbial electrochemical fluidized bed reactor (ME-FBR), with an electroconductive anodic bed made of activated carbon particles for treating a brewery wastewater. Under a batch operating mode, acetate and propionate consumption rates were 13-fold and 2.4-fold higher, respectively, when the fluidized anode was polarized (0.2 V) with respect to open circuit conditions. Operating in a continuous mode, this system could effectively treat the brewery effluent at organic loading rates (OLR) over 1.7 kg m^-3_NRV d^-1 and with removal efficiencies of 95 ± 1.4% (hydraulic retention time of 1 day and an influent of 1.7 g-COD L^-1). The coulombic efficiency values highly depended upon the OLR applied, and varied from a 56 ± 15% to 10 ± 1%. Fluorescence in situ hybridization (FISH) analysis revealed a relative high abundance of Geobacter species (ca. 20%), and clearly showed a natural microbial stratification. Interestingly, the Geobacter cluster was highly enriched in the innermost layers of the biofilm (thickness of 10 μm), which were in contact with the electroconductive particles of bed, whereas the rest of bacteria were located in the outermost layers. To our knowledge, this is the first time that such a clear microbial stratification has been observed on an anode-respiring biofilm. Our results revealed the relevant role of Geobacter in switching between the electrode and other microbial communities performing metabolic reactions in the outermost environment of the biofilm.

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.

Electroactive Biofilms for Sensing: Reflections and Perspectives

Microbial electrochemistry has from the onset been recognized for its sensing potential due to the microbial ability to enhance signals through metabolic cascades, its relative selectivity toward substrates, and the higher stability conferred by the microbial ability to self-replicate. The greatest challenge has been to achieve stable and efficient transduction between a microorganism and an electrode surface. Over the past decades, a new kind of microbial architecture has been observed to spontaneously develop on polarized electrodes: the electroactive biofilm (EAB). The EAB conducts electrons over long distances and performs quasi-reversible electron transfer on conventional electrode surfaces. It also possesses self-regenerative properties. In only a few years, EABs have inspired considerable research interest for use as biosensors for environmental or bioprocess monitoring. Multiple challenges still need to be overcome before implementation at larger scale of this new kind of biosensors can be realized. This perspective first introduces the specific characteristics of the EAB with respect to other electrochemical biosensors. It summarizes the sensing applications currently proposed for EABs, stresses their limitations, and suggests strategies toward potential solutions. Conceptual prospects to engineer EABs for sensing purposes are also discussed.

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.

Thermally activated long range electron transport in living biofilms

Microbial biofilms grown utilizing electrodes as metabolic electron acceptors or donors are a new class of biomaterials with distinct electronic properties. Here we report that electron transport through living electrode-grown Geobacter sulfurreducens biofilms is a thermally activated process with incoherent redox conductivity. The temperature dependency of this process is consistent with electron-transfer reactions involving hemes of c-type cytochromes known to play important roles in G. sulfurreducens extracellular electron transport. While incoherent redox conductivity is ubiquitous in biological systems at molecular-length scales, it is unprecedented over distances it appears to occur through living G. sulfurreducens biofilms, which can exceed 100 microns in thickness.