Long-distance electron transfer by G. sulfurreducens biofilms results in accumulation of reduced c-type cytochromes

Critical transport rates that limit the performance of microbial electrochemistry technologies

Microbial electrochemistry technologies (METs) take advantage of the connection of microorganisms with electrodes. In the classic case of a microbial anode, the maximization of current density produced is often the goal. But, current production is dependent on many transport processes occurring, which can be rate-limiting. These include the fluxes of electron donor and acceptor, the ionic flux, the acidity and alkalinity fluxes at anode and cathode respectively, the electron transport flux at the biofilm, and the reactant/product crossover flux. Associated with these fluxes are inherent concentration gradients that can affect performance. This critical review provides an analysis on how these transport processes have hindered the development of METs, and how MET designs have evolved as more knowledge of these transport limitations is gained. Finally, suggestions are provided on how to design MET systems taking into consideration critical transport processes that are intimately linked to the current produced.

Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires

Electricity generation by Geobacter sulfurreducens biofilms grown on electrodes involves matrix-associated electron carriers, such as c-type cytochromes. Yet, the contribution of the biofilm's conductive pili remains uncertain, largely because pili-defective mutants also have cytochrome defects. Here we report that a pili-deficient mutant carrying an inactivating mutation in the pilus assembly motor PilB has no measurable defects in cytochrome expression, yet forms anode biofilms with reduced electroactivity and is unable to grow beyond a threshold distance (∼10 μm) from the underlying electrode. The defects are similar to those of a Tyr3 mutant, which produces poorly conductive pili. The results support a model in which the conductive pili permeate the biofilms to wire the cells to the conductive biofilm matrix and the underlying electrode, operating coordinately with cytochromes until the biofilm reaches a threshold thickness that limits the efficiency of the cytochrome pathway but not the functioning of the conductive pili network.

The roles played by cytochromes and conductive filamentous appendages (pili) in the electrical conductivity of Geobacter bacterial biofilms are controversial. Here, Steidl et al. present evidence that both mechanisms cooperate in thin biofilms, while pili are important for conductivity across thicker biofilms.

Redox Conductivity of Current-Producing Mixed Species Biofilms

While most biological materials are insulating in nature, efficient extracellular electron transfer is a critical property of biofilms associated with microbial electrochemical systems and several microorganisms are capable of establishing conductive aggregates and biofilms. Though construction of these conductive microbial networks is an intriguing and important phenomenon in both natural and engineered systems, few studies have been published related to conductive biofilms/aggregates and their conduction mechanisms, especially in mixed-species environments. In the present study, current-producing mixed species biofilms exhibited high conductivity across non-conductive gaps. Biofilm growth observed on the inactive electrode contributed to overall power outputs, suggesting that an electrical connection was established throughout the biofilm assembly. Electrochemical gating analysis of the biofilms over a range of potentials (-600-200 mV, vs. Ag/AgCl) resulted in a peak-manner response with maximum conductance of 3437 ± 271 μS at a gate potential of -360 mV. Following removal of the electron donor (acetate), a 96.6% decrease in peak conductivity was observed. Differential responses observed in the absence of an electron donor and over varying potentials suggest a redox driven conductivity mechanism in mixed-species biofilms. These results demonstrated significant differences in biofilm development and conductivity compared to previous studies using pure cultures.

A miniaturized microbial fuel cell with three-dimensional graphene macroporous scaffold anode demonstrating a record power density of over 10,000 W m(-3)

A microbial fuel cell (MFC) is a bio-inspired renewable energy converter which directly converts biomass into electricity. This is accomplished via the unique extracellular electron transfer (EET) of a specific species of microbe called the exoelectrogen. Many studies have attempted to improve the power density of MFCs, yet the reported power density is still nearly two orders of magnitude lower than other power sources/converters. Such a low performance can primarily be attributed to two bottlenecks: (i) ineffective electron transfer from microbes located far from the anode and (ii) an insufficient buffer supply to the biofilm. This work takes a novel approach to mitigate these two bottlenecks by integrating a three-dimensional (3D) macroporous graphene scaffold anode in a miniaturized MFC. This implementation has delivered the highest power density reported to date in all MFCs of over 10,000 W m(-3). The miniaturized configuration offers a high surface area to volume ratio and improved mass transfer of biomass and buffers. The 3D graphene macroporous scaffold warrants investigation due to its high specific surface area, high porosity, and excellent conductivity and biocompatibility which facilitates EET and alleviates acidification in the biofilm. Consequently, the 3D scaffold houses an extremely thick and dense biofilm from the Geobacter-enriched culture, delivering an areal/volumetric current density of 15.51 A m(-2)/31,040 A m(-3) and a power density of 5.61 W m(-2)/11,220 W m(-3), a 3.3 fold increase when compared to its planar two-dimensional (2D) control counterparts.

Analysis of electron transfer dynamics in mixed community electroactive microbial biofilms

Electrochemically active microbial biofilms are capable to produce electric current when grown onto electrodes. This work investigates the dynamics of electron transfer inside the biofilm as well as at the biofilm/electrode interface.

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.

Enhanced Shewanella biofilm promotes bioelectricity generation

Electroactive biofilms play essential roles in determining the power output of microbial fuel cells (MFCs). To engineer the electroactive biofilm formation of Shewanella oneidensis MR-1, a model exoelectrogen, we herein heterologously overexpressed a c-di-GMP biosynthesis gene ydeH in S. oneidensis MR-1, constructing a mutant strain in which the expression of ydeH is under the control of IPTG-inducible promoter, and a strain in which ydeH is under the control of a constitutive promoter. Such engineered Shewanella strains had significantly enhanced biofilm formation and bioelectricity generation. The MFCs inoculated with these engineered strains accomplished a maximum power density of 167.6 ± 3.6 mW/m(2) , which was ∼ 2.8 times of that achieved by the wild-type MR-1 (61.0 ± 1.9 mW/m(2) ). In addition, the engineered strains in the bioelectrochemical system at poised potential of 0.2 V vs. saturated calomel electrode (SCE) generated a stable current density of 1100 mA/m(2) , ∼ 3.4 times of that by wild-type MR-1 (320 mA/m(2) ).

Electron acceptor-dependent respiratory and physiological stratifications in biofilms

Bacterial respiration is an essential driving force in biogeochemical cycling and bioremediation processes. Electron acceptors respired by bacteria often have solid and soluble forms that typically coexist in the environment. It is important to understand how sessile bacteria attached to solid electron acceptors respond to ambient soluble alternative electron acceptors. Microbial fuel cells (MFCs) provide a useful tool to investigate this interaction. In MFCs with Shewanella decolorationis, azo dye was used as an alternative electron acceptor in the anode chamber. Different respiration patterns were observed for biofilm and planktonic cells, with planktonic cells preferred to respire with azo dye while biofilm cells respired with both the anode and azo dye. The additional azo respiration dissipated the proton accumulation within the anode biofilm. There was a large redox potential gap between the biofilms and anode surface. Changing cathodic conditions caused immediate effects on the anode potential but not on the biofilm potential. Biofilm viability showed an inverse and respiration-dependent profile when respiring with only the anode or azo dye and was enhanced when respiring with both simultaneously. These results provide new insights into the bacterial respiration strategies in environments containing multiple electron acceptors and support an electron-hopping mechanism within Shewanella electrode-respiring biofilms.

A Geobacter strain isolated from rice paddy soil with higher bioelectricity generation capability in comparison to Geobacter sulfurreducens PCA

A novel electrochemically active strain D-8 was successfully isolated from rice paddy soil. The strain D-8 can use more carbon sources and show higher current density thanG. sulfurreducensPCA. It might be a promising bioanodic organism in MFCs.

Towards the understanding of the effect of oxygen on the electrocatalytic activity of microbial biofilms: a spectroelectrochemical study

Metal-respiring bacteria oxidize an organic substrate and transfer the liberated electrons to the electrode. Molecular oxygen interrupts the current flow by cutting the electrical circuit wiring the cell metabolism to the electrode.

Methods for understanding microbial community structures and functions in microbial fuel cells: a review

Microbial fuel cells (MFCs) employ microorganisms to recover electric energy from organic matter. However, fundamental knowledge of electrochemically active bacteria is still required to maximize MFCs power output for practical applications. This review presents microbiological and electrochemical techniques to help researchers choose the appropriate methods for the MFCs study. Pre-genomic and genomic techniques such as 16S rRNA based phylogeny and metagenomics have provided important information in the structure and genetic potential of electrode-colonizing microbial communities. Post-genomic techniques such as metatranscriptomics allow functional characterizations of electrode biofilm communities by quantifying gene expression levels. Isotope-assisted phylogenetic analysis can further link taxonomic information to microbial metabolisms. A combination of electrochemical, phylogenetic, metagenomic, and post-metagenomic techniques offers opportunities to a better understanding of the extracellular electron transfer process, which in turn can lead to process optimization for power output.

Influence of acidic pH on hydrogen and acetate production by an electrosynthetic microbiome

Production of hydrogen and organic compounds by an electrosynthetic microbiome using electrodes and carbon dioxide as sole electron donor and carbon source, respectively, was examined after exposure to acidic pH (∼5). Hydrogen production by biocathodes poised at −600 mV vs. SHE increased>100-fold and acetate production ceased at acidic pH, but ∼5–15 mM (catholyte volume)/day acetate and>1,000 mM/day hydrogen were attained at pH ∼6.5 following repeated exposure to acidic pH. Cyclic voltammetry revealed a 250 mV decrease in hydrogen overpotential and a maximum current density of 12.2 mA/cm² at −765 mV (0.065 mA/cm² sterile control at −800 mV) by the Acetobacterium-dominated community. Supplying −800 mV to the microbiome after repeated exposure to acidic pH resulted in up to 2.6 kg/m³/day hydrogen (≈2.6 gallons gasoline equivalent), 0.7 kg/m³/day formate, and 3.1 kg/m³/day acetate ( = 4.7 kg CO₂ captured).

Abundance of the multiheme c-type cytochrome OmcB increases in outer biofilm layers of electrode-grown Geobacter sulfurreducens

When Geobacter sulfurreducens utilizes an electrode as its electron acceptor, cells embed themselves in a conductive biofilm tens of microns thick. While environmental conditions such as pH or redox potential have been shown to change close to the electrode, less is known about the response of G. sulfurreducens to growth in this biofilm environment. To investigate whether respiratory protein abundance varies with distance from the electrode, antibodies against an outer membrane multiheme cytochrome (OmcB) and cytoplasmic acetate kinase (AckA) were used to determine protein localization in slices spanning ∼25 µm-thick G. sulfurreducens biofilms growing on polished electrodes poised at +0.24 V (vs. Standard Hydrogen Electrode). Slices were immunogold labeled post-fixing, imaged via transmission electron microscopy, and digitally reassembled to create continuous images allowing subcellular location and abundance per cell to be quantified across an entire biofilm. OmcB was predominantly localized on cell membranes, and 3.6-fold more OmcB was detected on cells 10–20 µm distant from the electrode surface compared to inner layers (0–10 µm). In contrast, acetate kinase remained constant throughout the biofilm, and was always associated with the cell interior. This method for detecting proteins in intact conductive biofilms supports a model where the utilization of redox proteins changes with depth.

Real-time measurements of the redox states of c-type cytochromes in electroactive biofilms: a confocal resonance Raman Microscopy study

Confocal Resonance Raman Microscopy (CRRM) was used to probe variations of redox state of c-type cytochromes embedded in living mixed-culture electroactive biofilms exposed to different electrode polarizations, under potentiostatic and potentiodynamic conditions. In the absence of the metabolic substrate acetate, the redox state of cytochromes followed the application of reducing and oxidizing electrode potentials. Real-time monitoring of the redox state of cytochromes during cyclic voltammetry (CV) in a potential window where cytochromes reduction occurs, evidenced a measurable time delay between the oxidation of redox cofactors probed by CV at the electrode interface, and oxidation of distal cytochromes probed by CRRM. This delay was used to tentatively estimate the diffusivity of electrons through the biofilm. In the presence of acetate, the resonance Raman spectra of young (10 days, j = 208±49 µA cm⁻²) and mature (57 days, j = 267±73 µA cm⁻²) biofilms show that cytochromes remained oxidized homogeneously even at layers as far as 70 µm from the electrode, implying the existence of slow metabolic kinetics that do not result in the formation of a redox gradient inside the biofilm during anode respiration. However, old biofilms (80 days, j = 190±37 µA cm⁻²) with thickness above 100 µm were characterized by reduced catalytic activity compared to the previous developing stages. The cytochromes in these biofilm were mainly in the reduced redox state, showing that only aged mixed-culture biofilms accumulate electrons during anode respiration. These results differ substantially from recent observations in pure Geobacter sulfurreducens electroactive biofilms, in which accumulation of reduced cytochromes is already observed in thinner biofilms, thus suggesting different bottlenecks in current production for mixed-culture and G. sulfurreducens biofilms.

Physiological stratification in electricity-producing biofilms of Geobacter sulfurreducens

The elucidation of mechanisms and limitations in electrode respiration by electroactive biofilms is significant for the development of rapidly emerging clean energy production and wastewater treatment technologies. In Geobacter sulfurreducens biofilms, the controlling steps in current production are thought to be the metabolic activity of cells, but still remain to be determined. By quantifying the DNA, RNA, and protein content during the long-term growth of biofilms on polarized graphite electrodes, we show in this work that current production becomes independent of DNA accumulation immediately after a maximal current is achieved. Indeed, the mean respiratory rate of biofilms rapidly decreases after this point, which indicates the progressive accumulation of cells that do not contribute to current production or contribute to a negligible extent. These results support the occurrence of physiological stratification within biofilms as a consequence of respiratory limitations imposed by limited biofilm conductivity.

Light-driven microbial dissimilatory electron transfer to hematite

Excitation of hematite with visible light promotes electron transfer from the dissimilatory metal-reducing speciesGeobacter sulfurreducensto the hematite surface.

Coupling dark metabolism to electricity generation using photosynthetic cocultures

We investigated the role of green sulfur bacteria inlight-responsive electricity generation in microbial electrochemical cells (MXCs). We operated MXCs containing either monocultures or defined cocultures of previously enriched phototrophic Chlorobium and anode-respiring Geobacter under anaerobic conditions in the absence of electron donor. Monoculture control MXCs containing Geobacter or Chlorobium neither responded to light nor produced current, respectively. Instead, light-responsive current generation occurred only in coculture MXCs. Current increased above background levels only in the dark and declined slowly over 96 h. This pattern suggested that Chlorobium exhausted intracellular glycogen reserves via dark fermentation to supply an electron donor, presumably acetate, to Geobacter. With medium containing sulfide as the sole photosynthetic electron donor, current generation had a similar and reproducible negative light response. To investigate whether this metabolic interaction also occurred without an electrode, we performed coculture experiments in batch serum bottles. In this setup, sulfide served as the sole electron donor, whose oxidation by Chlorobium was required to provide S(0) as the electron acceptor to Geobacter. Copies of Geobacter 16S rDNA increased approximately 14-fold in batch bottle cocultures containing sulfide compared to those lacking sulfide, and did not decline after termination of sulfide feeding. These results suggest that products of both photosynthesis and dark fermentation by Chlorobium were sufficient both to yield an electrochemical response by Geobacter biofilms, and to promote Geobacter growthin batch cocultures. Our work expands upon the fusion of MXCs with coculture techniques and reinforces the utility of microbial electrochemistry for sensitive, real-time monitoring of microbial interactions in which a metabolic intermediate can be converted to electrical current.

Generation of high current densities by pure cultures of anode-respiring Geoalkalibacter spp. under alkaline and saline conditions in microbial electrochemical cells

Anode-respiring bacteria (ARB) generate electric current in microbial electrochemical cells (MXCs) by channeling electrons from the oxidation of organic substrates to an electrode. Production of high current densities by monocultures in MXCs has resulted almost exclusively from the activity of Geobacter sulfurreducens, a neutrophilic freshwater Fe(III)-reducing bacterium and the highest-current-producing member documented for the Geobacteraceae family of the Deltaproteobacteria. Here we report high current densities generated by haloalkaliphilic Geoalkalibacter spp., thus broadening the capability for high anode respiration rates by including other genera within the Geobacteraceae. In this study, acetate-fed pure cultures of two related Geoalkalibacter spp. produced current densities of 5.0 to 8.3 and 2.4 to 3.3 A m⁻² under alkaline (pH 9.3) and saline (1.7% NaCl) conditions, respectively. Chronoamperometric studies of halophilic Glk. subterraneus DSM 23483 and alkaliphilic Glk. ferrihydriticus DSM 17813 suggested that cells performed long-range electron transfer through electrode-attached biofilms and not through soluble electron shuttles. Glk. ferrihydriticus also oxidized ethanol directly to produce current, with maximum current densities of 5.7 to 7.1 A m⁻² and coulombic efficiencies of 84 to 95%. Cyclic voltammetry (CV) elicited a sigmoidal response with characteristic onset, midpoint, and saturation potentials, while CV performed in the absence of an electron donor suggested the involvement of redox molecules in the biofilm that were limited by diffusion. These results matched those previously reported for actively respiring Gb. sulfurreducens biofilms producing similar current densities (~5 to 9 A m⁻²).

This study establishes the highest current densities ever achieved by pure cultures of anode-respiring bacteria (ARB) under alkaline and saline conditions in microbial electrochemical cells (MXCs) and provides the first electrochemical characterization of the genus Geoalkalibacter. Production of high current densities among the Geobacteraceae is no longer exclusive to Geobacter sulfurreducens, suggesting greater versatility for this family in fundamental and applied microbial electrochemical cell (MXC) research than previously considered. Additionally, this work raises the possibility that different members of the Geobacteraceae have conserved molecular mechanisms governing respiratory extracellular electron transfer to electrodes. Thus, the capacity for high current generation may exist in other uncultivated members of this family. Advancement of MXC technology for practical uses must rely on an expanded suite of ARB capable of using different electron donors and producing high current densities under various conditions. Geoalkalibacter spp. can potentially broaden the practical capabilities of MXCs to include energy generation and waste treatment under expanded ranges of salinity and pH.

Production of fuels and chemicals from waste by microbiomes

The demand for chemicals and fuels will continue to grow simultaneously with the costly requirement to treat solid waste, wastewater, and regarding climate change, carbon dioxide. A dual benefit is at hand if waste could be converted to valuable chemicals. The application of stable chemical producing microbiomes adapted to these waste streams may turn this challenge into an opportunity.

Limitations for current production in Geobacter sulfurreducens biofilms

Devices that exploit electricity produced by electroactive bacteria such as Geobacter sulfurreducens have not yet been demonstrated beyond the laboratory scale. The current densities are far from the maximum that the bacteria can produce because fundamental properties such as the mechanism of extracellular electron transport and factors limiting cell respiration remain unclear. In this work, a strategy for the investigation of electroactive biofilms is presented. Numerical modeling of the response of G. sulfurreducens biofilms cultured on a rotating disk electrode has allowed for the discrimination of different limiting steps in the process of current production within a biofilm. The model outputs reveal that extracellular electron transport limits the respiration rate of the cells furthest from the electrode to the extent that cell division is not possible. The mathematical model also demonstrates that recent findings such as the existence of a redox gradient in actively respiring biofilms can be explained by an electron hopping mechanism but not when considering metallic-like conductivities.

Long-range electron transport in Geobacter sulfurreducens biofilms is redox gradient-driven

Geobacter spp. can acquire energy by coupling intracellular oxidation of organic matter with extracellular electron transfer to an anode (an electrode poised at a metabolically oxidizing potential), forming a biofilm extending many cell lengths away from the anode surface. It has been proposed that long-range electron transport in such biofilms occurs through a network of bound redox cofactors, thought to involve extracellular matrix c-type cytochromes, as occurs for polymers containing discrete redox moieties. Here, we report measurements of electron transport in actively respiring Geobacter sulfurreducens wild type biofilms using interdigitated microelectrode arrays. Measurements when one electrode is used as an anode and the other electrode is used to monitor redox status of the biofilm 15 μm away indicate the presence of an intrabiofilm redox gradient, in which the concentration of electrons residing within the proposed redox cofactor network is higher farther from the anode surface. The magnitude of the redox gradient seems to correlate with current, which is consistent with electron transport from cells in the biofilm to the anode, where electrons effectively diffuse from areas of high to low concentration, hopping between redox cofactors. Comparison with gate measurements, when one electrode is used as an electron source and the other electrode is used as an electron drain, suggests that there are multiple types of redox cofactors in Geobacter biofilms spanning a range in oxidation potential that can engage in electron transport. The majority of these redox cofactors, however, seem to have oxidation potentials too negative to be involved in electron transport when acetate is the electron source.