On the relationship between long-distance and heterogeneous electron transfer in electrode-grown Geobacter sulfurreducens biofilms

Enhanced Microbial Protein Production from CO2 and Air by a MoS2 Catalyzed Bioelectrochemical System

Carbon dioxide can be relatively easily reduced to organic matter in a bioelectrochemical system (BES). However, due to insufficient reduction force from in-situ hydrogen evolution, it is difficult for nitrogen reduction. In this study, MoS2 was firstly used as an electrocatalyst for the simultaneous reduction of CO2 and N2 to produce microbial protein (MP) in a BES. Cell dry weight (CDW) could reach 0.81±0.04 g/L after 14 d operation at -0.7 V (vs. RHE), which was 108±3 % higher than that from non-catalyst control group (0.39±0.01 g/L). The produced protein had a better amino acid profile in the BES than that in a direct hydrogen system (DHS), particularly for proline (Pro). Besides, MoS2 promoted the growth of bacterial cell on an electrode and improved the biofilm extracellular electron transfer (EET) by microscopic observation and electrochemical characterization of MoS2 biocathode. The composition of the microbial community and the relative abundance of functional enzymes revealed that MoS2 as an electrocatalyst was beneficial for enriching Xanthobacter and enhancing CO2 and N2 reduction by electrical energy. These results demonstrated that an efficient strategy to improve MP production of BES is to use MoS2 as an electrocatalyst to shift amino acid profile and microbial community.

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.

Performance of a combined electrotrophic and electrogenic biofilm operated under long-term, continuous cycling

OBJECTIVES

Evaluate electrochemically active biofilms as high energy density rechargeable microbial batteries toward providing persistent power in applications where traditional battery technology is limiting (, remote monitoring applications).

RESULTS

Here we demonstrated that an electrochemically active biofilm was able to store and release electrical charge for alternating charge/discharge cycles of up to 24 h periodicity (50% duty cycle) with no significant decrease in average current density (0.16 ± 0.04 A/m2) for over 600 days. However, operation at 24 h periodicity for > 50 days resulted in a sharp decrease in the current to nearly zero. This current crash was recoverable by decreasing the periodicity. Overall, the coulombic efficiency remained near unity within experimental error (102 ± 3%) for all of the tested cycling periods. Electrochemical characterization here suggests that electron transfer occurs through multiple routes, likely a mixture of direct and mediated mechanisms.

CONCLUSIONS

These results indicate that bidirectional electrogenic/electrotrophic biofilms are capable of efficient charge storage/release over a wide range of cycling frequency and may eventually enable development of sustainable, high energy density rechargeable batteries.

Improving electroautotrophic ammonium production from nitrogen gas by simultaneous carbon dioxide fixation in a dual-chamber microbial electrolysis cell

Microbial electrosynthesis is a promising technology for high-value added products generation from organic and inorganic waste. In this work, autotrophic dual-chamber microbial electrolysis cells (MECs) were set up for N2 fixation at -0.9 V vs Ag/AgCl (sat. KCl) cathodic potential under ambient conditions. Higher NH4+ production yield (average value of 0.35 µmol h-1 cm-2, normalized to cathode surface area) and higher faradaic efficiency (FE, 20.25%) were obtained with intermittent addition of N2 and CO2, while the yield and FE were only 0.018 µmol h-1 cm-2 and 4.21% in the absence of CO2. Furthermore, cyclic voltammograms (CV) explained the bioelectrochemical behavior of N2 reduction was coupled with CO2 reduction in the autotrophic MECs. Microbial community analysis and functional prediction in the cathodic chamber revealed that Xanthobacter and Hydrogenophaga played as producers for N2 and CO2 fixation and Pannonibacter acting as a decomposer for converting organic nitrogen to ammonium. This work not only provided an optional bioelectrocatalytic method for N2 fixation with negative CO2-emissions but also revealed the mechanism of simultaneous fixation of N2 and CO2 via Calvin cycle in autotrophic MECs.

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.

Evidence of a Streamlined Extracellular Electron Transfer Pathway from Biofilm Structure, Metabolic Stratification, and Long-Range Electron Transfer Parameters

A strain of Geobacter sulfurreducens, an organism capable of respiring solid extracellular substrates, lacking four of five outer membrane cytochrome complexes (extABCD⁺ strain) grows faster and produces greater current density than the wild type grown under identical conditions. To understand cellular and biofilm modifications in the extABCD⁺ strain responsible for this increased performance, biofilms grown using electrodes as terminal electron acceptors were sectioned and imaged using electron microscopy to determine changes in thickness and cell density, while parallel biofilms incubated in the presence of nitrogen and carbon isotopes were analyzed using NanoSIMS (nanoscale secondary ion mass spectrometry) to quantify and localize anabolic activity. Long-distance electron transfer parameters were measured for wild-type and extABCD⁺ biofilms spanning 5-μm gaps. Our results reveal that extABCD⁺ biofilms achieved higher current densities through the additive effects of denser cell packing close to the electrode (based on electron microscopy), combined with higher metabolic rates per cell compared to the wild type (based on increased rates of ¹⁵N incorporation). We also observed an increased rate of electron transfer through extABCD⁺ versus wild-type biofilms, suggesting that denser biofilms resulting from the deletion of unnecessary multiheme cytochromes streamline electron transfer to electrodes. The combination of imaging, physiological, and electrochemical data confirms that engineered electrogenic bacteria are capable of producing more current per cell and, in combination with higher biofilm density and electron diffusion rates, can produce a higher final current density than the wild type.

IMPORTANCE Current-producing biofilms in microbial electrochemical systems could potentially sustain technologies ranging from wastewater treatment to bioproduction of electricity if the maximum current produced could be increased and current production start-up times after inoculation could be reduced. Enhancing the current output of microbial electrochemical systems has been mostly approached by engineering physical components of reactors and electrodes. Here, we show that biofilms formed by a Geobacter sulfurreducens strain producing ∼1.4× higher current than the wild type results from a combination of denser cell packing and higher anabolic activity, enabled by an increased rate of electron diffusion through the biofilms. Our results confirm that it is possible to engineer electrode-specific G. sulfurreducens strains with both faster growth on electrodes and streamlined electron transfer pathways for enhanced current production.

Nanoliter scale electrochemistry of natural and engineered electroactive bacteria

Bacterial extracellular electron transfer (EET) is envisioned for use in applied biotechnologies, necessitating electrochemical characterization of natural and engineered electroactive biofilms under conditions similar to the target application, including small-scale biosensing or biosynthesis platforms, which is often distinct from standard 100 mL-scale stirred-batch bioelectrochemical test platforms used in the laboratory. Here, we adapted an eight chamber, nanoliter volume (500 nL) electrochemical flow cell to grow biofilms of both natural (Biocathode MCL community, Marinobacter atlanticus, and Shewanella oneidensis MR1) or genetically modified (S. oneidensis ΔMtr and S. oneidensis ΔMtr + pLB2) electroactive bacteria on electrodes held at a constant potential. Maximum current density achieved by unmodified strains was similar between the nano- and milliliter-scale reactors. However, S. oneidensis biofilms engineered to activate EET upon exposure to 2,4-diacetylphloroglucinol (DAPG) produced current at wild-type levels in the stirred-batch reactor, but not in the nanoliter flow cell. We hypothesize this was due to differences in mass transport of DAPG, naturally-produced soluble redox mediators, and oxygen between the two reactor types. Results presented here demonstrate, for the first time, nanoliter scale chronoamperometry and cyclic voltammetry of a range of electroactive bacteria in a three-electrode reactor system towards development of miniaturized, and potentially high throughput, bioelectrochemical platforms.

How does electron transfer occur in microbial fuel cells?

Microbial fuel cells (MFCs) have emerged as a promising technology for sustainable wastewater treatment coupled with electricity generation. A MFC is a device that uses microbes as catalysts to convert chemical energy present in biomass into electrical energy. Among the various mechanisms that drive the operation of a MFC, extracellular electron transfer (EET) to the anode is one of the most important. Exoelectrogenic bacteria can natively transfer electrons to a conducting surface like the anode. The mechanisms employed for electron transfer can either be direct transfer via conductive pili or nanowires, or mediated transfer that involves either naturally secreted redox mediators like flavins and pyocyanins or artificially added mediators like methylene blue and neutral red. EET is a mechanism wherein microorganisms extract energy for growth and maintenance from their surroundings and transfer the resulting electrons to the anode to generate current. The efficiency of these electron transfer mechanisms is dependent not only on the redox potentials of the species involved, but also on microbial oxidative metabolism that liberates electrons. Attempts at understanding the electron transfer mechanisms will boost efforts in giving rise to practical applications. This article covers the various electron transfer mechanisms involved between microbes and electrodes in microbial fuel cells and their applications.

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.

Application of electrochemical surface plasmon resonance (ESPR) to the study of electroactive microbial biofilms

Electrochemical surface plasmon resonance (ESPR) monitors faradaic processes optically by the change in refractive index that occurs with a change in redox state at the electrode surface. Here we apply ESPR to investigate the anode-grown Geobacter sulfurreducens biofilm (GSB), a model system used to study electroactive microbial biofilms (EABFs) which perform electrochemical reactions using electrodes as metabolic electron acceptors or donors. A substantial body of evidence indicates that electron transfer reactions among hemes of c-type cytochromes (c-Cyt) play major roles in the extracellular electron transfer (EET) pathways that connect intracellular metabolic processes of cells in an EABF to the electrode surface. The results reported here reveal that when the potential of the electrode is changed from relatively oxidizing (0.40 V vs. SHE) to reducing (-0.55 V vs. SHE) and then back to oxidizing, 70% of c-Cyt residing closest to the biofilm/electrode (within hundreds of nm from the electrode surface) appear to remain trapped in the reduced state, requiring as long as 12 hours to be re-oxidized. c-Cyt storing electrons cannot contribute to EET, yet turnover current resulting from cellular oxidation of acetate coupled with EET to the electrode surface is unaffected. This suggests that a relatively small fraction of c-Cyt residing closest to the biofilm/electrode interface is involved in EET while the majority store electrons. The results also reveal that biomass density at the biofilm/electrode interface increases rapidly during lag phase, reaching its maximum value at the onset of exponential biofilm growth when turnover current begins to rapidly increase.