Influence of Anode Potentials on Current Generation and Extracellular Electron Transfer Paths of Geobacter Species

Electrochemical performance of Paenibacillus profundus YoMME encapsulated in alginate polymer

Gram-positive bacterium Paenibacillus profundus YoMME, entrapped in an alginate polymer onto graphite paper, preserves its extracellular electron transfer capabilities. A current density of up to 30 mA m^-2 was generated at an applied potential of -200 mV (vs. SHE). Fivefold higher initial current density values were recorded at applied potentials of +220 mV and +600 mV. The electrochemical behavior of the encapsulated bioelectrodes has been also explored by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy and evaluated by the parameters of the best-fitted equivalent electric circuit model. Over time the bacteria grow and divide within the alginate matrix, which affects considerably the capacitance and the charge transfer resistance of the coating. The impedance spectra follow the dynamics of the bacterial culture development within the alginate polymer and are useful for the prediction of the bioelectrode performance. A current density of 150 mA m^-2 was achieved when the alginate was functionalized by a mixture of the redox dyes thiazolyl blue (MTT) formazan and phenazine methosulfate (PMS). It is supposed that the added artificial mediators facilitate the electron transfer from the bacteria to the electrode surface by forming conductive cascade conduits through the alginate matrix.

An electrogenetic toggle switch model

Synthetic biology uses molecular biology to implement genetic circuits that perform computations. These circuits can process inputs and deliver outputs according to predefined rules that are encoded, often entirely, into genetic parts. However, the field has recently begun to focus on using mechanisms beyond the realm of genetic parts for engineering biological circuits. We analyse the use of electrogenic processes for circuit design and present a model for a merged genetic and electrogenetic toggle switch operating in a biofilm attached to an electrode. Computational simulations explore conditions under which bistability emerges in order to identify the circuit design principles for best switch performance. The results provide a basis for the rational design and implementation of hybrid devices that can be measured and controlled both genetically and electronically.

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.

Pulse-opencircuit voltammetry: A novel method characterizes bioanode performance from microbe-electrode interfacial processes

Bioanode is a key component of bioelectrochemical systems, but the methods characterizing its resistance distribution are lacked. We propose a novel pulse-opencircuit voltammetry (POV) based on the analytical principle clarified from the electron flow pathways of microbe-electrode interfacial processes (MEIPs). A dual-cathode cell is designed to provide an experimental platform for ensuring precise data acquisition of bioanodes. This POV method enables to measure steady state polarization curves and ohmic potential loss curves by integrating potentiostatic discharge and current interruption techniques. They determines reaction resistance (R_B,act) and ohmic resistance (R_B,ohm) of biofilm with the assistance of impedance spectroscopy measuring material resistance. The results of various bioanodes demonstrate that R_B,act is the principal limiting factor and its value relies on catabolism state. Whilst R_B,ohm is relevant to extracellular electron transfer behaviors. They are two useful indicators of the dynamic evaluation of biofilm. We anticipate that this method together with the cell platform is accessible to users and has wide applications in bioanode construction and electroactive bacteria investigation.

Theoretical characterisation of electron tunnelling from granular activated carbon to electron accepting organisms in direct interspecies electron transfer

Direct interspecies electron transfer (DIET) has been identified as an efficient metabolism between symbiotically interacting organisms. One method of DIET uses conductive materials (e.g., granular activated carbon (GAC)) as a medium to shuttle electrons from electron donating organisms (eg., Geobacter metallireducens) to electron accepting organisms (e.g., Geobacter sulfurreducens and Methanosarcina barkeri). Conductive materials such as GAC, become negatively charged in DIET processes due to reduction by electron donating organisms. This high excess electron density in GAC leads to quantum tunnelling of electrons being a significant electron transfer mechanism for DIET. Thus, a theoretical model obeying the Wentzel–Kramers–Brillouin (WKB) approximation and Fermi–Dirac statistics was developed and simulated. In the model, the electron tunnelling transfer barrier was described by an effective rectangular barrier. The result of our 1D tunnelling simulations indicates that within 29.4 nm of the GAC, tunnelling can sufficiently supply electrons from GAC to G. sulfurreducens and M. barkeri. The phenomenon of tunnelling may also have significance as a stimulant of chemotaxis for G. sulfurreducens and other electron accepting microbes when attempting to adsorb onto GAC. This study sheds light on quantum tunnelling’s significant potential in both bacterium and archaeon DIET-centric processes.

Suppressing peatland methane production by electron snorkeling through pyrogenic carbon in controlled laboratory incubations

Northern peatlands are experiencing more frequent and severe fire events as a result of changing climate conditions. Recent studies show that such a fire-regime change imposes a direct climate-warming impact by emitting large amounts of carbon into the atmosphere. However, the fires also convert parts of the burnt biomass into pyrogenic carbon. Here, we show a potential climate-cooling impact induced by fire-derived pyrogenic carbon in laboratory incubations. We found that the accumulation of pyrogenic carbon reduced post-fire methane production from warm (32 °C) incubated peatland soils by 13–24%. The redox-cycling, capacitive, and conductive electron transfer mechanisms in pyrogenic carbon functioned as an electron snorkel, which facilitated extracellular electron transfer and stimulated soil alternative microbial respiration to suppress methane production. Our results highlight an important, but overlooked, function of pyrogenic carbon in neutralizing forest fire emissions and call for its consideration in the global carbon budget estimation.

Warmer and drier conditions are increasing the frequency of forest fires, which in turn produce pyrogenic carbon. Here the authors show that accumulation of pyrogenic carbon can suppress post-fire methane production in northern peatlands and can effectively buffer fire-derived greenhouse gas emissions.

Resorcinol as a highly efficient aromatic electron donor in bioelectrochemical system

Bioelectrochemical systems (BESs) have been known as a promising technology for accelerating aromatic contaminants degradation and energy recovery. However, most existing studies concentrate on aromatics metabolized through a benzoyl-CoA pathway while those metabolized through other pathways are limited. In this work, resorcinol, a typical aromatic contaminant as well as a key central intermediate (other than benzoyl-CoA) involved in aromatics anaerobic biodegradation, was studied in BESs. Unlike the general impression of the relatively poor organic-to-current performance in the aromatics driven BESs, high efficiencies for resorcinol-fed BESs were observed with a current density and coulombic efficiency of up to 0.26 ± 0.05 mAcm^-2 and 74.3 ± 10.7%, respectively. The higher performance likely correlates to the readily fermentable property of resorcinol. Analysis of microbial communities in the biofilm suggests a syntrophic interaction between resorcinol-degrading bacteria (RDB) and anode-respiring bacteria (ARB) was involved in current generation. Additional tests involving the removal of accumulated acetate through fast resorcinol feeding indicates that a mechanism based on direct utilization of resorcinol for current generation may also exist. This study extends the knowledge for the fate of aromatics in BESs and indicates that aromatics entering into the resorcinol metabolic pathway can be treated efficiently with good energy recovery efficiency in BESs.

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.

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.

Physiological potential of extracellular polysaccharide in promoting Geobacter biofilm formation and extracellular electron transfer

Geobacter sulfurreducens biofilms have promising applications in renewable energy, pollutant bioremediation, and bioelectronic applications. Genetically manipulating G. sulfurreducens biofilms is an effective strategy to improve the capacity of extracellular electron transfer (EET). Extracellular polysaccharide, a sticky component surrounding microbes, plays an important role in EET. Herein, we constructed a mutant of G. sulfurreducens strain PCA overexpressing the gene GSU1501 (part of the ATP-dependent exporter of the polysaccharide biosynthesis gene operon), designated strain PCA-1501, to increase EET capacity. Experimental results showed that the overexpression of GSU1501 increased extracellular polysaccharide secretion by 25.5%, which promoted the formation of biofilm with higher thickness and viability, as well as the content of extracellular c-type cytochromes. Compared with the control strain, the mutant showed a higher capacity of Fe(III) oxide reduction and current generation (increased by 20.4% and 22.2%, respectively). Interestingly, the overexpression of GSU1501 hindered the pili formation by reducing the transcription level of pilA; a compensatory relationship between extracellular polysaccharide and pili in promoting biofilm formation deserves further investigation. This study provides a feasible method to promote the EET capacity of G. sulfurreducens biofilms, which benefit their bioelectrochemical applications.

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.

Direct Interspecies Electron Transfer Mediated by Graphene Oxide-Based Materials

Conductive materials are known to promote direct interspecies electron transfer (DIET) by electrically bridging microbial cells. Previous studies have suggested that supplementation of graphene oxide (GO) based materials, including GO, and reduced GO (rGO), to anaerobic microbial communities, can promote DIET. This promotion mechanism is thought to be involved in electron transfer via rGO or biologically formed rGO. However, concrete evidence that rGO directly promotes DIET is still lacking. Furthermore, the effects of the physicochemical properties of GO-based materials on DIET efficiency have not been elucidated. In the current work, we investigated whether chemically and biologically reduced GO compounds can promote DIET in a defined model coculture system, and also examined the effects of surface properties on DIET-promoting efficiency. Supplementation of GO to a defined DIET coculture composed of an ethanol-oxidizing electron producer Geobacter metallireducens and a methane-producing electron consumer Methanosarcina barkeri promoted methane production from ethanol. X-ray photoelectron spectroscopy revealed that GO was reduced to rGO during cultivation by G. metallireducens activity. The stoichiometry of methane production from ethanol and the isotope labeling experiments clearly showed that biologically reduced GO induced DIET-mediated syntrophic methanogenesis. We also assessed the DIET-promoting efficiency of chemically reduced GO and its derivatives, including hydrophilic amine-functionalized rGO (rGO-NH₂) and hydrophobic octadecylamine-functionalized rGO (rGO-ODA). While all tested rGO derivatives induced DIET, the rGO derivatives with higher hydrophilicity showed higher DIET-promoting efficiency. Optical microscope observation revealed that microbial cells, in particular, G. metallireducens, more quickly adhered to more hydrophilic GO-based materials. The superior ability to recruit microbial cells is a critical feature of the higher DIET-promoting efficiency of the hydrophilic materials. This study demonstrates that biologically and chemically reduced GO can promote DIET-mediated syntrophic methanogenesis. Our results also suggested that the surface hydrophilicity (i.e., affinity toward microbial cells) is one of the important determinants of the DIET-promoting efficiencies. These observations will provide useful guidance for the selection of conductive particles for the improvement of methanogenesis in anaerobic digesters.

Enhancing Anaerobic Digestion: The Effect of Carbon Conductive Materials

Anaerobic digestion is a well-known technology which has been extensively studied to improve its performance and yield biogas from substrates. The application of different types of pre-treatments has led to an increase in biogas production but also in global energy demand. However, in recent years the use of carbon conductive materials as supplement for this process has been studied resulting in an interesting way for improving the performance of anaerobic digestion without greatly affecting its energy demand. This review offers an introduction to this interesting approach and covers the different experiences performed on the use of carbon conductive materials proposing it as a feasible alternative for the production of energy from biomass, considering also the integration of anaerobic digestion and thermal valorisation.

Comparative metatranscriptomics reveals extracellular electron transfer pathways conferring microbial adaptivity to surface redox potential changes

Some microbes can capture energy through redox reactions with electron flow to solid-phase electron acceptors, such as metal-oxides or poised electrodes, via extracellular electron transfer (EET). While diverse oxide minerals, exhibiting different surface redox potentials, are widely distributed on Earth, little is known about how microbes sense and use the minerals. Here we show electrochemical, metabolic, and transcriptional responses of EET-active microbial communities established on poised electrodes to changes in the surface redox potentials (as electron acceptors) and surrounding substrates (as electron donors). Combination of genome-centric stimulus-induced metatranscriptomics and metabolic pathway investigation revealed that nine Geobacter/Pelobacter microbes performed EET activity differently according to their preferable surface potentials and substrates. While the Geobacter/Pelobacter microbes coded numerous numbers of multi-heme c-type cytochromes and conductive e-pili, wide variations in gene expression were seen in response to altering surrounding substrates and surface potentials, accelerating EET via poised electrode or limiting EET via an open circuit system. These flexible responses suggest that a wide variety of EET-active microbes utilizing diverse EET mechanisms may work together to provide such EET-active communities with an impressive ability to handle major changes in surface potential and carbon source availability.

Transcriptomic, Proteomic, and Bioelectrochemical Characterization of an Exoelectrogen Geobacter soli Grown With Different Electron Acceptors

The ability of Geobacter species to transfer electrons outside cells enables them to play an important role in biogeochemical and bioenergy processes. Our knowledge of the extracellular electron transfer (EET) process in the genus Geobacter is mainly from the study of G. sulfurreducens, and in order to fully investigate the EET mechanisms in the genus Geobacter, other Geobacter species should also be considered. This study focused on the EET of Geobacter soli GSS01, which exhibited a capability of reducing insoluble Fe(III) oxides and generating electrical current comparable with G. sulfurreducens PCA. Electrochemical characterization, including cyclic voltammetry, differential pulse voltammetry, and electrochemical in situ FTIR spectra, revealed that different redox proteins contributed to the electrochemical behaviors of G. soli and G. sulfurreducens. Based on comparative transcriptomic and proteomic analyses, OmcS was the most upregulated protein in both G. soli and G. sulfurreducens cells grown with insoluble Fe(III) oxides vs. soluble electron acceptor. However, the proteins including OmcE and PilA that were previously reported as being important for EET in G. sulfurreducens were downregulated or unchanged in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, and many proteins that were upregulated in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, such as OmcN, are not important for EET in G. sulfurreducens. We also identified 30 differentially expressed small RNAs (sRNAs) in G. soli cells grown with different acceptors. Taken together, these findings help to understand the versatile EET mechanisms that exist in the genus Geobacter and point to the possibility of sRNA in modulating EET gene expression.