Reaching full potential: bioelectrochemical systems for storing renewable energy in chemical bonds

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Extracellular electron uptake from a cathode by the lactic acid bacterium Lactiplantibacillus plantarum

A subset of microorganisms that perform respiration can endogenously utilize insoluble electron donors, such as Fe(II) or a cathode, in a process called extracellular electron transfer (EET). However, it is unknown whether similar endogenous EET can be performed by primarily fermentative species like lactic acid bacteria. We report for the first time electron uptake from a cathode by Lactiplantibacillus plantarum, a primarily fermentative bacteria found in the gut of mammals and in fermented foods. L. plantarum consumed electrons from a cathode and coupled this oxidation to the reduction of both an endogenous organic (pyruvate) and an exogenous inorganic electron acceptor (nitrate). This electron uptake from a cathode reroutes glucose fermentation toward lactate degradation and provides cells with a higher viability upon sugar exhaustion. Moreover, the associated genes and cofactors indicate that this activity is mechanistically different from that one employed by lactic acid bacteria to reduce an anode and to perform respiration. Our results expand our knowledge of the diversity of electroactive species and of the metabolic and bioenergetic strategies used by lactic acid bacteria.

Recent Advances In Microbe-Photocatalyst Hybrid Systems for Production of Bulk Chemicals: A Review

Solar-driven biocatalysis technologies can combine inorganic photocatalytic materials with biological catalysts to convert CO₂, light, and water into chemicals, offering the promise of high energy efficiency and a broader product scope than that of natural photosynthesis. Solar energy is the most abundant renewable energy source on earth, but it cannot be directly utilized by current industrial microorganisms. Therefore, the establishment of a solar-driven bio-catalysis platform, a bridge between solar energy and heterotrophic microorganisms, can dramatically increase carbon flux in biomanufacturing systems and consequently may revolutionize the biorefinery. This review first discusses the main applications of microbe-photocatalyst hybrid (MPH) systems in biorefinery processes. Then, various strategies to improve the electron transfer by microorganisms at the inorganic photocatalytic material interface are discussed, especially biohybrid systems based on autotrophic or heterotrophic bacteria and photocatalytic materials. Finally, we discuss the current challenges and offer potential solutions for the development of MPH systems.

Transition roadmap for thermophilic carbon dioxide microbial electrosynthesis: Testing with real exhaust gases and operational control for a scalable design

Human activities release more carbon dioxide (CO₂) into the atmosphere than the natural process can remove. This study attempts to address the main challenges for the thermophilic (50 °C) bioelectrochemical conversion of CO₂ into acetate. First, real gaseous emissions were tested with mixed microbial consortia, which had no substantial influence on production rates (difference of 2.5%). Subsequently, a bench-scale system (TRL 4-5) was designed and launched to control key operational variables. Fixing the current at 1.3 A m^-2, CO₂ was reduced at a rate of 2.21 kg CO₂ kg^-1 acetate, while the electricity consumption was 2.07 kWh kg^-1, the most efficient value so far. The results suggest that the operation with real effluents is feasible and the proposed design is energy efficient, but the right balance between maximising current densities without compromising the biocompatibility with catalysts will determine the transition from laboratory scale towards its implementation in the market.

Modular configurations of living biomaterials incorporating nano-based artificial mediators and synthetic biology to improve bioelectrocatalytic performance: A review

Currently, the industrial application of bioelectrochemical systems (BESs) that are incubated with natural electrochemically active microbes (EABs) is limited due to inefficient extracellular electron transfer (EET) by natural EABs. Notably, recent studies have identified several novel living biomaterials comprising highly efficient electron transfer systems allowing unparalleled proficiency of energy conversion. Introduction of these biomaterials into BESs could fundamentally increase their utilization for a wide range of applications. This review provides a comprehensive assessment of recent advancements in the design of living biomaterials that can be exploited to enhance bioelectrocatalytic performance. Further, modular configurations of abiotic and biotic components promise a powerful enhancement through integration of nano-based artificial mediators and synthetic biology. Herein, recent advancements in BESs are synthesized and assessed, including heterojunctions between conductive nanomaterials and EABs, in-situ hybrid self-assembly of EABs and nano-sized semiconductors, cytoprotection in biohybrids, synthetic biological modifications of EABs and electroactive biofilms. Since living biomaterials comprise a broad range of disciplines, such as molecular biology, electrochemistry and material sciences, full integration of technological advances applied in an interdisciplinary framework will greatly enhance/advance the utility and novelty of BESs. Overall, emerging fundamental knowledge concerning living biomaterials provides a powerful opportunity to markedly boost EET efficiency and facilitate the industrial application of BESs to meet global sustainability challenges/goals.

Bioelectrosynthesis systems

Bioelectrosynthesis (BES) systems exploit extracellular electron transport pathways to augment cellular metabolism. This strategy can be used to improve the economic viability of bio-based syntheses versus conventional methods, most notably petrochemical-based syntheses. It also has the potential to reduce the carbon footprint of biomanufacturing processes. Efficient channeling of cathode-derived electrons towards biosynthesis requires a better understanding of the biological mechanisms of electron transport as well as detailed evaluation of all aspects of process performance. More advanced solutions may deploy cell free systems that use ex situ generated reducing equivalents to improve economic performance.

Electronic control of redox reactions inside Escherichia coli using a genetic module

Microorganisms regulate the redox state of different biomolecules to precisely control biological processes. These processes can be modulated by electrochemically coupling intracellular biomolecules to an external electrode, but current approaches afford only limited control and specificity. Here we describe specific electrochemical control of the reduction of intracellular biomolecules in Escherichia coli through introduction of a heterologous electron transfer pathway. E. coli expressing cymAmtrCAB from Shewanella oneidensis MR-1 consumed electrons directly from a cathode when fumarate or nitrate, both intracellular electron acceptors, were present. The fumarate-triggered current consumption occurred only when fumarate reductase was present, indicating all the electrons passed through this enzyme. Moreover, CymAMtrCAB-expressing E. coli used current to stoichiometrically reduce nitrate. Thus, our work introduces a modular genetic tool to reduce a specific intracellular redox molecule with an electrode, opening the possibility of electronically controlling biological processes such as biosynthesis and growth in any microorganism.

Potential of Utilization of Renewable Energy Technologies in Gulf Countries

This critical review report highlights the enormous potentiality and availability of renewable energy sources in the Gulf region. The earth suffers from extreme air pollution, climate changes, and extreme problems due to the enormous usage of underground carbon resources applications materialized in industrial, transport, and domestic sectors. The countries under Gulf Cooperation Council, i.e., Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates, mainly explore those underground carbon resources for crude oil extraction and natural gas production. As a nonrenewable resource, these are bound to be exhausted in the near future. Hence, this review discusses the importance and feasibility of renewable sources in the Gulf region to persuade the scientific community to launch and explore renewable sources to obtain the maximum benefit in electric power generation. In most parts of the Gulf region, solar and wind energy sources are abundantly available. However, attempts to harness those resources are very limited. Furthermore, in this review report, innovative areas of advanced research (such as bioenergy, biomass) were proposed for the Gulf region to extract those resources at a higher magnitude to generate surplus power generation. Overall, this report clearly depicts the current scenario, current power demand, currently installed capacities, and the future strategies of power production from renewable power sources (viz., solar, wind, tidal, biomass, and bioenergy) in each and every part of the Gulf region.

Bespoke Biomolecular Wires for Transmembrane Electron Transfer: Spontaneous Assembly of a Functionalized Multiheme Electron Conduit

Shewanella oneidensis exchanges electrons between cellular metabolism and external redox partners in a process that attracts much attention for production of green electricity (microbial fuel cells) and chemicals (microbial electrosynthesis). A critical component of this pathway is the outer membrane spanning MTR complex, a biomolecular wire formed of the MtrA, MtrB, and MtrC proteins. MtrA and MtrC are decaheme cytochromes that form a chain of close-packed hemes to define an electron transfer pathway of 185 Å. MtrA is wrapped inside MtrB for solubility across the outer membrane lipid bilayer; MtrC sits outside the cell for electron exchange with external redox partners. Here, we demonstrate tight and spontaneous in vitro association of MtrAB with separately purified MtrC. The resulting complex is comparable with the MTR complex naturally assembled by Shewanella in terms of both its structure and rates of electron transfer across a lipid bilayer. Our findings reveal the potential for building bespoke electron conduits where MtrAB combines with chemically modified MtrC, in this case, labeled with a Ru-dye that enables light-triggered electron injection into the MtrC heme chain.

Microbial electrosynthesis: is it sustainable for bioproduction of acetic acid?

Microbial electrosynthesis (MES) is an innovative technology for electricity driven microbial reduction of carbon dioxide (CO2) to useful multi-carbon compounds. This study assesses the cradle-to-gate environmental burdens associated with acetic acid (AA) production via MES using graphene functionalized carbon felt cathode. The analysis shows that, though the environmental impact for the production of the functionalized cathode is substantially higher when compared to carbon felt with no modification, the improved productivity of the process helps in reducing the overall impact. It is also shown that, while energy used for extraction of AA is the key environmental hotspot, ion-exchange membrane and reactor medium (catholyte & anolyte) are other important contributors. A sensitivity analysis, describing four different scenarios, considering either continuous or fed-batch operation, is also described. Results show that even if MES productivity can be theoretically increased to match the highest space time yield reported for acetogenic bacteria in a continuous gas fermenter (148 g L-1 d-1), the environmental impact of AA produced using MES systems would still be significantly higher than that produced using a fossil-based process. Use of fed-batch operation and renewable (solar) energy sources do help in reducing the impact, however, the low production rates and overall high energy requirement makes large-scale implementation of such systems impractical. The analysis suggests a minimum threshold production rate of 4100 g m-2 d-1, that needs to be achieved, before MES could be seen as a sustainable alternative to fossil-based AA production.

Recent developments and key barriers to microbial CO2 electrobiorefinery

The electrochemical conversion of CO₂ can include renewable surplus electricity storage and CO₂ utilisation. This review focuses on the microbial CO₂ electrobiorefinery based on microbial electrosynthesis (MES) which merges electrochemical and microbial conversion to produce biofuels and higher-value chemicals. In this review, recent developments are discussed about bioelectrochemical conversion of CO₂ into biofuels and chemicals in MES via microbial CO₂-fixation and electricity utilisation reactions. In addition, this review examines technical approaches to overcome the current limitations of MES including the following: engineering of the biocathode, application of electron mediators, and reactor optimisation, among others. An in-depth discussion of strategies for the CO₂ electrobiorefinery is presented, including the integration of the biocathode with inorganic catalysts, screening of novel electroactive microorganisms, and metabolic engineering to improve target productivity from CO₂.

Cytochrome c Reductase is a Key Enzyme Involved in the Extracellular Electron Transfer Pathway towards Transition Metal Complexes in Pseudomonas Putida

Mediator-based extracellular electron transfer (EET) pathways can balance the redox metabolism of microbes. However, such electro-biosynthesis processes are constrained by the unknown underlying EET mechanisms. In this paper, Pseudomonas putida was studied to systematically investigate its EET pathway to transition metal complexes (i. e., [Fe(CN)6 ]3-/4- and [Co(bpy)3 ]3+/2+ ; bpy=2,2'-bipyridyl) under anaerobic conditions. Comparative proteomics showed the aerobic respiratory components were upregulated in a bioelectrochemical system without oxygen, suggesting their potential contribution to EET. Further tests found inhibiting cytochrome c oxidase activity by NaN3 and NADH dehydrogenase by rotenone did not significantly change the current output. However, the EET pathway was completely blocked, while cytochrome c reductase activity was inhibited by antimycin A. Although it cannot be excluded that cytochrome c and the periplasmic subunit of cytochrome c oxidase donate electrons to the transition metal complexes, these results strongly demonstrate that cytochrome c reductase is a key complex for the EET pathway.

His/Met heme ligation in the PioA outer membrane cytochrome enabling light-driven extracellular electron transfer by Rhodopseudomonas palustris TIE-1

A growing number of bacterial species are known to move electrons across their cell envelopes. Naturally this occurs in support of energy conservation and carbon-fixation. For biotechnology it allows electron exchange between bacteria and electrodes in microbial fuel cells and during microbial electrosynthesis. In this context Rhodopseudomonas palustris TIE-1 is of much interest. These bacteria respond to light by taking electrons from their external environment, including electrodes, to drive CO₂-fixation. The PioA cytochrome, that spans the bacterial outer membrane, is essential for this electron transfer and yet little is known about its structure and electron transfer properties. Here we reveal the ten c-type hemes of PioA are redox active across the window +250 to -400 mV versus Standard Hydrogen Electrode and that the hemes with most positive reduction potentials have His/Met and His/H₂O ligation. These chemical and redox properties distinguish PioA from the more widely studied family of MtrA outer membrane decaheme cytochromes with ten His/His ligated hemes. We predict a structure for PioA in which the hemes form a chain spanning the longest dimension of the protein, from Heme 1 to Heme 10. Hemes 2, 3 and 7 are identified as those most likely to have His/Met and/or His/H₂O ligation. Sequence analysis suggests His/Met ligation of Heme 2 and/or 7 is a defining feature of decaheme PioA homologs from over 30 different bacterial genera. His/Met ligation of Heme 3 appears to be less common and primarily associated with PioA homologs from purple non-sulphur bacteria belonging to the alphaproteobacteria class.

Organic Microbial Electrochemical Transistor Monitoring Extracellular Electron Transfer

Extracellular electron transfer (EET) denotes the process of microbial respiration with electron transfer to extracellular acceptors and has been exploited in a range of microbial electrochemical systems (MESs). To further understand EET and to optimize the performance of MESs, a better understanding of the dynamics at the microscale is needed. However, the real-time monitoring of EET at high spatiotemporal resolution would require sophisticated signal amplification. To amplify local EET signals, a miniaturized bioelectronic device, the so-called organic microbial electrochemical transistor (OMECT), is developed, which includes Shewanella oneidensis MR-1 integrated onto organic electrochemical transistors comprising poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) combined with poly(vinyl alcohol) (PVA). Bacteria are attached to the gate of the transistor by a chronoamperometric method and the successful attachment is confirmed by fluorescence microscopy. Monitoring EET with the OMECT configuration is achieved due to the inherent amplification of the transistor, revealing fast time-responses to lactate. The limits of detection when using microfabricated gates as charge collectors are also investigated. The work is a first step toward understanding and monitoring EET in highly confined spaces via microfabricated organic electronic devices, and it can be of importance to study exoelectrogens in microenvironments, such as those of the human microbiome.

Transport and Electrochemical Characteristics of CJMCED Homogeneous Cation Exchange Membranes in Sodium Chloride, Calcium Chloride, and Sodium Sulfate Solutions

Recently developed and produced by Hefei Chemjoy Polymer Material Co. Ltd., homogeneous CJMC-3 and CJMC-5 cation-exchange membranes (CJMCED) are characterized. The membrane conductivity in NaCl, Na2SO4, and CaCl2 solutions, permeability in respect to the NaCl and CaCl2 diffusion, transport numbers, current-voltage curves (CVC), and the difference in the pH (DpH) of the NaCl solution at the desalination compartment output and input are examined for these membranes in comparison with a well-studied commercial Neosepta CMX cation-exchange membrane produced by Astom Corporation, Japan. It is found that the conductivity, CVC (at relatively low voltages), and water splitting rate (characterized by DpH) for both CJMCED membranes are rather close to these characteristics for the CMX membrane. However, the diffusion permeability of the CJMCED membranes is significantly higher than that of the CMX membrane. This is due to the essentially more porous structure of the CJMCED membranes; the latter reduces the counterion permselectivity of these membranes, while allowing much easier transport of large ions, such as anthocyanins present in natural dyes of fruit and berry juices. The new membranes are promising for use in electrodialysis demineralization of brackish water and natural food solutions.

Advancing the fundamental understanding and practical applications of photo-bioelectrocatalysis

Photo-bioelectrocatalysis combines the natural and highly sophisticated process of photosynthesis in biological entities with an abiotic electrode surface, to perform semi-artificial photosynthesis. However, challenges must be overcome, from the establishment and understanding of the photoexcited electron harvesting process at the electrode to the electrochemical characterization of these biotic/abiotic systems, and their subsequent tuning for enhancing energy generation (chemical and/or electrical). This Feature Article discusses the various approaches utilized to tackle these challenges, particularly focusing on powerful multi-disciplinary approaches for understanding and improving photo-bioelectrocatalysis. Among them is the combination of experimental evidence and quantum mechanical calculations, the use of bioinformatics to understand photo-bioelectrocatalysis at a metabolic level, or bioengineering to improve and facilitate photo-bioelectrocatalysis. Key aspects for the future development of photo-bioelectrocatalysis are presented alongside future research needs and promising applications of semi-artificial photosynthesis.

A hybrid cyt c maturation system enhances the bioelectrical performance of engineered Escherichia coli by improving the rate-limiting step

Bioelectronic devices can use electron flux to enable communication between biotic components and abiotic electrodes. We have modified Escherichia coli to electrically interact with electrodes by expressing the cytochrome c from Shewanella oneidensis MR-1. However, we observe inefficient electrical performance, which we hypothesize is due to the limited compatibility of the E. coli cytochrome c maturation (Ccm) systems with MR-1 cytochrome c. Here we test whether the bioelectronic performance of E. coli can be improved by constructing hybrid Ccm systems containing protein domains from both E. coli and S. oneidensis MR-1. The hybrid CcmH increased cytochrome c expression by increasing the abundance of CymA 60%, while only slightly changing the abundance of the other cytochromes c. Electrochemical measurements showed that the overall current from the hybrid ccm strain increased 121% relative to the wildtype ccm strain, with an electron flux per cell of 12.3 ± 0.3 fA·cell^-1. Additionally, the hybrid ccm strain doubled its electrical response with the addition of exogenous flavin, and quantitative analysis of this demonstrates CymA is the rate-limiting step in this electron conduit. These results demonstrate that this hybrid Ccm system can enhance the bioelectrical performance of the cyt c expressing E. coli, allowing the construction of more efficient bioelectronic devices.

Cultivating electroactive microbes-from field to bench

Electromicrobiology is an emerging field investigating and exploiting the interaction of microorganisms with insoluble electron donors or acceptors. Some of the most recently categorized electroactive microorganisms became of interest to sustainable bioengineering practices. However, laboratories worldwide typically maintain electroactive microorganisms on soluble substrates, which often leads to a decrease or loss of the ability to effectively exchange electrons with solid electrode surfaces. In order to develop future sustainable technologies, we cannot rely solely on existing lab-isolates. Therefore, we must develop isolation strategies for environmental strains with electroactive properties superior to strains in culture collections. In this article, we provide an overview of the studies that isolated or enriched electroactive microorganisms from the environment using an anode as the sole electron acceptor (electricity-generating microorganisms) or a cathode as the sole electron donor (electricity-consuming microorganisms). Next, we recommend a selective strategy for the isolation of electroactive microorganisms. Furthermore, we provide a practical guide for setting up electrochemical reactors and highlight crucial electrochemical techniques to determine electroactivity and the mode of electron transfer in novel organisms.

Synergistic improvement of Shewanella loihica PV-4 extracellular electron transfer using a TiO2@TiN nanocomposite

Extracellular electron transfer (EET) allows microorganisms to perform anaerobic respiration using insoluble electron acceptors, including minerals and electrodes. EET-based applications require efficient electron transfer between living and non-living systems. To improve EET efficiency, the TiO₂@TiN nanocomposite was used to form hybrid biofilms with Shewanella loihica PV-4 (PV-4). Chronoamperometry showed that peak current was increased 4.6-fold via the addition of the TiO₂@TiN nanocomposite. Different biofilms were further tested in a dual-chamber microbial fuel cell. The PV-4 biofilm resulted a maximum power density of 33.4 mW/m², while the hybrid biofilm of the TiO₂@TiN nanocomposite with PV-4 yielded a 92.8% increase of power density. Electrochemical impedance spectroscopy analyses showed a lower electron-transfer resistance in the hybrid biofilm. Biological measurements revealed that both flavin secretion and cytochrome c expression were increased when the TiO₂@TiN nanocomposite presented. These results demonstrated that the TiO₂@TiN nanocomposite could synergistically enhance the EET of PV-4 through altering its metabolism. Our findings provide a new strategy for optimizing biotic-abiotic interactions in bioelectrochemical systems.

The emerging science of electrosynbionics

Dramatic changes in electricity generation, use and storage are needed to keep pace with increasing demand while reducing carbon dioxide emissions. There is great potential for application of bioengineering in this area. We have the tools to re-engineer biological molecules and systems, and a significant amount of research and development is being carried out on technologies such as biophotovoltaics, biocapacitors, biofuel cells and biobatteries. However, there does not seem to be a satisfactory overarching term to describe this area, and I propose a new word-'electrosynbionics'. This is to be defined as: the creation of engineered devices that use components derived from or inspired by biology to perform a useful electrical function. Here, the phrase 'electrical function' is taken to mean the generation, use and storage of electricity, where the primary charge carriers may be either electrons or ions. 'Electrosynbionics' is distinct from 'bioelectronics', which normally relates to applications in sensing, computing or electroceuticals. Electrosynbionic devices have the potential to solve challenges in electricity generation, use and storage by exploiting or mimicking some of the desirable attributes of biological systems, including high efficiency, benign operating conditions and intricate molecular structures.

Progress and Prospects of Bioelectrochemical Systems: Electron Transfer and Its Applications in the Microbial Metabolism

Bioelectrochemical systems are revolutionary new bioengineering technologies which integrate microorganisms or enzymes with the electrochemical method to improve the reducing or oxidizing metabolism. Generally, the bioelectrochemical systems show the processes referring to electrical power generation or achieving the reducing reaction with a certain potential poised by means of electron transfer between the electron acceptor and electron donor. Researchers have focused on the selection and optimization of the electrode materials, design of electrochemical device, and screening of electrochemically active or inactive model microorganisms. Notably, all these means and studies are related to electron transfer: efflux and consumption. Thus, here we introduce the basic concepts of bioelectrochemical systems, and elaborate on the extracellular and intracellular electron transfer, and the hypothetical electron transfer mechanism. Also, intracellular energy generation and coenzyme metabolism along with electron transfer are analyzed. Finally, the applications of bioelectrochemical systems and the prospect of microbial electrochemical technologies are discussed.