Engineering microbial electrocatalysis for chemical and fuel production

Metabolic engineering strategies to enable microbial electrosynthesis utilization of CO2: recent progress and challenges

Microbial electrosynthesis (MES) is a promising technology that mainly utilizes microbial cells to convert CO2 into value-added chemicals using electrons provided by the cathode. However, the low electron transfer rate is a solid bottleneck hindering the further application of MES. Thus, as an effective strategy, genetic tools play a key role in MES for enhancing the electron transfer rate and diversity of production. We describe a set of genetic strategies based on fundamental characteristics and current successes and discuss their functional mechanisms in driving microbial electrocatalytic reactions to fully comprehend the roles and uses of genetic tools in MES. This paper also analyzes the process of nanomaterial application in extracellular electron transfer (EET). It provides a technique that combines nanomaterials and genetic tools to increase MES efficiency, because nanoparticles have a role in the production of functional genes in EET although genetic tools can subvert MES, it still has issues with difficult transformation and low expression levels. Genetic tools remain one of the most promising future strategies for advancing the MES process despite these challenges.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Highly selective butanol production by manipulating electron flow via cathodic electro-fermentation

Butanol production by solventogenic Clostridia shows great potential to combat the energy crisis, but is still challenged by low butanol selectivity and high downstream cost. In this study, a novel cathodic electro-fermentation (CEF) system mediated by methyl viologen (MV) was proposed and sequentially optimized to obtain highly selective butanol production. Under the optimal conditions (-0.60 V cathode potential, 0.50 mM MV, 30 g/L glucose), 7.17 ± 0.55 g/L butanol production were achieved with the yield of 0.32 ± 0.02 g/g. With the supplement of 4 g/L butyric acid as co-substrate, butanol production further improved to 13.14 ± 1.14 g/L with butanol yield and selectivity as high as 0.43 ± 0.01 g/g and 90.44 ± 1.66%, respectively. The polarized electrode enabled the unbalanced fermentation towards butanol formation and MV further inhibited hydrogen production, both of which contributed to the high-level butanol production and selectivity. The MV-mediated CEF system is a promising approach for cost-effective bio-butanol production.

Detoxification mechanisms of electroactive microorganisms under toxicity stress: A review

Remediation of environmental toxic pollutants has attracted extensive attention in recent years. Microbial bioremediation has been an important technology for removing toxic pollutants. However, microbial activity is also susceptible to toxicity stress in the process of intracellular detoxification, which significantly reduces microbial activity. Electroactive microorganisms (EAMs) can detoxify toxic pollutants extracellularly to a certain extent, which is related to their unique extracellular electron transfer (EET) function. In this review, the extracellular and intracellular aspects of the EAMs’ detoxification mechanisms are explored separately. Additionally, various strategies for enhancing the effect of extracellular detoxification are discussed. Finally, future research directions are proposed based on the bottlenecks encountered in the current studies. This review can contribute to the development of toxic pollutants remediation technologies based on EAMs, and provide theoretical and technical support for future practical engineering applications.

Synthetic Biology Tool Development Advances Predictable Gene Expression in the Metabolically Versatile Soil Bacterium Rhodopseudomonas palustris

Harnessing the unique biochemical capabilities of non-model microorganisms would expand the array of biomanufacturing substrates, process conditions, and products. There are non-model microorganisms that fix nitrogen and carbon dioxide, derive energy from light, catabolize methane and lignin-derived aromatics, are tolerant to physiochemical stresses and harsh environmental conditions, store lipids in large quantities, and produce hydrogen. Model microorganisms often only break down simple sugars and require low stress conditions, but they have been engineered for the sustainable manufacture of numerous products, such as fragrances, pharmaceuticals, cosmetics, surfactants, and specialty chemicals, often by using tools from synthetic biology. Transferring complex pathways has proven to be exceedingly difficult, as the cofactors, cellular conditions, and energy sources necessary for this pathway to function may not be present in the host organism. Utilization of unique biochemical capabilities could also be achieved by engineering the host; although, synthetic biology tools developed for model microbes often do not perform as designed in other microorganisms. The metabolically versatile Rhodopseudomonas palustris CGA009, a purple non-sulfur bacterium, catabolizes aromatic compounds derived from lignin in both aerobic and anaerobic conditions and can use light, inorganic, and organic compounds for its source of energy. R. palustris utilizes three nitrogenase isozymes to fulfill its nitrogen requirements while also generating hydrogen. Furthermore, the bacterium produces two forms of RuBisCo in response to carbon dioxide/bicarbonate availability. While this potential chassis harbors many beneficial traits, stable heterologous gene expression has been problematic due to its intrinsic resistance to many antibiotics and the lack of synthetic biology parts investigated in this microbe. To address these problems, we have characterized gene expression and plasmid maintenance for different selection markers, started a synthetic biology toolbox specifically for the photosynthetic R. palustris, including origins of replication, fluorescent reporters, terminators, and 5′ untranslated regions, and employed the microbe’s endogenous plasmid for exogenous protein production. This work provides essential synthetic biology tools for engineering R. palustris’ many unique biochemical processes and has helped define the principles for expressing heterologous genes in this promising microbe through a methodology that could be applied to other non-model microorganisms.

A 2D Modelling Approach for Predicting the Response of a Two-Chamber Microbial Fuel Cell to Substrate Concentration and Electrolyte Conductivity Changes

Bioelectrochemical systems have been the focus of extensive research due to their unique advantages of converting the chemical energy stored in waste to electricity. To acquire a better understanding and optimize these systems, modelling has been employed. A 2D microbial fuel cell (MFC) model was developed using the finite element software Comsol Multiphysics® (version 5.2), simulating a two-chamber MFC operating in batch mode. By solving mass and charge balance equations along with Monod–Butler–Volmer kinetics, the operation of the MFC was simulated. The model accurately describes voltage output and substrate consumption in the MFC. The computational results were compared with experimental data, thus validating the model. The voltage output and substrate consumption originating from the model were in agreement with the experimental data for two different cases (100 Ω, 1000 Ω external resistances). A polarization curve was extracted from the model by shifting the external resistance gradually, calculating a similar maximum power (47 mW/m2) to the observed experimental one (49 mW/m2). The validated model was used to predict the MFC response to varying initial substrate concentrations (0.125–4 g COD/L) and electrolyte conductivity (0.04–100 S/m) in order to determine the optimum operating conditions.

Empower C1: Combination of Electrochemistry and Biology to Convert C1 Compounds

The idea to somehow combine electrical current and biological systems is not new. It was subject of research as well as of science fiction literature for decades. Nowadays, in times of limited resources and the need to capture greenhouse gases like CO₂, this combination gains increasing interest, since it might allow to use C1 compounds and highly oxidized compounds as substrate for microbial production by "activating" them with additional electrons. In this chapter, different possibilities to combine electrochemistry and biology to convert C1 compounds into useful products will be discussed. The chapter first shows electrochemical conversion of C1 compounds, allowing the use of the product as substrate for a subsequent biosynthesis in uncoupled systems, further leads to coupled systems of biology and electrochemical conversion, and finally reaches the discipline of bioelectrosynthesis, where electrical current and C1 compounds are directly converted by microorganisms or enzymes. This overview will give an idea about the potentials and challenges of combining electrochemistry and biology to convert C1 molecules.

Anodic electro-fermentation: Empowering anaerobic production processes via anodic respiration

In nature as well as in industrial microbiology, all microorganisms need to achieve redox balance. Their redox state and energy conservation highly depend on the availability of a terminal electron acceptor, for example oxygen in aerobic production processes. Under anaerobic conditions in the absence of an electron acceptor, redox balance is achieved via the production of reduced carbon-compounds (fermentation). An alternative strategy to artificially stabilize microbial redox and energy state is the use of anodic electro-fermentation (AEF). This emerging biotechnology empowers respiration under anaerobic conditions using the anode of a bioelectrochemical system as an undepletable terminal electron acceptor. Electrochemical control of redox metabolism and energy conservation via AEF can steer the carbon metabolism towards a product of interest and avoid the need for continuous and cost-inefficient supply of oxygen as well as the production of mixed reduced by-products, as is the case in aerobic production and fermentation processes, respectively. The great challenge for AEF is to establish efficient extracellular electron transfer (EET) from the microbe to the anode and link it to central carbon metabolism to enhance the synthesis of a target product. This article reviews the advantages and challenges of AEF, EET mechanisms, microbial energy gain, and discusses the rational choice of substrate-product couple as well as the choice of microbial catalyst. Besides, it discusses the potential of the industrial model-organism Bacillus subtilis as a promising candidate for AEF, which has not been yet considered for such an application. This prospective review contributes to a better understanding of how industrial microbiology can benefit from AEF and analyses key-factors required to successfully implement AEF processes. Overall, this work aims to advance the young research field especially by critically revisiting the fundamental aspects of AEF.

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.

Strategies for improving the electroactivity and specific metabolic functionality of microorganisms for various microbial electrochemical technologies

Electroactive microorganisms, which possess extracellular electron transfer (EET) capabilities, are the basis of microbial electrochemical technologies (METs) such as microbial fuel and electrolysis cells. These are considered for several applications ranging from the energy-efficient treatment of waste streams to the production of value-added chemicals and fuels, bioremediation, and biosensing. Various aspects related to the microorganisms, electrodes, separators, reactor design, and operational or process parameters influence the overall functioning of METs. The most fundamental and critical performance-determining factor is, however, the microorganism-electrode interactions. Modification of the electrode surfaces and microorganisms for optimizing their interactions has therefore been the major MET research focus area over the last decade. In the case of microorganisms, primarily their EET mechanisms and efficiencies along with the biofilm formation capabilities, collectively considered as microbial electroactivity, affect their interactions with the electrodes. In addition to electroactivity, the specific metabolic or biochemical functionality of microorganisms is equally crucial to the target MET application. In this article, we present the major strategies that are used to enhance the electroactivity and specific functionality of microorganisms pertaining to both anodic and cathodic processes of METs. These include simple physical methods based on the use of heat and magnetic field along with chemical, electrochemical, and growth media amendment approaches to the complex procedure-based microbial bioaugmentation, co-culture, and cell immobilization or entrapment, and advanced toolkit-based biofilm engineering, genetic modifications, and synthetic biology strategies. We further discuss the applicability and limitations of these strategies and possible future research directions for advancing the highly promising microbial electrochemistry-driven biotechnology.

Boosting Heterologous Phenazine Production in Pseudomonas putida KT2440 Through the Exploration of the Natural Sequence Space

Phenazine-1-carboxylic acid (PCA) and its derivative pyocyanin (PYO) are natural redox mediators in bioelectrochemical systems and have the potential to enable new bioelectrochemical production strategies. The native producer Pseudomonas aeruginosa harbors two identically structured operons in its genome, which encode the enzymes responsible for PCA synthesis [phzA1-G1 (operon 1), phzA2-G2 (operon 2)]. To optimize heterologous phenazines production in the biotech host Pseudomonas putida KT2440, we compared PCA production from both operons originating from P. aeruginosa strain PAO1 (O1.phz1 and O1.phz2) as well as from P. aeruginosa strain PA14 (14.phz1 and 14.phz2). Comparisons of phenazine synthesis and bioelectrochemical activity were performed between heterologous constructs with and without the combination with the genes phzM and phzS required to convert PCA to PYO. Despite a high amino acid homology of all enzymes of more than 97%, P. putida harboring 14.phz2 produced 4-times higher PCA concentrations (80 μg/mL), which resulted in 3-times higher current densities (12 μA/cm²) compared to P. putida 14.phz1. The respective PCA/PYO producer containing the 14.phz2 operon was the best strain with 80 μg/mL PCA, 11 μg/mL PYO, and 22 μA/cm² current density. Tailoring phenazine production also resulted in improved oxygen-limited metabolic activity of the bacterium through enhanced anodic electron discharge. To elucidate the reason for this superior performance, a detailed structure comparison of the PCA-synthesizing proteins has been performed. The here presented characterization and optimization of these new strains will be useful to improve electroactivity in P. putida for oxygen-limited biocatalysis.

How Thermophilic Gram-Positive Organisms Perform Extracellular Electron Transfer: Characterization of the Cell Surface Terminal Reductase OcwA

Thermophilic Gram-positive organisms were recently shown to be a promising class of organisms to be used in bioelectrochemical systems for the production of electrical energy. These organisms present a thick peptidoglycan layer that was thought to preclude them to perform extracellular electron transfer (i.e., exchange catabolic electrons with solid electron acceptors outside the cell). In this paper, we describe the structure and functional mechanisms of the multiheme cytochrome OcwA, the terminal reductase of the Gram-positive bacterium Thermincola potens JR found at the cell surface of this organism. The results presented here show that this protein can take the role of a respiratory “Swiss Army knife,” allowing this organism to grow in environments with soluble and insoluble substrates. Moreover, it is shown that it is unrelated to terminal reductases found at the cell surface of other electroactive organisms. Instead, OcwA is similar to terminal reductases of soluble electron acceptors. Our data reveal that terminal oxidoreductases of soluble and insoluble substrates are evolutionarily related, providing novel insights into the evolutionary pathway of multiheme cytochromes.

Extracellular electron transfer is the key process underpinning the development of bioelectrochemical systems for the production of energy or added-value compounds. Thermincola potens JR is a promising Gram-positive bacterium to be used in these systems because it is thermophilic. In this paper, we describe the structural and functional properties of the nonaheme cytochrome OcwA, which is the terminal reductase of this organism. The structure of OcwA, determined at 2.2-Å resolution, shows that the overall fold and organization of the hemes are not related to other metal reductases and instead are similar to those of multiheme cytochromes involved in the biogeochemical cycles of nitrogen and sulfur. We show that, in addition to solid electron acceptors, OcwA can also reduce soluble electron shuttles and oxyanions. These data reveal that OcwA can work as a multipurpose respiratory enzyme allowing this organism to grow in environments with rapidly changing availability of terminal electron acceptors without the need for transcriptional regulation and protein synthesis.

Reversing an Extracellular Electron Transfer Pathway for Electrode-Driven Acetoin Reduction

Microbial electrosynthesis is an emerging technology with the potential to simultaneously store renewably generated energy, fix carbon dioxide, and produce high-value organic compounds. However, limited understanding of the route of electrons into the cell remains an obstacle to developing a robust microbial electrosynthesis platform. To address this challenge, we leveraged the native extracellular electron transfer pathway in Shewanella oneidensis MR-1 to connect an extracellular electrode with an intracellular reduction reaction. The system uses native Mtr proteins to transfer electrons from an electrode to the inner membrane quinone pool. Subsequently, electrons are transferred from quinones to NAD⁺ by native NADH dehydrogenases. This reverse functioning of NADH dehydrogenases is thermodynamically unfavorable; therefore, we added a light-driven proton pump (proteorhodopsin) to generate proton-motive force to drive this activity. Finally, we use reduction of acetoin to 2,3-butanediol via a heterologous butanediol dehydrogenase (Bdh) as an electron sink. Bdh is an NADH-dependent enzyme; therefore, observation of acetoin reduction supports our hypothesis that cathodic electrons are transferred to intracellular NAD⁺. Multiple lines of evidence indicate proper functioning of the engineered electrosynthesis system: electron flux from the cathode is influenced by both light and acetoin availability, and 2,3-butanediol production is highest when both light and a poised electrode are present. Using a hydrogenase-deficient S. oneidensis background strain resulted in a stronger correlation between electron transfer and 2,3-butanediol production, suggesting that hydrogen production is an off-target electron sink in the wild-type background. This system represents a promising step toward a genetically engineered microbial electrosynthesis platform and will enable a new focus on synthesis of specific compounds using electrical energy.

Microbial Electrosynthesis I: Pure and Defined Mixed Culture Engineering

In the past 6 years, microbial bioelectrochemistry has strongly increased in attraction and audience when expanding from mainly environmental technology applications to biotechnology. In particular, the promise to combine electrosynthesis with microbial catalysis opens attractive approaches for new sustainable redox-cofactor recycling, redox-balancing, or even biosynthesis processes. Much of this promise is still not fulfilled, but it has opened and fueled entirely new research areas in this discipline. Activities in designing, tailoring, and applying specific microbial catalysts as pure or defined co-cultures for defined target bioproductions are greatly accelerating. This chapter gives an overview of the current progress as well as the emerging trends in molecular and ecological engineering of defined microbial biocatalysts to prepare them for evolving microbial electrosynthesis processes. In addition, the multitude of microbial electrosynthetic processes with complex undefined mixed cultures is covered by ter Heijne et al. (Adv Biochem Eng Biotechnol. https://doi.org/10.1007/10_2017_15 , 2017). Graphical Abstract.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

Engineering an electroactive Escherichia coli for the microbial electrosynthesis of succinate from glucose and CO2

BACKGROUND

Electrochemical energy is a key factor of biosynthesis, and is necessary for the reduction or assimilation of substrates such as CO₂. Previous microbial electrosynthesis (MES) research mainly utilized naturally electroactive microbes to generate non-specific products.

RESULTS

In this research, an electroactive succinate-producing cell factory was engineered in E. coli T110(pMtrABC, pFccA-CymA) by expressing mtrABC, fccA and cymA from Shewanella oneidensis MR-1, which can utilize electricity to reduce fumarate. The electroactive T110 strain was further improved by incorporating a carbon concentration mechanism (CCM). This strain was fermented in an MES system with neutral red as the electron carrier and supplemented with HCO₃⁺, which produced a succinate yield of 1.10 mol/mol glucose-a 1.6-fold improvement over the parent strain T110.

CONCLUSIONS

The strain T110(pMtrABC, pFccA-CymA, pBTCA) is to our best knowledge the first electroactive microbial cell factory engineered to directly utilize electricity for the production of a specific product. Due to the versatility of the E. coli platform, this pioneering research opens the possibility of engineering various other cell factories to utilize electricity for bioproduction.

Expanding the product spectrum of value added chemicals in microbial electrosynthesis through integrated process design-A review

Microbial electrosynthesis (MES) is a novel microbial electrochemical technology proposed for chemicals production with the storage of sustainable energy. However, the practical application of MES is currently restricted by the limited low market value of products in one-step conversion process, mostly acetate. A theme that is pervasive throughout this review is the challenges associated with the expanded product spectrum. Several recent research efforts to improve acetate production, using novel reactor configuration, renewable power supply, and various 3-D cathode are summarized. The importance of genetic modification, two-step hybrid process, as well as input substrates other than CO₂ are highlighted in this review as the future research paths for higher value chemicals production. At last, how to integrate MES with existing biochemicals processes is proposed. Definitely, more studies are encouraged to evaluate the overall performances and economic efficiency of these integrated process designs to make MES more competitive.

Salinity-gradient energy driven microbial electrosynthesis of value-added chemicals from CO2 reduction

Biological conversion of CO₂ to value-added chemicals and biofuels has emerged as an attractive strategy to address the energy and environmental concerns caused by the over-reliance on fossil fuels. In this study, an innovative microbial reverse-electrodialysis electrolysis cell (MREC), which combines the strengths of reverse electrodialysis (RED) and microbial electrosynthesis technology platforms, was developed to achieve efficient CO₂-to-value chemicals bioconversion by using the salinity gradient energy as driven energy sources. In the MREC, maximum acetate and ethanol concentrations of 477.5 ± 33.2 and 46.2 ± 8.2 mg L^-1 were obtained at the cathode, catalyzed by Sporomusa ovata with production rates of 165.79 ± 11.52 and 25.11 ± 4.46 mmol m^-2 d^-1, respectively. Electron balance analysis indicates that 94.4 ± 3.9% of the electrons derived from wastewater and salinity gradient were recovered in acetate and ethanol. This work for the first time proved the potential of innovative MREC configuration has the potential as an efficient technology platform for simultaneous CO₂ capture and electrosynthesis of valuable chemicals.

Construction of a Geobacter Strain With Exceptional Growth on Cathodes

Insoluble extracellular electron donors are important sources of energy for anaerobic respiration in biogeochemical cycling and in diverse practical applications. The previous lack of a genetically tractable model microorganism that could be grown to high densities under anaerobic conditions in pure culture with an insoluble extracellular electron donor has stymied efforts to better understand this form of respiration. We report here on the design of a strain of Geobacter sulfurreducens, designated strain ACL, which grows as thick (ca. 35 μm) confluent biofilms on graphite cathodes poised at -500 mV (versus Ag/AgCl) with fumarate as the electron acceptor. Sustained maximum current consumption rates were >0.8 A/m², which is >10-fold higher than the current consumption of the wild-type strain. The improved function on the cathode was achieved by introducing genes for an ATP-dependent citrate lyase, completing the complement of enzymes needed for a reverse TCA cycle for the synthesis of biosynthetic precursors from carbon dioxide. Strain ACL provides an important model organism for elucidating the mechanisms for effective anaerobic growth with an insoluble extracellular electron donor and may offer unique possibilities as a chassis for the introduction of synthetic metabolic pathways for the production of commodities with electrons derived from electrodes.

Microbial electrocatalysis: Redox mediators responsible for extracellular electron transfer

Redox mediator plays an important role in extracellular electron transfer (EET) in many environments wherein microbial electrocatalysis occurs actively. Because of the block of cell envelope and the low difference of redox potential between the intracellular and extracellular surroundings, the proceeding of EET depends mainly on the help of a variety of mediators that function as an electron carrier or bridge. In this Review, we will summarize a wide range of redox mediators and further discuss their functional mechanisms in EET that drives a series of microbial electrocatalytic reactions. Studying these mediators adds to our knowledge of how charge transport and electrochemical reactions occur at the microorganism-electrode interface. This understanding would promote the widespread applications of microbial electrocatalysis in microbial fuel cells, bioremediation, bioelectrosynthesis, biomining, nanomaterial productions, etc. These improved applications will greatly benefit the sustainable development of the environmental-friendly biochemical industries.

Balancing cellular redox metabolism in microbial electrosynthesis and electro fermentation - A chance for metabolic engineering

More and more microbes are discovered that are capable of extracellular electron transfer, a process in which they use external electrodes as electron donors or acceptors for metabolic reactions. This feature can be used to overcome cellular redox limitations and thus optimizing microbial production. The technologies, termed microbial electrosynthesis and electro-fermentation, have the potential to open novel bio-electro production platforms from sustainable energy and carbon sources. However, the performance of reported systems is currently limited by low electron transport rates between microbes and electrodes and our limited ability for targeted engineering of these systems due to remaining knowledge gaps about the underlying fundamental processes. Metabolic engineering offers many opportunities to optimize these processes, for instance by genetic engineering of pathways for electron transfer on the one hand and target product synthesis on the other hand. With this review, we summarize the status quo of knowledge and engineering attempts around chemical production in bio-electrochemical systems from a microbe perspective. Challenges associated with the introduction or enhancement of extracellular electron transfer capabilities into production hosts versus the engineering of target compound synthesis pathways in natural exoelectrogens are discussed. Recent advances of the research community in both directions are examined critically. Further, systems biology approaches, for instance using metabolic modelling, are examined for their potential to provide insight into fundamental processes and to identify targets for metabolic engineering.

Extracellular electron transfer in acetogenic bacteria and its application for conversion of carbon dioxide into organic compounds

Acetogenic bacteria (i.e., acetogens) produce acetate from CO₂ during anaerobic chemoautotrophic growth. Because acetogens fix CO₂ with high energy efficiency, they have been investigated as biocatalysts of CO₂ conversion into valuable chemicals. Recent studies revealed that some acetogens are capable of extracellular electron transfer (EET), which enables electron exchange between microbial cells and extracellular solid materials. Thus, acetogens are promising candidates as biocatalysts in recently developed bioelectrochemical technologies, including microbial electrosynthesis (MES), in which useful chemicals are biologically produced from CO₂ using electricity as the energy source. In microbial photoelectrosynthesis, a variant of MES technology, the conversion of CO₂ into organic compounds is achieved using light as the sole energy source without an external power supply. In this mini-review, we introduce the general features of bioproduction and EET of acetogens and describe recent progress and future prospects of MES technologies based on the EET capability of acetogens.

How to Sustainably Feed a Microbe: Strategies for Biological Production of Carbon-Based Commodities with Renewable Electricity

As interest and application of renewable energy grows, strategies are needed to align the asynchronous supply and demand. Microbial metabolisms are a potentially sustainable mechanism for transforming renewable electrical energy into biocommodities that are easily stored and transported. Acetogens and methanogens can reduce carbon dioxide to organic products including methane, acetic acid, and ethanol. The library of biocommodities is expanded when engineered metabolisms of acetogens are included. Typically, electrochemical systems are employed to integrate renewable energy sources with biological systems for production of carbon-based commodities. Within these systems, there are three prevailing mechanisms for delivering electrons to microorganisms for the conversion of carbon dioxide to reduce organic compounds: (1) electrons can be delivered to microorganisms via H₂ produced separately in a electrolyzer, (2) H₂ produced at a cathode can convey electrons to microorganisms supported on the cathode surface, and (3) a cathode can directly feed electrons to microorganisms. Each of these strategies has advantages and disadvantages that must be considered in designing full-scale processes. This review considers the evolving understanding of each of these approaches and the state of design for advancing these strategies toward viability.

Characterization of Clostridium ljungdahlii OTA1: a non-autotrophic hyper ethanol-producing strain

A Clostridium ljungdahlii lab-isolated spontaneous-mutant strain, OTA1, has been shown to produce twice as much ethanol as the C. ljungdahlii ATCC 55383 strain when cultured in a mixotrophic medium containing fructose and syngas. Whole-genome sequencing identified four unique single nucleotide polymorphisms (SNPs) in the C. ljungdahlii OTA1 genome. Among these, two SNPs were found in the gene coding for AcsA and HemL, enzymes involved in acetyl-CoA formation from CO/CO₂. Homology models of the respective mutated enzymes revealed alterations in the size and hydrogen bonding of the amino acids in their active sites. Failed attempts to grow OTA1 autotrophically suggested that one or both of these mutated genes prevented acetyl-CoA synthesis from CO/CO₂, demonstrating that its activity was required for autotrophic growth by C. ljungdahlii. An inoperable Wood-Ljungdahl pathway resulted in higher CO₂ and ethanol yields and lower biomass and acetate yields compared to WT for multiple growth conditions including heterotrophic and mixotrophic conditions. The two other SNPs identified in the C. ljungdahlii OTA1 genome were in genes coding for transcriptional regulators (CLJU_c09320 and CLJU_c18110) and were found to be responsible for deregulated expression of co-localized arginine catabolism and 2-deoxy-D-ribose catabolism genes. Growth medium supplementation experiments suggested that increased arginine metabolism and 2-deoxy-D-ribose were likely to have minor effects on biomass and fermentation product yields. In addition, in silico flux balance analysis simulating mixotrophic and heterotrophic conditions showed no change in flux to ethanol when flux through HemL was changed whereas limited flux through AcsA increased the ethanol flux for both simulations. In characterizing the effects of the SNPs identified in the C. ljungdahlii OTA1 genome, a non-autotrophic hyper ethanol-producing strain of C. ljungdahlii was identified that has utility for further physiology and strain performance studies and as a biocatalyst for industrial applications.

Photoreduction of Shewanella oneidensis Extracellular Cytochromes by Organic Chromophores and Dye-Sensitized TiO2

The transfer of photoenergized electrons from extracellular photosensitizers across a bacterial cell envelope to drive intracellular chemical transformations represents an attractive way to harness nature's catalytic machinery for solar-assisted chemical synthesis. In Shewanella oneidensis MR-1 (MR-1), trans-outer-membrane electron transfer is performed by the extracellular cytochromes MtrC and OmcA acting together with the outer-membrane-spanning porin⋅cytochrome complex (MtrAB). Here we demonstrate photoreduction of solutions of MtrC, OmcA, and the MtrCAB complex by soluble photosensitizers: namely, eosin Y, fluorescein, proflavine, flavin, and adenine dinucleotide, as well as by riboflavin and flavin mononucleotide, two compounds secreted by MR-1. We show photoreduction of MtrC and OmcA adsorbed on Ru^II -dye-sensitized TiO₂ nanoparticles and that these protein-coated particles perform photocatalytic reduction of solutions of MtrC, OmcA, and MtrCAB. These findings provide a framework for informed development of strategies for using the outer-membrane-associated cytochromes of MR-1 for solar-driven microbial synthesis in natural and engineered bacteria.

"Hot" acetogenesis

Thermophilic microorganisms as well as acetogenic bacteria are both considered ancient. Interestingly, only a few species of bacteria, all belonging to the family Thermoanaerobacteraceae, are described to conserve energy from acetate formation with hydrogen as electron donor and carbon dioxide as electron acceptor. This review reflects the metabolic differences between Moorella spp., Thermoanaerobacter kivui and Thermacetogenium phaeum, with focus on the biochemistry of autotrophic growth and energy conservation. The potential of these thermophilic acetogens for biotechnological applications is discussed briefly.

Carbon recovery by fermentation of CO-rich off gases - Turning steel mills into biorefineries

Technological solutions to reduce greenhouse gas (GHG) emissions from anthropogenic sources are required. Heavy industrial processes, such as steel making, contribute considerably to GHG emissions. Fermentation of carbon monoxide (CO)-rich off gases with wild-type acetogenic bacteria can be used to produce ethanol, acetate, and 2,3-butanediol, thereby, reducing the carbon footprint of heavy industries. Here, the processes for the production of ethanol from CO-rich off gases are discussed and a perspective on further routes towards an integrated biorefinery at a steel mill is given. Recent achievements in genetic engineering as well as integration of other biotechnology platforms to increase the product portfolio are summarized. Already, yields have been increased and the portfolio of products broadened. To develop a commercially viable process, however, the extraction from dilute product streams is a critical step and alternatives to distillation are discussed. Finally, another critical step is waste(water) treatment with the possibility to recover resources.

Expanding the molecular toolkit for the homoacetogen Clostridium ljungdahlii

Increasing interest in homoacetogenic bacteria for the production of biochemicals and biofuels requisites the development of new genetic tools for these atypical production organisms. An attractive host for the conversion of synthesis gas or electricity into multi-carbon compounds is Clostridium ljungdahlii. So far only limited achievements in modifying this organism towards the production of industrially relevant compounds have been made. Therefore, there is still a strong need for developing new and optimizing existing genetic tools to efficiently access its metabolism. Here, we report on the development of a stable and reproducible transformation protocol that is applicable to C. ljungdahlii and several other clostridial species. Further, we demonstrate the functionality of a temperature-sensitive origin of replication in combination with a fluorescence marker system as important tools for future genetic engineering of this host for microbial bioproduction.

¹³C Pathway Analysis for the Role of Formate in Electricity Generation by Shewanella Oneidensis MR-1 Using Lactate in Microbial Fuel Cells

Microbial fuel cell (MFC) is a promising technology for direct electricity generation from organics by microorganisms. The type of electron donors fed into MFCs affects the electrical performance, and mechanistic understanding of such effects is important to optimize the MFC performance. In this study, we used a model organism in MFCs, Shewanella oneidensis MR-1, and (13)C pathway analysis to investigate the role of formate in electricity generation and the related microbial metabolism. Our results indicated a synergistic effect of formate and lactate on electricity generation, and extra formate addition on the original lactate resulted in more electrical output than using formate or lactate as a sole electron donor. Based on the (13)C tracer analysis, we discovered decoupled cell growth and electricity generation in S. oneidensis MR-1 during co-utilization of lactate and formate (i.e., while the lactate was mainly metabolized to support the cell growth, the formate was oxidized to release electrons for higher electricity generation). To our best knowledge, this is the first time that (13)C tracer analysis was applied to study microbial metabolism in MFCs and it was demonstrated to be a valuable tool to understand the metabolic pathways affected by electron donors in the selected electrochemically-active microorganisms.

Nature of the Surface-Exposed Cytochrome-Electrode Interactions in Electroactive Biofilms of Desulfuromonas acetoxidans

Metal-respiring bacteria are microorganisms capable of oxidizing organic pollutants present in wastewater and transferring the liberated electrons to an electrode. This ability has led to their application as catalysts in bioelectrochemical systems (BESs), a sustainable technology coupling bioremediation to electricity production. Crucial for the functioning of these BESs is a complex protein architecture consisting of several surface-exposed multiheme proteins, called outer membrane cytochromes, wiring the cell metabolism to the electrode. Although the role of these proteins has been increasingly understood, little is known about the protein-electrode interactions and their impact on the performance of BESs. In this study, we used surface-enhanced resonance Raman spectroscopy in combination with electrochemical techniques to unravel the nature of the protein-electrode interaction for the outer membrane cytochrome OmcB from Desulfuromonas acetoxidans (Dace). Comparing the spectroelectrochemical properties of OmcB bound directly to the electrode surface with those of the same protein embedded inside an electroactive biofilm, we have shown that the surface-exposed cytochromes of Dace biofilms are in direct contact with the electrode surface. Even if direct binding causes protein denaturation, the biofilm possesses the ability to minimize the extent of the damage maximizing the amount of cells in direct electrical communication with the electrode.

Biotechnological Aspects of Microbial Extracellular Electron Transfer

Extracellular electron transfer (EET) is a type of microbial respiration that enables electron transfer between microbial cells and extracellular solid materials, including naturally-occurring metal compounds and artificial electrodes. Microorganisms harboring EET abilities have received considerable attention for their various biotechnological applications, in addition to their contribution to global energy and material cycles. In this review, current knowledge on microbial EET and its application to diverse biotechnologies, including the bioremediation of toxic metals, recovery of useful metals, biocorrosion, and microbial electrochemical systems (microbial fuel cells and microbial electrosynthesis), were introduced. Two potential biotechnologies based on microbial EET, namely the electrochemical control of microbial metabolism and electrochemical stimulation of microbial symbiotic reactions (electric syntrophy), were also discussed.

Simplifying microbial electrosynthesis reactor design

Microbial electrosynthesis, an artificial form of photosynthesis, can efficiently convert carbon dioxide into organic commodities; however, this process has only previously been demonstrated in reactors that have features likely to be a barrier to scale-up. Therefore, the possibility of simplifying reactor design by both eliminating potentiostatic control of the cathode and removing the membrane separating the anode and cathode was investigated with biofilms of Sporomusa ovata. S. ovata reduces carbon dioxide to acetate and acts as the microbial catalyst for plain graphite stick cathodes as the electron donor. In traditional ‘H-cell’ reactors, where the anode and cathode chambers were separated with a proton-selective membrane, the rates and columbic efficiencies of microbial electrosynthesis remained high when electron delivery at the cathode was powered with a direct current power source rather than with a potentiostat-poised cathode utilized in previous studies. A membrane-less reactor with a direct-current power source with the cathode and anode positioned to avoid oxygen exposure at the cathode, retained high rates of acetate production as well as high columbic and energetic efficiencies. The finding that microbial electrosynthesis is feasible without a membrane separating the anode from the cathode, coupled with a direct current power source supplying the energy for electron delivery, is expected to greatly simplify future reactor design and lower construction costs.

Engineering mediator-based electroactivity in the obligate aerobic bacterium Pseudomonas putida KT2440

Pseudomonas putida strains are being developed as microbial production hosts for production of a range of amphiphilic and hydrophobic biochemicals. P. putida's obligate aerobic growth thereby can be an economical and technical challenge because it requires constant rigorous aeration and often causes reactor foaming. Here, we engineered a strain of P. putida KT2440 that can produce phenazine redox-mediators from Pseudomonas aeruginosa to allow partial redox balancing with an electrode under oxygen-limited conditions. P. aeruginosa is known to employ its phenazine-type redox mediators for electron exchange with an anode in bioelectrochemical systems (BES). We transferred the seven core phenazine biosynthesis genes phzA-G and the two specific genes phzM and phzS required for pyocyanin synthesis from P. aeruginosa on two inducible plasmids into P. putida KT2440. The best clone, P. putida pPhz, produced 45 mg/L pyocyanin over 25 h of growth, which was visible as blue color formation and is comparable to the pyocyanin production of P. aeruginosa. This new strain was then characterized under different oxygen-limited conditions with electrochemical redox control and changes in central energy metabolism were evaluated in comparison to the unmodified P. putida KT2440. In the new strain, phenazine synthesis with supernatant concentrations up to 33 μg/mL correlated linearly with the ability to discharge electrons to an anode, whereby phenazine-1-carboxylic acid served as the dominating redox mediator. P. putida pPhz sustained strongly oxygen-limited metabolism for up to 2 weeks at up to 12 μA/cm(2) anodic current density. Together, this work lays a foundation for future oxygen-limited biocatalysis with P. putida strains.

Electricity-driven metabolic shift through direct electron uptake by electroactive heterotroph Clostridium pasteurianum

Although microbes directly accepting electrons from a cathode have been applied for CO₂ reduction to produce multicarbon-compounds, a high electron demand and low product concentration are critical limitations. Alternatively, the utilization of electrons as a co-reducing power during fermentation has been attempted, but there must be exogenous mediators due to the lack of an electroactive heterotroph. Here, we show that Clostridium pasteurianum DSM 525 simultaneously utilizes both cathode and substrate as electron donors through direct electron transfer. In a cathode compartment poised at +0.045 V vs. SHE, a metabolic shift in C. pasteurianum occurs toward NADH-consuming metabolite production such as butanol from glucose (20% shift in terms of NADH consumption) and 1,3-propandiol from glycerol (21% shift in terms of NADH consumption). Notably, a small amount of electron uptake significantly induces NADH-consuming pathways over the stoichiometric contribution of the electrons as reducing equivalents. Our results demonstrate a previously unknown electroactivity and metabolic shift in the biochemical-producing heterotroph, opening up the possibility of efficient and enhanced production of electron-dense metabolites using electricity.