Electro-biocatalytic production of formate from carbon dioxide using an oxygen-stable whole cell biocatalyst

From formate oxidation to CO₂ reduction: The role of formate dehydrogenase in sustainable carbon utilization

The escalation of global climate change and environmental degradation has made it imperative to develop innovative strategies to mitigate carbon dioxide (CO₂) emissions and enhance its utilization. Formate dehydrogenase (FDH) is a key enzyme capable of catalyzing the reversible conversion between CO₂ and formate. Due to its critical role in sustainable carbon recycling processes, FDH has garnered significant attention in recent times. This review offers a thorough analysis of FDH, emphasizing its dual function of converting one carbon (C1) substrates and providing reducing power. Recent advancements in utilizing FDH for CO₂ reduction, both in vitro and in vivo, underscoring its potential to facilitate carbon capture and conversion under mild conditions. Additionally, this review discusses the limitations of FDH in C1 metabolism and proposes targeted strategies to address these challenges. Future research should focus on achieving a balance between energy production and carbon assimilation, mediated by FDH activity. Ultimately, this work aims to offer both theoretical insights and practical guidance, advancing microbial engineering for CO₂ reduction and resource recycling, and contributing to the development of sustainable carbon utilization technologies.

A novel engineered strain of Methylorubrum extorquens for methylotrophic production of glycolic acid

The conversion of CO2 into methanol depicts one of the most promising emerging renewable routes for the chemical and biotech industry. Under this regard, native methylotrophs have a large potential for converting methanol into value-added products but require targeted engineering approaches to enhance their performances and to widen their product spectrum. Here we use a systems-based approach to analyze and engineer M. extorquens TK 0001 for production of glycolic acid. Application of constraint-based metabolic modeling reveals the great potential of M. extorquens for that purpose, which is not yet described in literature. In particular, a superior theoretical product yield of 1.0 C-molGlycolic acid C-molMethanol-1 is predicted by our model, surpassing theoretical yields of sugar fermentation. Following this approach, we show here that strain engineering is viable and present 1st generation strains producing glycolic acid via a heterologous NADPH-dependent glyoxylate reductase. It was found that lactic acid is a surprising by-product of glycolic acid formation in M. extorquens, most likely due to a surplus of available NADH upon glycolic acid synthesis. Finally, the best performing strain was tested in a fed-batch fermentation producing a mixture of up to total 1.2 g L-1 glycolic acid and lactic acid. Several key performance indicators of our glycolic acid producer strain are superior to state-of-the-art synthetic methylotrophs. The presented results open the door for further strain engineering of the native methylotroph M. extorquens and pave the way to produce two promising biopolymer building blocks from green methanol, i.e., glycolic acid and lactic acid.

Boosting the Microbial Electrosynthesis of Formate by Shewanella oneidensis MR-1 with an Ionic Liquid Cosolvent

Microbial electrosynthesis (MES) is a rapidly growing technology at the forefront of sustainable chemistry, leveraging the ability of microorganisms to catalyze electrochemical reactions to synthesize valuable compounds from renewable energy sources. The reduction of CO2 is a major target application for MES, but research in this area has been stifled, especially with the use of direct electron transfer (DET)-based microbial systems. The major fundamental hurdle that needs to be overcome is the low efficiency of CO2 reduction largely attributed to minimal microbial access to CO2 owing to its low solubility in the electrolyte. With their tunable physical properties, ionic liquids present a potential solution to this challenge and have previously shown promise in facilitating efficient CO2 electroreduction by increasing the CO2 solubility. However, the use of ionic liquids in MES remains unexplored. In this study, we investigated the role of 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) using Shewanella oneidensis MR-1 as a model DET strain. Electrochemical investigations demonstrated the ability of S. oneidensis MR-1 biocathodes to directly convert CO2 to formate with a faradaic efficiency of 34.5 ± 26.1%. The addition of [EMIM][Ac] to the system significantly increased cathodic current density and enhanced the faradaic efficiency to 94.5 ± 4.3% while concurrently amplifying the product yield from 34 ± 23 μM to 366 ± 34 μM. These findings demonstrate that ionic liquids can serve as efficient, biocompatible cosolvents for microbial electrochemical reduction of CO2 to value-added products, holding promise for more robust applications of MES.

Shewanella oneidensis: Biotechnological Application of Metal-Reducing Bacteria

What is an unconventional organism in biotechnology? The γ-proteobacterium Shewanella oneidensis might fall into this category as it was initially established as a laboratory model organism for a process that was not seen as potentially interesting for biotechnology. The reduction of solid-state extracellular electron acceptors such as iron and manganese oxides is highly relevant for many biogeochemical cycles, although it turned out in recent years to be quite relevant for many potential biotechnological applications as well. Applications started with the production of nanoparticles and dramatically increased after understanding that electrodes in bioelectrochemical systems can also be used by these organisms. From the potential production of current and hydrogen in these systems and the development of biosensors, the field expanded to anode-assisted fermentations enabling fermentation reactions that were - so far - dependent on oxygen as an electron acceptor. Now the field expands further to cathode-dependent production routines. As a side product to all these application endeavors, S. oneidensis was understood more and more, and our understanding and genetic repertoire is at eye level to E. coli. Corresponding to this line of thought, this chapter will first summarize the available arsenal of tools in molecular biology that was established for working with the organism and thereafter describe so far established directions of application. Last but not least, we will highlight potential future directions of work with the unconventional model organism S. oneidensis.

A Comprehensive Review of CO2 Hydrogenation into Formate/Formic Acid Catalyzed by Whole Cell Bacteria

The increasing levels of carbon dioxide (CO2) in the atmosphere, primarily due to the use of fossil fuels, pose a significant threat to the environment and necessitate urgent action to mitigate climate change. Carbon capture and utilization technologies that can convert CO2 into economically valuable compounds have gained attention as potential solutions. Among these technologies, biocatalytic CO2 hydrogenation using bacterial whole cells shows promise for the efficient conversion of CO2 into formate, a valuable chemical compound. Although it was discovered nearly a century ago, comprehensive reviews focusing on the utilization of whole-cell bacteria as the biocatalyst in this area remain relatively limited. Therefore, this review provides an analysis of the progress, strategies, and key findings in this field. It covers the use of living cells, resting cells, or genetically modified bacteria as biocatalysts to convert CO2 into formate, either naturally or with the integration of electrochemical and protochemical techniques as sources of protons and electrons. By consolidating the current knowledge in this field, this review article aims to serve as a valuable resource for researchers and practitioners interested in understanding the recent progress, challenges, and potential applications of bacterial whole cell catalyzed CO2 hydrogenation into formate.

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.

Development of Cupriavidus necator H16 as a host for heterologous production of formate dehydrogenase I of Methylorubrum extorquens: Possibilities and limitations

The discovery of formate dehydrogenase (Me-FDH1) from Methylorubrum extorquens has provided an avenue for sustainable CO2 fixation and utilization. However, the mass production of Me-FDH1 is challenging due to the presence of its unique tungsto-bis-metalopterin guanine dinucleotide (W-bis-MGD) cofactor, limiting its practical applications. In this study, C. necator H16 is proposed as a host for the large-scale production of Me-FDH1, utilizing fructose as a carbon source and its inherent machinery for cofactor synthesis. In a minimal salt medium, C. necator H16 could produce active Me-FDH1, which exhibited a specific activity of 80 to 100 U/mg for CO2 conversion to formate. In fed batch bioreactor experiments, approximately 50 g CDW/L (cell dry weight/L) and 10,000 U/L Me-FDH1 were achieved within 50 h. This study highlights C. necator H16 as the recombinant host for Me-FDH1, paving the way for the future development of efficient mass-production methods for this crucial enzyme.

Influence of carbon-based cathodes on biofilm composition and electrochemical performance in soil microbial fuel cells

Increasing energy demands and environmental pollution concerns press for sustainable and environmentally friendly technologies. Soil microbial fuel cell (SMFC) technology has great potential for carbon-neutral bioenergy generation and self-powered electrochemical bioremediation. In this study, an in-depth assessment on the effect of several carbon-based cathode materials on the electrochemical performance of SMFCs is provided for the first time. An innovative carbon nanofibers electrode doped with Fe (CNFFe) is used as cathode material in membrane-less SMFCs, and the performance of the resulting device is compared with SMFCs implementing either Pt-doped carbon cloth (PtC), carbon cloth, or graphite felt (GF) as the cathode. Electrochemical analyses are integrated with microbial analyses to assess the impact on both electrogenesis and microbial composition of the anodic and cathodic biofilm. The results show that CNFFe and PtC generate very stable performances, with a peak power density (with respect to the cathode geometric area) of 25.5 and 30.4 mW m^-2, respectively. The best electrochemical performance was obtained with GF, with a peak power density of 87.3 mW m^-2. Taxonomic profiling of the microbial communities revealed differences between anodic and cathodic communities. The anodes were predominantly enriched with Geobacter and Pseudomonas species, while cathodic communities were dominated by hydrogen-producing and hydrogenotrophic bacteria, indicating H₂ cycling as a possible electron transfer mechanism. The presence of nitrate-reducing bacteria, combined with the results of cyclic voltammograms, suggests microbial nitrate reduction occurred on GF cathodes. The results of this study can contribute to the development of effective SMFC design strategies for field implementation.

Renewable Hydrogen Production and Storage Via Enzymatic Interconversion of CO2 and Formate with Electrochemical Cofactor Regeneration

The urgent need to reduce CO2 emissions has motivated the development of CO2 capture and utilization technologies. An emerging application is CO2 transformation into storage chemicals for clean energy carriers. Formic acid (FA), a valuable product of CO2 reduction, is an excellent hydrogen carrier. CO2 conversion to FA, followed by H2 release from FA, are conventionally chemically catalyzed. Biocatalysts offer a highly specific and less energy-intensive alternative. CO2 conversion to formate is catalyzed by formate dehydrogenase (FDH), which usually requires a cofactor to function. Several FDHs have been incorporated in bioelectrochemical systems where formate is produced by the biocathode and the cofactor is electrochemically regenerated. H2 production from formate is also catalyzed by several microorganisms possessing either formate hydrogenlyase or hydrogen-dependent CO2 reductase complexes. Combination of these two processes can lead to a CO2 -recycling cycle for H2 production, storage, and release with potentially lower environmental impact than conventional methods.

Structure and function relationship of formate dehydrogenases: an overview of recent progress

Formate dehydrogenases (FDHs) catalyze the two-electron oxidation of formate to carbon dioxide. FDHs can be divided into several groups depending on their subunit composition and active-site metal ions. Metal-dependent (Mo- or W-containing) FDHs from prokaryotic organisms belong to the superfamily of molybdenum enzymes and are members of the dimethylsulfoxide reductase family. In this short review, recent progress in the structural analysis of FDHs together with their potential biotechnological applications are summarized.

Direct Biocatalytic Processes for CO2 Capture as a Green Tool to Produce Value-Added Chemicals

Direct biocatalytic processes for CO₂ capture and transformation in value-added chemicals may be considered a useful tool for reducing the concentration of this greenhouse gas in the atmosphere. Among the other enzymes, carbonic anhydrase (CA) and formate dehydrogenase (FDH) are two key biocatalysts suitable for this challenge, facilitating the uptake of carbon dioxide from the atmosphere in complementary ways. Carbonic anhydrases accelerate CO₂ uptake by promoting its solubility in water in the form of hydrogen carbonate as the first step in converting the gas into a species widely used in carbon capture storage and its utilization processes (CCSU), particularly in carbonation and mineralization methods. On the other hand, formate dehydrogenases represent the biocatalytic machinery evolved by certain organisms to convert CO₂ into enriched, reduced, and easily transportable hydrogen species, such as formic acid, via enzymatic cascade systems that obtain energy from chemical species, electrochemical sources, or light. Formic acid is the basis for fixing C₁-carbon species to other, more reduced molecules. In this review, the state-of-the-art of both methods of CO₂ uptake is assessed, highlighting the biotechnological approaches that have been developed using both enzymes.

Lignocellulosic biomass valorization via bio-photo/electro hybrid catalytic systems

Lignocellulosic biomass valorization is regarded as a promising approach to alleviate energy crisis and achieve carbon neutrality. Bioactive enzymes have attracted great attention and been commonly applied for biomass valorization owing to their high selectivity and catalytic efficiency under environmentally benign reaction conditions. Same as biocatalysis, photo-/electro-catalysis also happens at mild conditions (i.e., near ambient temperature and pressure). Therefore, the combination of these different catalytic approaches to benefit from their resulting synergy is appealing. In such hybrid systems, harness of renewable energy from the photo-/electro-catalytic compartment can be combined with the unique selectivity of biocatalysts, therefore providing a more sustainable and greener approach to obtain fuels and value-added chemicals from biomass. In this review, we firstly introduce the pros/cons, classifications, and the applications of photo-/electro-enzyme coupled systems. Then we focus on the fundamentals and comprehensive applications of the most representative biomass-active enzymes including lytic polysaccharide monooxygenases (LPMOs), glucose oxidase (GOD)/dehydrogenase (GDH) and lignin peroxidase (LiP), together with other biomass-active enzymes in the photo-/electro- enzyme coupled systems. Finally, we propose current deficiencies and future perspectives of biomass-active enzymes to be applied in the hybrid catalytic systems for global biomass valorization.

Recent Applications and Strategies to Enhance Performance of Electrochemical Reduction of CO2 Gas into Value-Added Chemicals Catalyzed by Whole-Cell Biocatalysts

Carbon dioxide (CO2) is one of the major greenhouse gases that has been shown to cause global warming. Decreasing CO2 emissions plays an important role to minimize the impact of climate change. The utilization of CO2 gas as a cheap and sustainable source to produce higher value-added chemicals such as formic acid, methanol, methane, and acetic acid has been attracting much attention. The electrochemical reduction of CO2 catalyzed by whole-cell biocatalysts is a promising process for the production of value-added chemicals because it does not require costly enzyme purification steps and the supply of exogenous cofactors such as NADH. This study covered the recent applications of the diversity of microorganisms (pure cultures such as Shewanella oneidensis MR1, Sporomusa species, and Clostridium species and mixed cultures) as whole-cell biocatalysts to produce a wide range of value-added chemicals including methane, carboxylates (e.g., formate, acetate, butyrate, caproate), alcohols (e.g., ethanol, butanol), and bioplastics (e.g., Polyhydroxy butyrate). Remarkably, this study provided insights into the molecular levels of the proteins/enzymes (e.g., formate hydrogenases for CO2 reduction into formate and electron-transporting proteins such as c-type cytochromes) of microorganisms which are involved in the electrochemical reduction of CO2 into value-added chemicals for the suitable application of the microorganism in the chemical reduction of CO2 and enhancing the catalytic efficiency of the microorganisms toward the reaction. Moreover, this study provided some strategies to enhance the performance of the reduction of CO2 to produce value-added chemicals catalyzed by whole-cell biocatalysts.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Recent trends of biotechnological production of polyhydroxyalkanoates from C1 carbon sources

Growing concerns over the use of limited fossil fuels and their negative impacts on the ecological niches have facilitated the exploration of alternative routes. The use of conventional plastic material also negatively impacts the environment. One such green alternative is polyhydroxyalkanoates, which are biodegradable, biocompatible, and environmentally friendly. Recently, researchers have focused on the utilization of waste gases particularly those belonging to C1 sources derived directly from industries and anthropogenic activities, such as carbon dioxide, methane, and methanol as the substrate for polyhydroxyalkanoates production. Consequently, several microorganisms have been exploited to utilize waste gases for their growth and biopolymer accumulation. Methylotrophs such as Methylobacterium organophilum produced highest amount of PHA up to 88% using CH₄ as the sole carbon source and 52-56% with CH₃OH. On the other hand Cupriavidus necator, produced 71-81% of PHA by utilizing CO and CO₂ as a substrate. The present review shows the potential of waste gas valorization as a promising solution for the sustainable production of polyhydroxyalkanoates. Key bottlenecks towards the usage of gaseous substrates obstructing their realization on a large scale and the possible technological solutions were also highlighted. Several strategies for PHA production using C1 gases through fermentation and metabolic engineering approaches are discussed. Microbes such as autotrophs, acetogens, and methanotrophs can produce PHA from CO₂, CO, and CH₄. Therefore, this article presents a vision of C1 gas into bioplastics are prospective strategies with promising potential application, and aspects related to the sustainability of the system.

Winogradsky Bioelectrochemical System as a Novel Strategy to Enrich Electrochemically Active Microorganisms from Arsenic-Rich Sediments

Bioelectrochemical systems (BESs) have been extensively studied for treatment and remediation. However, BESs have the potential to be used for the enrichment of microorganisms that could replace their natural electron donor or acceptor for an electrode. In this study, Winogradsky BES columns with As-rich sediments extracted from an Andean watershed were used as a strategy to enrich lithotrophic electrochemically active microorganisms (EAMs) on electrodes (i.e., cathodes). After 15 months, Winogradsky BESs registered power densities up to 650 μWcm^-2. Scanning electron microscopy and linear sweep voltammetry confirmed microbial growth and electrochemical activity on cathodes. Pyrosequencing evidenced differences in bacterial composition between sediments from the field and cathodic biofilms. Six EAMs from genera Herbaspirillum, Ancylobacter, Rhodococcus, Methylobacterium, Sphingomonas, and Pseudomonas were isolated from cathodes using a lithoautotrophic As oxidizers culture medium. These results suggest that the tested Winogradsky BES columns result in an enrichment of electrochemically active As-oxidizing microorganisms. A bioelectrochemical boost of centenarian enrichment approaches, such as the Winogradsky column, represents a promising strategy for prospecting new EAMs linked with the biogeochemical cycles of different metals and metalloids.

Fabrication of a biocathode for formic acid production upon the immobilization of formate dehydrogenase from Candida boidinii on a nanoporous carbon

The immobilization of the non-metallic enzyme formate dehydrogenase from Candida boidinii (CbFDH) into a nanoporous carbon with appropriate pore structure was explored for the bioelectrochemical conversion of CO₂ to formic acid (FA). Higher FA production rates were obtained upon immobilization of CbFDH compared to the performance of the enzyme in solution, despite the lower nominal CbFDH to NADH (β-nicotinamide adenine dinucleotide reduced) cofactor ratio and the lower amount of enzyme immobilized. The co-immobilization of the enzyme and a rhodium complex as mediator in the nanoporous carbon allowed the electrochemical regeneration of the cofactor. Preparative electrosynthesis of FA carried out on biocathodes of relatively large dimensions (ca. 3 cm × 2 cm) confirmed the higher production rate of FA for the immobilized enzyme. Furthermore, the incorporation of a Nafion binder in the biocathodes did not modify the immobilization extent of the CbFDH in the carbon support. Coulombic efficiencies close to 46% were obtained for the electrosynthesis carried out at -0.8 V for the biocathodes prepared using the lowest Nafion binder content and the co-immobilized enzyme and rhodium redox mediator. Although these values may yet be improved, they confirm the feasibility of these biocathodes in larger scales (6 cm²) beyond most common electrode dimensions reported in the literature (ca. a few mm²).

Applications of Synthetic Biotechnology on Carbon Neutrality Research: A Review on Electrically Driven Microbial and Enzyme Engineering

With the advancement of science, technology, and productivity, the rapid development of industrial production, transportation, and the exploitation of fossil fuels has gradually led to the accumulation of greenhouse gases and deterioration of global warming. Carbon neutrality is a balance between absorption and emissions achieved by minimizing carbon dioxide (CO2) emissions from human social productive activity through a series of initiatives, including energy substitution and energy efficiency improvement. Then CO2 was offset through forest carbon sequestration and captured at last. Therefore, efficiently reducing CO2 emissions and enhancing CO2 capture are a matter of great urgency. Because many species have the natural CO2 capture properties, more and more scientists focus their attention on developing the biological carbon sequestration technique and further combine with synthetic biotechnology and electricity. In this article, the advances of the synthetic biotechnology method for the most promising organisms were reviewed, such as cyanobacteria, Escherichia coli, and yeast, in which the metabolic pathways were reconstructed to enhance the efficiency of CO2 capture and product synthesis. Furthermore, the electrically driven microbial and enzyme engineering processes are also summarized, in which the critical role and principle of electricity in the process of CO2 capture are canvassed. This review provides detailed summary and analysis of CO2 capture through synthetic biotechnology, which also pave the way for implementing electrically driven combined strategies.

The challenges of using NAD+-dependent formate dehydrogenases for CO2 conversion

In recent years, CO₂ reduction and utilization have been proposed as an innovative solution for global warming and the ever-growing energy and raw material demands. In contrast to various classical methods, including chemical, electrochemical, and photochemical methods, enzymatic methods offer a green and sustainable option for CO₂ conversion. In addition, enzymatic hydrogenation of CO₂ into platform chemicals could be used to produce economically useful hydrogen storage materials, making it a win-win strategy. The thermodynamic and kinetic stability of the CO₂ molecule makes its utilization a challenging task. However, Nicotine adenine dinucleotide (NAD⁺)-dependent formate dehydrogenases (FDHs), which have high selectivity and specificity, are attractive catalysts to overcome this issue and convert CO₂ into fuels and renewable chemicals. It is necessary to improve the stability, cofactor necessity, and CO₂ conversion efficiency of these enzymes, such as by combining them with appropriate hybrid systems. However, metal-independent, NAD⁺-dependent FDHs, and their CO₂ reduction activity have received limited attention to date. This review outlines the CO₂ reduction ability of these enzymes as well as their properties, reaction mechanisms, immobilization strategies, and integration with electrochemical and photochemical systems for the production of formic acid or formate. The biotechnological applications of FDH, future perspectives, barriers to CO₂ reduction with FDH, and aspects that must be further developed are briefly summarized. We propose that constructing hybrid systems that include NAD⁺-dependent FDHs is a promising approach to convert CO₂ and strengthen the sustainable carbon bio-economy.

Metabolic engineering of Methylorubrum extorquens AM1 for poly (3-hydroxybutyrate-co-3-hydroxyvalerate) production using formate

Formate is a promising environmentally friendly and sustainable feedstock synthesized from syngas or carbon dioxide. Methylorubrum extorquens is a type II methylotroph that can use formate as a carbon source. It accumulates polyhydroxyalkanoates (PHAs) inside the cell, mainly producing poly-3-hydroxybutyrate (PHB), a degradable biopolymer. Owing to its high melting point and stiff nature, however, mechanical property improvement is warranted in the form of copolymerization. To produce the PHA copolymer, poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), the endogenous gene phaC was deleted and the pathway genes bktB, phaJ1, and phaC2, with broader substrate specificities, were heterologously expressed. To improve the incorporation of 3-hydroxyvalerate (3HV), the expression level of bktB was improved by untranslated region (UTR) engineering, and the endogenous gene phaA was deleted. The engineered M. extorquens produced PHBV with 8.9% 3HV using formate as the sole carbon source. In addition, when propionate and butyrate were supplemented, PHBVs with 3HV portions of up to 70.6% were produced. This study shows that a PHBV copolymer with a high proportion of 3HV can be synthesized using formate, a C1 carbon source, through metabolic engineering and supplementation with short-chain fatty acids.

The Future of Carbon Dioxide Chemistry

The utilization of carbon dioxide as building block for chemicals or source of carbon for energy products has been explored for over 40 years now, with varying allure. In correspondence with oil-crises, the use of CO₂ has come into the spotlight, soon set aside when the crisis was over due to the low price of fossil carbon and the convenience of using established technologies. Nowadays, there is a continuous shift from fossil-C-based to perennial (solar, wind, geothermal, hydro-power) energy-driven processes that will also have a great potential to convert large amounts of carbon dioxide. The integration of biotechnology and catalysis will be a key player towards the utilization of CO₂ in several different applications, reducing both the extraction of fossil carbon and the carbon transfer to the atmosphere.

Abatement of the membrane biofouling: Performance of an in-situ integrated bioelectrochemical-ultrafiltration system

The practical applications of membrane-based water treatment techniques are constrained by the problem of membrane fouling. Various studies have revealed that interactions between extracellular polymeric substances (EPS) and the membrane surface determine the extent of irreversible fouling. Herein, we describe a novel bioelectrochemical system (BES) integrated with an ultrafiltration (UF) membrane in order to provide an enhanced antifouling property. It was found that the integrated BES membrane system had a superior performance compared to a conventional (control) UF system, as manifested by a much lower development of transmembrane pressure. The BES significantly reduced microbial viability in the membrane tank and the imposed electrode potential contributed to the degradation of biopolymers, which favored the alleviation of membrane fouling. Notably, the electron transfer between the acclimated microorganisms and the conductive membrane in the BES integrated system exhibited an increasing trend with the operation time, indicating a gradual increase in microbial electrical activity. Correspondingly, the accumulation of extracellular polymeric substances (EPS) on the membrane surface of the BES integrated system showed a substantial decrease compared to the control system, which could be attributed to a series of synergistic effects induced by the BES integration. The differences in the microbial diversity between the control and the BES integrated system revealed the microbial selectivity of the poised potential. Specifically, microbial strains with relatively high EPS production, like the genus of Zoogloea and Methyloversatilis, were reduced significantly in the BES integrated system, while the expression of the electroactive bacteria was promoted, which facilitated extracellular electron transfer (EET) and therefore the bioelectrochemical reactions. Overall, this study has presented a feasible and promising new approach for membrane fouling mitigation during the process of water treatment.

Elevated conversion of CO2 to versatile formate by a newly discovered formate dehydrogenase from Rhodobacter aestuarii

Due to climate change, recent research interests have increased towards CO₂ utilization as a strategy to mitigate the atmospheric CO₂ level. Herein, we aimed to explore formate dehydrogenases (FDHs) from chemoautotroph to discover an efficient and O₂-tolerant biocatalyst for catalyzing the CO₂ reduction to a versatile formate. Through genome-mining and phylogenetic analysis, the FDH from Rhodobacter aestuarii (RaFDH) was newly discovered as a promising O₂-tolernat CO₂ reductase and was successfully expressed in Escherichia coli. In this study, the optimum conditions and turnover rates of RaFDH were examined for CO₂ reduction and formate oxidation. In particular, the RaFDH-driven CO₂ reduction far surpassed the formate oxidation with a turnover rate of 48.3 and 15.6 min^-1, respectively. The outstanding superiority of RaFDH towards CO₂ reduction can be applicable for constructing a feasible electroenzymatic system that produce a versatile formate from CO₂ as a cheap, abundant, and renewable resource.

Strategies for Bioelectrochemical CO2 Reduction

Atmospheric CO₂ is a cheap and abundant source of carbon for synthetic applications. However, the stability of CO₂ makes its conversion to other carbon compounds difficult and has prompted the extensive development of CO₂ reduction catalysts. Bioelectrocatalysts are generally more selective, highly efficient, can operate under mild conditions, and use electricity as the sole reducing agent. Improving the communication between an electrode and a bioelectrocatalyst remains a significant area of development. Through the examples of CO₂ reduction catalyzed by electroactive enzymes and whole cells, recent advancements in this area are compared and contrasted.

Recent Advances in Power-to-X Technology for the Production of Fuels and Chemicals

Environmental issues related to greenhouse gas emissions are progressively pushing the transition toward fossil-free energy scenario, in which renewable energies such as solar and wind power will unavoidably play a key role. However, for this transition to succeed, significant issues related to renewable energy storage have to be addressed. Power-to-X (PtX) technologies have gained increased attention since they actually convert renewable electricity to chemicals and fuels that can be more easily stored and transported. H2 production through water electrolysis is a promising approach since it leads to the production of a sustainable fuel that can be used directly in hydrogen fuel cells or to reduce carbon dioxide (CO2) in chemicals and fuels compatible with the existing infrastructure for production and transportation. CO2 electrochemical reduction is also an interesting approach, allowing the direct conversion of CO2 into value-added products using renewable electricity. In this review, attention will be given to technologies for sustainable H2 production, focusing on water electrolysis using renewable energy as well as on its remaining challenges for large scale production and integration with other technologies. Furthermore, recent advances on PtX technologies for the production of key chemicals (formic acid, formaldehyde, methanol and methane) and fuels (gasoline, diesel and jet fuel) will also be discussed with focus on two main pathways: CO2 hydrogenation and CO2 electrochemical reduction.

Enhanced Bio-Electrochemical Reduction of Carbon Dioxide by Using Neutral Red as a Redox Mediator

Microbial electrosynthetic cells containing Methylobacterium extorquens were studied for the reduction of CO2 to formate by direct electron injection and redox mediator-assisted approaches, with CO2 as the sole carbon source. The formation of a biofilm on a carbon felt (CF) electrode was achieved while applying a constant potential of -0.75 V versus Ag/AgCl under CO2 -saturated conditions. During the biofilm growth period, continuous H2 evolution was observed. The long-term performance for CO2 reduction of the biofilm with and without neutral red as a redox mediator was studied by an applied potential of -0.75 V versus Ag/AgCl. The neutral red was introduced into the systems in two different ways: homogeneous (dissolved in solution) and heterogeneous (electropolymerized onto the working electrode). The heterogeneous approach was investigated in the microbial system, for the first time, where the CF working electrode was coated with poly(neutral red) by the oxidative electropolymerization thereof. The formation of poly(neutral red) was characterized by spectroscopic techniques. During long-term electrolysis up to 17 weeks, the formation of formate was observed continuously with an average Faradaic efficiency of 4 %. With the contribution of neutral red, higher formate accumulation was observed. Moreover, the microbial electrosynthetic cell was characterized by means of electrochemical impedance spectroscopy to obtain more information on the CO2 reduction mechanism.

Efficient and Selective Electrochemically Driven Enzyme-Catalyzed Reduction of Carbon Dioxide to Formate using Formate Dehydrogenase and an Artificial Cofactor

Increasing levels of carbon dioxide in the atmosphere and the growing need for energy necessitate a shift toward reliance on renewable energy sources and the utilization of carbon dioxide. Thus, producing carbonaceous fuel by the electrochemical reduction of carbon dioxide has been very appealing. We have focused on addressing the principal challenges of poor selectivity and poor energy efficiency in the electrochemical reduction of carbon dioxide. We have demonstrated here a viable pathway for the efficient and continuous electrochemical reduction of CO₂ to formate using the metal-independent enzyme type of formate dehydrogenase (FDH) derived from C andida boidinii yeast. This type of FDH is attractive because it is commercially produced. In natural metabolic processes, this type of metal-independent FDH oxidizes formate to carbon dioxide using NAD⁺ as a cofactor. We show that FDH can catalyze the reverse process to generate formate when the natural cofactor NADH is replaced with an artificial cofactor, the methyl viologen radical cation. The methyl viologen radical cation is generated in situ, electrochemically. Our approach relies on the special properties of methyl viologen as a "unidirectional" redox cofactor for the conversion of CO₂ to formate. Methyl viologen (in the oxidized form) does not catalyze formate oxidation, while the methyl viologen radical cation is an effective cofactor for the reduction of carbon dioxide. Thus, although the thermodynamic driving force is favorable for the oxidized form of methyl viologen to oxidize formate to carbon dioxide, the kinetic factors are not favorable. Only the reverse reaction of carbon dioxide reduction to formate is kinetically viable with the cofactor, methyl viologen radical cation. Binding free energy calculated from atomistic molecular dynamics (MD) simulations consolidate our understanding of these special binding properties of the methyl viologen radical cation and its ability to facilitate the two-electron reduction of carbon dioxide to formate in metal-independent FDH. By carrying out the reactions in a novel three-compartment cell, we have demonstrated the continuous production of formate at high energy efficiency and yield. This cell configuration uses judiciously selected ion-exchange membranes to separate the reaction compartments to preserve the yields of the methyl viologen radical cation and formate. By the electroregeneration of the methyl viologen radical cation at -0.44 V versus the normal hydrogen electrode, we could produce formate at 20 mV negative to the reversible electrode potential for carbon dioxide reduction to formate. Our results are in sharp contrast to the large overpotentials of -800 to -1000 mV required on metal catalysts, vindicating the selectivity and kinetic facility provided by FDH. Formate yields as high as 97% ± 1% could be realized by avoiding the adventitious reoxidation of the methyl viologen radical cation by molecular oxygen. We anticipate that the insights from the electrochemical studies and the MD simulations to be useful in redesigning the metal-independent FDH and alternate artificial cofactors to achieve even higher rates of conversion.

Adaptively evolved Escherichia coli for improved ability of formate utilization as a carbon source in sugar-free conditions

BACKGROUND

Formate converted from CO₂ reduction has great potential as a sustainable feedstock for biological production of biofuels and biochemicals. Nevertheless, utilization of formate for growth and chemical production by microbial species is limited due to its toxicity or the lack of a metabolic pathway. Here, we constructed a formate assimilation pathway in Escherichia coli and applied adaptive laboratory evolution to improve formate utilization as a carbon source in sugar-free conditions.

RESULTS

The genes related to the tetrahydrofolate and serine cycles from Methylobacterium extorquens AM1 were overexpressed for formate assimilation, which was proved by the ¹³C-labeling experiments. The amino acids detected by GC/MS showed significant carbon labeling due to biomass production from formate. Then, 150 serial subcultures were performed to screen for evolved strains with improved ability to utilize formate. The genomes of evolved mutants were sequenced and the mutations were associated with formate dehydrogenation, folate metabolism, and biofilm formation. Last, 90 mg/L of ethanol production from formate was achieved using fed-batch cultivation without addition of sugars.

CONCLUSION

This work demonstrates the effectiveness of the introduction of a formate assimilation pathway, combined with adaptive laboratory evolution, to achieve the utilization of formate as a carbon source. This study suggests that the constructed E. coli could serve as a strain to exploit formate and captured CO₂.

Photoreduction of CO2 with a Formate Dehydrogenase Driven by Photosystem II Using a Semi-artificial Z-Scheme Architecture

Solar-driven coupling of water oxidation with CO₂ reduction sustains life on our planet and is of high priority in contemporary energy research. Here, we report a photoelectrochemical tandem device that performs photocatalytic reduction of CO₂ to formate. We employ a semi-artificial design, which wires a W-dependent formate dehydrogenase (FDH) cathode to a photoanode containing the photosynthetic water oxidation enzyme, Photosystem II, via a synthetic dye with complementary light absorption. From a biological perspective, the system achieves a metabolically inaccessible pathway of light-driven CO₂ fixation to formate. From a synthetic point of view, it represents a proof-of-principle system utilizing precious-metal-free catalysts for selective CO₂-to-formate conversion using water as an electron donor. This hybrid platform demonstrates the translatability and versatility of coupling abiotic and biotic components to create challenging models for solar fuel and chemical synthesis.

Bioelectrochemical conversion of CO2 to value added product formate using engineered Methylobacterium extorquens

The conversion of carbon dioxide to formate is a fundamental step for building C1 chemical platforms. Methylobacterium extorquens AM1 was reported to show remarkable activity converting carbon dioxide into formate. Formate dehydrogenase 1 from M. extorquens AM1 (MeFDH1) was verified as the key responsible enzyme for the conversion of carbon dioxide to formate in this study. Using a 2% methanol concentration for induction, microbial harboring the recombinant MeFDH1 expressing plasmid produced the highest concentration of formate (26.6 mM within 21 hours) in electrochemical reactor. 60 μM of sodium tungstate in the culture medium was optimal for the expression of recombinant MeFDH1 and production of formate (25.7 mM within 21 hours). The recombinant MeFDH1 expressing cells showed maximum formate productivity of 2.53 mM/g-wet cell/hr, which was 2.5 times greater than that of wild type. Thus, M. extorquens AM1 was successfully engineered by expressing MeFDH1 as recombinant enzyme to elevate the production of formate from CO₂ after elucidating key responsible enzyme for the conversion of CO₂ to formate.

Mediator-free enzymatic electrosynthesis of formate by the Methanococcus maripaludis heterodisulfide reductase supercomplex

Electrosynthesis of formate is a promising technology to convert CO₂ and electricity from renewable sources into a biocompatible, soluble, non-flammable, and easily storable compound. In the model methanogen Methanococcus maripaludis, uptake of cathodic electrons was shown to proceed indirectly via formation of formate or H₂ by undefined, cell-derived enzymes. Here, we identified that the multi-enzyme heterodisulfide reductase supercomplex (Hdr-SC) of M. maripaludis is capable of direct electron uptake and catalyzes rapid H₂ and formate formation in electrochemical reactors (-800 mV vs Ag/AgCl) and in Fe(0) corrosion assays. In Fe(0) corrosion assays and electrochemical reactors, purified Hdr-SC primarily catalyzed CO₂ reduction to formate with a coulombic efficiency of 90% in the electrochemical cells for 5 days. Thus, this report identified the first enzyme that stably catalyzes the mediator-free electrochemical reduction of CO₂ to formate, which can serve as the basis of an enzyme electrode for sustained electrochemical production of formate.

A continuous system for biocatalytic hydrogenation of CO2 to formate

In this work a novel bioprocess for hydrogenation of CO₂ to formate was developed, using whole cell catalysis by a sulfate-reducing bacterium. Three Desulfovibrio species were tested (D. vulgaris Hildenborough, D. alaskensis G20, and D. desulfuricans ATCC 27774), of which D. desulfuricans showed the highest activity, producing 12mM of formate in batch, with a production rate of 0.09mMh^-1. Gene expression analysis indicated that among the three formate dehydrogenases and five hydrogenases, the cytoplasmic FdhAB and the periplasmic [FeFe] HydAB are the main enzymes expressed in D. desulfuricans in these conditions. The new bioprocess for continuous formate production by D. desulfuricans had a maximum specific formate production rate of 14mMg_dcw^-1h^-1, and more than 45mM of formate were obtained with a production rate of 0.40mMh^-1. This is the first report of a continuous process for biocatalytic formate production.

Electro-stimulated microbial factory for value added product synthesis

Interplay of charge between bacteria and electrode has led to emergence of bioelectrochemical systems which leads to applications such as production of electricity, wastewater treatment, bioremediation and production of value added products. Many electroactive bacteria have been identified that have unique external electron transport systems. Coupling of electron transport with carbon metabolism has opened a new approach of carbon dioxide sequestration. The electron transport mechanism involves various cellular and sub cellular molecules. The outer membrane cytochromes, Mtr-complex and Ech-complex are few key molecules involved in electron transport in many electrogenic bacteria. Few cytochrome independent acetogenic electroactive bacteria were also discovered using Rnf complex to transport electrons. For improved productivity, an efficient bioreactor design is mandatory. It should encompass all certain critical issues such as microbial cell retention, charge dissipation, separators and simultaneous product recovery.

A review of harvesting clean fuels from enzymatic CO2 reduction

This review has summarised single enzyme, multi enzymatic and semiconducting nanomaterial integrated enzymatic systems for CO₂ conversion to clean fuels.

Biocatalysis for the application of CO2 as a chemical feedstock

Biocatalysts, capable of efficiently transforming CO2 into other more reduced forms of carbon, offer sustainable alternatives to current oxidative technologies that rely on diminishing natural fossil-fuel deposits. Enzymes that catalyse CO2 fixation steps in carbon assimilation pathways are promising catalysts for the sustainable transformation of this safe and renewable feedstock into central metabolites. These may be further converted into a wide range of fuels and commodity chemicals, through the multitude of known enzymatic reactions. The required reducing equivalents for the net carbon reductions may be drawn from solar energy, electricity or chemical oxidation, and delivered in vitro or through cellular mechanisms, while enzyme catalysis lowers the activation barriers of the CO2 transformations to make them more energy efficient. The development of technologies that treat CO2-transforming enzymes and other cellular components as modules that may be assembled into synthetic reaction circuits will facilitate the use of CO2 as a renewable chemical feedstock, greatly enabling a sustainable carbon bio-economy.