The progress and outlook of bioelectrocatalysis for the production of chemicals, fuels and materials

Proteomic and metabolic analysis of Moorella thermoacetica-g-C3N4 nanocomposite system for artificial photosynthesis

Artificial photosynthesis by microbe-semiconductor biohybrid systems has been demonstrated as a valuable strategy in providing sustainable energy and in carbon fixation. However, most of the developed biohybrid systems for light harvesting employ heavy metal materials, especially cadmium sulfide (CdS), which normally cause environmental pollution and restrict the widespread of the systems. Herein, we constructed an environmentally friendly biohybirid system based on a typical acetogenic bacteria, Moorella thermoacetica, coupling with a carbon-based semiconductor, graphitic carbon nitride (g-C3N4), to realize light-driven carbon fixation. The proposed biohybrid system displayed outstanding acetate productivity with a quantum yield of 2.66 ± 0.43 %. Non-targeted proteomic analysis indicated that the physiological activity of the bacteria was improved, coupling with the non-toxic material. We further proposed the mechanisms of energy generation, electron transfer and CO2 fixation of the irradiated biohybrid system by proteomic and metabolomic characterization. With the photoelectron generated in g-C3N4 under illumination, CO2 is finally converted to acetate via the Wood-Ljungdahl pathway (WLP). Other associated pathways were also proved to be activated, providing extra energy or substrates for acetate production. The study reveals that the future focus of the development of biohybrid systems for light harvesting can be on the metal-free biocompatible material, which can activate the expression of the key enzymes involved in the electron transfer and carbon metabolism under light irradiation.

Blended nexus molecules promote CO2 to l-tyrosine conversion

Using CO2 as the primary feedstock offers the potential for high-value utilization of CO2 while forging sustainable pathways for producing valuable natural products, such as l-tyrosine. Cascade catalysis is a promising approach but limited by stringent purity demands of nexus molecules. We developed an abiotic/biotic cascade catalysis using blended nexus molecules for l-tyrosine synthesis. Specifically, we begin by constructing a solid-state reactor to reduce CO2 electrochemically, yielding a mixture of acetic acid and ethanol, which serves as the blended nexus molecules. Subsequently, we use genetic engineering to introduce an ethanol utilization pathway and a tyrosine producing pathway to Escherichia coli to facilitate l-tyrosine production. The ethanol pathway synergistically cooperated with the acetic acid pathway, boosting l-tyrosine production rate (nearly five times higher compared to the strain without ethanol utilization pathway) and enhancing carbon efficiency. Our findings demonstrate that using blended nexus molecules could potentially offer a more favorable strategy for the cascade catalysis aimed at producing valuable natural products.

Waste Valorization in a Sustainable Bio-Based Economy: The Road to Carbon Neutrality

The development of sustainable chemistry underlying the quest to minimize and/or valorize waste in the carbon-neutral manufacture of chemicals is followed over the last four to five decades. Both chemo- and biocatalysis have played an indispensable role in this odyssey. in particular developments in protein engineering, metagenomics and bioinformatics over the preceding three decades have played a crucial supporting role in facilitating the widespread application of both whole cell and cell-free biocatalysis. The pressing need, driven by climate change mitigation, for a drastic reduction in greenhouse gas (GHG) emissions, has precipitated an energy transition based on decarbonization of energy and defossilization of organic chemicals production. The latter involves waste biomass and/or waste CO2 as the feedstock and green electricity generated using solar, wind, hydroelectric or nuclear energy. The use of waste polysaccharides as feedstocks will underpin a renaissance in carbohydrate chemistry with pentoses and hexoses as base chemicals and bio-based solvents and polymers as environmentally friendly downstream products. The widespread availability of inexpensive electricity and solar energy has led to increasing attention for electro(bio)catalysis and photo(bio)catalysis which in turn is leading to myriad innovations in these fields.

Engineered Cell Elongation Promotes Extracellular Electron Transfer of Shewanella Oneidensis

To investigate how cell elongation impacts extracellular electron transfer (EET) of electroactive microorganisms (EAMs), the division of model EAM Shewanella oneidensis (S. oneidensis) MR-1 is engineered by reducing the formation of cell divisome. Specially, by blocking the translation of division proteins via anti-sense RNAs or expressing division inhibitors, the cellular length and output power density are all increased. Electrophysiological and transcriptomic results synergistically reveal that the programmed cell elongation reinforces EET by enhancing NADH oxidation, inner-membrane quinone pool, and abundance of c-type cytochromes. Moreover, cell elongation enhances hydrophobicity due to decreased cell-surface polysaccharide, thus facilitates the initial surface adhesion stage during biofilm formation. The output current and power density all increase in positive correction with cellular length. However, inhibition of cell division reduces cell growth, which is then restored by quorum sensing-based dynamic regulation of cell growth and elongation phases. The QS-regulated elongated strain thus enables a cell length of 143.6 ± 40.3 µm (72.6-fold of that of S. oneidensis MR-1), which results in an output power density of 248.0 ± 10.6 mW m-2 (3.41-fold of that of S. oneidensis MR-1) and exhibits superior potential for pollutant treatment. Engineering cellular length paves an innovate avenue for enhancing the EET of EAMs.

Metal-free functionalized carbon nitride as a photocatalyst driven by sunlight for acetal synthesis and selective regeneration of NAD(P)H cofactor

Nicotinamide Adenine Dinucleotide Phosphate (NAD(P)H) plays an important role in numerous biologically significant redox reactions. The photochemical restoration of its oxidized form (NAD(P)+) under physiological conditions is intriguing in the context of integrated photo and catalysis. Herein, we report the functionalized graphitic carbon-based solar light active photocatalyst by doping boron and fluorine in the native graphitic carbon nitride (GCN) (nonfunctionalized) for the regeneration of enzymatically visible light active coenzyme and in photo-acetalization reactions. The metal-free functionalized photocatalyst systems such as BFGCN-x leads to higher yield NADH and NADPH regeneration. They are also capable of catalyzing acetal reactions in the absence of any Lewis and Bronsted acids. The current research endeavor provides the advancement and the application of functionalized GCN-based photocatalysts for NADH (61.89%), NADPH (59.84%) regeneration, and photo-acetalization reactions.

Navigating the landscape of enzyme design: from molecular simulations to machine learning

Global environmental issues and sustainable development call for new technologies for fine chemical synthesis and waste valorization. Biocatalysis has attracted great attention as the alternative to the traditional organic synthesis. However, it is challenging to navigate the vast sequence space to identify those proteins with admirable biocatalytic functions. The recent development of deep-learning based structure prediction methods such as AlphaFold2 reinforced by different computational simulations or multiscale calculations has largely expanded the 3D structure databases and enabled structure-based design. While structure-based approaches shed light on site-specific enzyme engineering, they are not suitable for large-scale screening of potential biocatalysts. Effective utilization of big data using machine learning techniques opens up a new era for accelerated predictions. Here, we review the approaches and applications of structure-based and machine-learning guided enzyme design. We also provide our view on the challenges and perspectives on effectively employing enzyme design approaches integrating traditional molecular simulations and machine learning, and the importance of database construction and algorithm development in attaining predictive ML models to explore the sequence fitness landscape for the design of admirable biocatalysts.

Anchoring Ultrafine β-Mo2C Clusters Inside Porous Co-NC Using MOFs for Electric-Powered Coproduction of Valuable Chemicals

Electroredox of organics provides a promising and green approach to producing value-added chemicals. However, it remains a grand challenge to achieve high selectivity of desired products simultaneously at two electrodes, especially for non-isoelectronic transfer reactions. Here a porous heterostructure of Mo2C@Co-NC is successfully fabricated, where subnanometre β-Mo2C clusters (<1 nm, ≈10 wt%) are confined inside porous Co, N-doped carbon using metalorganic frameworks. It is found that Co species not only promote the formation of β-Mo2C but also can prevent it from oxidation by constructing the heterojunctions. As noted, the heterostructure achieves >96% yield and 92% Faradaic efficiency (FE) for aldehydes in anodic alcohol oxidation, as well as >99.9% yield and 96% FE for amines in cathodal nitrocompounds reduction in 1.0 M KOH. Precise control of the reaction kinetics of two half-reactions by the electronic interaction between β-Mo2C and Co is a crucial adjective. Density functional theory (DFT) gives in-depth mechanistic insight into the high aldehyde selectivity. The work guides authors to reveal the electrooxidation nature of Mo2C at a subnanometer level. It is anticipated that the strategy will provide new insights into the design of highly effective bifunctional electrocatalysts for the coproduction of more complex fine chemicals.

Synergistic material-microbe interface toward deeper anaerobic defluorination

Per- and polyfluoroalkyl substances (PFAS), particularly the perfluorinated ones, are recalcitrant to biodegradation. By integrating an enrichment culture of reductive defluorination with biocompatible electrodes for the electrochemical process, a deeper defluorination of a C6-perfluorinated unsaturated PFAS was achieved compared to the biological or electrochemical system alone. Two synergies in the bioelectrochemical system were identified: i) The in-series microbial-electrochemical defluorination and ii) the electrochemically enabled microbial defluorination of intermediates. These synergies at the material-microbe interfaces surpassed the limitation of microbial defluorination and further turned the biotransformation end products into less fluorinated products, which could be less toxic and more biodegradable in the environment. This material-microbe hybrid system brings opportunities in the bioremediation of PFAS driven by renewable electricity and warrants future research on mechanistic understanding of defluorinating and electroactive microorganisms at the material-microbe interface for system optimizations.

Integrated Proteomics and Metabolomics Reveal Altered Metabolic Regulation of Xanthobacter autotrophicus under Electrochemical Water-Splitting Conditions

Biological-inorganic hybrid systems are a growing class of technologies that combine microorganisms with materials for a variety of purposes, including chemical synthesis, environmental remediation, and energy generation. These systems typically consider microorganisms as simple catalysts for the reaction of interest; however, other metabolic activity is likely to have a large influence on the system performance. The investigation of biological responses to the hybrid environment is thus critical to the future development and optimization. The present study investigates this phenomenon in a recently reported hybrid system that uses electrochemical water splitting to provide reducing equivalents to the nitrogen-fixing bacteria Xanthobacter autotrophicus for efficient reduction of N2 to biomass that may be used as fertilizer. Using integrated proteomic and metabolomic methods, we find a pattern of differentiated metabolic regulation under electrochemical water-splitting (hybrid) conditions with an increase in carbon fixation products glycerate-3-phosphate and acetyl-CoA that suggests a high energy availability. We further report an increased expression of proteins of interest, namely, those responsible for nitrogen fixation and assimilation, which indicate increased rates of nitrogen fixation and support previous observations of faster biomass accumulation in the hybrid system compared to typical planktonic growth conditions. This work complicates the inert catalyst view of biological-inorganic hybrids while demonstrating the power of multiomics analysis as a tool for deeper understanding of those systems.

Sustainable media feedstocks for cellular agriculture

The global food system is shifting towards cellular agriculture, a second domestication marked by cultivating microorganisms and tissues for sustainable food production. This involves tissue engineering, precision fermentation, and microbial biomass fermentation to establish food value chains independent of traditional agriculture. However, these techniques rely on growth media sourced from agricultural, chemical (fossil fuels), and mining supply chains, raising concerns about land use competition, emissions, and resource depletion. Fermentable sugars, nitrogen, and phosphates are key ingredients derived from starch crops, energy-intensive fossil fuel based processes, and finite phosphorus resources, respectively. This review explores sustainable alternatives to reduce land use and emissions associated with cellular agriculture media ingredients. Sustainable alternatives to first generation sugars (lignocellulosic substrates, sidestreams, and gaseous feedstocks), sustainable nitrogen sources (sidestreams, green ammonia, biological nitrogen fixation), and efficient use of phosphates are reviewed. Especially cellulosic sugars, gaseous chemoautotrophic feedstocks, green ammonia, and phosphate recycling are the most promising technologies but economic constraints hinder large-scale adoption, necessitating more efficient processes and cost reduction. Collaborative efforts are vital for a biotechnological future grounded in sustainable feedstocks, mitigating competition with agricultural land and emissions.

Hydrodynamics-Controlled Single-Particle Electrocatalysis

Electrocatalysis is considered promising in renewable energy conversion and storage, yet numerous efforts rely on catalyst design to advance catalytic activity. Herein, a hydrodynamic single-particle electrocatalysis methodology is developed by integrating collision electrochemistry and microfluidics to improve the activity of an electrocatalysis system. As a proof-of-concept, hydrogen evolution reaction (HER) is electrocatalyzed by individual palladium nanoparticles (Pd NPs), with the development of microchannel-based ultramicroelectrodes. The controlled laminar flow enables the precise delivery of Pd NPs to the electrode-electrolyte interface one by one. Compared to the diffusion condition, hydrodynamic collision improves the number of active sites on a given electrode by 2 orders of magnitude. Furthermore, forced convection enables the enhancement of proton mass transport, thereby increasing the electrocatalytic activity of each single Pd NP. It turns out that the improvement in mass transport increases the reaction rate of HER at individual Pd NPs, thus a phase transition without requiring a high overpotential. This study provides new avenues for enhancing electrocatalytic activity by altering operating conditions, beyond material design limitations.

Harnessing iron‑sulfur enzymes for synthetic biology

Reactions catalysed by iron-sulfur (Fe-S) enzymes appear in a variety of biosynthetic pathways that produce valuable natural products. Harnessing these biosynthetic pathways by expression in microbial cell factories grown on an industrial scale would yield enormous economic and environmental benefits. However, Fe-S enzymes often become bottlenecks that limits the productivity of engineered pathways. As a consequence, achieving the production metrics required for industrial application remains a distant goal for Fe-S enzyme-dependent pathways. Here, we identify and review three core challenges in harnessing Fe-S enzyme activity, which all stem from the properties of Fe-S clusters: 1) limited Fe-S cluster supply within the host cell, 2) Fe-S cluster instability, and 3) lack of specialized reducing cofactor proteins often required for Fe-S enzyme activity, such as enzyme-specific flavodoxins and ferredoxins. We highlight successful methods developed for a variety of Fe-S enzymes and electron carriers for overcoming these difficulties. We use heterologous nitrogenase expression as a grand case study demonstrating how each of these challenges can be addressed. We predict that recent breakthroughs in protein structure prediction and design will prove well-suited to addressing each of these challenges. A reliable toolkit for harnessing Fe-S enzymes in engineered metabolic pathways will accelerate the development of industry-ready Fe-S enzyme-dependent biosynthesis pathways.

Biocompatible Cu/NiMo Composite Electrocatalyst for Hydrogen Evolution Reaction in Microbial Electrosynthesis; Unveiling the Self-Detoxification Effect of Cu

H2-driven microbial electrosynthesis (MES) is an emerging bioelectrochemical technology that enables the production of complex compounds from CO2. Although the performance of microbial fermentation in the MES system is closely related to the H2 production rate, high-performing metallic H2-evolving catalysts (HEC) generate cytotoxic H2O2 and metal cations from undesirable side reactions, severely damaging microorganisms. Herein, a novel design for self-detoxifying metallic HEC, resulting in biologically benign H2 production, is reported. Cu/NiMo composite HEC suppresses H2O2 evolution by altering the O2 reduction kinetics to a four-electron pathway and subsequently decomposes the inevitably generated H2O2 in sequential catalytic and electrochemical pathways. Furthermore, in situ generated Cu-rich layer at the surface prevents NiMo from corroding and releasing cytotoxic Ni cations. Consequently, the Cu/NiMo composite HEC in the MES system registers a 50% increase in the performance of lithoautotrophic bacterium Cupriavidus necator H16, for the conversion of CO2 to a biopolymer, poly(3-hydroxybutyrate). This work successfully demonstrates the concept of self-detoxification in designing biocompatible materials for bioelectrochemical applications as well as MES systems.

Atomic Pyridinic Nitrogen as Highly Active Metal-Free Coordination Sites at the Biotic-Abiotic Interface for Bio-Electrochemical CO2 Reduction

Bio-electrochemical conversion of anthropogenic CO2 into value-added products using cost-effective metal-free catalysts represents a promising strategy for sustainable fuel production. Herein, N-doped carbon nanosheets synthesized via pyrolysis of the zeolitic-imidazolate framework (ZIF) are developed for constructing efficient biohybrids to facilitate CO2-to-CH4 conversion. The microbial enrichment and bio-interfacial charge transfer are significantly affected by the proportion of the co-existed graphitic-N, pyridinic-N, and pyrrolic-N in the defective carbon nanosheets. It is unfolded that pyridinic-N and pyrrolic-N with the doped N atoms near the edge can significantly enhance the adsorption of their adjacent C atoms toward O, leading to improved microbe enrichment. Especially, pyridinic-N which can provide one p electron to the aromatic π system, greatly enhances the electron-donating capability of the carbon nanosheets to the microorganisms. Correspondingly, due to its largest amount of pyridinic-N doping, the N-doped carbon nanosheets derived from ZIF pyrolysis at 900 °C (denoted 900-NC) achieve the highest methane production of ≈215.7 mmol m-2 day-1 with a high selectivity (Faradaic efficiency = ≈94.2%) at -0.9 V versus Ag/AgCl. This work demonstrates the effectiveness of N-doped carbon catalysts for bio-electrochemical CO2 fixation and contributes to the understanding of N functionalities toward microbiome response and biotic-abiotic charge transfer in various bio-electrochemical systems.

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.

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

Advances in Biological Wastewater Treatment Processes: Focus on Low-Carbon Energy and Resource Recovery in Biorefinery Context

Advancements in biological wastewater treatment with sustainable and circularity approaches have a wide scope of application. Biological wastewater treatment is widely used to remove/recover organic pollutants and nutrients from a diverse wastewater spectrum. However, conventional biological processes face challenges, such as low efficiency, high energy consumption, and the generation of excess sludge. To overcome these limitations, integrated strategies that combine biological treatment with other physical, chemical, or biological methods have been developed and applied in recent years. This review emphasizes the recent advances in integrated strategies for biological wastewater treatment, focusing on their mechanisms, benefits, challenges, and prospects. The review also discusses the potential applications of integrated strategies for diverse wastewater treatment towards green energy and resource recovery, along with low-carbon fuel production. Biological treatment methods, viz., bioremediation, electro-coagulation, electro-flocculation, electro-Fenton, advanced oxidation, electro-oxidation, bioelectrochemical systems, and photo-remediation, are summarized with respect to non-genetically modified metabolic reactions. Different conducting materials (CMs) play a significant role in mass/charge transfer metabolic processes and aid in enhancing fermentation rates. Carbon, metal, and nano-based CMs hybridization in different processes provide favorable conditions to the fermentative biocatalyst and trigger their activity towards overcoming the limitations of the conventional process. The emerging field of nanotechnology provides novel additional opportunities to surmount the constraints of conventional process for enhanced waste remediation and resource valorization. Holistically, integrated strategies are promising alternatives for improving the efficiency and effectiveness of biological wastewater treatment while also contributing to the circular economy and environmental protection.

Paradigm shift in Nutrient-Energy-Water centered sustainable wastewater treatment system through synergy of bioelectrochemical system and anaerobic digestion

With advancements in research and the necessity of improving the performance of bioelectrochemical system (BES), coupling anaerobic digestion (AD) with BES is crucial for energy gain from wastewater and bioremediation. Hybridization of BES-AD concept opens new avenues for pollutant degradation, carbon capture and nutrient-resource recovery from wastewater. The strength of merging BES-AD lies in synergy, and this approach was employed to differentiate fads from strategies with the potential for full-scale implementation and making it an energy-positive system. The integration of BES and AD system increases the overall performance and complexity of combined system and the cost of operation. From a technical standpoint, the primary determinants of BES-AD feasibility for field applications are the scalability and economic viability. High potential market for such integrated system attract industrial partners for more industrial trials and investment before commercialization. However, BES-AD with high energy efficacy and negative economics demands performance boost.

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.

Implementation of cytochrome c proteins and carbon nanotubes hybrids in bioelectrodes towards bioelectrochemical systems applications

Multiheme cytochrome c (Cyt c) can function as a redox protein on electrode to accomplish bioelectrocatalysis. However, the direct electron transfer (DET) between the redox site of Cyt c and electrode is low due to the large coupling distance. A close proximity or a connection pathway from the deeply buried active site to the protein surface can be established by modifying the electrode with carbon nanotubes (CNTs) to improve the DET. Therefore, the isolated Cyt c has been assembled or casted with CNTs by various processes to form Cyt c-CNTs bioelectrodes that can be further applied to biosensing and bioanalysis. These strategies can be transplanted to the fabrication of biofilm-CNTs based electrodes by complexing the out membrane (OM) Cyt c of natural electricigen with CNTs to realize the application of the electrochemical properties of "in vivo" Cyt c to bioelectrochemical systems (BESs). This review intends to highlight the preparation strategies of bioelectrodes that have been well studied in electrochemical biosensors and improving approaches of the DET from the CNTs surface to Cyt c in their hybrids. The efficient fabrication processes of the biofilm-CNTs based electrodes that can be considered as "in vivo" Cyt c-CNTs based electrodes for BES designs are also summarized, aiming to provide an inspiration source and a reference to the related studies of BES downstream.

Integration of Enzyme and Covalent Organic Frameworks: From Rational Design to Applications

Manufacturing is undergoing profound transformations, among which green biomanufacturing with low energy consumption, high efficiency, and sustainability is becoming one of the major trends. However, enzymes, as the "core chip" of biomanufacturing, are often handicapped in their application by their high cost, low operational stability, and nonreusability. Immobilization of enzymes is a technology that binds or restricts enzymes in a certain area with solid materials, allows them to still carry out their unique catalytic reaction, and allows them to be recycled and reused. Compared with free enzymes, immobilized enzymes boast numerous advantages such as enhanced storage stability, ease of separation, reusability, and controlled operation. Currently, commonly used supports for enzyme immobilization (e.g., mesoporous silica, sol-gel hydrogels, and porous polymer) can effectively improve enzyme stability and reduce product inhibition. However, they still face drawbacks such as potential leaching or conformational change during immobilization and poor machining performance. Especially, most enzyme carrier solid materials possess disordered structures, inevitably introducing deficiencies such as low loading capacity, hindered mass transfer, and unclear structure-property relationships. Additionally, it remains a notable challenge to meticulously design immobilization systems tailored to the specific characteristics of enzyme/reaction. Therefore, there is a significant demand for reliable solid materials to overcome the above challenges. Crystalline porous materials, particularly covalent organic frameworks (COFs), have garnered significant interest as a promising platform for immobilizing enzymes due to their unique properties, such as their crystalline nature, high porosity, accessible active sites, versatile synthetic conditions, and tunable structure. COFs create a stabilizing microenvironment that protects enzymes from denaturation and significantly enhances reusability. Nevertheless, some challenges still remain, including difficulties in loading large enzymes, reduced enzyme activities, and the limited functionality of carriers. Therefore, it is essential to develop innovative carriers and novel strategies to broaden the methods of immobilizing enzymes, enabling their application across a more diverse array of fields.The integration of enzymes with advanced porous materials for intensified performance and diverse applications is still in its infancy, and our group has done a series of pioneering works. This Account presents a comprehensive overview of recent research progress made by our group, including (i) the development of innovative enzyme immobilization strategies utilizing COFs to make the assembly and integration of enzymes and carriers more effective; (ii) rational design and construction of functional carriers for enzyme immobilization using COFs; and (iii) extensions of immobilized enzyme applications based on COFs from industrial catalysis to biomedicine and chiral separation. The integration of enzymes with functional crystalline materials offers mutual benefits and results in a performance that surpasses what either component can achieve individually. Additionally, immobilized enzymes exhibit enhanced functionality and intriguing characteristics that differ from those of free enzymes. Consistent with our research philosophy centered on integration, platform development, and engineering application, this Account addresses the critical challenges associated with enzyme immobilization using COFs while extending the applications of COFs and proposing future design principles for biomanufacturing and enzyme industry.

Machine Learning in Bioelectrocatalysis

At present, the global energy crisis and environmental pollution coexist, and the demand for sustainable clean energy has been highly concerned. Bioelectrocatalysis that combines the benefits of biocatalysis and electrocatalysis produces high-value chemicals, clean biofuel, and biodegradable new materials. It has been applied in biosensors, biofuel cells, and bioelectrosynthesis. However, there are certain flaws in the application process of bioelectrocatalysis, such as low accuracy/efficiency, poor stability, and limited experimental conditions. These issues can possibly be solved using machine learning (ML) in recent reports although the combination of them is still not mature. To summarize the progress of ML in bioelectrocatalysis, this paper first introduces the modeling process of ML, then focuses on the reports of ML in bioelectrocatalysis, and ultimately makes a summary and outlook about current issues and future directions. It is believed that there is plenty of scope for this interdisciplinary research direction.

A Dual-Functional MXene-Based Bioanode for Wearable Self-Charging Biosupercapacitors

As a reliable energy-supply platform for wearable electronics, biosupercapacitors combine the characteristics of biofuel cells and supercapacitors to harvest and store the energy from human's sweat. However, the bulky preparation process and deep embedding of enzyme active sites in bioelectrodes usually limit the energy-harvesting process, retarding the practical power-supply sceneries especially during the complicated in vivo motion. Herein, a MXene/single-walled carbon nanotube/lactate oxidase hierarchical structure as the dual-functional bioanode is designed, which can not only provide a superior 3D catalytic microenvironment for enzyme accommodation to harvest energy from sweat, but also offers sufficient capacitance to store energy via the electrical double-layer capacitor. A wearable biosupercapacitor is fabricated in the "island-bridge" structure with a composite bioanode, active carbon/Pt cathode, polyacrylamide hydrogel substrate, and liquid metal conductor. The device exhibits an open-circuit voltage of 0.48 V and the high power density of 220.9 µW cm-2 at 0.5 mA cm-2 . The compact conformal adhesion with skin is successfully maintained under stretching/bending conditions. After repeatedly stretching the devices, there is no significant power attenuation in pulsed output. The unique bioelectrode structure and attractive energy harvesting/storing properties demonstrate the promising potential of this biosupercapacitor as a micro self-powered platform of wearable electronics.

Development of a novel magnetic metal-organic framework for the immobilization of short-chain dehydrogenase for the asymmetric reduction of pro-chiral ketone

Short-chain dehydrogenase/reductase (SDR) acts as a biocatalyst in the synthesis of chiral alcohols with high optical purity. Herein, we achieved immobilization via crosslinking on novel magnetic metal-organic framework nanoparticles with a three-layer shell structure (Fe3O4@PDA@Cu (PABA)). The results of scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and energy dispersive X-ray spectroscopy confirmed the morphology and cross-linking property of immobilized SDR, which was more durable, stable, and reusable and exhibited better kinetic performance than free enzyme. The SDR and glucose dehydrogenase (GDH) were co-immobilized and then used for the asymmetric reduction of COBE and ethyl 2-oxo-4-phenylbutanoate (OPBE). These finding suggest that enzymes immobilized on novel MOF nanoparticles can serve as promising biocatalysts for asymmetric reduction prochiral ketones into chiral alcohols.

Interfacial Electron Transfer in Chemical and Biological Transformation of Pollutants in Environmental Catalysis

Interfacial electron transfer (IET) is essential for chemical and biological transformation of pollutants, operative across diverse lengths and time scales. This Perspective presents an array of multiscale molecular simulation methodologies, supplemented by in situ monitoring and imaging techniques, serving as robust tools to decode IET enhancement mechanisms such as interface molecular modification, catalyst coordination mode, and atomic composition regulation. In addition, three IET-based pollutant transformation systems, an electrocatalytic oxidation system, a bioelectrochemical spatial coupling system, and an enzyme-inspired electrocatalytic system, were developed, demonstrating a high effect in transforming and degrading pollutants. To improve the effectiveness and scalability of IET-based strategies, the refinement of these systems is necessitated through rigorous research and theoretical exploration, particularly in the context of practical wastewater treatment scenarios. Future endeavors aim to elucidate the synergy between biological and chemical modules, edit the environmental functional microorganisms, and harness machine learning for designing advanced environmental catalysts to boost efficiency. This Perspective highlights the powerful potential of IET-focused environmental remediation strategies, emphasizing the critical role of interdisciplinary research in addressing the urgent global challenge of water pollution.

Co-NC@Co-NP hierarchical nanoforest steering charge exchange efficiency at biotic-abiotic interface for microbial electrochemical carbon reduction

Converting anthropogenic carbon dioxide (CO2) to value-added products using bio-electrochemical conversions represents a promising strategy for producing sustainable fuel. However, the reaction kinetics are hindered by insufficient attachment of microorganisms and limited charge extraction at the bioinorganic interface. A hierarchical nanoforest with doped cobalt‑nitrogen-doped carbon covering cobalt nanoparticle (Co-NC@Co-NP) was integrated with a CO2-to-CH4 conversion microbiome for methane production to address these shortcomings. In-situ nanoforests were developed on the nanosheet by chemical vapor deposition with Co nanoparticles catalyzed. The bio-nanowire-like carbon nanotubes enhanced the electrostatic force for microbe enrichment via the tip effect, providing a maximum of 3.6-fold electron-receiving microbes to utilize reducing equivalents. The Co-NC@Co-NP enhanced the direct electron transfer between microbes and electrodes, reducing the adoption of energy barriers for heme-like proteins. Thus, the optimized electron transfer pathway improved selectivity by a factor of 2.0 compared to the pristine nanosheet biohybrid. Furthermore, the adjusted microbial community structure provided sufficient methanogenesis genes to match the strong electron flow, achieving maximal methane production rates (311.1 mmol/m2/day at -0.9 V vs. Ag/AgCl), 8.62 times higher than those of the counterpart nanosheet biohybrid (36.06 mmol/m2/day). This work demonstrates a comprehensive assessment of biotic-abiotic energy transfer, which may serve as a guiding principle for designing efficient bio-electrochemical systems.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Electrical-biological hybrid system for carbon efficient isobutanol production

We have developed an electrical-biological hybrid system wherein an engineered microorganism consumes electrocatalytically produced formate from CO2 to supplement the bioproduction of isobutanol, a valuable fuel chemical. Biological CO2 sequestration is notoriously slow compared to electrochemical CO2 reduction, while electrochemical methods struggle to generate carbon-carbon bonds which readily form in biological systems. A hybrid system provides a promising method for combining the benefits of both biology and electrochemistry. Previously, Escherichia coli was engineered to assimilate formate and CO2 in central metabolism using the reductive glycine pathway. In this work, we have shown that chemical production in E. coli can benefit from single carbon substrates when equipped with the RGP. By installing the RGP and the isobutanol biosynthetic pathway into E. coli and by further genetic modifications, we have generated a strain of E. coli that can consume formate and produce isobutanol at a yield of >100% of theoretical maximum from glucose. Our results demonstrate that carbon produced from electrocatalytically reduced CO2 can bolster chemical production in E. coli. This study shows that E. coli can be engineered towards carbon efficient methods of chemical production.

Metabolic regulation of Shewanella oneidensis for microbial electrosynthesis: From extracellular to intracellular

Shewanella oneidensis MR-1 (S. oneidensis MR-1) has been shown to benefit from microbial electrosynthesis (MES) due to its exceptional electron transfer efficiency. In this study, genes involved in both extracellular electron uptake (EEU) and intracellular CO2 conversion processes were examined and regulated to enhance MES performance. The key genes identified for MES in the EEU process were mtrB, mtrC, mtrD, mtrE, omcA and cctA. Overexpression of these genes resulted in 1.5-2.1 times higher formate productivity than that of the wild-type strains (0.63 mmol/(L·μg protein)), as 0.94-1.61 mmol/(L·μg protein). In the intracellular CO2 conversion process, overexpression of the nadE, nadD, nadR, nadV, pncC and petC genes increased formate productivity 1.3-fold-3.4-fold. Moreover, overexpression of the formate dehydrogenase genes fdhA1, fdhB1 and fdhX1 in modified strains led to a 2.3-fold-3.1-fold increase in formate productivity compared to wild-type strains. The co-overexpression of cctA, fdhA1 and nadV in the mutant strain resulted in 5.59 times (3.50 mmol/(L·μg protein)) higher formate productivity than that of the wild-type strains. These findings revealed that electrons of MES derived from the electrode were utilized in the energy module for synthesizing ATP and NADH, followed by the synthesis of formate in formate dehydrogenase by the combinatorial effects of ATP, NADH, electrons and CO2. The results provide new insights into the mechanism of MES in S. oneidensis MR-1 and pave the way for genetic improvements that could facilitate the further application of MES.

Impact of sodium pyruvate on the electrochemical reduction of NAD+ biomimetics

Biomimetics of nicotinamide adenine dinucleotide (mNADH) are promising cost-effective alternatives to their natural counterpart for biosynthetic applications; however, attempts to recycle mNADH often rely on coenzymes or precious metal catalysts. Direct electrolysis is an attractive approach for recycling mNADH, but electrochemical reduction of the oxidized mimetic (mNAD+) primarily results in the formation of an enzymatically inactive dimer. Herein, we find that aqueous electrochemical reduction of an NAD+ mimetic, 1-n-butyl-3-carbamoylpyridinium bromide (1+), to its enzymatically active form, 1,4-dihydro-1-n-butyl nicotinamide (1H), is favored in the presence of sodium pyruvate as a supporting electrolyte. Maximum formation of 1H is achieved in the presence of a large excess of pyruvate in combination with a large excess of a co-supporting electrolyte. Formation of 1H is found to be favored at pH 7, with an optimized product ratio of ∼50/50 dimer/1H observed by cyclic voltammetry. Furthermore, sodium pyruvate is shown to promote electroreductive generation of the 1,4-dihydro form of several additional mNADH as well as NADH itself. This method provides a general strategy for regenerating 1,4-dihydro-nicotinamide mimetics of NADH from their oxidized forms.

Unexpected metabolic rewiring of CO2 fixation in H2-mediated materials-biology hybrids

A hybrid approach combining water-splitting electrochemistry and H2-oxidizing, CO2-fixing microorganisms offers a viable solution for producing value-added chemicals from sunlight, water, and air. The classic wisdom without thorough examination to date assumes that the electrochemistry in such a H2-mediated process is innocent of altering microbial behavior. Here, we report unexpected metabolic rewiring induced by water-splitting electrochemistry in H2-oxidizing acetogenic bacterium Sporomusa ovata that challenges such a classic view. We found that the planktonic S. ovata is more efficient in utilizing reducing equivalent for ATP generation in the materials-biology hybrids than cells grown with H2 supply, supported by our metabolomic and proteomic studies. The efficiency of utilizing reducing equivalents and fixing CO2 into acetate has increased from less than 80% of chemoautotrophy to more than 95% under electroautotrophic conditions. These observations unravel previously underappreciated materials' impact on microbial metabolism in seemingly simply H2-mediated charge transfer between biotic and abiotic components. Such a deeper understanding of the materials-biology interface will foster advanced design of hybrid systems for sustainable chemical transformation.

Model-Driven Design of Redox Mediators: Quantifying the Impact of Quinone Structure on Bioelectrocatalytic Activity with Glucose Oxidase

Successful application of emerging bioelectrocatalysis technologies depends upon an efficient electrochemical interaction between redox enzymes as biocatalysts and conductive electrode surfaces. One approach to establishing such enzyme-electrode interfaces utilizes small redox-active molecules to act as electron mediators between an enzyme-active site and the electrode surface. While redox mediators have been successfully used in bioelectrocatalysis applications ranging from enzymatic electrosynthesis to enzymatic biofuel cells, they are often selected using a guess-and-check approach. Herein, we identify structure-function relationships in redox mediators that describe the bimolecular rate constant for its reaction with a model enzyme, glucose oxidase (GOx). Based on a library of quinone-based redox mediators, a quantitative structure-activity relationship (QSAR) model is developed to describe the importance of mediator redox potential and projected molecular area as two key parameters for predicting the activity of quinone/GOx-based electroenzymatic systems. Additionally, rapid scan stopped-flow spectrophotometry was used to provide fundamental insights into the kinetics and the stoichiometry of reactions between different quinones and the flavin adenine dinucleotide (FAD+/FADH2) cofactor of GOx. This work provides a critical foundation for both designing new enzyme-electrode interfaces and understanding the role that quinone structure plays in altering electron flux in electroenzymatic reactions.

High-performance flower-like and biocompatible nickel-coated Fe3O4@SiO2 magnetic nanoparticles decorated on a graphene electrocatalyst for the oxygen evolution reaction

The electrocatalytic oxygen evolution reaction (OER) plays a crucial role in renewable clean energy conversion technologies and has developed into an important direction in the field of advanced energy, becoming the focus of basic research and industrial development. Herein, we report the synthesis and application of flower-like nickel-coated Fe3O4@SiO2 magnetic nanoparticles decorated on a graphene electrocatalyst for the OER that exhibit high efficiency and robust durability. The catalysts were optimized using a rotating ring-disk electrode to test their oxygen evolution properties in 1.0 M KOH solution. Importantly, owing to the high specific surface area and conductivity of C3N4 and graphene, the as-synthesized Fe3O4@SiO2@NiO/graphene/C3N4 exhibits a small Tafel slope of 40.46 mV dec-1, low overpotential of 288 mV at 10 mA cm-2, and robust OER durability within a prolonged test period of 100 h. The cytotoxicity of Fe3O4@SiO2, Fe3O4@SiO2@NiO, and Fe3O4@SiO2@NiO/graphene/C3N4 was evaluated in HeLa and MC3T3-E1 cells, demonstrating that they are efficient and biocompatible catalysts for the OER. Owing to its excellent electrocatalytic efficiency and eco-friendliness, Fe3O4@SiO2@NiO/graphene/C3N4 has considerable potential as a new multifunctional composite for large-scale applications in catalysis, biology, medicine, and high-efficiency hydrogen production.

High-Rate CO2 Electrolysis to Formic Acid over a Wide Potential Window: An Electrocatalyst Comprised of Indium Nanoparticles on Chitosan-Derived Graphene

Realizing industrial-scale production of HCOOH from the CO2 reduction reaction (CO2 RR) is very important, but the current density as well as the electrochemical potential window are still limited to date. Herein, we achieved this by integration of chemical adsorption and electrocatalytic capabilities for the CO2 RR via anchoring In nanoparticles (NPs) on biomass-derived substrates to create In/X-C (X=N, P, B) bifunctional active centers. The In NPs/chitosan-derived N-doped defective graphene (In/N-dG) catalyst had outstanding performance for the CO2 RR with a nearly 100 % Faradaic efficiency (FE) of HCOOH across a wide potential window. Particularly, at 1.2 A ⋅ cm-2 high current density, the FE of HCOOH was as high as 96.0 %, and the reduction potential was as low as -1.17 V vs RHE. When using a membrane electrode assembly (MEA), a pure HCOOH solution could be obtained at the cathode without further separation and purification. The FE of HCOOH was still up to 93.3 % at 0.52 A ⋅ cm-2 , and the HCOOH production rate could reach 9.051 mmol ⋅ h-1  ⋅ cm-2 . Our results suggested that the defects and multilayer structure in In/N-dG could not only enhance CO2 chemical adsorption capability, but also trigger the formation of an electron-rich catalytic environment around In sites to promote the generation of HCOOH.

An n-Type Conjugated Oligoelectrolyte Mimics Transmembrane Electron Transport Proteins for Enhanced Microbial Electrosynthesis

Interfacing bacteria as biocatalysts with an electrode provides the basis for emerging bioelectrochemical systems that enable sustainable energy interconversion between electrical and chemical energy. Electron transfer rates at the abiotic-biotic interface are, however, often limited by poor electrical contacts and the intrinsically insulating cell membranes. Herein, we report the first example of an n-type redox-active conjugated oligoelectrolyte, namely COE-NDI, which spontaneously intercalates into cell membranes and mimics the function of endogenous transmembrane electron transport proteins. The incorporation of COE-NDI into Shewanella oneidensis MR-1 cells amplifies current uptake from the electrode by 4-fold, resulting in the enhanced bio-electroreduction of fumarate to succinate. Moreover, COE-NDI can serve as a "protein prosthetic" to rescue current uptake in non-electrogenic knockout mutants.

Engineering Outer Membrane Vesicles to Increase Extracellular Electron Transfer of Shewanella oneidensis

Outer membrane vesicles (OMVs) of Gram-negative bacteria play an essential role in cellular physiology. The underlying regulatory mechanism of OMV formation and its impact on extracellular electron transfer (EET) in the model exoelectrogenShewanella oneidensis MR-1 remain unclear and have not been reported. To explore the regulatory mechanism of OMV formation, we used the CRISPR-dCas9 gene repression technology to reduce the crosslink between the peptidoglycan (PG) layer and the outer membrane, thus promoting the OMV formation. We screened the target genes that were potentially beneficial to the outer membrane bulge, which were classified into two modules: PG integrity module (Module 1) and outer membrane component module (Module 2). We found that downregulation of the penicillin-binding protein-encoding gene pbpC for peptidoglycan integrity (Module 1) and the N-acetyl-d-mannosamine dehydrogenase-encoding gene wbpP involved in lipopolysaccharide synthesis (Module 2) exhibited the highest production of OMVs and enabled the highest output power density of 331.3 ± 1.2 and 363.8 ± 9.9 mW m-2, 6.33- and 6.96-fold higher than that of the wild-typeS. oneidensis MR-1 (52.3 ± 0.6 mW m-2), respectively. To elucidate the specific impacts of OMV formation on EET, OMVs were isolated and quantified for UV-visible spectroscopy and heme staining characterization. Our study showed that abundant outer membrane c-type cytochromes (c-Cyts) including MtrC and OmcA and periplasmic c-Cyts were exposed on the surface or inside of OMVs, which were the vital constituents responsible for EET. Meanwhile, we found that the overproduction of OMVs could facilitate biofilm formation and increase biofilm conductivity. To the best of our knowledge, this study is the first to explore the mechanism of OMV formation and its correlation with EET of S. oneidensis, which paves the way for further study of OMV-mediated EET.

Advances in mechanisms and engineering of electroactive biofilms

Electroactive biofilms (EABs) are electroactive microorganisms (EAMs) encased in conductive polymers that are secreted by EAMs and formed by the accumulation and cross-linking of extracellular polysaccharides, proteins, nucleic acids, lipids, and other components. EABs are present in the form of multicellular aggregates and play a crucial role in bioelectrochemical systems (BESs) for diverse applications, including biosensors, microbial fuel cells for renewable bioelectricity production and remediation of wastewaters, and microbial electrosynthesis of valuable chemicals. However, naturally occurred EABs are severely limited owing to their low electrical conductivity that seriously restrict the electron transfer efficiency and practical applications. In the recent decade, synthetic biology strategies have been adopted to elucidate the regulatory mechanisms of EABs, and to enhance the formation and electrical conductivity of EABs. Based on the formation of EABs and extracellular electron transfer (EET) mechanisms, the synthetic biology-based engineering strategies of EABs are summarized and reviewed as follows: (i) Engineering the structural components of EABs, including strengthening the synthesis and secretion of structural elements such as polysaccharides, eDNA, and structural proteins, to improve the formation of biofilms; (ii) Enhancing the electron transfer efficiency of EAMs, including optimizing the distribution of c-type cytochromes and conducting nanowire assembly to promote contact-based EET, and enhancing electron shuttles' biosynthesis and secretion to promote shuttle-mediated EET; (iii) Incorporating intracellular signaling molecules in EAMs, including quorum sensing systems, secondary messenger systems, and global regulatory systems, to increase the electron transfer flux in EABs. This review lays a foundation for the design and construction of EABs for diverse BES applications.

Untangling the Effects of Doping Carbon with Diverse Heteroatoms on the Bioelectrochemistry of Glucose Oxidase

Great enthusiasm for doping carbon materials with nonmetallic heteroatoms for promoting electrical contact of redox enzymes with electrodes in bioelectronics has been aroused. However, systematic studies of different heteroatoms on enzyme activities are still lacking. Herein, choosing glucose oxidase (GOD) as a model enzyme, carbon nanotubes (CNTs) are used as electron carriers to evaluate the effects of heteroatoms' species on the direct electron transfer and catalytic activities of GOD. Experimental data demonstrate that phosphorus (P)-doped CNTs provide the most intimate electrical contact with GOD compared to other elements (B, N, and S) doping, delivering a 3-fold increase in rate constant (k_s, 2.1 s^-1) and an enhanced turnover rate (k_cat, 2.74 × 10^-9 M cm^-2 s^-1) in comparison with CNTs. Meanwhile, theoretical modeling clarifies that the active center of GOD interacts more strongly with P-doped CNTs and maintains their conformation well compared to other CNTs. This study will help to understand the mechanism of heteroatom doping of carbon on the enzymatic electron transfer and shed light on the design of efficient bioelectrocatalytic interfaces.

Mechano-control of Extracellular Electron Transport Rate via Modification of Inter-heme Coupling in Bacterial Surface Cytochrome

Bacterial outer-membrane multi-heme cytochromes (OMCs) mediate extracellular electron transport (EET). While heme alignment dictates the rate of EET, control of inter-heme coupling in a single OMC remains challenging, especially in intact cells. Given that OMCs diffuse and collide without aggregation on the cell surface, the overexpression of OMCs could increase such mechanical stress to impact the OMCs' protein structure. Here, the heme coupling is modified via mechanical interactions among OMCs by controlling their concentrations. Employment of whole-cell circular dichroism (CD) spectra of genetically engineered Escherichia coli reveals that the OMC concentration significantly impacts the molar CD and redox property of OMCs, resulting in a 4-fold change of microbial current production. The overexpression of OMCs increased the conductive current across the biofilm on an interdigitated electrode, indicating that a higher concentration of OMCs causes more lateral inter-protein electron hopping via collision on the cell surface. The present study would open a novel strategy to increase microbial current production by mechanically enhancing the inter-heme coupling.

Active and stable alcohol dehydrogenase-assembled hydrogels via synergistic bridging of triazoles and metal ions

Biocatalysis is increasingly replacing traditional methods of manufacturing fine chemicals due to its green, mild, and highly selective nature, but biocatalysts, such as enzymes, are generally costly, fragile, and difficult to recycle. Immobilization provides protection for the enzyme and enables its convenient reuse, which makes immobilized enzymes promising heterogeneous biocatalysts; however, their industrial applications are limited by the low specific activity and poor stability. Herein, we report a feasible strategy utilizing the synergistic bridging of triazoles and metal ions to induce the formation of porous enzyme-assembled hydrogels with increased activity. The catalytic efficiency of the prepared enzyme-assembled hydrogels toward acetophenone reduction is 6.3 times higher than that of the free enzyme, and the reusability is confirmed by the high residual catalytic activity after 12 cycles of use. A near-atomic resolution (2.1 Å) structure of the hydrogel enzyme is successfully analyzed via cryogenic electron microscopy, which indicates a structure-property relationship for the enhanced performance. In addition, the possible mechanism of gel formation is elucidated, revealing the indispensability of triazoles and metal ions, which guides the use of two other enzymes to prepare enzyme-assembled hydrogels capable of good reusability. The described strategy can pave the way for the development of practical catalytic biomaterials and immobilized biocatalysts.

Lumichrome from the photolytic riboflavin acts as an electron shuttle in microbial photoelectrochemical systems

Riboflavin has been proposed to serve as an electron shuttle in photoelectrochemical systems. However, riboflavin was also observed for abiotic photolysis under illumination. Such conflicting reports raise the necessity for further investigation. In this study, riboflavin secreted by Rhodopseudomonas palustris was studied to clarify its stability and electron shuttle function under illumination. The data of high-performance liquid chromatography-mass spectrometry showed that the riboflavin was photolyzed to lumichrome in microbial photoelectrochemical systems. In addition, the anodic current increased by 75% after adding lumichrome compared with that of the control; it further demonstrated that lumichrome, not riboflavin, as an electron shuttle could facilitate microbial electron transfer. This study clarifies the mechanism of the interface process in microbial photoelectrochemical systems.

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

Electricity-driven microbial metabolism relies on the extracellular electron transfer (EET) process between microbes and electrodes and provides promise for resource recovery from wastewater and industrial discharges. Over the past decades, tremendous efforts have been dedicated to designing electrocatalysts and microbes, as well as hybrid systems to push this approach toward industrial adoption. This paper summarizes these advances in order to facilitate a better understanding of electricity-driven microbial metabolism as a sustainable waste-to-resource solution. Quantitative comparisons of microbial electrosynthesis and abiotic electrosynthesis are made, and the strategy of electrocatalyst-assisted microbial electrosynthesis is critically discussed. Nitrogen recovery processes including microbial electrochemical N₂ fixation, electrocatalytic N₂ reduction, dissimilatory nitrate reduction to ammonium (DNRA), and abiotic electrochemical nitrate reduction to ammonia (Abio-NRA) are systematically reviewed. Furthermore, the synchronous metabolism of carbon and nitrogen using hybrid inorganic-biological systems is discussed, including advanced physicochemical, microbial, and electrochemical characterizations involved in this field. Finally, perspectives for future trends are presented. The paper provides valuable insights on the potential contribution of electricity-driven microbial valorization of waste carbon and nitrogen toward a green and sustainable society.

Systematic Full-Cycle Engineering Microbial Biofilms to Boost Electricity Production in Shewanella oneidensis

Electroactive biofilm plays a crucial rule in the electron transfer efficiency of microbial electrochemical systems (MES). However, the low ability to form biofilm and the low conductivity of the formed biofilm substantially limit the extracellular electron transfer rate of microbial cells to the electrode surfaces in MES. To promote biofilm formation and enhance biofilm conductivity, we develop synthetic biology approach to systematically engineer Shewanella oneidensis, a model exoelectrogen, via modular manipulation of the full-cycle different stages of biofilm formation, namely, from initial contact, cell adhesion, and biofilm growth stable maturity to cell dispersion. Consequently, the maximum output power density of the engineered biofilm reaches 3.62 ± 0.06 W m-2, 39.3-fold higher than that of the wild-type strain of S. oneidensis, which, to the best our knowledge, is the highest output power density that has ever been reported for the biofilms of the genetically engineered Shewanella strains.

Screening of natural phenazine producers for electroactivity in bioelectrochemical systems

Mediated extracellular electron transfer (EET) might be a great vehicle to connect microbial bioprocesses with electrochemical control in stirred-tank bioreactors. However, mediated electron transfer to date is not only much less efficient but also much less studied than microbial direct electron transfer to an anode. For example, despite the widespread capacity of pseudomonads to produce phenazine natural products, only Pseudomonas aeruginosa has been studied for its use of phenazines in bioelectrochemical applications. To provide a deeper understanding of the ecological potential for the bioelectrochemical exploitation of phenazines, we here investigated the potential electroactivity of over 100 putative diverse native phenazine producers and the performance within bioelectrochemical systems. Five species from the genera Pseudomonas, Streptomyces, Nocardiopsis, Brevibacterium and Burkholderia were identified as new electroactive bacteria. Electron discharge to the anode and electric current production correlated with the phenazine synthesis of Pseudomonas chlororaphis subsp. aurantiaca. Phenazine-1-carboxylic acid was the dominant molecule with a concentration of 86.1 μg/ml mediating an anodic current of 15.1 μA/cm² . On the other hand, Nocardiopsis chromatogenes used a wider range of phenazines at low concentrations and likely yet-unknown redox compounds to mediate EET, achieving an anodic current of 9.5 μA/cm² . Elucidating the energetic and metabolic usage of phenazines in these and other species might contribute to improving electron discharge and respiration. In the long run, this may enhance oxygen-limited bioproduction of value-added compounds based on mediated EET mechanisms.

Tapping the potential of Gram-positive bacteria for bioelectrochemical applications

Bacteria that perform extracellular electron transfer (EET) are central to redox-driven biotechnologies, including microbial fuel cells, bioremediation, and bioelectrosynthesis. However, engineerable EET strains have been restricted to well-characterized, Gram-negative model species. Light et al. identified a previously unknown but widely conserved EET pathway in the Gram-positive bacterium Listeria monocytogenes.

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.

Modular Engineering Strategy to Redirect Electron Flux into the Electron-Transfer Chain for Enhancing Extracellular Electron Transfer in Shewanella oneidensis

Efficient extracellular electron transfer (EET) of exoelectrogens is critical for practical applications of various bioelectrochemical systems. However, the low efficiency of electron transfer remains a major bottleneck. In this study, a modular engineering strategy, including broadening the sources of the intracellular electron pool, enhancing intracellular nicotinamide adenine dinucleotide (NADH) regeneration, and promoting electron release from electron pools, was developed to redirect electron flux into the electron transfer chain in Shewanella oneidensis MR-1. Among them, four genes include gene SO1522 encoding a lactate transporter for broadening the sources of the intracellular electron pool, gene gapA encoding a glyceraldehyde-3-phosphate dehydrogenase and gene mdh encoding a malate dehydrogenase in the central carbon metabolism for enhancing intracellular NADH regeneration, and gene ndh encoding NADH dehydrogenase on the inner membrane for releasing electrons from intracellular electron pools into the electron-transport chain. Upon assembly of the four genes, electron flux was directly redirected from the electron donor to the electron-transfer chain, achieving 62% increase in intracellular NADH levels, which resulted in a 3.5-fold enhancement in the power density from 59.5 ± 3.2 mW/m² (wild type) to 270.0 ± 12.7 mW/m² (recombinant strain). This study confirmed that redirecting electron flux from the electron donor to the electron-transfer chain is a viable approach to enhance the EET rate of S. oneidensis.

Photodriven Chemical Synthesis by Whole-Cell-Based Biohybrid Systems: From System Construction to Mechanism Study

By simulating natural photosynthesis, the desirable high-value chemical products and clean fuels can be sustainably generated with solar energy. Whole-cell-based photosensitized biohybrid system, which innovatively couples the excellent light-harvesting capacity of semiconductor materials with the efficient catalytic ability of intracellular biocatalysts, is an appealing interdisciplinary creature to realize photodriven chemical synthesis. In this review, we summarize the constructed whole-cell-based biohybrid systems in different application fields, including carbon dioxide fixation, nitrogen fixation, hydrogen production, and other chemical synthesis. Moreover, we elaborate the charge transfer mechanism studies of representative biohybrids, which can help to deepen the current understanding of the synergistic process between photosensitizers and microorganisms, and provide schemes for building novel biohybrids with less electron transfer resistance, advanced productive efficiency, and functional diversity. Further exploration in this field has the prospect of making a breakthrough on the biotic-abiotic interface that will provide opportunities for multidisciplinary research.

Applying Dynamic Magnetic Field To Promote Anaerobic Digestion via Enhancing the Electron Transfer of a Microbial Respiration Chain

Electrochemical methods have been reported to strengthen anaerobic digestion, but the continuous electrical power supply and the complicated electrode installed inside the digester have restricted it from practical use. In this study, a dynamic magnetic field (DMF) was placed outside a digester to induce an electromotive force to electrically promote anaerobic digestion. With the applied DMF, an electromotive force of 0.14 mV was generated in the anaerobic sludge, and a 65.02% methane increment was obtained from the anaerobic digestion of waste-activated sludge. Experiments on each stage of anaerobic digestion showed that acidification and methanogenesis that involve electron transfer of respiration chains were promoted with the DMF, while solubilization and hydrolysis less related to respiration chains were not enhanced. Further analysis indicated that the induced electromotive force polarized the protein-like substances in the sludge to increase the conductivity and capacitance of the sludge. Electrotrophic methanogens (Methanothrix) and exoelectrogens (Exiguobacterium) were enriched with DMF. The kinetic isotope effect test confirmed that electron transfer was accelerated with DMF. Consistently, the concentration ratio of co-enzymes (NADH/NAD⁺ and F₄₂₀H₂/F₄₂₀₎ that reflects the electron exchange with respiration chains significantly increased. Applying the DMF seemed a more accessible strategy to electrically strengthen anaerobic digestion.