A genetic approach for microbial electrosynthesis system as biocommodities production platform

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.

A novel electrolytic gas lift reactor for efficient microbial electrosynthesis of acetate from carbon dioxide

The low current density impedes the practical application of microbial electrosynthesis for CO2 fixation. Engineering the reactor design is an effective way to increase the current density, especially for H2-mediated microbial electrosynthesis reactors. The electrolytic bubble column microbial electrosynthesis reactor has shown great potential for scaling up, but the mixing and gas mass transfer still need to be enhanced to further increase the current density. Here, we introduced an inner draft tube to the bubble column to tackle the problem. The addition of draft tube resulted in a 76.6% increase in the volumetric mass transfer coefficient (kLa) of H2, a 40% increase in the maximum current density (337 A/m2) and a 72% increase in average acetate production rate (3.1 g/L/d). The computational fluid dynamics simulations showed that the addition of draft tube enhanced mixing efficiency by enabling a more ordered cyclic flow pattern and a more uniform gas/liquid distribution. These results indicate that the electro-bubble column reactor with draft tube holds great potential for industrial implementation.

Recent progress in the characterization and application of exo-electrogenic microorganisms

Exo-electrogenic microorganisms are characterized by their special metabolic capability of transferring metabolic electrons out of their cell, into insoluble external electron acceptors such as iron or manganese oxides and electrodes, or vice versa take up electron from electrodes. Their conventional application is primarily limited to microbial fuel cells for electrical power generation and microbial electrolysis cells for the production of value-added products such as biohydrogen, biomethane and hydrogen peroxide. The utility of exo-electrogenic organisms has expanded into many other applications in recent times. Such examples include microbial desalination cells, microbial electro-synthesis cells producing value-added chemicals such as bio-butanol and their applications in other carbon sequestration technologies. Additionally, electrochemically-active organisms are now beginning to be employed in biosensor applications for environmental monitoring. Additionally, the utility of biocathodes in bio-electrochemical systems is also a novel application in catalyzing the cathodic oxygen reduction reaction to enhance their electrochemical performance. Advances have also been made in the expansion and use of other organisms such as the usage of photosynthetic microorganisms for the fabrication of self-sustained bio-electrochemical systems. This review attempts to provide a comprehensive picture of the state-of the art of exo-electrogenic organisms and their novel utility in bioelectrochemical systems.

Effect of anode material and dispersal limitation on the performance and biofilm community in microbial electrolysis cells

In a microbial electrolysis cell (MEC), the oxidization of organic compounds is facilitated by an electrogenic biofilm on the anode surface. The biofilm community composition determines the function of the system. Both deterministic and stochastic factors affect the community, but the relative importance of different factors is poorly understood. Anode material is a deterministic factor as materials with different properties may select for different microorganisms. Ecological drift is a stochastic factor, which is amplified by dispersal limitation between communities. Here, we compared the effects of three anode materials (graphene, carbon cloth, and nickel) with the effect of dispersal limitation on the function and biofilm community assembly. Twelve MECs were operated for 56 days in four hydraulically connected loops and shotgun metagenomic sequencing was used to analyse the microbial community composition on the anode surfaces at the end of the experiment. The anode material was the most important factor affecting the performance of the MECs, explaining 54-80 % of the variance observed in peak current density, total electric charge generation, and start-up lag time, while dispersal limitation explained 10-16 % of the variance. Carbon cloth anodes had the highest current generation and shortest lag time. However, dispersal limitation was the most important factor affecting microbial community structure, explaining 61-98 % of the variance in community diversity, evenness, and the relative abundance of the most abundant taxa, while anode material explained 0-20 % of the variance. The biofilms contained nine Desulfobacterota metagenome-assembled genomes (MAGs), which made up 64-89 % of the communities and were likely responsible for electricity generation in the MECs. Different MAGs dominated in different MECs. Particularly two different genotypes related to Geobacter benzoatilyticus competed for dominance on the anodes and reached relative abundances up to 83 %. The winning genotype was the same in all MECs that were hydraulically connected irrespective of anode material used.

Evidence of competition between electrogens shaping electroactive microbial communities in microbial electrolysis cells

In single-chamber microbial electrolysis cells (MECs), organic compounds are oxidized at the anode, liberating electrons that are used for hydrogen evolution at the cathode. Microbial communities on the anode and cathode surfaces and in the bulk liquid determine the function of the MEC. The communities are complex, and their assembly processes are poorly understood. We investigated MEC performance and community composition in nine MECs with a carbon cloth anode and a cathode of carbon nanoparticles, titanium, or stainless steel. Differences in lag time during the startup of replicate MECs suggested that the initial colonization by electrogenic bacteria was stochastic. A network analysis revealed negative correlations between different putatively electrogenic Deltaproteobacteria on the anode. Proximity to the conductive anode surface is important for electrogens, so the competition for space could explain the observed negative correlations. The cathode communities were dominated by hydrogen-utilizing taxa such as Methanobacterium and had a much lower proportion of negative correlations than the anodes. This could be explained by the diffusion of hydrogen throughout the cathode biofilms, reducing the need to compete for space.

Sewage-Water Treatment and Sewage-Sludge Management with Power Production as Bioenergy with Carbon Capture System: A Review

Sewage-water treatment comprehends primary, secondary, and tertiary steps to produce reusable water after removing sewage contaminants. However, a sewage-water treatment plant is typically a power and energy consumer and produces high volumes of sewage sludge mainly generated in the primary and secondary steps. The use of more efficient anaerobic digestion of sewage water with sewage sludge can produce reasonable flowrates of biogas, which is shown to be a consolidated strategy towards the energy self-sufficiency and economic feasibility of sewage-water treatment plants. Anaerobic digestion can also reduce the carbon footprint of energy sources since the biogas produced can replace fossil fuels for electricity generation. In summary, since the socio-economic importance of sewage treatment is high, this review examined works that contemplate: (i) improvements of sewage-water treatment plant bioenergy production and economic performances; (ii) the exploitation of technology alternatives for the energy self-sufficiency of sewage-water treatment plants; (iii) the implementation of new techniques for sewage-sludge management aiming at bioenergy production; and (iv) the implementation of sewage-water treatment with bioenergy production and carbon capture and storage.

Valorisation of CO2 into Value-Added Products via Microbial Electrosynthesis (MES) and Electro-Fermentation Technology

Microbial electrocatalysis reckons on microbes as catalysts for reactions occurring at electrodes. Microbial fuel cells and microbial electrolysis cells are well-known in this context; both prefer the oxidation of organic and inorganic matter for producing electricity. Notably, the synthesis of high energy-density chemicals (fuels) or their precursors by microorganisms using bio-cathode to yield electrical energy is called Microbial Electrosynthesis (MES), giving an exceptionally appealing novel way for producing beneficial products from electricity and wastewater. This review accentuates the concept, importance and opportunities of MES, as an emerging discipline at the nexus of microbiology and electrochemistry. Production of organic compounds from MES is considered as an effective technique for the generation of various beneficial reduced end-products (like acetate and butyrate) as well as in reducing the load of CO2 from the atmosphere to mitigate the harmful effect of greenhouse gases in global warming. Although MES is still an emerging technology, this method is not thoroughly known. The authors have focused on MES, as it is the next transformative, viable alternative technology to decrease the repercussions of surplus carbon dioxide in the environment along with conserving energy.

An insight into the bioelectrochemical photoreduction of CO2 to value-added chemicals

Goal of sustainable carbon neutral economy can be achieved by designing an efficient CO₂ reduction system to generate biofuels, in particular, by mimicking the mechanism of natural photosynthesis using semiconducting nanomaterials interfaced with electroactive bacteria (EAB) in a photosynthetic microbial electrosynthesis (PMES) system. This review paper presents an overview of the recent advancements in the biohybrid photoanode and photocathode materials. We discuss the reaction mechanism observed at photoanode and photocathode to enhance our understanding on the solar driven MES. We extend the discussion by showcasing the potential activity of EABs toward high selectivity and production rates for desirable products by manipulating their genomic sequence. Additionally, the critical challenges associated in scaling up the PMES system including the strategies for diminution of reactive oxygen species, low solubility of CO₂ in the typical electrolytes, low selectivity of product species are presented along with the suggestions of alternative strategies to achieve economically viable generation of (bio)commodities.

Recent developments and key barriers to microbial CO2 electrobiorefinery

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

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

Microbial electrosynthesis: Towards sustainable biorefineries for production of green chemicals from CO2 emissions

Decarbonisation of the economy has become a priority at the global level, and the resulting legislative pressure is pushing the chemical and energy industries away from fossil fuels. Microbial electrosynthesis (MES) has emerged as a promising technology to promote this transition, which will further benefit from the decreasing cost of renewable energy. However, several technological challenges need to be addressed before the MES technology can reach its maturity. The aim of this review is to critically discuss the bottlenecks hampering the industrial adoption of MES, considering the whole production process (from the CO₂ source to the marketable products), and indicate future directions. A flexible stack design, with flat or tubular MES modules and direct CO₂ supply, is required for site-specific decentralised applications. The experience gained for scaling-up electrochemical cells (e.g. electrolysers) can serve as a guideline for realising pilot MES stacks to be technologically and economically evaluated in industrially relevant conditions. Maximising CO₂ abatement rate by targeting high-rate production of acetate can promote adoption of MES technology in the short term. However, the development of a replicable and robust strategy for production and in-line extraction of higher-value products (e.g. caproic acid and hexanol) at the cathode, and meaningful exploitation of the currently overlooked anodic reactions, can further boost MES cost-effectiveness. Furthermore, the use of energy storage and smart electronics can alleviate the fluctuations of renewable energy supply. Despite the unresolved challenges, the flexible MES technology can be applied to decarbonise flue gas from different sources, to upgrade industrial and wastewater treatment plants, and to produce a wide array of green and sustainable chemicals. The combination of these benefits can support the industrial adoption of MES over competing technologies.

Research on the electrocatalytic reduction of CO2 by microorganisms with a nano-titanium carburizing electrode

The reduction of CO₂ to organics using microbial electrosynthesis (MES) is currently a popular research direction in the environmental field. In this study, we evaluated the effect of the electrode material on the production of organics from CO₂ in microbial electrosynthesis with a mixed-culture biocathode. The electrode material is an important factor influencing electron transfer, since it directly affects the efficiency of CO₂ reduction. In this study, we compared the performance of a graphite electrode and a metal-based carbon hybrid material electrode for the electro-reduction of CO_2. The cathode potential was set to -0.8 V (vs Ag/AgCl). When the cathode material was changed from a graphite electrode to a nano-titanium carburizing electrode, the current density of MES increased from 1.66 ± 0.2 A·m^-2 to 2.75 ± 0.2 A·m^-2, acetate accumulation increased from 127 mg/L to 234 mg/L, butyrate accumulation increased from 46 mg/L to 86.5 mg/L, and the total electron recovery of MES increased to nearly 70%. The results show that improving electrode performance can effectively improve the efficiency of MES for reducing CO₂. Metal-based carbon hybrid materials have good biological affinity and stability and also have good electrochemical performance.

Microbial electrosynthesis feasibility evaluation at high bicarbonate concentrations with enriched homoacetogenic biocathode

An enrichment methodology was developed for a homoacetogenic biocathode that is able to function at high concentrations of bicarbonates for the microbial electrosynthesis (MES) of acetate from carbon dioxide. The study was performed in two stages; enrichment of consortia in serum bottles and the development of a biocathode in MES. A homoacetogenic consortium was sequentially grown under increasing concentrations of bicarbonate, in serum bottles, at room temperature. The acetate production rate was found to increase with the increase in the bicarbonate concentration and evidenced a maximum production rate of 260 mg/L d^-1 (15 g HCO₃^-/L). On the contrary, carbon conversion efficiency decreased with the increase in the bicarbonate concentration, which evidenced a maximum at 2.5 g HCO₃^-/L (90.16%). Following a further increase in the bicarbonate concentration up to 20 g HCO₃^-/L, a visible inhibition was registered with respect to the acetate production rate and the carbon conversion efficiency. Well adapted biomass from 15 g HCO₃^-/L was used to develop biocathodic catalyst for MES. An effective biocathode was developed after 4 cycles of operation, during which acetate production was improved gradually, evidencing a maximum production rate of 24.53 mg acetate L^-1 d^-1 (carbon conversion efficiency, 47.72%). Compared to the enrichment stage, the carbon conversion efficiency and the rate of acetate production in MES were found to be low. The production of acetate induced a change in the catholyte pH, from neutral conditions towards acidic conditions.

Mo2C-induced hydrogen production enhances microbial electrosynthesis of acetate from CO2 reduction

BACKGROUND

Microbial electrosynthesis (MES) is a biocathode-driven process, in which electroautotrophic microorganisms can directly uptake electrons or indirectly via H₂ from the cathode as energy sources and CO₂ as only carbon source to produce chemicals.

RESULTS

This study demonstrates that a hydrogen evolution reaction (HER) catalyst can enhance MES performance. An active HER electrocatalyst molybdenum carbide (Mo₂C)-modified electrode was constructed for MES. The volumetric acetate production rate of MES with 12 mg cm^-2 Mo₂C was 0.19 ± 0.02 g L^-1 day^-1, which was 2.1 times higher than that of the control. The final acetate concentration reached 5.72 ± 0.6 g L^-1 within 30 days, and coulombic efficiencies of 64 ± 0.7% were yielded. Furthermore, electrochemical study, scanning electron microscopy, and microbial community analyses suggested that Mo₂C can accelerate the release of hydrogen, promote the formation of biofilms and regulate the mixed microbial flora.

CONCLUSION

Coupling a HER catalyst to a cathode of MES system is a promising strategy for improving MES efficiency.

Fe3O4/granular activated carbon as an efficient three-dimensional electrode to enhance the microbial electrosynthesis of acetate from CO2

Microbial electrosynthesis (MES) allows the transformation of CO₂ into value-added products by coupling with renewable energy.

On-going applications of Shewanella species in microbial electrochemical system for bioenergy, bioremediation and biosensing

Microbial electrochemical system (MES) has attracted ever-growing interest as a promising platform for renewable energy conversion and bioelectrochemical remediation. Shewanella species, the dissimilatory metal reduction model bacteria with versatile extracellular electron transfer (EET) strategies, are the well-received microorganisms in diverse MES devices for various practical applications as well as microbial EET mechanism investigation. Meanwhile, the available genomic information and the unceasing established gene-editing toolbox offer an unprecedented opportunity to boost the applications of Shewanella species in MES. This review thoroughly summarizes the status quo of the applications of Shewanella species in microbial fuel cells for bioelectricity generation, microbial electrosynthesis for biotransformation of valuable chemicals and bioremediation of environment-hazardous pollutants with synoptical discussion on their EET mechanism. Recent advances in rational design and genetic engineering of Shewanella strains for either promoting the MES performance or broadening their applications are surveyed. Moreover, some emerging applications beyond electricity generation, such as biosensing and biocomputing, are also documented. The challenges and perspectives for Shewanella-based MES are also discussed elaborately for the sake of not only discovering new scientific lights on microbial extracellular respiratory but also propelling practical applications.

Fluidized granular activated carbon electrode for efficient microbial electrosynthesis of acetate from carbon dioxide

The electricity-driven bioreduction of carbon dioxide to multi-carbon organic compounds, particularly acetate, has been achieved in microbial electrosynthesis (MES). MES performance can be limited by the amount of cathode surface area available for biofilm formation and slow substrate mass transfer. Here, a fluidized three-dimensional electrode, containing granular activated carbon (GAC) particles, was constructed via MES. The volumetric acetate production rate increased by 2.8 times through MES with 16 g L^-1 GAC (0.14 g L^-1 d^-1) compared with that of the control (no GAC), and the final acetate concentration reached 3.92 g L^-1 within 24 days. Electrochemical, scanning electron microscopy, and microbial community analyses suggested that GAC might improve the performance of MES by accelerating direct and indirect (via H₂) electron transfer because GAC could provide a high electrode surface and a favorable mass transport. This study attempted to improve the efficiency of MES and presented promising opportunities for MES scale-up.

Water Splitting-Biosynthetic Hybrid System for CO2 Conversion using Nickel Nanoparticles Embedded in N-Doped Carbon Nanotubes

CO₂ reduction has drawn increasing attention owing to the concern of global warming. Water splitting-biosynthetic hybrid systems are novel and efficient approaches for CO₂ conversion. Intimate coupling of electrocatalysts and biosynthesis requires the catalysts possess both high catalytic performance and excellent biocompatibility, which is a bottleneck of developing such catalysts. Here, a complex of Ni nanoparticles embedded in N-doped carbon nanotubes (Ni@N-C) is synthesized as a hydrogen evolution reaction electrocatalyst and is coupled with a hydrogen oxidizing autotroph, Cupriavidus necator H16, to convert CO₂ to poly-β-hydroxybutyrate. In Ni@N-C, the Ni nanoparticles are encapsulated in N-C nanotubes, which prevents bacteria from direct contact with Ni and inhibits Ni²⁺ leaching. As a result, Ni@N-C exhibits excellent biocompatibility and stability. This work demonstrates that electrocatalysts and biosynthesis can be intimately coupled through rational catalyst design.

Weak electricigens: A new avenue for bioelectrochemical research

Electroactivity appears to be a phylogenetically diverse trait independent of cell wall classification, with both Gram-negative and Gram-positive electricigens reported. While numerous electricigens have been observed, the majority of research focuses on a select group of highly electroactive species. Under favorable conditions, many microorganisms can be considered electroactive, either through their own mechanisms or exogenously-added mediators, producing a weak current. Such microbes should not be dismissed based on their modest electroactivity. Rather, they may be key to understanding what drives extracellular electron transfer in response to transient limitations of electron acceptor or donor, with implications for the study of pathogens and industrial bioprocesses. Due to their low electroactivity, such populations are difficult to grow in bioelectrochemical systems and characterise with electrochemistry. Here, a critical review of recent research on weak electricigens is provided, with a focus on the methodology and the overall relevance to microbial ecology and bioelectrochemical systems.