Metabolic Reconstruction and Modeling Microbial Electrosynthesis

Microbial electrosynthesis from CO2 reaches productivity of syngas and chain elongation fermentations

Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO2) to valuable carbon-based chemicals. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here, we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO2 to even-chain C2-C6 carboxylic acids over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-scale and industrial syngas (CO-H2-CO2) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO2 reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale up the technology.

Autotrophic degradation of sulfamethoxazole using sulfate-reducing biocathode in microbial photo-electrolysis system

Sulfamethoxazole is a representative of sulfonamide antibiotic pollutants. This study aims to investigate the degradation pathways of sulfamethoxazole and the response of microbial communities using the autotrophic biocathode in microbial photo-electrolysis systems (MPESs). Sulfamethoxazole with an initial concentration of 2 mg L-1 was degraded into small molecule propanol within 6 h with the biocathode. Elemental sulfur (S0) was detected in the cathode chamber, accounting for 57 % of the removed sulfate. The conversion from sulfate to S0 indicated that autotrophic microorganisms might adopt a novel pathway for sulfamethoxazole removal in the MPES. In the abiotic cathode, sulfamethoxazole degradation rate was 0.09 mg L-1 h-1 with the electrochemistry process. However, sulfamethoxazole was converted to products that still contain benzene rings, including p-aminothiophenol, 3-amino-5-methylisoxazole, and sulfonamide. The microbial community analysis indicated that the synergistic interaction of Desulfovibrio and Acetobacterium promoted the autotrophic degradation of sulfamethoxazole. The results suggested that autotrophic microorganisms may play an important role in the environmental transformation of sulfamethoxazole.

Effect of different hydrogen evolution rates at cathode on bioelectrochemical reduction of CO2 to acetate

Microbial electrosynthesis (MES) offers a promising approach for converting CO2 into valuable chemicals such as acetate. However, the relative low conversion rate severely limits its practical application. This study investigated the impact of different hydrogen evolution rates on the conversion rate of CO2 to acetate in the MES system. Three potentials (-0.8 V, -0.9 V and -1.0 V) corresponding to various hydrogen evolution rates were set and analyzed, revealing an optimal hydrogen evolution rate, yielding a maximum acetate formation rate of 1410.9 mg/L and 73.5 % coulomb efficiency. The electrochemical findings revealed that an optimal hydrogen evolution rate facilitated the formation of an electroactive biofilm. The microbial community of the cathode biofilm highlighted key genera, including Clostridium and Acetobacterium, which played essential roles in electrosynthesis within the MES system. Notably, a low hydrogen evolution rate failed to provide sufficient energy for the electrochemical reduction of CO2 to acetate, while a high rate led to cathode alkalinization, impeding the reaction and causing significant energy wastage. Therefore, maintaining an appropriate hydrogen evolution rate is crucial for the development of mature electroactive biofilms and achieving optimal performance in the MES system.

Housing of electrosynthetic biofilms using a roll-up carbon veil electrode increases CO2 conversion and faradaic efficiency in microbial electrosynthesis cells

Electrode-driven microbial electron transfer enables the conversion of CO2 into multi-carbon compounds. The electrosynthetic biofilms grow slowly on the surface and are highly susceptible to operational influences, such as hydrodynamic shear stress. In this study, a cylindrical roll-up carbon felt electrode was developed as a novel strategy to protect biofilms from shear stress within the reactor. The fabricated electrode allowed hydrogen bubble formation inside the structure, which enabled microbes to uptake hydrogen and convert CO2 to multi-carbon organic compounds. The roll-up electrode exhibited faster start-up and biofilm formation than the conventional linear shape carbon felt. The acetate yield and cathodic faradaic efficiency increased by 80% and 34%, respectively, and the bioelectrochemical stability was improved significantly. The roll-up structure increased biofilm development per unit electrode surface by three to five-fold. The roll-up configuration improved biofilm formation on the electrode, which enhanced the performance of microbial electrosynthesis-based CO2 valorization.

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.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Impact analysis of operating conditions on carbon dioxide reduction in microbial electrosynthesis: Insight into the substance utilization and microbial response

Microbial electrosynthesis (MES) is facing a series of problems including low energy utilization and production efficiency of high value-added products, which seriously hinder its practical application. In this study, a more practical direct current power source was used and the anaerobic activated sludge from wastewater treatment plants was inoculated to construct the acetic acid-producing MES. The operating conditions of acetic acid production were further optimized and the specific mechanisms involving the substance utilization and microbial response were revealed. The optimum conditions were the potential of 3.0 V and pH 6.0. Under these conditions, highly electroactive biofilms formed and all kinds of substances were effectively utilized. In addition, dominant bacteria (Acetobacterium, Desulfovibrio, Sulfuricurvum, Sulfurospirillum, and Fusibacter) had high abundances. Under optimal conditions, acetic acid-forming characteristic genera (Acetobacterium) had the highest relative abundance (Biocathode-25.82 % and Suspension-17.24 %). This study provided references for the optimal operating conditions of MES and revealed the corresponding mechanisms.

Microbial electrosynthesis of acetate from CO2 in three-chamber cells with gas diffusion biocathode under moderate saline conditions

The industrial adoption of microbial electrosynthesis (MES) is hindered by high overpotentials deriving from low electrolyte conductivity and inefficient cell designs. In this study, a mixed microbial consortium originating from an anaerobic digester operated under saline conditions (∼13 g L^-1 NaCl) was adapted for acetate production from bicarbonate in galvanostatic (0.25 mA cm^-2) H-type cells at 5, 10, 15, or 20 g L^-1 NaCl concentration. The acetogenic communities were successfully enriched only at 5 and 10 g L^-1 NaCl, revealing an inhibitory threshold of about 6 g L^-1 Na⁺. The enriched planktonic communities were then used as inoculum for 3D printed, three-chamber cells equipped with a gas diffusion biocathode. The cells were fed with CO₂ gas and operated galvanostatically (0.25 or 1.00 mA cm^-2). The highest production rate of 55.4 g m^-2 d^-1 (0.89 g L^-1 d^-1), with 82.4% Coulombic efficiency, was obtained at 5 g L^-1 NaCl concentration and 1 mA cm^-2 applied current, achieving an average acetate production of 44.7 kg MWh^-1. Scanning electron microscopy and 16S rRNA sequencing analysis confirmed the formation of a cathodic biofilm dominated by Acetobacterium sp. Finally, three 3D printed cells were hydraulically connected in series to simulate an MES stack, achieving three-fold production rates than with the single cell at 0.25 mA cm^-2. This confirms that three-chamber MES cells are an efficient and scalable technology for CO₂ bio-electro recycling to acetate and that moderate saline conditions (5 g L^-1 NaCl) can help reduce their power demand while preserving the activity of acetogens.

A meta-analysis of acetogenic and methanogenic microbiomes in microbial electrosynthesis

A meta-analysis approach was used, to study the microbiomes of biofilms and planktonic communities underpinning microbial electrosynthesis (MES) cells. High-throughput DNA sequencing of 16S rRNA gene amplicons has been increasingly applied to understand MES systems. In this meta-analysis of 22 studies, we find that acetogenic and methanogenic MES cells share 80% of a cathodic core microbiome, and that different inoculum pre-treatments strongly affect community composition. Oxygen scavengers were more abundant in planktonic communities, and several key organisms were associated with operating parameters and good cell performance. We suggest Desulfovibrio sp. play a role in initiating early biofilm development and shaping microbial communities by catalysing H₂ production, to sustain either Acetobacterium sp. or Methanobacterium sp. Microbial community assembly became more stochastic over time, causing diversification of the biofilm (cathodic) community in acetogenic cells and leading to re-establishment of methanogens, despite inoculum pre-treatments. This suggests that repeated interventions may be required to suppress methanogenesis.

Enhanced CO2 reduction and acetate synthesis in autotrophic biocathode by N-Hexanoyl-L-homoserine lactone (C6HSL)-based quorum-sensing regulation

The aim of this study was to investigate the ecological role of quorum-sensing signaling molecule on the autotrophic biocathode for CO₂ reduction and acetate synthesis. As a typical quorum-sensing signaling molecule, N-Hexanoyl-L-homoserine lactone (C6HSL) was used to regulate the construction of cathode biofilm. Results showed that the maximum acetate production from CO₂ reduction improved by 94.8%, and the maximum Faraday efficiency of the microbial electrosynthesis system enhanced by 71.7%, with the regulation of C6HSL. Electrochemical analyses indicated that higher electrochemical activity and lower charge resistance of biocathode were obtained with C6HSL than without C6HSL. Confocal laser scanning microscopy and electron inhibitor experiment suggested that exogenous C6HSL increased living biomass in the biofilm and facilitated the electron transfer pathway related to NADH dehydrogenase-CoQ and proton motive force. With the C6HSL regulation, the relative abundance of hydrogen producers (e.g., Desulfovibrio and Desulfomicrobium) increased, contributing to the improved performance of autotrophic biocathode.

Bio-augmentation with dissimilatory nitrate reduction to ammonium (DNRA) driven sulfide-oxidizing bacteria enhances the durability of nitrate-mediated souring control

Biological souring (producing sulfide) is a global challenge facing anaerobic water bodies, especially the oil reservoir fluids. Nitrate injection has demonstrated great potential in souring control, and dissimilatory nitrate reduction to ammonium (DNRA) bacteria was proposed to play crucial roles in the process. How to durably control souring with nitrate amendment, however, remains undiscovered. Herein, Gordonia sp. TD-4, a DNRA-driven sulfide-oxidizing bacterium, was used to elucidate the effects of bio-augmentation with DNRA bacteria on the durability of nitrate-mediated souring control. The results revealed that nitrate amendment combined with bio-augmentation with TD-4 after souring could effectively control souring and enhance the durability of nitrate-mediated souring control, while nitrate amendment before souring failed to persistently control souring. Nitrate amendment before and after souring resulted in different evolution dynamics of nitrate-reducing bacteria. Denitrifying bacteria were enriched in reactors amended with nitrate before souring or in dissolved sulfide exhausted reactors amended with nitrate after souring. The heterotrophic denitrifying activity of denitrifying bacteria, however, decreased the durability of nitrate-mediated souring control. Comparative and functional genomics analysis identified potential niche adaptation mechanisms (autotrophic and heterotrophic nitrate/nitrite reduction, including DNRA and denitrification) of predominant SRB in nitrate-amended environments, which were responsible for the rapid resumption of sulfide accumulation after the depletion of nitrate and nitrite. Pulsed injection of nitrate combined with bio-augmentation with DNRA-driven sulfide-oxidizing bacteria was proposed as a potential method to enhance the durability of nitrate-mediated souring control. The findings were innovatively applied to simultaneous bio-demulsification and souring control of emulsified and sour produced water from the petroleum industry.

Sporomusa ovata as Catalyst for Bioelectrochemical Carbon Dioxide Reduction: A Review Across Disciplines From Microbiology to Process Engineering

Sporomusa ovata is a bacterium that can accept electrons from cathodes to drive microbial electrosynthesis (MES) of acetate from carbon dioxide. It is the biocatalyst with the highest acetate production rate described. Here we review the research on S. ovata across different disciplines, including microbiology, biochemistry, engineering, and materials science, to summarize and assess the state-of-the-art. The improvement of the biocatalytic capacity of S. ovata in the last 10 years, using different optimization strategies is described and discussed. In addition, we propose possible electron uptake routes derived from genetic and experimental data described in the literature and point out the possibilities to understand and improve the performance of S. ovata through genetic engineering. Finally, we identify current knowledge gaps guiding further research efforts to explore this promising organism for the MES field.

CuO/g-C3N4 heterojunction photocathode enhances the microbial electrosynthesis of acetate through CO2 reduction

Microbial electrosynthesis (MES) is a novel CO₂ utilization technology. Biocatalysts in this process may use electrons obtained from a photovoltaic system to reduce CO₂ to chemicals and realize energy conversion from solar energy to chemical energy. The photoelectric material CuO/g-C₃N₄ was directly introduced into the MES system using mixed culture as biocatalyst in this study. CuO/g-C₃N₄ can effectively absorb light and presents satisfactory electron and hole separation ability. Photogenerated electrons from CuO/g-C₃N₄ enhanced the electron transfer rate and reduced cathodic charge transfer resistance. CuO/g-C₃N₄ mainly improved the electron supply of electroautotrophic microorganisms through direct electron transfer rather than indirect electron transfer via hydrogen. Photogenerated holes can combine electrons from anode and provide extra driving force to improve the MES performance. Furthermore, the CuO/g-C₃N₄ photocathode also improved the biocatalytic activity by increasing the total amount of biocatalyst and regulating cathodic microbial community composition. Acetate production rate in MES with the CuO/g-C₃N₄ photocathode was 2.6 times higher than that of the control group. This study provides a new strategy for semiconductor photocathodes to improve the MES performance with mixed culture.

Cathodic biofilms - A prerequisite for microbial electrosynthesis

Cathodic biofilms have an important role in CO₂ bio-reduction to carboxylic acids and biofuels in microbial electrosynthesis (MES) cells. However, robust and resilient electroactive biofilms for an efficient CO₂ conversion are difficult to achieve. In this review, the fundamentals of cathodic biofilm formation, including energy conservation, electron transfer and development of catalytic biofilms, are presented. In addition, strategies for improving cathodic biofilm formation, such as the selection of electrode and carrier materials, cell design and operational conditions, are described. The knowledge gaps are individuated, and possible solutions are proposed to achieve stable and productive biofilms in MES cathodes.

Dual cathode configuration and headspace gas recirculation for enhancing microbial electrosynthesis using Sporomusa ovata

High-rate production of acetate and other value-added products from the reduction of CO₂ in microbial electrosynthesis (MES) using acetogens can be achieved with high reducing power where H₂ appears as a key electron mediator. H₂ evolution using metal cathodes can enhance the availability of H₂ to support high-rate microbial reduction of CO₂. Due to the low solubility of H₂, the availability of H₂ remains limited to the bacteria. In this study, we investigated the performances of Sporomusa ovata for CO₂ reduction when dual cathodes were used together in an MES, one was regular carbon cathode, and the other was a titanium mesh that allows higher hydrogen evolution. The dual cathode configuration was investigated in two sets of MES, one set had the usual S. ovata inoculated graphite rod, and another set had a synthetic biofilm-imprinted carbon cloth. Additionally, the headspace gas in MES was recirculated to increase the H₂ availability to the bacteria in suspension. High-rate CO₂ reduction was observed at -0.9 V vs Ag/AgCl with dual cathode configuration as compared to single cathodes. High titers of acetate (up to ∼11 g/L) with maximum instantaneous rates of 0.68-0.7 g/L/d at -0.9 V vs Ag/AgCl were observed, which are higher than the production rates reported in the literatures for S. ovata using MES with surface modified cathodes. A high H₂ availability supported the high-rate acetate production from CO₂ with diminished electricity input.

Continuous H2/CO2 fermentation for acetic acid production under transient and continuous sulfide inhibition

Waste gas fermentation powered by renewable H₂ is reaching kiloton scale. The presence of sulfide, inherent to many waste gases, can cause inhibition, requiring additional gas treatment. In this work, acetogenesis and methanogenesis inhibition by sulfide were studied in a 10-L mixed-culture fermenter, supplied with CO₂ and connected with a water electrolysis unit for electricity-powered H₂ supply. Three cycles of inhibition (1.3 mM total dissolved sulfide (TDS)) and recovery were applied, then the fermenter was operated at 0.5 mM TDS for 35 days. During operation at 0.5 mM TDS the acetate production rate reached 7.1 ± 1.5 mmol C L^-1 d^-1. Furthermore, 43.7 ± 15.6% of the electrons, provided as H₂, were distributed to acetate and 7.7 ± 4.1% to butyrate, the second most abundant fermentation product. Selectivity of sulfide as inhibitor was demonstrated by a 7 days lag-phase of methanogenesis recovery, compared to 48 h for acetogenesis and by the less than 1% electrons distribution to CH₄, under 0.5 mM TDS. The microbial community was dominated by Eubacterium, Proteiniphilum and an unclassified member of the Eggerthellaceae family. The taxonomic diversity of the community decreased and conversely the phenotypic diversity increased, during operation. This work illustrated the scale-up potential of waste gas fermentations, by elucidating the effect of sulfide as a common gas impurity, and by demonstrating continuous, potentially renewable supply of electrons.

nZVI Impacts Substrate Conversion and Microbiome Composition in Chain Elongation From D- and L-Lactate Substrates

Medium-chain carboxylates (MCC) derived from biomass biorefining are attractive biochemicals to uncouple the production of a wide array of products from the use of non-renewable sources. Biological conversion of biomass-derived lactate during secondary fermentation can be steered to produce a variety of MCC through chain elongation. We explored the effects of zero-valent iron nanoparticles (nZVI) and lactate enantiomers on substrate consumption, product formation and microbiome composition in batch lactate-based chain elongation. In abiotic tests, nZVI supported chemical hydrolysis of lactate oligomers present in concentrated lactic acid. In fermentation experiments, nZVI created favorable conditions for either chain-elongating or propionate-producing microbiomes in a dose-dependent manner. Improved lactate conversion rates and n-caproate production were promoted at 0.5-2 g nZVI⋅L^-1 while propionate formation became relevant at ≥ 3.5 g nZVI⋅L^-1. Even-chain carboxylates (n-butyrate) were produced when using enantiopure and racemic lactate with lactate conversion rates increased in nZVI presence (1 g⋅L^-1). Consumption of hydrogen and carbon dioxide was observed late in the incubations and correlated with acetate formation or substrate conversion to elongated products in the presence of nZVI. Lactate racemization was observed during chain elongation while isomerization to D-lactate was detected during propionate formation. Clostridium luticellarii, Caproiciproducens, and Ruminococcaceae related species were associated with n-valerate and n-caproate production while propionate was likely produced through the acrylate pathway by Clostridium novyi. The enrichment of different potential n-butyrate producers (Clostridium tyrobutyricum, Lachnospiraceae, Oscillibacter, Sedimentibacter) was affected by nZVI presence and concentrations. Possible theories and mechanisms underlying the effects of nZVI on substrate conversion and microbiome composition are discussed. An outlook is provided to integrate (bio)electrochemical systems to recycle (n)ZVI and provide an alternative reducing power agent as durable control method.

A General Model for Biofilm-Driven Microbial Electrosynthesis of Carboxylates From CO2

Up to now, computational modeling of microbial electrosynthesis (MES) has been underexplored, but is necessary to achieve breakthrough understanding of the process-limiting steps. Here, a general framework for modeling microbial kinetics in a MES reactor is presented. A thermodynamic approach is used to link microbial metabolism to the electrochemical reduction of an intracellular mediator, allowing to predict cellular growth and current consumption. The model accounts for CO₂ reduction to acetate, and further elongation to n-butyrate and n-caproate. Simulation results were compared with experimental data obtained from different sources and proved the model is able to successfully describe microbial kinetics (growth, chain elongation, and product inhibition) and reactor performance (current density, organics titer). The capacity of the model to simulate different system configurations is also shown. Model results suggest CO₂ dissolved concentration might be limiting existing MES systems, and highlight the importance of the delivery method utilized to supply it. Simulation results also indicate that for biofilm-driven reactors, continuous mode significantly enhances microbial growth and might allow denser biofilms to be formed and higher current densities to be achieved.

Analysis of a Methanogen and an Actinobacterium Dominating the Thermophilic Microbial Community of an Electromethanogenic Biocathode

Electromethanogenesis refers to the bioelectrochemical synthesis of methane from CO₂ by biocathodes. In an electromethanogenic system using thermophilic microorganisms, metagenomic analysis along with quantitative real-time polymerase chain reaction and fluorescence in situ hybridization revealed that the biocathode microbiota was dominated by the methanogen Methanothermobacter sp. strain EMTCatA1 and the actinobacterium Coriobacteriaceae sp. strain EMTCatB1. RNA sequencing was used to compare the transcriptome profiles of each strain at the methane-producing biocathodes with those in an open circuit and with the methanogenesis inhibitor 2-bromoethanesulfonate (BrES). For the methanogen, genes related to hydrogenotrophic methanogenesis were highly expressed in a manner similar to those observed under H₂-limited conditions. For the actinobacterium, the expression profiles of genes encoding multiheme c-type cytochromes and membrane-bound oxidoreductases suggested that the actinobacterium directly takes up electrons from the electrode. In both strains, various stress-related genes were commonly induced in the open-circuit biocathodes and biocathodes with BrES. This study provides a molecular inventory of the dominant species of an electromethanogenic biocathode with functional insights and therefore represents the first multiomics characterization of an electromethanogenic biocathode.

Anode Surface Bioaugmentation Enhances Deterministic Biofilm Assembly in Microbial Fuel Cells

Microbial fuel cells (MFCs) generate energy while aiding the biodegradation of waste through the activity of an electroactive mixed biofilm. Metabolic cooperation is essential for MFCs' efficiency, especially during early colonization. Thus, examining specific ecological processes that drive the assembly of anode biofilms is highly important for shortening startup times and improving MFC performance, making this technology cost-effective and sustainable. Here, we use metagenomics to show that bioaugmentation of the anode surface with a taxonomically defined electroactive consortium, dominated by Desulfuromonas, resulted in an extremely rapid current density generation. Conversely, the untreated anode surface resulted in a highly stochastic and slower biofilm assembly. Remarkably, an efficient anode colonization process was obtained only if wastewater was added, leading to a nearly complete replacement of the bioaugmented community by Geobacter lovleyi Although different approaches to improve MFC startup have been investigated, we propose that only the combination of anode bioaugmentation with wastewater inoculation can reduce stochasticity. Such an approach provides the conditions that support the growth of specific newly arriving species that positively support the fast establishment of a highly functional anode biofilm.IMPORTANCE Mixed microbial communities play important roles in treating wastewater, in producing renewable energy, and in the bioremediation of pollutants in contaminated environments. While these processes are well known, especially the community structure and biodiversity, how to efficiently and robustly manage microbial community assembly remains unknown. Moreover, it has been shown that a high degree of temporal variation in microbial community composition and structure often occurs even under identical environmental conditions. This heterogeneity is directly related to stochastic processes involved in microbial community organization, similarly during the initial stages of biofilm formation on surfaces. In this study, we show that anode surface pretreatment alone is not sufficient for a substantial improvement in startup times in microbial fuel cells (MFCs), as previously thought. Rather, we have discovered that the combination of applying a well-known consortium directly on the anode surface together with wastewater (including the bacteria that they contain) is the optimized management scheme. This allowed a selected colonization process by the wastewater species, which improved the functionality relative to that of untreated systems.

The ModelSEED Biochemistry Database for the integration of metabolic annotations and the reconstruction, comparison and analysis of metabolic models for plants, fungi and microbes

For over 10 years, ModelSEED has been a primary resource for the construction of draft genome-scale metabolic models based on annotated microbial or plant genomes. Now being released, the biochemistry database serves as the foundation of biochemical data underlying ModelSEED and KBase. The biochemistry database embodies several properties that, taken together, distinguish it from other published biochemistry resources by: (i) including compartmentalization, transport reactions, charged molecules and proton balancing on reactions; (ii) being extensible by the user community, with all data stored in GitHub; and (iii) design as a biochemical 'Rosetta Stone' to facilitate comparison and integration of annotations from many different tools and databases. The database was constructed by combining chemical data from many resources, applying standard transformations, identifying redundancies and computing thermodynamic properties. The ModelSEED biochemistry is continually tested using flux balance analysis to ensure the biochemical network is modeling-ready and capable of simulating diverse phenotypes. Ontologies can be designed to aid in comparing and reconciling metabolic reconstructions that differ in how they represent various metabolic pathways. ModelSEED now includes 33,978 compounds and 36,645 reactions, available as a set of extensible files on GitHub, and available to search at https://modelseed.org/biochem and KBase.

The ModelSEED Biochemistry Database for the integration of metabolic annotations and the reconstruction, comparison and analysis of metabolic models for plants, fungi and microbes

For over ten years, ModelSEED has been a primary resource for the construction of draft genome-scale metabolic models based on annotated microbial or plant genomes. Now being released, the biochemistry database serves as the foundation of biochemical data underlying ModelSEED and KBase. The biochemistry database embodies several properties that, taken together, distinguish it from other published biochemistry resources by: (i) including compartmentalization, transport reactions, charged molecules and proton balancing on reactions;; (ii) being extensible by the user community, with all data stored in GitHub; and (iii) design as a biochemical “Rosetta Stone” to facilitate comparison and integration of annotations from many different tools and databases. The database was constructed by combining chemical data from many resources, applying standard transformations, identifying redundancies, and computing thermodynamic properties. The ModelSEED biochemistry is continually tested using flux balance analysis to ensure the biochemical network is modeling-ready and capable of simulating diverse phenotypes. Ontologies can be designed to aid in comparing and reconciling metabolic reconstructions that differ in how they represent various metabolic pathways. ModelSEED now includes 33,978 compounds and 36,645 reactions, available as a set of extensible files on GitHub, and available to search at https://modelseed.org and KBase.

Propionate Production by Bioelectrochemically-Assisted Lactate Fermentation and Simultaneous CO2 Recycling

Production of volatile fatty acids (VFAs), fundamental building blocks for the chemical industry, depends on fossil fuels but organic waste is an emerging alternative substrate. Lactate produced from sugar-containing waste streams can be further processed to VFAs. In this study, electrofermentation (EF) in a two-chamber cell is proposed to enhance propionate production via lactate fermentation. At an initial pH of 5, an applied potential of −1 V vs. Ag/AgCl favored propionate production over butyrate from 20 mM lactate (with respect to non-electrochemical control incubations), due to the pH buffering effect of the cathode electrode, with production rates up to 5.9 mM d^–1 (0.44 g L^–1 d^–1). Microbial community analysis confirmed the enrichment of propionate-producing microorganisms, such as Tyzzerella sp. and Propionibacterium sp. Organisms commonly found in microbial electrosynthesis reactors, such as Desulfovibrio sp. and Acetobacterium sp., were also abundant at the cathode, indicating their involvement in recycling CO₂ produced by lactate fermentation into acetate, as confirmed by stoichiometric calculations. Propionate was the main product of lactate fermentation at substrate concentrations up to 150 mM, with a highest production rate of 12.9 mM d^–1 (0.96 g L^–1 d^–1) and a yield of 0.48 mol mol^–1 lactate consumed. Furthermore, as high as 81% of the lactate consumed (in terms of carbon) was recovered as soluble product, highlighting the potential for EF application with high-carbon waste streams, such as cheese whey or other food wastes. In summary, EF can be applied to control lactate fermentation toward propionate production and to recycle the resulting CO₂ into acetate, increasing the VFA yield and avoiding carbon emissions and addition of chemicals for pH control.

Microbial Electrosynthesis: Where Do We Go from Here?

The valorization of CO₂ to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions.

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.

Carboxylic acids production and electrosynthetic microbial community evolution under different CO2 feeding regimens

Microbial electrosynthesis (MES) is a potential technology for CO₂ recycling, but insufficient information is available on the microbial interactions underpinning electrochemically-assisted reactions. In this study, a MES reactor was operated for 225 days alternately with bicarbonate or CO₂ as carbon source, under batch or continuous feeding regimens, to evaluate the response of the microbial communities, and their productivity, to dynamic operating conditions. A stable acetic acid production rate of 9.68 g m^-2 d^-1, and coulombic efficiency up to 40%, was achieved with continuous CO₂ sparging, higher than the rates obtained with bicarbonate (0.94 g m^-2 d^-1) and CO₂ under fed-batch conditions (2.54 g m^-2 d^-1). However, the highest butyric acid production rate (0.39 g m^-2 d^-1) was achieved with intermittent CO₂ sparging. The microbial community analyses focused on differential amplicon sequence variants (ASVs), allowing detection of ASVs significantly different across consecutive samples. This analysis, combined with co-occurence network analysis, and cyclic voltammetry, indicated that hydrogen-mediated acetogenesis was carried out by Clostridium, Eubacterium and Acetobacterium, whereas Oscillibacter and Caproiciproducens were involved in butyric acid production. The cathodic community was spatially inhomogeneous, with potential electrotrophs, such as Sulfurospirillum and Desulfovibrio, most prevalent near the current collector. The abundance of Sulfurospirillum positively correlated with that of Acetobacterium, supporting the syntrophic metabolism of both organisms.

Parameters influencing the development of highly conductive and efficient biofilm during microbial electrosynthesis: the importance of applied potential and inorganic carbon source

Cathode-driven applications of bio-electrochemical systems (BESs) have the potential to transform CO2 into value-added chemicals using microorganisms. However, their commercialisation is limited as biocathodes in BESs are characterised by slow start-up and low efficiency. Understanding biosynthesis pathways, electron transfer mechanisms and the effect of operational variables on microbial electrosynthesis (MES) is of fundamental importance to advance these applications of a system that has the capacity to convert CO2 to organics and is potentially sustainable. In this work, we demonstrate that cathodic potential and inorganic carbon source are keys for the development of a dense and conductive biofilm that ensures high efficiency in the overall system. Applying the cathodic potential of -1.0 V vs. Ag/AgCl and providing only gaseous CO2 in our system, a dense biofilm dominated by Acetobacterium (ca. 50% of biofilm) was formed. The superior biofilm density was significantly correlated with a higher production yield of organic chemicals, particularly acetate. Together, a significant decrease in the H2 evolution overpotential (by 200 mV) and abundant nifH genes within the biofilm were observed. This can only be mechanistically explained if intracellular hydrogen production with direct electron uptake from the cathode via nitrogenase within bacterial cells is occurring in addition to the commonly observed extracellular H2 production. Indeed, the enzymatic activity within the biofilm accelerated the electron transfer. This was evidenced by an increase in the coulombic efficiency (ca. 69%) and a 10-fold decrease in the charge transfer resistance. This is the first report of such a significant decrease in the charge resistance via the development of a highly conductive biofilm during MES. The results highlight the fundamental importance of maintaining a highly active autotrophic Acetobacterium population through feeding CO2 in gaseous form, which its dominance in the biocathode leads to a higher efficiency of the system.

Extracellular electron uptake by autotrophic microbes: physiological, ecological, and evolutionary implications

Microbes exchange electrons with their extracellular environment via direct or indirect means. This exchange is bidirectional and supports essential microbial oxidation–reduction processes, such as respiration and photosynthesis. The microbial capacity to use electrons from insoluble electron donors, such as redox-active minerals, poised electrodes, or even other microbial cells is called extracellular electron uptake (EEU). Autotrophs with this capability can thrive in nutrient and soluble electron donor-deficient environments. As primary producers, autotrophic microbes capable of EEU greatly impact microbial ecology and play important roles in matter and energy flow in the biosphere. In this review, we discuss EEU-driven autotrophic metabolisms, their mechanism and physiology, and highlight their ecological, evolutionary, and biotechnological implications.

Defining Genomic and Predicted Metabolic Features of the Acetobacterium Genus

Acetogens are anaerobic bacteria capable of fixing CO₂ or CO to produce acetyl-CoA and ultimately acetate using the Wood-Ljungdahl pathway (WLP). This autotrophic metabolism plays a major role in the global carbon cycle and, if harnessed, can help reduce greenhouse gas emissions. Overall, the data presented here provide a framework for examining the ecology and evolution of the Acetobacterium genus and highlight the potential of these species as a source for production of fuels and chemicals from CO₂ feedstocks.

Acetogens are anaerobic bacteria capable of fixing CO₂ or CO to produce acetyl coenzyme A (acetyl-CoA) and ultimately acetate using the Wood-Ljungdahl pathway (WLP). Acetobacterium woodii is the type strain of the Acetobacterium genus and has been critical for understanding the biochemistry and energy conservation in acetogens. Members of the Acetobacterium genus have been isolated from a variety of environments or have had genomes recovered from metagenome data, but no systematic investigation has been done on the unique and various metabolisms of the genus. To gain a better appreciation for the metabolic breadth of the genus, we sequenced the genomes of 4 isolates (A. fimetarium, A. malicum, A. paludosum, and A. tundrae) and conducted a comparative genome analysis (pan-genome) of 11 different Acetobacterium genomes. A unifying feature of the Acetobacterium genus is the carbon-fixing WLP. The methyl (cluster II) and carbonyl (cluster III) branches of the Wood-Ljungdahl pathway are highly conserved across all sequenced Acetobacterium genomes, but cluster I encoding the formate dehydrogenase is not. In contrast to A. woodii, all but four strains encode two distinct Rnf clusters, Rnf being the primary respiratory enzyme complex. Metabolism of fructose, lactate, and H₂:CO₂ was conserved across the genus, but metabolism of ethanol, methanol, caffeate, and 2,3-butanediol varied. Additionally, clade-specific metabolic potential was observed, such as amino acid transport and metabolism in the psychrophilic species, and biofilm formation in the A. wieringae clade, which may afford these groups an advantage in low-temperature growth or attachment to solid surfaces, respectively.

IMPORTANCE Acetogens are anaerobic bacteria capable of fixing CO₂ or CO to produce acetyl-CoA and ultimately acetate using the Wood-Ljungdahl pathway (WLP). This autotrophic metabolism plays a major role in the global carbon cycle and, if harnessed, can help reduce greenhouse gas emissions. Overall, the data presented here provide a framework for examining the ecology and evolution of the Acetobacterium genus and highlight the potential of these species as a source for production of fuels and chemicals from CO₂ feedstocks.

Enhanced electron transfer on microbial electrosynthesis biocathode by polypyrrole-coated acetogens

Extracellular electron transfer (EET) is a significant pathway to transport electrons between bacteria and electrode in microbial electrosynthesis systems (MESs). To enhance EET in the MES, a high-conductivity polymer, polypyrrole (PPy), was coated on the surface of mixed culture acetogens in situ and the PPy-coated bacteria were inoculated on the cathode of MES. The charge transfer resistance of PPy-coated biocathode was 33%-70% of that with PPy-uncoated. Acetate production rate and Faradic efficiency in PPy-coated biocathodes increased by 3 to 6 times. After 960 h operation, Acetobacterium, Desulfovibrio, and Acinetobacter dominate the community on the coated and uncoated biocathode. Quinone loop and NADH dehydrogenase to ubiquinone were involved in electron transfer pathway of biocathode and stimulated by PPy coating. Low-level expression of C-type cytochromes on biocathode indicated its less important role in inward EET. The study provided useful information for applications of high-conductivity chemicals in microbial electrosynthesis.

Perovskite-Based Multifunctional Cathode with Simultaneous Supplementation of Substrates and Electrons for Enhanced Microbial Electrosynthesis of Organics

Microbial electrosynthesis (MES) is an electricity-driven technology for the microbial reduction of CO₂ to organic commodities. However, the limited solubility of CO₂ in a solution and the inefficient electron transfer make it impossible for microorganisms to obtain an efficient surface for catalytic interaction, thus resulting in the low efficiency of MES. To address this, we introduce a multifunctional perovskite-based cathode material Pr_0.5(Ba_0.5Sr_0.5)_0.5Co_0.8Fe_0.2O_3-δ-carbon felt (Pr_0.5BSCF-CF), which provides a simultaneously significant increase in CO₂ absorption and hydrogen production. As a result, the volumetric acetate production rate of MES obtained by Pr_0.5BSCF-CF is 0.24 ± 0.01 g L^-1 day^-1, and it achieves a maximum acetate titer of 13.74 ± 0.20 g L^-1 within 70 days. An adequate supply of CO₂ and H₂ also provides a sufficient amount of substrates and energy for the self-replication of the biocatalysts in the MES reactor. This effect not only increases the amount of biocatalysts but also optimizes the functions of the biocatalysts; the above benefits further improve the production efficiency of the MES system. This strategy demonstrates that the development of perovskite-based multifunctional cathodes with a simultaneous supplementation of substrates and electrons is a promising approach toward improving the MES efficiency.

Microbial electrosynthesis from CO2: Challenges, opportunities and perspectives in the context of circular bioeconomy

Recycling CO₂ into organic products through microbial electrosynthesis (MES) is attractive from the perspective of circular bioeconomy. However, several challenges need to be addressed before scaling-up MES systems. In this review, recent advances in electrode materials, microbe-catalyzed CO₂ reduction and MES energy consumption are discussed in detail. Anode materials are briefly reviewed first, with several strategies proposed to reduce the energy input for electron generation and enhance MES bioeconomy. This was followed by discussions on MES cathode materials and configurations for enhanced chemolithoautotroph growth and CO₂ reduction. Various chemolithoautotrophs, effective for CO₂ reduction and diverse bioproduct formation, on MES cathode were also discussed. Finally, research efforts on developing cost-effective process for bioproduct extraction from MES are presented. Future perspectives to improve product formation and reduce energy cost are discussed to realize the application of the MES as a chemical production platform in the context of building a circular economy.

From an extremophilic community to an electroautotrophic production strain: identifying a novel Knallgas bacterium as cathodic biofilm biocatalyst

Coupling microbial electrosynthesis to renewable energy sources can provide a promising future technology for carbon dioxide conversion. However, this technology suffers from a limited number of suitable biocatalysts, resulting in a narrow product range. Here, we present the characterization of the first thermoacidophilic electroautotrophic community using chronoamperometric, metagenomic, and ¹³C-labeling analyses. The cathodic biofilm showed current consumption of up to -80 µA cm^-2 over a period of 90 days (-350 mV vs. SHE). Metagenomic analyses identified members of the genera Moorella, Desulfofundulus, Thermodesulfitimonas, Sulfolobus, and Acidianus as potential primary producers of the biofilm, potentially thriving via an interspecies sulfur cycle. Hydrogenases seem to be key for cathodic electron uptake. An isolation campaign led to a pure culture of a Knallgas bacterium from this community. Growth of this organism on cathodes led to increasing reductive currents over time. Transcriptomic analyses revealed a distinct gene expression profile of cells grown at a cathode. Moreover, pressurizable flow cells combined with optical coherence tomography allowed an in situ observation of cathodic biofilm growth. Autotrophic growth was confirmed via isotope analysis. As a natural polyhydroxybutyrate (PHB) producer, this novel species, Kyrpidia spormannii, coupled the production of PHB to CO₂ fixation on cathode surfaces.

Microbiome for the Electrosynthesis of Chemicals from Carbon Dioxide

The price for renewable electricity is rapidly decreasing, and the availability of such energy is expected to increase in the coming years. This is a welcomed outcome considering that mitigation of climate disruption due to the use of fossil carbon is reaching a critical stage. However, the economy will remain dependent on carbon-based chemicals and the problem of electricity storage persists. Therefore, the development of electrosynthetic processes that convert electricity and CO₂ into chemicals and energy dense fuels, perhaps even food, would be desirable. Electrochemistry has been applied to the manufacture of many valuable products and at a large industrial scale, but it is difficult to produce multicarbon chemicals from CO₂ by chemistry alone. Being that the biological world possesses expertise at the construction of C-C bonds, it is being examined in conjunction with electrochemistry to discover new ways of synthesizing chemicals from electricity and CO₂. One approach is microbial electrosynthesis. This Account describes the development of a microbial electrosynthesis system by the authors. A biocathode consisting of a carbon-based electrode and a microbial community produced short chain fatty acids, primarily acetate. The device works by electrolysis of water, but microbes facilitate electron transfer from the cathode while reducing CO₂ by the Wood-Ljungdahl pathway possessed by an Acetobacterium sp. While this acetogenic microorganism dominates the microbiome growing on the cathode surface, 13 total species of microbes overall were ecologically selected on the cathode and genomes for each have been assembled. The combined species may contribute to the stability of the microbiome, a common feature of naturally selected microbial communities. The microbial electrosynthesis system was demonstrated to operate continuously at a cathode for more than 2 years and could also be used with intermittent power, thus demonstrating the stability of the microbiome living at the cathode. In addition to the description of reactor design and startup procedures, the possible mechanisms of electron transfer are described in this Account. While mysteries remain to be solved, much evidence indicates that the microbiome may facilitate electron transfer by supplying catalyst(s) external to the bacterial cells and onto the cathode surface. This may be in the form of a hydrogen-producing catalyst that enhances hydrogen generation by an inert carbon-based electrode. Through the enrichment of the electrosynthetic microbiome along with several modifications in reactor design and operation, the productivity and efficiency were improved. In addition to the intrinsic value of the current products, coupling the process with a secondary stage might be used to produce more valuable products from the acetic acid stream such as lipids, biocrude oil, or higher value food supplements. Alternatively, additional work on the mechanism of electron transfer, reactor design/operation, and modification of the microbes through synthetic biology, particularly to enhance carbon efficiency into higher value chemicals, are the needed next steps to advance microbial electrosynthesis so that it may be used to transform renewable electrons and CO₂ directly into products and help solve the problem of climate disruption.

Extracellular Electron Uptake by Acetogenic Bacteria: Does H2 Consumption Favor the H2 Evolution Reaction on a Cathode or Metallic Iron?

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H2 partial pressures by microbial H2 consumption, which thermodynamically favors the H2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H2 partial pressure affects the H2 evolution onset potential and the H2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H2 consumption characteristics, i.e., H2 threshold (thermodynamic limit for H2 consumption) and H2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H2 partial pressures (lower H2 threshold, higher H2 affinity) more strongly increase the H2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Enrichment of Marinobacter sp. and Halophilic Homoacetogens at the Biocathode of Microbial Electrosynthesis System Inoculated With Red Sea Brine Pool

Homoacetogens are efficient CO₂ fixing bacteria using H₂ as electron donor to produce acetate. These organisms can be enriched at the biocathode of microbial electrosynthesis (MES) for electricity-driven CO₂ reduction to acetate. Studies exploring homoacetogens in MES are mainly conducted using pure or mix-culture anaerobic inocula from samples with standard environmental conditions. Extreme marine environments host unique microbial communities including homoacetogens that may have unique capabilities due to their adaptation to harsh environmental conditions. Anaerobic deep-sea brine pools are hypersaline and metalliferous environments and homoacetogens can be expected to live in these environments due to their remarkable metabolic flexibility and energy-efficient biosynthesis. However, brine pools have never been explored as inocula for the enrichment of homacetogens in MES. Here we used the saline water from a Red Sea brine pool as inoculum for the enrichment of halophilic homoacetogens at the biocathode (-1 V vs. Ag/AgCl) of MES. Volatile fatty acids, especially acetate, along with hydrogen gas were produced in MES systems operated at 25 and 10% salinity. Acetate concentration increased when MES was operated at a lower salinity ∼3.5%, representing typical seawater salinity. Amplicon sequencing and genome-centric metagenomics of matured cathodic biofilm showed dominance of the genus Marinobacter and phylum Firmicutes at all tested salinities. Seventeen high-quality draft metagenome-assembled genomes (MAGs) were extracted from the biocathode samples. The recovered MAGs accounted for 87 ± 4% of the quality filtered sequence reads. Genome analysis of the MAGs suggested CO₂ fixation via Wood-Ljundahl pathway by members of the phylum Firmicutes and the fixed CO₂ was possibly utilized by Marinobacter sp. for growth by consuming O₂ escaping from the anode to the cathode for respiration. The enrichment of Marinobacter sp. with homoacetogens was only possible because of the specific cathodic environment in MES. These findings suggest that in organic carbon-limited saline environments, Marinobacter spp. can live in consortia with CO₂ fixing bacteria such as homoacetogens, which can provide them with fixed carbon as a source of carbon and energy.

Recovery of elemental sulfur with a novel integrated bioelectrochemical system with an electrochemical cell

Several industrial activities produce wastewater with high sulfate content that can cause significant environmental issues. Although bioelectrochemical systems (BESs) have recently been studied for the treatment of sulfate contained in this wastewater, the recovery of elemental sulfur with BESs is still in its beginnings. This work proposes a new reactor configuration named BES-EC, consisting of the coupling of a BES with an electrochemical cell (EC), to treat this type of wastewater and recover elemental sulfur. The reactor consisted of four electrodes: i) an abiotic anode, ii) a biocathode for the autotrophic sulfate reduction, iii) an anode of an electrochemical cell (EC) for the partial oxidation of sulfide to elemental sulfur (the biocathode and the EC anode were placed in the same chamber) and iv) an abiotic EC cathode. Several cathode potentials and sulfate loads were tested, obtaining high sulfate removal rates (up to 888 mg SO₄^2--S L^-1 d^-1 at -0.9 V vs. SHE with a specific energy consumption of 9.18 ± 0.80 kWh kg^-1 SO₄^2--S). Exceptionally high theoretical elemental sulfur production rates (up to 498 mg S⁰-S L^-1 d^-1) were achieved with the EC controlled at a current density of 2.5 A m^-2. Electron recovery around 80% was observed throughout most of the operation of the integrated system. In addition, short experiments were performed at different current densities, observing that sulfate removal did not increase proportionally to the higher applied current density. However, when the BES was controlled at 30 A m^-2 and the EC at 7.5 A m^-2, the proportion of elemental sulfur produced corresponded to 92.9 ± 1.9% of all sulfate removed.

Continuous n-valerate formation from propionate and methanol in an anaerobic chain elongation open-culture bioreactor

BACKGROUND

Chain elongation forms a new platform technology for the circular production of biobased chemicals from renewable carbon and energy sources. This study aimed to develop a continuous methanol-based chain elongation process for the open-culture production of a new-generation biofuel precursor and potential platform chemical: n-valerate. Propionate was used as a substrate for chain elongation to n-valerate in an anaerobic open-culture bioreactor. In addition, the co-production of n- and iso-butyrate in addition to n-valerate via, respectively, acetate and propionate elongation was investigated.

RESULTS

n-Valerate was produced during batch and continuous experiments with a pH in the range 5.5-5.8 and a hydraulic retention time of 95 h. Decreasing the pH from 5.8 to 5.5 caused an increase of the selectivity for n-valerate formation (from 58 up to 70 wt%) during methanol-based propionate elongation. n-Valerate and both n- and iso-butyrate were produced during simultaneous methanol-based elongation of propionate and acetate. Propionate was within the open-culture preferred over acetate as a substrate with 10-30% more consumption. Increasing the methanol concentration in the influent (from 250 to 400 mM) resulted in a higher productivity (from 45 to 58 mmol C/L/day), but a lower relative product selectivity (from 49 to 43 wt%) of n-valerate. The addition of acetate as a substrate did not change the average n-valerate productivities. Within the continuous bioreactor experiments, 6 to 17 wt% of formed products was methane. The microbial community during all steady-states in both methanol-based elongation bioreactors was dominated by species related to Clostridium luticellarii and Candidatus Methanogranum. C. luticellarii is the main candidate for n-valerate formation from methanol and propionate.

CONCLUSIONS

n-Valerate was for the first time proven to be produced from propionate and methanol by an open-culture bioreactor. Methanogenic activity can be inhibited by decreasing the pH, and the n-valerate productivity can be improved by increasing the methanol concentration. The developed process can be integrated with various biorefinery processes from thermochemical, (bio)electrochemical, photovoltaic and microbial technologies. The findings from this study form a useful tool to steer the process of biological production of chemicals from biomass and other carbon and energy sources.

Electro-driven methanogenic microbial community diversity and variability in the electron abundant niche

The underlying dynamics of microbial (bacteria and archaea) communities ecologically responding to an applied potential are critical to achieving the goal of enhancing bioenergy recovery but are not sufficiently understood. We built a MEC-AD mode that increased methane production rate by several times (max. 3.8 times) during the startup period compared to control AD, changed the absence or presence of external voltage to provide the pre-, dur-, and post- samples for microbial analysis. From a time and spatially dependent community analysis of electrode-respiring bacteria and methanogens, the corresponding Geobacter developed under the influence of external voltage, pairing with methanogens in the anodic and cathodic biofilm to generate methane. Additionally, at the cathode, the Alkaliphilus (basophilic bacteria) also correspondingly shifted alongside the change of external voltage. The mcrA sequencing confirmed a change in the dominant microbe from acetoclastic (mostly Methanosarcina mazei LYC) to hydrogenotrophic methanogens (mostly basophilic Methanobacterium alcaliphilum) at the cathode with 0.8 V voltage. Overall, the external voltage not only enriched the functional microbes including electrogens and methanogens but also indirectly shifted the composition of the bacterial and archaeal community via disturbing the pH condition. The predictive functional profiling indicated that the cathodic methanogenesis principally followed the metabolism pathway of the hydrogenotrophic methanogens, suggesting the F420 co-enzyme could be the key mediate for electron transfer. All data suggested that the electric stimulation would change and maintain the micro-environmental conditions to shift the bacterial/archaeal community.