Electro-fermentation: Sustainable bioproductions steered by electricity

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.

Microbial electrosynthesis: opportunities for microbial pure cultures

Microbial electrosynthesis (MES) is an emerging technology that couples renewable electricity to microbial production processes. Although advances in MES performance have been driven largely by microbial mixed cultures, we see a great limitation in the diversity, and hence value, of products that can be achieved in undefined mixed cultures. By contrast, metabolic control of pure cultures and genetic engineering could greatly expand the scope of MES, and even of broader electrobiotechnology, to include targeted high-value products. To leverage this potential, we advocate for more efforts and activities to develop engineered electroactive microbes for synthesis, and we highlight the need for a standardized electrobioreactor infrastructure that allows the establishment and engineering of electrobioprocesses with these novel biocatalysts.

Electrochemical promotion of organic waste fermentation: Research advances and prospects

The current methods of treating organic waste suffer from limited resource usage and low product value. Research and development of value-added products emerges as an unavoidable trend for future growth. Electro-fermentation (EF) is a technique employed to stimulate cell proliferation, expedite microbial metabolism, and enhance the production of value-added products by administering minute voltages or currents in the fermentation system. This method represents a novel research direction lying at the crossroads of electrochemistry and biology. This article documents the current progress of EF for a range of value-added products, including gaseous fuels, organic acids, and other organics. It also presents novel value-added products, such as 1,3-propanediol, 3-hydroxypropionic acid, succinic acid, acrylic acid, and lysine. The latest research trends suggest a focus on EF for cogeneration of value-added products, studying microbial community structure and electroactive bacteria, exploring electron transfer mechanisms in EF systems, developing effective methods for nutrient recovery of nitrogen and phosphorus, optimizing EF conditions, and utilizing biosensors and artificial neural networks in this area. In this paper, an analysis is conducted on the challenges that currently exist regarding the selection of conductive materials, optimization of electrode materials, and development of bioelectrochemical system (BES) coupling processes in EF systems. The aim is to provide a reference for the development of more efficient, advanced, and value-added EF technologies. Overall, this paper aims to provide references and ideas for the development of more efficient and advanced EF technology.

Sustainable Production of Biofuels and Biochemicals via Electro-Fermentation Technology

The energy crisis and climate change are two of the most concerning issues for human beings nowadays. For that reason, the scientific community is focused on the search for alternative biofuels to conventional fossil fuels as well as the development of sustainable processes to develop a circular economy. Bioelectrochemical processes have been demonstrated to be useful for producing bioenergy and value-added products from several types of waste. Electro-fermentation has gained great attention in the last few years due to its potential contribution to biofuel and biochemical production, e.g., hydrogen, methane, biopolymers, etc. Conventional fermentation processes pose several limitations in terms of their practical and economic feasibility. The introduction of two electrodes in a bioreactor allows the regulation of redox instabilities that occur in conventional fermentation, boosting the overall process towards a high biomass yield and enhanced product formation. In this regard, key parameters such as the type of culture, the nature of the electrodes as well as the operating conditions are crucial in order to maximize the production of biofuels and biochemicals via electro-fermentation technology. This article comprises a critical overview of the benefits and limitations of this emerging bio-electrochemical technology and its contribution to the circular economy.

Full quantitative resource utilization of raw mustard waste through integrating a comprehensive approach for producing hydrogen and soil amendments

BACKGROUND

Pickled mustard, the largest cultivated vegetable in China, generates substantial waste annually, leading to significant environmental pollution due to challenges in timely disposal, leading to decomposition and sewage issues. Consequently, the imperative to address this concern centers on the reduction and comprehensive resource utilization of raw mustard waste (RMW). To achieve complete and quantitative resource utilization of RMW, this study employs novel technology integration for optimizing its higher-value applications.

RESULTS

Initially, subcritical hydrothermal technology was applied for rapid decomposition, with subsequent ammonia nitrogen removal via zeolite. Thereafter, photosynthetic bacteria, Rhodopseudomonas palustris, were employed to maximize hydrogen and methane gas production using various fermentation enhancement agents. Subsequent solid-liquid separation yielded liquid fertilizer from the fermented liquid and soil amendment from solid fermentation remnants. Results indicate that the highest glucose yield (29.6 ± 0.14) was achieved at 165-173℃, with a total sugar content of 50.2 g/L and 64% glucose proportion. Optimal ammonia nitrogen removal occurred with 8 g/L zeolite and strain stable growth at 32℃, with the highest OD600 reaching 2.7. Several fermentation promoters, including FeSO4, Neutral red, Na2S, flavin mononucleotide, Nickel titanate, Nickel oxide, and Mixture C, were evaluated for hydrogen production. Notably, Mixture C resulted in the maximum hydrogen production (756 mL), a production rate of 14 mL/h, and a 5-day stable hydrogen production period. Composting experiments enhanced humic acid content and organic matter (OM) by 17% and 15%, respectively.

CONCLUSIONS

This innovative technology not only expedites RMW treatment and hydrogen yield but also substantially enriches soil fertility. Consequently, it offers a novel approach for low-carbon, zero-pollution RMW management. The study's double outcomes extend to large-scale RMW treatment based on the aim of full quantitative resource utilization of RMW. Our method provides a valuable reference for waste management in similar perishable vegetable plantations.

Superior anodic electro-fermentation by enhancing capacity for extracellular electron transfer

Anodic electro-fermentation (AEF), where an anode replaces the terminal electron acceptor, shows great promise. Recently a Lactococcus lactis strain blocked in NAD+ regeneration was demonstrated to use ferricyanide as an alternative electron acceptor to support fast growth, but the need for high concentrations of this non-regenerated electron acceptor limits practical applications. To address this, growth of this L. lactis strain, and an adaptively evolved (ALE) mutant with enhanced ferricyanide respiration capacity were investigated using an anode as electron acceptor in a bioelectrochemical system (BES) setup. Both strains grew well, however, the ALE mutant significantly faster. The ALE mutant almost exclusively generated 2,3-butanediol, whereas its parent strain mainly produced acetoin. The ALE mutant interacted efficiently with the anode, achieving a record high current density of 0.81 ± 0.05 mA/cm2. It is surprising that a Lactic Acid Bacterium, with fermentative metabolism, interacts so well with an anode, which demonstrates the potential of AEF.

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.

Electrobiochemical skills of Pseudomonas aeruginosa species that produce pyocyanin or pyoverdine for glycerol oxidation in a microbial fuel cell

Pseudomonas aeruginosa can produce pigments, which mediate external electron transfer (EET). Depending on the mediator, this species can be explored in bioelectrosystems to harvest energy or to obtain chemicals from residual organic compounds. This study has compared the performance of microbial fuel cells (MFCs) inoculated with a Pseudomonas aeruginosa isolate, namely EW603 or EW819, which produce pyocyanin and pyoverdine, respectively. The efficiency of these MFCs in glycerol, a typical residue of biodiesel production, were also compared. The MFCs exhibited different performances. The maximum voltage was 411 and 281 mV m², the power density was 40.1 and 21.3 mW m^-2, and the coulombic efficiency was 5.16 and 1.49% for MFC-EW603 and MFC-EW819, respectively. MFC-EW603 and MFC-EW819 achieved maximum current at 560 and 2200 Ω, at 141.2 and 91.3 mA m^-2, respectively. When the system was operated at the respective maximum current output, MFC-EW603 consumed the total glycerol content (11 mmol L^-1), and no products could be detected after 50 h. In turn, acetic and butyric acids were detected at the end of MFC-EW819 operation (75 h). The results suggested that P. aeruginosa metabolism can be steered in the MFC to generate current or microbial products depending on the pigment-producing strain and the conditions applied to the system, such as the external resistance. In addition, gene cluster pathways related to phenazine production (phzA and phzB) and other electrogenic-related genes (mexGHI-opmB) were identified in the strain genomes, supporting the findings. These results open new possibilities for using glycerol in bioelectrochemical systems.

Membrane engineering of Escherichia coli based on "Building bridges" and "Digging tunnels" to improve electro-fermentation of succinate

Succinate is the end product of anaerobic metabolism of Escherichia coli, and its over-production needs abundant reducing force. Electro-fermentation (EF) is a novel biotechnology to steer and control fermentative processes by supplying extra electrons. However, E.coli is a non-electroactive strain which needs the support of electron shuttle in EF. Here, membrane engineering strategies of "Building bridges" via screening direct electron transport pathway and "Digging tunnels" via screening membrane porins were developed to improve the transmembrane transport of electron during the cathodic electro-fermentation (CEF). As a result, the total electron quantity during electro-fermentation was increased from 1.21 mmol to 7.90 mmol, and succinate yield was increased by 23.3% when these strategies simultaneously were applied to the succinate candidate E. coli Suc260. Hence, this study provides a reference mode for designing and constructing non-electroactive bacteria for electro-fermentation of reductive metabolites.

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

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

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

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

Marine Biofilm Engineered to Produce Current in Response to Small Molecules

Engineered electroactive bacteria have potential applications ranging from sensing to biosynthesis. In order to advance the use of engineered electroactive bacteria, it is important to demonstrate functional expression of electron transfer modules in chassis adapted to operationally relevant conditions, such as non-freshwater environments. Here, we use the Shewanella oneidensis electron transfer pathway to induce current production in a marine bacterium, Marinobacter atlanticus, during biofilm growth in artificial seawater. Genetically encoded sensors optimized for use in Escherichia coli were used to control protein expression in planktonic and biofilm attached cells. Significant current production required the addition of menaquinone, which M. atlanticus does not produce, for electron transfer from the inner membrane to the expressed electron transfer pathway. Current through the S. oneidensis pathway in M. atlanticus was observed when inducing molecules were present during biofilm formation. Electron transfer was also reversible, indicating that electron transfer into M. atlanticus could be controlled. These results show that an operationally relevant marine bacterium can be genetically engineered for environmental sensing and response using an electrical signal.

Production of acetone, butanol, and ethanol by electro-fermentation with Clostridium saccharoperbutylacetonicum N1-4

This manuscript describes the effect of altering the extracellular redox potential during the production of acetone, butanol, and ethanol on a dual chamber H-type microbial fuel cell by fermenting glucose with Clostridium saccharoperbutylacetonicum N1-4. Extracellular redox potential modification was achieved by either supplementing the microbial broth with the redox agent NADH or by poising the cathode potential at -600 mV vs. Ag/AgCl. The addition of NADH was found to foment the production of acetone via fermentation of glucose. The addition of 200 mM of NADH to the catholyte rendered the highest production of acetone (2.4 g L^-1), thus outperforming the production of acetone by conventional fermentation means (control treatment) by a factor of 2.2. The experimental evidence gathered here, indicates that cathodic electro-fermentation of glucose favors the production of butanol. When poising the cathode potential at -600 mV vs Ag/AgCl (electro-fermentation), the largest production of butanol was achieved (5.8 g L^-1), outperforming the control treatment by a factor of 1.5. The production of ABE solvents and the electrochemical measurements demonstrate the electroactive properties of C. saccharoperbutylacetonicum N1-4 and illustrates the usefulness of bio-electrochemical systems to improve conventional fermentative processes.

Marinobacter: A case study in bioelectrochemical chassis evaluation

The junction of bioelectrochemical systems and synthetic biology opens the door to many potentially groundbreaking technologies. When developing these possibilities, choosing the correct chassis organism can save a great deal of engineering effort and, indeed, can mean the difference between success and failure. Choosing the correct chassis for a specific application requires a knowledge of the metabolic potential of the candidate organisms, as well as a clear delineation of the traits, required in the application. In this review, we will explore the metabolic and electrochemical potential of a single genus, Marinobacter. We will cover its strengths, (salt tolerance, biofilm formation and electrochemical potential) and weaknesses (insufficient characterization of many strains and a less developed toolbox for genetic manipulation) in potential synthetic electromicrobiology applications. In doing so, we will provide a roadmap for choosing a chassis organism for bioelectrochemical systems.

Boosting ethanol production rates from carbon dioxide in MES cells under optimal solventogenic conditions

Microbial Electrosynthesis (MES) has been widely applied for acetic acid (HA) production from CO₂ and electricity. Ethanol (EtOH) has a higher market value than HA, and wide application in industry and as a biofuel. However, it has only been obtained sporadically and at low concentrations, probably due to sub-optimal operating conditions. This study aimed at enhancing EtOH productivity in MES cells by jointly optimising key operation parameters, including pH, H₂ and CO₂ partial pressure (pH₂ and pCO₂), and HA concentration, to promote solventogenesis. Two H-type cells were operated in fed-batch mode at -0.8 V vs. SHE with CO₂ as the sole carbon source. A mixed culture, enriched with Clostridium ljungdahlii was used as the biocatalyst. The combination of low pH (<4.5) and pCO₂ (<0.3 atm), along with high HA concentration (about 6 g L^-1) and pH₂ (>3 atm), were mandatory conditions for maintaining an efficient solventogenic culture, dominated by Clostridium sp., capable of high-rate EtOH production. The maximum EtOH production rate was 10.95 g m^-2 d^-1, and a concentration of 5.28 g L^-1 was achieved. Up to 30 % of the electrons and 15.2 % of the carbon provided were directed towards EtOH production, and 28.1 kWh were required for the synthesis of 1 kg of EtOH from CO₂. These results highlight that strict conditions are required for a continuous, reliable, EtOH production in MES cells. Future investigation should focus on improving cell configuration to achieve EtOH production at higher current densities while minimizing the electric energy input.

Anode-assisted electro-fermentation with Bacillus subtilis under oxygen-limited conditions

BACKGROUND

Bacillus subtilis is generally regarded as a ubiquitous facultative anaerobe. Oxygen is the major electron acceptor of B. subtilis, and when oxygen is absent, B. subtilis can donate electrons to nitrate or perform fermentation. An anode electrode can also be used by microorganisms as the electron sink in systems called anodic electro-fermentation. The facultative anaerobic character of B. subtilis makes it an excellent candidate to explore with different electron acceptors, such as an anode. This study aimed to optimise industrial aerobic bioprocesses using alternative electron acceptors. In particular, different end product spectrum of B. subtilis with various electron acceptors, including anode from the electro-fermentation system, was investigated.

RESULTS

B. subtilis was grown using three electron acceptors, i.e. oxygen, nitrate and anode (poised at a potential of 0.7 V vs. standard hydrogen electrode). The results showed oxygen had a crucial role for cells to remain metabolically active. When nitrate or anode was applied as the sole electron acceptor anaerobically, immediate cell lysis and limited glucose consumption were observed. In anode-assisted electro-fermentation with a limited aeration rate, acetoin, as the main end product showed the highest yield of 0.78 ± 0.04 mol_product/mol_glucose, two-fold higher than without poised potential (0.39 ± 0.08 mol_product/mol_glucose).

CONCLUSIONS

Oxygen controls B. subtilis biomass growth, alternative electron acceptors utilisation and metabolites formation. Limited oxygen/air supply enabled the bacteria to donate excess electrons to nitrate or anode, leading to steered product spectrum. The anode-assisted electro-fermentation showed its potential to boost acetoin production for future industrial biotechnology applications.

Product spectrum analysis and microbial insights of medium-chain fatty acids production from waste biomass during liquor fermentation process: Effects of substrate concentrations and fermentation modes

Substrate toxicity would limit the upgrading of waste biomass to medium-chain fatty acids (MCFAs). In this work, two fermentation modes of electro-fermentation (EF) and traditional fermentation (TF) with different concentration of liquor fermentation waste (20%, 40%, 60%) were used for MCFAs production as well as mechanism investigation. The highest caproate (4.04 g/L) and butyrate (13.96 g/L) concentrations were obtained by EF at 40% substrate concentration. TF experiments showed that the substrate concentration above 40% severely inhibited ethanol oxidation and products formation. Compared with TF mode, the total substrates consumption and product yields under EF mode were significantly increased by 2.6%-43.5% and 54.0%-83.0%, respectively. Microbial analysis indicated that EF effectively alleviated substrate toxicity and enriched chain elongation bacteria, particularly Clostridium_sensu_stricto 12, thereby promoting ethanol oxidation and products formation. Caproiciproducens tolerated high-concentration substrates to ensure normal lactate metabolism. This study provides a new way to produce MCFAs from high concentration wastewater.

Transition roadmap for thermophilic carbon dioxide microbial electrosynthesis: Testing with real exhaust gases and operational control for a scalable design

Human activities release more carbon dioxide (CO₂) into the atmosphere than the natural process can remove. This study attempts to address the main challenges for the thermophilic (50 °C) bioelectrochemical conversion of CO₂ into acetate. First, real gaseous emissions were tested with mixed microbial consortia, which had no substantial influence on production rates (difference of 2.5%). Subsequently, a bench-scale system (TRL 4-5) was designed and launched to control key operational variables. Fixing the current at 1.3 A m^-2, CO₂ was reduced at a rate of 2.21 kg CO₂ kg^-1 acetate, while the electricity consumption was 2.07 kWh kg^-1, the most efficient value so far. The results suggest that the operation with real effluents is feasible and the proposed design is energy efficient, but the right balance between maximising current densities without compromising the biocompatibility with catalysts will determine the transition from laboratory scale towards its implementation in the market.