Extracellular electron transfer in acetogenic bacteria and its application for conversion of carbon dioxide into organic compounds

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.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Exploring Acetogenesis in Firmicutes: From Phylogenetic Analysis to Solid Medium Cultivation with Solid-Phase Electrochemical Isolation Equipments

This study introduces a groundbreaking approach for the exploration and utilization of electrotrophic acetogens, essential for advancing microbial electrosynthesis systems (MES). Our initial focus was the development of Solid-Phase Electrochemical Isolation Equipment (SPECIEs), a novel cultivation method for isolating electrotrophic acetogens directly from environmental samples on a solid medium. SPECIEs uses electrotrophy as a selection pressure, successfully overcoming the traditional cultivation method limitations and enabling the cultivation of diverse microbial communities with enhanced specificity towards acetogens. Following the establishment of SPECIEs, we conducted a genome-based phylogenetic analysis using the Genome Taxonomy Database (GTDB) to identify potential electrotrophic acetogens within the Firmicutes phylum and its related lineages. Subsequently, we validated the electrotrophic capabilities of selected strains under electrode-oxidizing conditions in a liquid medium. This sequential approach, integrating innovative cultivation techniques with detailed phylogenetic analysis, paves the way for further advances in microbial cultivation and the identification of new biocatalysts for sustainable energy applications.

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

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

Electron transfer of extremophiles in bioelectrochemical systems

The interaction of bacteria and archaea with electrodes is a relatively new research field which spans from fundamental to applied research and influences interdisciplinary research in the fields of microbiology, biochemistry, biotechnology as well as process engineering. Although a substantial understanding of electron transfer processes between microbes and anodes and between microbes and cathodes has been achieved in mesophilic organisms, the mechanisms used by microbes under extremophilic conditions are still in the early stages of discovery. Here, we review our current knowledge on the biochemical solutions that evolved for the interaction of extremophilic organisms with electrodes. To this end, the available knowledge on pure cultures of extremophilic microorganisms has been compiled and the study has been extended with the help of bioinformatic analyses on the potential distribution of different electron transfer mechanisms in extremophilic microorganisms.

The oxygen dilemma: The challenge of the anode reaction for microbial electrosynthesis from CO2

Microbial electrosynthesis (MES) from CO2 provides chemicals and fuels by driving the metabolism of microorganisms with electrons from cathodes in bioelectrochemical systems. These microorganisms are usually strictly anaerobic. At the same time, the anode reaction of bioelectrochemical systems is almost exclusively water splitting through the oxygen evolution reaction (OER). This creates a dilemma for MES development and engineering. Oxygen penetration to the cathode has to be excluded to avoid toxicity and efficiency losses while assuring low resistance. We show that this dilemma derives a strong need to identify novel reactor designs when using the OER as an anode reaction or to fully replace OER with alternative oxidation reactions.

Syngas fermentation and microbial electrosynthesis integration as a single process unit

Industrially relevant syngas (15 % CO, 15% H₂, 20% N₂ in 50% CO₂) fermentation and microbial electrosynthesis were integrated as a single process unit in open and closed-circuit modes. This study examined the impact of electrochemical reducing power from -50 to -400 mV on the acetic acid synthesis and CO inhibition on fermentation. -150 mV vs. Ag/AgCl (3.0 NaCl) was identified as the lowest benchmark potential for improved acetic acid synthesis rate (0.263 mmol L^-1h^-1), which is 15-fold higher than the open circuit mode's rate. No significant inhibition by CO in the fermentation was observed, while 60% of the gas was consumed. Anodic potential above 2.0 V substantially lowered the product formation. Superseding the fermentation medium with fresh inoculum through a fed-batch operation helped lower the anodic potential.

Electricity production and key exoelectrogens in a mixed-culture psychrophilic microbial fuel cell at 4 °C

The electricity production via psychrophilic microbial fuel cell (PMFC) for wastewater treatment in cold regions offers an alternative to avoid the unwanted methane dissolution of traditional anaerobic fermentation. But, it is seldom reported by mixed-culture, especially closed to 0 °C. Thus, a two-chamber mixed-culture PMFC at 4 °C was successfully operated in this study using acetate as an electron donor. The main results demonstrated a good performance of PMFC, including the maximum voltage of 513 mV at 1000 Ω, coulombic efficiency of 53%, and power density of 689 mW/m². The cyclic voltammetry curves of enriched biofilm showed a direct electron transfer pathway. These good performances of mixed-culture PMFC were due to the high psychrophilic activity of enriched biofilm, including exoelectrogens genera of Geobacter (6.1%), Enterococcus (17.5%), and Clostridium_sensu_stricto_12 (3.8%). Consequently, a mixed-culture PMFC provides a reasonable strategy to enrich exoelectrogens with high activity. For low-temperature regions, the mixed-culture PMFC involved biotechnologies shall benefit energy generation and valuable chemical production in the future. KEY POINTS: • PMFC showed a maximum voltage of around 513 mV under a resistance of 1000 Ω. • The coulombic efficiency was 53% and the max power density was 689 mW/m². • Geobacter, Enterococcus, and Clostridium_sensu_stricto_12 were key exoelectrogens.

Effect of Humin and Chemical Factors on CO2-Fixing Acetogenesis and Methanogenesis

Acetogenesis and methanogenesis have attracted attention as CO₂-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO₂-fixing acetogenesis as the sole electron donor. Here, using two CO₂-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H₂) as electron donors for acetogenesis, either separately or simultaneously, but with a preference for the electron use from humin. The acetogenic activity was accelerated 14 times by FeS at 0.2 g/L as the optimal concentration, while being inhibited by MgSO₄ at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H₂ as electron donor, suggesting that humin was not a universal electron donor for acetogenesis. For methanogenesis, both consortia did not utilize extracellular electrons from humin unless H₂ was present. The methanogenesis was promoted by FeS at 0.2 g/L or higher concentrations, especially without humin, and with NaCl at 2 g/L or higher concentrations regardless of the presence of humin, while no significant effect was observed with MgSO₄. Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by Clostridia, while Methanobacterium was the methanogen utilizing humin with H₂.

Engineering Yeast Yarrowia lipolytica for Methanol Assimilation

Conferring methylotrophy on industrial microorganisms would enable the production of diverse products from one-carbon feedstocks and contribute to establishing a low-carbon society. Rebuilding methylotrophs, however, requires a thorough metabolic refactoring and is highly challenging. Only recently was synthetic methylotrophy achieved in model microorganisms─Escherichia coli and baker's yeast Saccharomyces cerevisiae. Here, we have engineered industrially important yeast Yarrowia lipolytica to assimilate methanol. Through rationally constructing a chimeric assimilation pathway, rewiring the native metabolism for improved precursor supply, and laboratory evolution, we improved the methanol assimilation from undetectable to a level of 1.1 g/L per 72 h and enabled methanol-supported cellular maintenance. By transcriptomic analysis, we further found that fine-tuning of methanol assimilation and ribulose monophosphate/xylulose monophosphate (RuMP/XuMP) regeneration and strengthening formate dehydrogenation and the serine pathway were beneficial for methanol assimilation. This work paves the way for creating synthetic methylotrophic yeast cell factories for low-carbon economy.

Engineering Wired Life: Synthetic Biology for Electroactive Bacteria

Electroactive bacteria produce or consume electrical current by moving electrons to and from extracellular acceptors and donors. This specialized process, known as extracellular electron transfer, relies on pathways composed of redox active proteins and biomolecules and has enabled technologies ranging from harvesting energy on the sea floor, to chemical sensing, to carbon capture. Harnessing and controlling extracellular electron transfer pathways using bioengineering and synthetic biology promises to heighten the limits of established technologies and open doors to new possibilities. In this review, we provide an overview of recent advancements in genetic tools for manipulating native electroactive bacteria to control extracellular electron transfer. After reviewing electron transfer pathways in natively electroactive organisms, we examine lessons learned from the introduction of extracellular electron transfer pathways into Escherichia coli. We conclude by presenting challenges to future efforts and give examples of opportunities to bioengineer microbes for electrochemical applications.

Engineering microbial metabolic energy homeostasis for improved bioproduction

Metabolic energy (ME) homeostasis is essential for the survival and proper functioning of microbial cell factories. However, it is often disrupted during bioproduction because of inefficient ME supply and excessive ME consumption. In this review, we propose strategies, including reinforcement of the capacity of ME-harvesting systems in autotrophic microorganisms; enhancement of the efficiency of ME-supplying pathways in heterotrophic microorganisms; and reduction of unessential ME consumption by microbial cells, to address these issues. This review highlights the potential of biotechnology in the engineering of microbial ME homeostasis and provides guidance for the higher efficient bioproduction of microbial cell factories.

Acetogenic bacteria utilize light-driven electrons as an energy source for autotrophic growth

To develop an efficient artificial photosynthesis system using acetogen-nanoparticle hybrids, the efficiency of the electron–hole pair generation of nanoparticles must be enhanced to demonstrate extracellular electron utilization by the acetogen. Here we verified that Clostridium autoethanogenum, an industrially relevant acetogen, could use electrons generated from size- and structure-controlled chemically synthesized cadmium sulfide nanoparticles displayed on the cell surface under light-exposure conditions. In addition, transcriptomic analysis showed that the electrons generated from nanoparticles were largely transported to the intracellular matrix via the metal ion or flavin-binding proteins. These results illustrate the potential to increase the CO₂-fixing efficiency of nanoparticle-based artificial photosynthesis by engineering cellular processes related to electron transfer generated from the cathode.

Acetogenic bacteria use cellular redox energy to convert CO₂ to acetate using the Wood–Ljungdahl (WL) pathway. Such redox energy can be derived from electrons generated from H₂ as well as from inorganic materials, such as photoresponsive semiconductors. We have developed a nanoparticle-microbe hybrid system in which chemically synthesized cadmium sulfide nanoparticles (CdS-NPs) are displayed on the cell surface of the industrial acetogen Clostridium autoethanogenum. The hybrid system converts CO₂ into acetate without the need for additional energy sources, such as H₂, and uses only light-induced electrons from CdS-NPs. To elucidate the underlying mechanism by which C. autoethanogenum uses electrons generated from external energy sources to reduce CO₂, we performed transcriptional analysis. Our results indicate that genes encoding the metal ion or flavin-binding proteins were highly up-regulated under CdS-driven autotrophic conditions along with the activation of genes associated with the WL pathway and energy conservation system. Furthermore, the addition of these cofactors increased the CO₂ fixation rate under light-exposure conditions. Our results demonstrate the potential to improve the efficiency of artificial photosynthesis systems based on acetogenic bacteria integrated with photoresponsive nanoparticles.

Hydraulic flow direction alters nutrients removal performance and microbial mechanisms in electrolysis-assisted constructed wetlands

In this study, an electrolysis-assisted down-flow constructed wetland (E-DFCW) was successfully established, and achieved simultaneously efficient removal of PO₄^3--P (93.6% ± 3.2%), NO₃^--N (97.1% ± 2.0%) and TN (80.6% ± 5.4%). When compared with electrolysis-assisted up-flow constructed wetland (E-UFCW), E-DFCW allowed significantly lower concentrations of PO₄^3--P, NO₃^--N, total Fe and SO₄^2--S in effluents. In addition, microbial community and functional genes prediction results indicated that hydraulic flow direction significantly altered microbial nitrogen, sulfur and carbon metabolisms in electrolysis-assisted constructed wetlands (E-CWs). Specifically, multi-path denitrification facilitated NO₃^--N reduction in cathodic chamber of E-DFCW, whereas autohydrogenotrophic denitrification might dominate NO₃^--N reduction in cathodic chamber of E-UFCW. More abundant and diverse denitrifiers in cathodic chamber of E-DFCW contributed to enhanced denitrification performance. Overall, this work provides microbial insights into multi-path nitrogen metabolisms in electrolysis-assisted denitrification systems in response to hydraulic flow direction.

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.

Semiquantitative Detection of Hydrogen-Associated or Hydrogen-Free Electron Transfer within Methanogenic Biofilm of Microbial Electrosynthesis

Hydrogen-entangled electron transfer has been verified as an important extracellular pathway of sharing reducing equivalents to regulate biofilm activities within a diversely anaerobic environment, especially in microbial electrosynthesis systems. However, with a lack of useful methods for in situ hydrogen detection in cathodic biofilms, the role of hydrogen involvement in electron transfer is still debatable. Here, a cathodic biofilm was constructed in CH₄-produced microbial electrosynthesis reactors, in which the hydrogen evolution dynamic was analyzed to confirm the presence of hydrogen-associated electron transfer near the cathode within a micrometer scale. Fluorescent in situ hybridization images indicated that a colocalized community of archaea and bacteria developed within a 58.10-μm-thick biofilm at the cathode, suggesting that the hydrogen gradient detected by the microsensor was consumed by the collaboration of bacteria and archaea. Coupling of a microsensor and cyclic voltammetry test further provided semiquantitative results of the hydrogen-associated contribution to methane generation (around 21.20% ± 1.57% at a potential of -0.5 V to -0.69 V). This finding provides deep insight into the mechanism of electron transfer in biofilm on conductive materials.IMPORTANCE Electron transfer from an electrode to biofilm is of great interest to the fields of microbial electrochemical technology, bioremediation, and methanogenesis. It has a promising potential application to boost more value-added products or pollutant degradation. Importantly, the ability of microbes to obtain electrons from electrodes and utilize them brings new insight into direct interspecies electron transfer during methanogenesis. Previous studies verified the direct pathway of electron transfer from the electrode to a pure-culture bacterium, but it was rarely reported how the methanogenic biofilm of mixed cultures shares electrons by a hydrogen-associated or hydrogen-free pathway. In the current study, a combination method of microsensor and cyclic voltammetry successfully semiquantified the role of hydrogen in electron transfer from an electrode to methanogenic biofilm.

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.

Biotechnological upcycling of plastic waste and other non-conventional feedstocks in a circular economy

The envisaged circular economy requires absolute carbon efficiency and in the long run abstinence from fossil feedstocks, and integration of industrial production with end-of-life waste management. Non-conventional feedstocks arising from industrial production and societal consumption such as CO₂ and plastic waste may soon enable manufacture of multiple products from simple bulk chemicals to pharmaceuticals using biotechnology. The change to these feedstocks could be faster than expected by many, especially if the true cost, including the carbon footprint of products, is considered. The efficiency of biotechnological processes can be improved through metabolic engineering, which can help fulfill the promises of the Paris agreement.

Extracellular Electron Transfer via Outer Membrane Cytochromes in a Methanotrophic Bacterium Methylococcus capsulatus (Bath)

Electron exchange reactions between microbial cells and solid materials, referred to as extracellular electron transfer (EET), have attracted attention in the fields of microbial physiology, microbial ecology, and biotechnology. Studies of model species of iron-reducing, or equivalently, current-generating bacteria such as Geobacter spp. and Shewanella spp. have revealed that redox-active proteins, especially outer membrane c-type cytochromes (OMCs), play a pivotal role in the EET process. Recent (meta)genomic analyses have revealed that diverse microorganisms that have not been demonstrated to have EET ability also harbor OMC-like proteins, indicating that EET via OMCs could be more widely preserved in microorganisms than originally thought. A methanotrophic bacterium Methylococcus capsulatus (Bath) was reported to harbor multiple OMC genes whose expression is elevated by Cu starvation. However, the physiological role of these genes is unknown. Therefore, in this study, we explored whether M. capsulatus (Bath) displays EET abilities via OMCs. In electrochemical analysis, M. capsulatus (Bath) generated anodic current only when electron donors such as formate were available, and could reduce insoluble iron oxides in the presence of electron donor compounds. Furthermore, the current-generating and iron-reducing activities of M. capsulatus (Bath) cells that were cultured in a Cu-deficient medium, which promotes high levels of OMC expression, were higher than those cultured in a Cu-supplemented medium. Anodic current production by the Cu-deficient cells was significantly suppressed by disruption of MCA0421, a highly expressed OMC gene, and by treatment with carbon monoxide (CO) gas (an inhibitor of c-type cytochromes). Our results provide evidence of EET in M. capsulatus (Bath) and demonstrate the pivotal role of OMCs in this process. This study raises the possibility that EET to solid compounds is a novel survival strategy of methanotrophic bacteria.

Electrotrophic activity and electrosynthetic acetate production by Desulfobacterium autotrophicum HRM2

Electroautotrophic microorganisms accept electrons from a cathode as source of reducing equivalents to drive CO₂ fixation by poorly understood mechanisms. Acetogenic bacteria were the first group found to possess the capability for electroautotrophic metabolism in pure culture with associated electrosynthesis of acetate as primary metabolite. Identification of additional electrotrophic species can contribute to our understanding of this unusual form of metabolism. Here, bioelectrochemical techniques, chemical analysis and microscopy were used to determine electrotrophic metabolism of Desulfobacterium autotrophicum HRM2. Chronoamperometry showed increasing current uptake over 21 days of incubation in duplicate bioelectrochemical system sets. Linear sweep voltammetry indicated peak current uptake at -243 mV. High performance liquid chromatography (HPLC) analysis quantified acetate accumulation in anaerobic minimal media containing inorganic carbon as sole carbon source, consistent with electrosynthesis. Scanning electron microscopy and live/dead staining by epifluorescence microscopy analysis indicated viable 1-2 μm cells after 76 days of cultivation under electroautotrophic conditions. The genome of Db. autotrophicum HRM2 is fully sequenced and, thus, could provide insight into the biochemical and physiological mechanisms by which electrotrophic cells utilize cathode-derived electrons. This research expands the diversity of facultative autotrophs capable of electrotrophic metabolism to include the sulfate-reducing marine bacterium Db. autotrophicum HRM2.

Electrochemical biotechnologies minimizing the required electrode assemblies

Microbial electrochemical systems (MESs) are expected to be put into practical use as an environmental technology that can support a future environmentally friendly society. However, conventional MESs present a challenge of inevitably increasing initial investment, mainly due to requirements for a large numbers of electrode assemblies. In this review, we introduce electrochemical biotechnologies that are under development and can minimize the required electrode assemblies. The novel biotechnologies, called electro-fermentation and indirect electro-stimulation, can drive specific microbial metabolism by electrochemically controlling intercellular and extracellular redox states, respectively. Other technologies, namely electric syntrophy and microbial photo-electrosynthesis, obviate the need for electrode assemblies, instead stimulating targeted reactions by using conductive particles to create new metabolic electron flows.