Extracellular Electron Transfer Capabilities Research Articles

Influence of shear stress on electroactive biofilm characteristics and performance in microbial fuel cells

This study has provided comprehensive insights into the intricate relationship between shear stress and the development, structure, and functionality of electroactive biofilms in Microbial Fuel Cells (MFCs). A multichannel microfluidic MFC reactors that created specific shear stress on the anode, were designed for the simultaneous study of multiple flow conditions using the same medium. Then, the evolution of the biofilm growth under different shear stress conditions (1, 5 and 10 mPa) were compared. The taxonomic and functional structure was studied by 16S rRNA gene and metagenomic sequencing and the physical biofilm characteristics were measured via fluorescence microscopy. The results demonstrate the pivotal role of shear stress in influencing the growth kinetics, electrical performance, and physical structure of anodic biofilms. Notably, the selection of specific EAB was observed to be shear stress-dependent, with a marked increase in specific EAB abundance as shear stress increased. The power density, while not directly correlated with the relative abundance of specific or nonspecific EAB, exhibited a strong linear relationship with biofilm coverage. This suggests that factors beyond the microbial composition, potentially including mass transport or electrochemical conditions, might be instrumental in determining electricity production. The functional metagenomic analysis further highlighted the complexities of extracellular electron transfer (EET) mechanisms in electroactive biofilm. While certain genes associated with EET in known species such as Geobacter and Shewanella were identified, the study also examined the limitations of solely relying on genetic markers to infer EET capabilities, emphasizing the need for complementary metaproteomic analyses. This study demonstrates the multifaceted impact of shear stress on electroactive biofilm and paves the way for future investigations aimed at harnessing the potential of electroactive biofilms in microbial fuel cell applications.

The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition.

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Enforcing energy consumption promotes microbial extracellular respiration for xenobiotic bioconversion.

Extracellular electron transfer (EET) empowers electrogens to catalyse the bioconversion of a wide range of xenobiotics in the environment. Synthetic bioengineering has proven effective in promoting EET output. However, conventional strategies mainly focus on modifications of EET-related genes or pathways, which leads to a bottleneck due to the intricate nature of electrogenic metabolic properties and intricate pathway regulation that remain unelucidated. Herein, we propose a novel EET pathway-independent approach, from an energy manipulation perspective, to enhance microbial EET output. The Controlled Hydrolyzation of ATP to Enhance Extracellular Respiration (CHEER) strategy promotes energy utilization and persistently reduces the intracellular ATP level in Shewanella oneidensis, a representative electrogenic microbe. This approach leads to the accelerated consumption of carbon substrate, increased biomass accumulation and an expanded intracellular NADH pool. Both microbial electrolysis cell and microbial fuel cell tests exhibit that the CHEER strain substantially enhances EET capability. Analysis of transcriptome profiles reveals that the CHEER strain considerably bolsters biomass synthesis and metabolic activity. When applied to the bioconversion of model xenobiotics including methyl orange, Cr(VI) and U(VI), the CHEER strain consistently exhibits enhanced removal efficiencies. This work provides a new perspective and a feasible strategy to enhance microbial EET for efficient xenobiotic conversion.

Microbial electrosynthetic nitrate reduction to ammonia by reversing the typical electron transfer pathway in Shewanella oneidensis

Ammonia production is a critical industrial process, and mild routes to recycle nitrates in wastewater could be a promising route to ammonia synthesis. In this study, ammonia production is demonstrated in a microbial electrosynthesis system with nitrate and an electrode as electron acceptors and donors, respectively. Based on the bidirectional extracellular electron transfer capability of Shewanella oneidensis MR-1, our microbial electrosynthetic system achieves a maximum ammonia production rate of 24.3 μg h−1·mg protein−1 with 82.5% selectivity and 33.1% cathodic efficiency and functions for several cycles over 30 days. Electrochemical analysis suggests that cytochromes c, flavins, and the flavin/c-cytochrome combination play a pivotal role. Charge transfer resistance weakens over the course of weeks, resulting in easier electron transfer. Parallel reaction monitoring proteomics suggest that reversing a typical “Mtr pathway” plays a role, and a dissimilatory nitrate to ammonia pathway is used. This work proposes a progressive route to carry out ammonia synthesis under mild conditions.

Developing a PAM-Flexible CRISPR-Mediated Dual-Deaminase Base Editor to Regulate Extracellular Electron Transport in Shewanella oneidensis.

Shewanella oneidensis MR-1 is a promising electroactive microorganism in environmental bioremediation, bioenergy generation, and bioproduct synthesis. Accelerating the extracellular electron transfer (EET) pathway that enables efficient electron exchange between microbes and extracellular substances is critical for improving its electrochemical properties. However, the potential genomic engineering strategies for enhancing EET capabilities are still limited. Here, we developed a clustered regularly interspaced short palindromic repeats (CRISPR)-mediated dual-deaminase base editing system, named in situ protospacer-adjacent motif (PAM)-flexible dual base editing regulatory system (iSpider), for precise and high-throughput genomic manipulation. The iSpider enabled simultaneous C-to-T and A-to-G conversions with high diversity and efficiency in S. oneidensis. By weakening DNA glycosylase-based repair pathway and tethering two copies of adenosine deaminase, the A-to-G editing efficiency was obviously improved. As a proof-of-concept study, the iSpider was adapted to achieve multiplexed base editing for the regulation of the riboflavin biosynthesis pathway, and the optimized strain showed an approximately three-fold increase in riboflavin production. Moreover, the iSpider was also applied to evolve the performance of an inner membrane component CymA implicated in EET, and one beneficial mutant facilitating electron transfer could be rapidly identified. Taken together, our study demonstrates that the iSpider allows efficient base editing in a PAM-flexible manner, providing insights into the design of novel genomic tools for Shewanella engineering.

Facile Biological-Based Synthesis of Size-Controlled Palladium Nanoclusters Anchored on the Surface of Geobacter sulfurreducens and Their Application in Electrocatalysis

Metal nanoclusters can be synthesized following the top-down or bottom-up approaches involving multiple chemical and/or physical steps where strong ligands, toxic chemicals, and high temperature and pressure are normally applied. In contrast, biological methods eliminate the use and generation of hazardous substances and do not require the application of high temperature and pressure during the synthesis process. Biological methods based on the extracellular electron transfer (EET) capability of electroactive bacteria (EAB) are considered a promising sustainable route for the synthesis of metal nanoparticles; however, a fine control of the size of the nanoparticles has not been achieved yet. Herein, we report a facile biological-based synthesis method of size-controlled palladium (Pd) nanoclusters using the EET capability of Geobacter sulfurreducens. By controlling the metal precursor concentrations and dosing and incubation times, we synthesized size-controlled Pd nanoclusters (1.0 ± 0.6 to 4.8 ± 1.4 nm) anchored on the surface of G. sulfurreducens. The as-synthesized Pd nanoclusters anchored on the surface of G. sulfurreducens cells (Pd/GS) were tested for their performance as bifunctional electrocatalysts for the overall alkaline water splitting reaction. Despite a very low mass loading of 0.002 mg Pd cm–2, the hybrid material (i.e., Pd/GS) showed exceptionally higher activity for hydrogen evolution reaction (HER) when compared to benchmark 10 wt % Pt/C and Pd/C. Similarly, Pd/GS showed higher activity for oxygen evolution reaction (OER) when compared to Pd/C and comparable OER activity when compared to benchmark IrO2. The findings in this study provide a promising sustainable route for designing size-controlled metal nanoclusters anchored on the surface of EAB as efficient and low-cost electrocatalysts for various applications.

Conductive carrier promotes synchronous biofilm formation and granulation of anammox bacteria

The extracellular electron transfer capability of some anaerobic ammonium oxidation (anammox) bacteria was confirmed in recent years. However, the effect of conductive carriers on the synchronous formation of anammox biofilm and granules is rarely reported. Anammox biofilm and granules with compact and stable structures accelerate the initiation and enhance the stability of the anammox process. In this study, we found that the conductive carbon fiber brush (CB) carrier promoted synchronous biofilm formation and granulation of anammox bacteria in the internal circulation immobilized blanket (ICIB) reactor. Compared with polyurethane sponge and zeolite carrier, the ICIB reactor packed with CB carrier can be operated under the highest total nitrogen loading rate of 6.53 kg-N/(m3·d) and maintain the effluents NH4+-N and NO2−-N at less than 1 mM. The volatile suspended solids concentration in the ICIB reactor packed with conductive carrier increased from 5.17 ± 0.40 g/L of inoculum sludge to 24.24 ± 1.20 g/L of biofilm, and the average particle size of granules increased from 222.09 µm to 879.80 µm in 150 days. Fluorescence in situ hybridization analysis showed that anammox bacteria prevailed in the biofilm and granules. The analysis of extracellular polymeric substances indicated that protein and humic acid-like substances played an important role in the formation of anammox biofilm and granules. Microbiome analysis showed that the relative abundance of Candidatus Jettenia was increased from 0.18% to 38.15% in the biofilm from CB carrier during start-up stage. This study provides a strategy for rapid anammox biofilm and granules enrichment and carrier selection of anammox process.

Electrochemical performance of Paenibacillus profundus YoMME encapsulated in alginate polymer

Gram-positive bacterium Paenibacillus profundus YoMME, entrapped in an alginate polymer onto graphite paper, preserves its extracellular electron transfer capabilities. A current density of up to 30 mA m−2 was generated at an applied potential of −200 mV (vs. SHE). Fivefold higher initial current density values were recorded at applied potentials of +220 mV and +600 mV. The electrochemical behavior of the encapsulated bioelectrodes has been also explored by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy and evaluated by the parameters of the best-fitted equivalent electric circuit model. Over time the bacteria grow and divide within the alginate matrix, which affects considerably the capacitance and the charge transfer resistance of the coating. The impedance spectra follow the dynamics of the bacterial culture development within the alginate polymer and are useful for the prediction of the bioelectrode performance. A current density of 150 mA m−2 was achieved when the alginate was functionalized by a mixture of the redox dyes thiazolyl blue (MTT) formazan and phenazine methosulfate (PMS). It is supposed that the added artificial mediators facilitate the electron transfer from the bacteria to the electrode surface by forming conductive cascade conduits through the alginate matrix.

Photosynthetic microbial fuel cells for methanol treatment using graphene electrodes

Photosynthetic microbial fuel cells (pMFC) represent a promising approach for treating methanol (CH3OH) wastewater. However, their use is constrained by a lack of knowledge on the extracellular electron transfer capabilities of photosynthetic methylotrophs, especially when coupled with metal electrodes. This study assessed the CH3OH oxidation capabilities of Rhodobacter sphaeroides 2.4.1 in two-compartment pMFCs. A 3D nickel (Ni) foam modified with plasma-grown graphene (Gr) was used as an anode, nitrate mineral salts media (NMS) supplemented with 0.1% CH3OH as anolyte, carbon brush as cathode, and 50 mM ferricyanide as catholyte. Two simultaneous pMFCs that used bare Ni foam and carbon felt served as controls. The Ni/Gr electrode registered a two-fold lower charge transfer resistance (0.005 kΩ cm2) and correspondingly 16-fold higher power density (141 mW/m2) compared to controls. The underlying reasons for the enhanced performance of R. sphaeroides at the graphene interface were discerned. The real-time polymerase chain reaction (PCR) analysis revealed the upregulation of cytochrome c oxidase, aa3 type, subunit I gene, and Flp pilus assembly protein genes in the sessile cells compared to their planktonic counterparts. The key EET pathways used for sustaining CH3OH oxidation were discussed.

Transcriptome Analysis to Identify Crucial Genes for Reinforcing Flavins-Mediated Extracellular Electron Transfer in Shewanella oneidensis.

Flavins serve as the electron mediators in Shewanella oneidensis, determining the extracellular electron transfer (EET) rate. Currently, metabolic engineering of flavins biosynthetic pathway has been studied for improving EET. However, the cellular response triggered by flavins that contribute to EET remains to be elucidated. In this study, the riboflavin-overproducing strain C5 (expressing the flavins synthetic genes in plasmid PYYDT) and the PYYDT strain (harboring the empty plasmid PYYDT) in the microbial fuel cells are applied for comparative transcriptomic analyses to investigate beneficial gene targets that could improve EET. From the differentially expressed genes, we select the significantly upregulated and downregulated genes for inverse engineering in S. oneidensis. The results show that overexpression of ahpC and ccpA, and inactivation of pubA, putB, and tonB are able to improve the EET capability. Combinatorial modulation of these five genes results in the recombinant strain CM4, achieving the maximum power density of 651.78 ± 124.60 mW/m2, 1.97 folds of the parental strain. These genes modulation is speculated to reduce the ROS damage and to promote cytochrome synthesis and heme accumulation, which coherently enhance EET. Our findings facilitate in-depth understanding of the mechanism of flavins-mediated EET and provide new insights in promoting EET of S. oneidensis for electricity generation.

Biological removal of sulfamethoxazole enhanced by S. oneidensis MR-1 via promoting NADH generation and electron transfer and consumption

The bio-removal efficiency of sulfamethoxazole (SMX) from wastewater is usually very poor. In this paper a new efficient method to biodegrade SMX was reported. The SMX biodegradation efficiency by Paracoccus denitrificans was observed to be remarkably enhanced from 48.9% to 94.2% after Shewanella oneidensis MR-1 addition. The mechanisms investigation revealed that P. denitrificans was the dominant microbe for SMX biodegradation. Although SMX biodegradation by S. oneidensis MR-1 alone was negligible, its presence advanced NADH generation. The proteomics assay revealed that the expression of key proteins relevant with complex I and III and cytochrome c in electron transfer chain were increased due to P. denitrificans acquiring iron from periplasm to cytoplasm being improved. In addition, the extracellular electron transfer capability was enhanced as S. oneidensis MR-1 not only produced flavin, but caused P. denitrificans to secret more extracellular polymeric substances. Further investigation indicated that the expression of key enzymes related to electron consumption in SMX biodegradation was up-regulated. Based on these findings, the pathways of S. oneidensis MR-1 promoting SMX biodegradation were proposed. As all nitrate could be removed with almost no nitrite accumulation, this study would also provide an attractive way for simultaneous bio-removal of multiple pollutants from wastewater.

Electrochemical Energy Storage: Harnessing the Extracellular Electron Transfer Capability of Geobacter sulfurreducens for Ambient Synthesis of Stable Bifunctional Single‐Atom Electrocatalyst for Water Splitting (Adv. Funct. Mater. 22/2021)

In article number 2010916, Srikanth Pedireddy, Pascal E. Saikaly, and co-workers report a facile synthesis strategy of single-atom catalysts (SACs) at room temperature by harnessing the extracellular electron transfer capability of Geobacter sulfurreducens. This strategy can be successfully extended for the synthesis of various transition metal SACs by merely altering the metal precursors. Without further heat treatment, the dried catalysts exhibit excellent electrocatalytic activity for oxygen and hydrogen evolution reactions.

Harnessing the Extracellular Electron Transfer Capability of Geobacter sulfurreducens for Ambient Synthesis of Stable Bifunctional Single‐Atom Electrocatalyst for Water Splitting

AbstractSingle‐atom metal (SA‐M) catalysts with high dispersion of active metal sites allow maximum atomic utilization. Conventional synthesis of SA‐M catalysts involves high‐temperature treatments, leading to low yield with a random distribution of atoms. Herein, a nature‐based facile method to synthesize SA‐M catalysts (M = Fe, Ir, Pt, Ru, Cu, or Pd) in a single step at ambient temperature, using the extracellular electron transfer capability of Geobacter sulfurreducens (GS), is presented. Interestingly, the SA‐M is coordinated to three nitrogen atoms adopting an MN3 on the surface of GS. Dry samples of SA‐Ir@GS without further heat treatment show exceptionally high activity for oxygen evolution reaction when compared to benchmark IrO2 catalyst and comparable hydrogen evolution reaction activity to commercial 10 wt% Pt/C. The SA‐Ir@GS exhibits the best water‐splitting performance compared to other SA‐M@GS, showing a low applied potential of 1.65 V to achieve 10 mA cm−2 in 1.0 M KOH with cycling over 5 h. The density functional calculations reveal that the large adsorption energy of H2O and moderate adsorption energies of reactants and reaction intermediates for SA‐Ir@GS favorably improve its activity. This synthesis method at room temperature provides a versatile platform for the preparation of SA‐M catalysts for various applications by merely altering the metal precursors.

Microbial electroactive biofilms dominated by Geoalkalibacter spp. from a highly saline\u2013alkaline environment

Understanding of the extreme microorganisms that possess extracellular electron transfer (EET) capabilities is pivotal to advance electromicrobiology discipline and to develop niche-specific microbial electrochemistry-driven biotechnologies. Here, we report on the microbial electroactive biofilms (EABs) possessing the outward EET capabilities from a haloalkaline environment of the Lonar lake. We used the electrochemical cultivation approach to enrich haloalkaliphilic EABs under 9.5 pH and 20 g/L salinity conditions. The electrodes controlled at 0.2 V vs. Ag/AgCl yielded the best-performing biofilms in terms of maximum bioelectrocatalytic current densities of 548 ± 23 and 437 ± 17 µA/cm2 with acetate and lactate substrates, respectively. Electrochemical characterization of biofilms revealed the presence of two putative redox-active moieties with the mean formal potentials of 0.183 and 0.333 V vs. Ag/AgCl, which represent the highest values reported to date for the EABs. 16S-rRNA amplicon sequencing of EABs revealed the dominance of unknown Geoalkalibacter sp. at ~80% abundance. Further investigations on the haloalkaliphilic EABs possessing EET components with high formal potentials might offer interesting research prospects in electromicrobiology.

Electromicrobiology and biotechnological applications of the exoelectrogens Geobacter and Shewanella spp.

Electromicrobiology is a sub-discipline of microbiology that investigates electrical interplay between microorganisms and redox active materials, such as electrodes and solid-phase minerals, and the mechanisms underlying microbial ability to exchange electrons with the redox active materials that are external to the microbial cells. The microorganisms with extracellular electron transfer capability are often referred to as exoelectrogens. Although exoelectrogens were documented in early 1900’s, discovery of the dissimilatory metal-reducing microorganisms Geobacter and Shewanella spp. in late 1980’s marked the beginning of modern electromicrobiology. Since then, thorough and rigorous studies have made Geobacter and Shewanella spp. the two best characterized groups of exoelectrogens. These include identification and characterization of the molecular mechanisms for exchanging electrons with electrodes by Geobacter sulfurreducens and Shewanella oneidensis . In addition, a variety of applications of Geobacter and Shewanella spp. in microbial fuel cells and electrobiosynthesis, such as maintenance of redox balance during fermentations and bioremediations, have also been developed. This review briefly discusses the molecular mechanisms by which G. sulfurreducens and S . oneidensis exchange electrons with electrodes and then focuses on biotechnological applications of Geobacter and Shewanella spp. in microbial fuel cells and electrobiosynthesis as well as the future directions of this research area.

Molecular underpinnings for microbial extracellular electron transfer during biogeochemical cycling of earth elements.

Microbial extracellular electron transfer (EET) is electron exchanges between the quinol/quinone pools in microbial cytoplasmic membrane and extracellular substrates. Microorganisms with EET capabilities are widespread in Earth hydrosphere, such as sediments of rivers, lakes and oceans, where they play crucial roles in biogeochemical cycling of key elements, including carbon, nitrogen, sulfur, iron and manganese. Over the past 12 years, significant progress has been made in mechanistic understanding of microbial EET at the molecular level. In this review, we focus on the molecular mechanisms underlying the microbial ability for extracellular redox transformation of iron, direct interspecies electron transfer as well as long distance electron transfer mediated by the cable bacteria in the hydrosphere.

Toward a better understanding of microbiologically influenced corrosion caused by sulfate reducing bacteria

Sulfate reducing bacteria (SRB) are often the culprits of microbiologically influenced corrosion (MIC) in anoxic environments because sulfate is a ubiquitous oxidant. MIC of carbon steel caused by SRB is the most intensively investigated topic in MIC because of its practical importance. It is also because biogenic sulfides complicate mechanistic SRB MIC studies, making SRB MIC of carbon steel is a long-lasting topic that has generated considerable confusions. It is expedient to think that biogenic H2S secreted by SRB acidifies the broth because it is an acid gas. However, this is not true because endogenous H2S gets its H+ from organic carbon oxidation and the fluid itself in the first place rather than an external source. Many people believe that biogenic H2S is responsible for SRB MIC of carbon steel. However, in recent years, well designed mechanistic studies provided evidence that contradicts this misconception. Experimental data have shown that cathodic electron harvest by an SRB biofilm from elemental iron via extracellular electron transfer (EET) for energy production by SRB is the primary cause. It has been demonstrated that when a mature SRB biofilm is subjected to carbon source starvation, it switches to elemental iron as an electron source and becomes more corrosive. It is anticipated that manipulations of EET related genes will provide genetic-level evidence to support the biocathode theory in the future. This kind of new advances will likely lead to new gene probes or transcriptomics tools for detecting corrosive SRB strains that possess high EET capabilities.

TiO2 Nanoparticle-Induced Nanowire Formation Facilitates Extracellular Electron Transfer

Semiconductor nanoparticles (NPs) have been reported to facilitate extracellular electron transfer (EET) by increasing the electrical conductivity of electroactive biofilms. However, the underlying molecular mechanisms remain unclear. In this study, we demonstrated the unique role of semiconductor titanium dioxide (TiO2) NPs in facilitating EET from Geobacter cells to an electrode. Compared to other NPs that include conductor carbon, semiconductor α-Fe2O3, and insulator SiO2, TiO2 NPs improved the bacterial EET capability most significantly, leading to an approximately 5.1-fold increase in microbial current generation. Cell morphology and gene analysis revealed that TiO2 NPs specifically induced the formation of conductive nanowires by stimulating pilA expression, which primarily contributed to the enhanced EET in biofilms. In addition, TiO2 NPs might compensate for the lack of a pili-associated c-cytochrome OmcS in the EET function. This finding had important implications not only for optimizing the perf...

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

Acetogenic bacteria (i.e., acetogens) produce acetate from CO2 during anaerobic chemoautotrophic growth. Because acetogens fix CO2 with high energy efficiency, they have been investigated as biocatalysts of CO2 conversion into valuable chemicals. Recent studies revealed that some acetogens are capable of extracellular electron transfer (EET), which enables electron exchange between microbial cells and extracellular solid materials. Thus, acetogens are promising candidates as biocatalysts in recently developed bioelectrochemical technologies, including microbial electrosynthesis (MES), in which useful chemicals are biologically produced from CO2 using electricity as the energy source. In microbial photoelectrosynthesis, a variant of MES technology, the conversion of CO2 into organic compounds is achieved using light as the sole energy source without an external power supply. In this mini-review, we introduce the general features of bioproduction and EET of acetogens and describe recent progress and future prospects of MES technologies based on the EET capability of acetogens.

Extracellular electron transfer mechanisms between microorganisms and minerals.

Electrons can be transferred from microorganisms to multivalent metal ions that are associated with minerals and vice versa. As the microbial cell envelope is neither physically permeable to minerals nor electrically conductive, microorganisms have evolved strategies to exchange electrons with extracellular minerals. In this Review, we discuss the molecular mechanisms that underlie the ability of microorganisms to exchange electrons, such as c-type cytochromes and microbial nanowires, with extracellular minerals and with microorganisms of the same or different species. Microorganisms that have extracellular electron transfer capability can be used for biotechnological applications, including bioremediation, biomining and the production of biofuels and nanomaterials.