Extracellular Electron Transfer Mechanisms Research Articles

典型胞外呼吸细菌的胞内电子转移机制研究进展

PDF HTML阅读 XML下载 导出引用 引用提醒 典型胞外呼吸细菌的胞内电子转移机制研究进展 DOI: 10.5846/stxb201412262581 作者: 作者单位: 北京师范大学水科学研究院,中国环境科学研究院环境基准与风险评估国家重点实验室;中国环境科学研究院地下水与环境系统创新基地;中国环境科学研究院地下水与环境系统创新基地,中国环境科学研究院环境基准与风险评估国家重点实验室;中国环境科学研究院地下水与环境系统创新基地;中国环境科学研究院地下水与环境系统创新基地,中国环境科学研究院环境基准与风险评估国家重点实验室;中国环境科学研究院地下水与环境系统创新基地;中国环境科学研究院地下水与环境系统创新基地,北京师范大学水科学研究院,中国环境科学研究院环境基准与风险评估国家重点实验室;中国环境科学研究院地下水与环境系统创新基地;中国环境科学研究院地下水与环境系统创新基地,中国环境科学研究院环境基准与风险评估国家重点实验室;中国环境科学研究院地下水与环境系统创新基地;中国环境科学研究院地下水与环境系统创新基地 作者简介: 通讯作者: 中图分类号: 基金项目: 国家杰出青年科学基金项目（51325804）；国家自然科学青年基金项目（51408573） Intracellular electron transfer mechanism of typical extracellular respiratory bacteria Author: Affiliation: College of Water Science,Beijing Normal University,Beijing,State Key Laboratory of Environmental Criteria and Risk Assessment,Chinese Research Academy of Environmental Sciences,Beijing,State Key Laboratory of Environmental Criteria and Risk Assessment,Chinese Research Academy of Environmental Sciences,Beijing,State Key Laboratory of Environmental Criteria and Risk Assessment,Chinese Research Academy of Environmental Sciences,Beijing,College of Water Science,Beijing Normal University,Beijing,State Key Laboratory of Environmental Criteria and Risk Assessment,Chinese Research Academy of Environmental Sciences,Beijing,State Key Laboratory of Environmental Criteria and Risk Assessment,Chinese Research Academy of Environmental Sciences,Beijing Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:胞外呼吸在污染物的降解转化和微生物产电过程中具有重要作用。微生物进行胞外呼吸时，其电子受体多以固态形式存在于胞外，氧化产生的电子必须通过电子传递链从胞内经细胞周质转移到外膜。S.oneidensis MR-1与G. Sulfurreducens作为微生物燃料电池中最常用的模式菌株，是现阶段研究最深入和系统的胞外呼吸细菌，其胞内电子传递过程目前研究最为清楚。这两种胞外呼吸细菌的电子传递需多种细胞色素c的参与，S.oneidensis MR-1位于内膜及周质上的细胞色素c-CymA和MtrA可将电子由内膜上的醌池通过周质到外膜蛋白MtrC和OmcA，MtrC和OmcA接收电子后可直接还原胞外受体，Type Ⅱ secretion system对外膜蛋白中的MtrC和OmcA起到了转运及定位的作用。而在G. sulfurreducens中，电子由MacA传递到PpcA，最终由外膜蛋白OmcB、OmcE、OmcS及OmcZ接受电子，并在Type Ⅳ pili的共同作用下将电子传递到胞外电子受体。本文最后指出目前对Shewanella与Geobacter胞内电子转移研究尚不清楚的地方提出展望。 Abstract:The reduction of humus and metals such as Fe and Mn is a challenge for microorganisms, as these substances have low water solubility and cannot enter into cell envelopes. Extracellular electron transfer is defined as the process through which electrons derived from the oxidation of electron donors are transferred from the inner membrane to the outer membrane of the cell to reduce an extracellular terminal electron acceptor. S. oneidensis MR-1 and G. sulfurreducens are the most frequently used organisms for extracellular respiratory bacteria experiments because they have developed electron transfer strategies that require mutiheme c-type cytochromes (c-Cyts). In S. oneidensis MR-1, multiheme c-Cyts, CymA, and MtrA are believed to transfer electrons from the inner membrane qunione/quinol pool through the periplasm to the outer membrane (OmcA, MtrC). The Type Ⅱ secretion system of S. oneidensis MR-1 was due to the direct involvement of translocation of MtrC and OmcA to the bacterial cell surface. The decaheme c-Cyts, MtrC, and OmcA can directly reduce the extracellular electron acceptors. Likewise, for G. sulfurreducens, MacA delivered electrons from the inner membrane to PpcA in periplasm, and PpcA subsequently transferred electrons to the OMCs (OmcB, OmcE, OmcS, and OmcZ) and Type Ⅳ pili that were hypothesized to relay the electrons to extracellular electron acceptors. This review summarizes the recent advances of extracellular electron transfer mechanisms with a focus on Shewanella and Geobacter. 参考文献 相似文献 引证文献

Harvesting Energy using Biocatalysts

<i>Harvesting Energy using Biocatalysts</i>

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.

Electromicrobiology: realities, grand challenges, goals and predictions.

SummaryElectromicrobiology is a subdiscipline of microbiology that involves extracellular electron transfer (EET) to (or from) insoluble electron active redox compounds located outside the outer membrane of the cell. These interactions can often be studied using electrochemical techniques which have provided novel insights into microbial physiology in recent years. The mechanisms (and variations) of outward EET are well understood for two model systems, Shewanella and Geobacter, both of which employ multihaem cytochromes to provide an electron conduit to the cell exterior. In contrast, little is known of the intricacies of inward EET, even in these model systems. Given the number of labs now working on EET, it seems likely that most of the mechanistic details will be understood in a few years for the model systems, and the many applications of electromicrobiology will continue to move forward. But emerging work, using electrodes as electron acceptors and donors is providing an abundance of new types of microbes capable of EET inward and/or outward: microbes that are clearly different from our known systems. The extent of this very diverse, and perhaps widely distributed and biogeochemically important ability needs to be determined to understand the mechanisms, importance, and raison d'etre of EET for microbial biology.

Electrochemical Surface Plasmon Resonance Fiber-Optic Sensor: In Situ Detection of Electroactive Biofilms.

Spectroelectrochemistry has been found to be an efficient technique for revealing extracellular electron transfer (EET) mechanism of electroactive biofilms (EABs). Herein, we propose a novel electrochemical surface plasmon resonance (EC-SPR) optical fiber sensor for monitoring EABs in situ. The sensor uses a tilted fiber Bragg grating (TFBG) imprinted in a commercial single-mode fiber and coated with nanoscale gold film for high-efficiency SPR excitation. The wavelength shift of the surface plasmon resonance (SPR) over the fiber surface clearly identifies the electrochemical activity of the surface localized (adjacent to the electrode interface) bacterial cells in EABs, which differs from the "bulk" detections of the conventional electrochemical measurements. A close relationship between the variations of redox state of the EABs and the changes of the SPR under potentiostatic conditions has been achieved, pointing to a new way to study the EET mechanism of the EABs. Benefiting from its compact size, high sensitivity, and ease of use, together with remote operation ability, the proposed sensor opens up a multitude of opportunities for monitoring EABs in various hard-to-reach environments.

The Low Conductivity of Geobacter uraniireducens Pili Suggests a Diversity of Extracellular Electron Transfer Mechanisms in the Genus Geobacter.

Studies on the mechanisms for extracellular electron transfer in Geobacter species have primarily focused on Geobacter sulfurreducens, but the poor conservation of genes for some electron transfer components within the Geobacter genus suggests that there may be a diversity of extracellular electron transport strategies among Geobacter species. Examination of the gene sequences for PilA, the type IV pilus monomer, in Geobacter species revealed that the PilA sequence of Geobacter uraniireducens was much longer than that of G. sulfurreducens. This is of interest because it has been proposed that the relatively short PilA sequence of G. sulfurreducens is an important feature conferring conductivity to G. sulfurreducens pili. In order to investigate the properties of the G. uraniireducens pili in more detail, a strain of G. sulfurreducens that expressed pili comprised the PilA of G. uraniireducens was constructed. This strain, designated strain GUP, produced abundant pili, but generated low current densities and reduced Fe(III) very poorly. At pH 7, the conductivity of the G. uraniireducens pili was 3 × 10-4 S/cm, much lower than the previously reported 5 × 10-2 S/cm conductivity of G. sulfurreducens pili at the same pH. Consideration of the likely voltage difference across pili during Fe(III) oxide reduction suggested that G. sulfurreducens pili can readily accommodate maximum reported rates of respiration, but that G. uraniireducens pili are not sufficiently conductive to be an effective mediator of long-range electron transfer. In contrast to G. sulfurreducens and G. metallireducens, which require direct contact with Fe(III) oxides in order to reduce them, G. uraniireducens reduced Fe(III) oxides occluded within microporous beads, demonstrating that G. uraniireducens produces a soluble electron shuttle to facilitate Fe(III) oxide reduction. The results demonstrate that Geobacter species may differ substantially in their mechanisms for long-range electron transport and that it is important to have information beyond a phylogenetic affiliation in order to make conclusions about the mechanisms by which Geobacter species are transferring electrons to extracellular electron acceptors.

Editorial: Current Challenges and Future Perspectives on Emerging Bioelectrochemical Technologies.

Editorial: Current Challenges and Future Perspectives on Emerging Bioelectrochemical Technologies.

Riboflavin-mediated extracellular electron transfer process involving Pachysolen tannophilus

Microbes can generate electricity in microbial fuel cells and transform contaminants in nature, utilizing extracellular electron transfer (EET). Fungi exist widely in environment but few studies have concerned an associated EET mechanism and so their application is limited. In this study electrochemical techniques were employed to study the extracellular reduction capability of fungi using a representative model viz. Pachysolen tannophilus; a solid electrode and soluble Cr(VI) were used as extracellular electron acceptors. Despite a thick cell wall, the yeast was proved to be electrochemically active and an obvious redox peak was observed at −0.41V in differential pulse voltammetry. Analysis of electrochemical and HPLC data led to the proposal of an EET with riboflavin as the electron transfer mediator. Riboflavin secreted by the yeast was also responsible for the transformation of Cr(VI) to less mobile and less toxic Cr(III). Further results demonstrated that the acidophilic P. tannophilus secreted riboflavin to a concentration of 0.26μmol/g biomass at pH 3, but only to 0.15-0.16μmol/g biomass at pHs between 4 and 7. The findings contribute to the understanding of biogeochemical processes and further contribute to innovative remediation of polluted environments.

Editorial: Wired for Life

Fossil fuels and fossil fuel derived chemicals are limited, non-renewable, and foreseen to run out in the near future. To find viable solutions for the production of renewable fuels and chemicals, significant efforts are being made by the scientific community. For example, some microorganisms have been shown to produce fuels or chemicals using renewable electricity charged electrodes as electron donors for their metabolism (Tremblay and Zhang). The ability to utilize insoluble substrates such as electrodes is due to the property of certain microorganisms to release or take up electrons from insoluble substrates, a process known as extracellular electron transfer (EET). During EET to a solid electron acceptor, cells dispose of excess electrons indirectly by exploiting shuttles, or directly via a network of conductive pili and cytochromes (Lovley et al., 2011). The same EET mechanisms are considered to assist microorganisms when taking up electrons from solid electron donors, however less is known about this process (Tremblay and Zhang). Microorganisms also use direct EET for syntrophic interspecies interactions between a donor species and an acceptor species (Kouzuma et al.; Shrestha and Rotaru). The papers in this research topic present insights into EET and how this process governs microbe–microbe, microbe–mineral, and microbe–electrodes interactions.

Microbial Fuel Cell Anode Materials: Supporting Biofilms of Geobacter Sulfurreducens

Bioelectrochemical systems (BESs) are interesting systems that combine electrochemical red-ox reaction with biological activity for generating electricity from organic compounds. In fact, the organic compounds are actually the fuel for the fuel cell in which bacteria on the anode degrade organic molecules and transfer the resulting electrons to the electrode surface. The electrons move through the external circuit generating useful electricity to power devices or sensors. At the cathode, an oxidant is reduced to complete the red-ox reaction. Generally oxygen is used due to its high potential and natural availability. Interestingly, it has been found that some bacteria, named exoelectrogens, are able to transfer electrons extracellularly to a solid support, generally called the anode electrode, if the substrate oxidation reaction occurs in absence of oxygen. The halfway potential represents the potential at which the electron transfer mechanism of an exoelectrogen is most favorable and thus outputs the most electricity. The more negative the halfway potential, the more energy can be produced. The goal of this project is to maximize microbial energy production considering different anode material-bacteria interactions. Carbonaceous-based materials are typically used as anode in BESs due to their simplicity, low-cost fabrication, high surface area, high mechanical strength, high chemical resistance to corrosion and biocompatibility. It has been shown previously that both surface chemistry and surface morphology can affect positively or negatively the bacteria attachment on a surface. Unfortunately, the electrical conductivity of carbonaceous materials is generally low compared to other materials and the durability is often negatively affected in long-term operation mainly due to material deterioration. Materials other than graphite have been proposed as suitable anode materials, but the effect of anode material on the underlying mechanism of extracellular electron transfer (EET) has not been yet addressed. Here, we measure electron transport properties of the model organism, Geobacter sulfurreducens, under turnover (with organic substrate) and nonturnover (without organic substrate) conditions, using an array of materials as working electrodes of an MFC (glassy carbon (GC), graphite (GR), gold (Au), platinum mesh (Pt), nickel (Ni) and indium tin oxide (ITO)). Experimentally, a 1L reactor that accommodates 6 working electrodes was used so all of the working electrodes could be tested under the same conditions with the same reference and counter electrodes. Ag/AgCl (3M KCl) was used as reference electrode while Pt was used as counter. Each material was used as a separate working electrode and connected to a single potentiostat (VMP3, Biologic, Inc., Knoxville, TN) channel. The reactor was operated using a three-electrode configuration at a set anode potential of +0.3 V (vs. Ag/AgCl) to study each material at stable fixed potential, as opposed to a floating potential observed for MFC anodes. Preliminary electrochemical tests produced cyclic voltammograms (CV) of all the materials under turnover and nonturnover conditions that displayed differences in slope and in the difference between halfway potential and formal potential, indicating that different materials yielded different electrochemical responses (Figure 1). The observed differences suggest that the bacteria are either using different electrochemical pathways to perform EET or that the material being used as the working electrode is influencing the environment and therefore altering the formal potential. We are currently conducting chemical measurements to characterize the working electrode surfaces along with a detailed study of the Geobacterbiofilm colonization. Finally, we will establish a relationship between the halfway potential and extracellular electron transfer dependence on the surface to which the biofilm is attached. Figure 1. Polarization of Geobacter sulfurreducens grown for 14 days on various materials [Preliminary Data] Figure 1

(Invited) Limitations in Electrochemically Active Biofilms

Almost a decade has passed since the rediscovery of microbial fuel cell research, yet the power generation of these devices has not advanced. This is mainly due to the research being focused solely on the improvement of power generation rather than on a fundamental understanding of electron transfer processes and their limitations in the biofilms grown on the anode and cathode. The chemical and electrochemical gradients in these biofilms play a critical role in electron transfer rates between cells and a solid electron acceptor or donor. Our research group has developed and used novel microelectrodes and in situ Nuclear Magnetic Resonance (NMR) imaging techniques to investigate electron transfer mechanisms in biofilms. The newly developed microelectrodes were successfully operated on polarized surfaces. Both NMR and microelectrode technologies allowed us to quantify metabolic and chemical variations in biofilms while the cells respired on polarized electrode surfaces. We combined our techniques with two new techniques – rotating disk electrode (RDE) and electrochemical quartz crystal balance (eQCM). The addition of both these techniques allowed us to differentiate mass transfer limitations from intra-biofilm electron transfer processes. When we attempted to identify limitations in anodic biofilms, first, we addressed the importance of local acetate concentrations within G. sulfurreducens biofilms using a novel acetate microelectrode which has better resolution than NMR. The microelectrode work confirmed our previous NMR findings and generated profiles on the order of µM. We should note that the microelectrode method developed is significantly more economical to use than NMR and can facilitate rapid in situ measurements of acetate in anoxic biofilms. Second, we integrated our previous RDE method with our eQCM methods to show that, on both a per current basis and a per mass basis, biofilm capacitance is a better indicator of biofilm performance than conductance.In particular, we compared the capacitance/conductance relationship of G. sulfurreducens biofilms to those of both polyaniline films and diffusing flavin systems. The results indicated that the tandem increase in capacitance and conductance is unique to the extracellular electron transfer mechanisms operating in G. sulfurreducens biofilms. Our local potential and current measurements indicated that the upper layers of the biofilms are always under a reducing condition. This knowledge also confirmed that the amount of cytochromes in the biofilm is not high enough or the biofilm conductivity is not sufficient to generate an oxidizing condition near the top of a biofilm. Finally, we extended the role of excess surface area in the scale-up of bioelectrochemical systems by examining biomass accumulation under this condition. We found that biomass continues to accumulate on the excess surface area electrodes with net arithmetic growth rates and acetate consumption during steady-state current. The linear growth rates constrained by ion transport limitations that we found may be important factors in understanding the current generation using high-surface-area 3D electrodes with advection. In conclusion, we demonstrated the importance of the metabolic inactivity of layers within G. sulfurreducens biofilms using microelectrodes and NMR. In addition, we verified these findings using electrochemical impedance spectroscopy (EIS) by demonstrating the pseudocapacitive behavior of biofilm impedance. We also demonstrated the feasibility of directly measuring biofilm inefficiencies via intra-biofilm electron transfer rates and local current depth profiles.

Acceleration of Extracellular Electron Transfer by Alternative Redox-Active Molecules to Riboflavin for Outer-Membrane Cytochrome c of Shewanella oneidensis MR-1

Extracellular electron transfer (EET) between outer membrane c-type cytochromes (OM c-Cyts) of Shewanella oneidensis MR-1 and anodic and cathodic electrodes is markedly enhanced by the presence of riboflavin that operates as a redox active center in OM c-Cyt protein OmcA. Here, to obtain insight into the EET mechanism via bound riboflavin in OmcA, we replaced riboflavin with two flavin-like polycyclic molecules, safranin (Saf) and anthraquinone-1-sulfonate (α-AQS), and examined the influence of the interaction with OmcA and the potential of these redox molecules for the rate of cathodic EET by MR-1 in vivo. The results of electrochemical assays with wild-type and mutant strains of MR-1 lacking OmcA showed that both Saf and α-AQS increased the cathodic current production for fumarate reduction at −0.45 V versus SHE as a bound cofactor in OmcA. Dissociation constant and enhancement factor analyses of Saf and α-AQS suggested that the N(5) atom in the isoalloxazine ring of riboflavin is important for the affi...

A Re-evaluation of Electron-Transfer Mechanisms in Microbial Electrochemistry: Shewanella Releases Iron that Mediates Extracellular Electron Transfer.

Exoelectrogenic bacteria can couple their metabolism to extracellular electron acceptors, including macroscopic electrodes, and this has applications in energy production, bioremediation and biosensing. Optimisation of these technologies relies on a detailed molecular understanding of extracellular electron‐transfer (EET) mechanisms, and Shewanella oneidensis MR‐1 (MR‐1) has become a model organism for such fundamental studies. Here, cyclic voltammetry was used to determine the relationship between the surface chemistry of electrodes (modified gold, ITO and carbon electrodes) and the EET mechanism. On ultra‐smooth gold electrodes modified with self‐assembled monolayers containing carboxylic‐acid‐terminated thiols, an EET pathway dominates with an oxidative catalytic onset at 0.1 V versus SHE. Addition of iron(II)chloride enhances the catalytic current, whereas the siderophore deferoxamine abolishes this signal, leading us to conclude that this pathway proceeds via an iron mediated electron transfer mechanism. The same EET pathway is observed at other electrodes, but the onset potential is dependent on the electrolyte composition and electrode surface chemistry. EET pathways with onset potentials above −0.1 V versus SHE have previously been ascribed to direct electron‐transfer (DET) mechanisms through the surface exposed decaheme cytochromes (MtrC/OmcA) of MR‐1. In light of the results reported here, we propose that the previously identified DET mechanism of MR‐1 needs to be re‐evaluated.

Divide-Conquer-Recombine Kinetic Monte Carlo Simulations of Electron Transfer in the Extracellular Redox Network of Shewanella oneidensis MR-1

Metal-reducing bacteria, such as Shewanella oneidensis MR-1, have evolved extracellular electron transfer (EET) mechanisms to external surfaces, allowing them to use abundant minerals as respiratory electron acceptors, instead of oxygen or other soluble oxidants that would normally diffuse inside cells. By performing EET at electrodes, such microbes can serve as biocatalysts for converting the energy stored in fuels to electricity, or vice versa, in renewable energy technologies. Furthermore, a biophysical understanding of this phenomenon may enable the transmission of signals at cellular/synthetic interfaces, creating new materials that combine the replication and self-repair of a natural system with the vast toolbox of nanotechnology.But how can a cell transfer electrons to surface micrometers away from the cell? Microbes overcome this hurdle by deploying multiheme cytochrome complexes that form 20-30 nm conduits through the periplasm and across the outer-membrane. More recently, we learned that this cytochrome network extends along micrometer-long membrane extensions called bacterial nanowires, and even over entire bacterial biofilms. Here we report a detailed computational study of the electron transfer dynamics in microbial cytochrome networks. Using atomistically-informed kinetic Monte Carlo (KMC) simulations, we calculate the electron transport rate through the Shewanella decaheme cytochrome MtrF, and find the result to match in vitro measurements and satisfy the in vivo cellular respiration rates on electrodes. To extend this approach to multi-cytochrome complexes, we present a hierarchical framework that predicts the structure of Mtr-Omc complexes, performs KMC simulations, and visualizes electron transport. We also present a divide-conquer-recombine KMC approach, and an implementation on parallel computers, to simulate EET over much larger structures such as entire bacterial nanowires. The electron flux is found to strongly depend on the cytochrome density, topology, and orientation on membrane.

Genome sequence of a dissimilatory Fe(III)-reducing bacterium Geobacter soli type strain GSS01T

Strain GSS01T (=KCTC 4545=MCCC 1 K00269) is the type strain of the species Geobacter soli. G. soli strain GSS01T is of interest due to its ability to reduce insoluble Fe(III) oxides with a wide range of electron donors. Here we describe some key features of this strain, together with the whole genome sequence and annotation. The genome of size 3,657,100 bp contains 3229 protein-coding and 54 RNA genes, including 2 16S rRNA genes. The genome of strain GSS01Tcontains 76 predicted cytochrome genes, 24 pilus assembly protein genes and several other genes, which were proposed to be related to the reduction of insoluble Fe(III) oxides. The genes associated with the electron donors and acceptors of strain GSS01T were predicted in the genome. Information gained from its sequence will be relevant to the future elucidation of extracellular electron transfer mechanism during the reduction of Fe(III) oxides.Electronic supplementary materialThe online version of this article (doi:10.1186/s40793-015-0117-7) contains supplementary material, which is available to authorized users.

Unraveling the electron transfer processes of a nanowire protein from Geobacter sulfurreducens

The extracellular electron transfer metabolism of Geobacter sulfurreducens is sustained by several multiheme c-type cytochromes. One of these is the dodecaheme cytochrome GSU1996 that belongs to a new sub-class of c-type cytochromes. GSU1996 is composed by four similar triheme domains (A–D). The C-terminal half of the molecule encompasses the domains C and D, which are connected by a small linker and the N-terminal half of the protein contains two domains (A and B) that form one structural unit. It was proposed that this protein works as an electrically conductive device in G. sulfurreducens, transferring electrons within the periplasm or to outer-membrane cytochromes. In this work, a novel strategy was applied to characterize in detail the thermodynamic and kinetic properties of the hexaheme fragment CD of GSU1996. This characterization revealed the electron transfer process of GSU1996 for the first time, showing that a heme at the edge of the C-terminal of the protein is thermodynamic and kinetically competent to receive electrons from physiological redox partners. This information contributes towards understanding how this new sub-class of cytochromes functions as nanowires, and also increases the current knowledge of the extracellular electron transfer mechanisms in G. sulfurreducens.

Complete Genome Sequence of Marinobacter sp. CP1, Isolated from a Self-Regenerating Biocathode Biofilm.

Marinobacter sp. CP1 was isolated from a self-regenerating and self-sustaining biocathode biofilm that can fix CO2 and generate electric current. We present the complete genome sequence of this strain, which consists of a circular 4.8-Mbp chromosome, to understand the mechanism of extracellular electron transfer in a microbial consortium.

Bidirectional microbial electron transfer: Switching an acetate oxidizing biofilm to nitrate reducing conditions

Up to date a few electroactive bacteria embedded in biofilms are described to catalyze both anodic and cathodic reactions in bioelectrochemical systems (i.e. bidirectional electron transfer). How these bacteria transfer electrons to or from the electrode is still uncertain. In this study the extracellular electron transfer mechanism of bacteria within an electroactive biofilm was investigated by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). First, a mature anodic electroactive biofilm was developed from an activated sludge sample (inoculum), acetate as electron donor and a poised electrode (+397mV vs. SHE). Later, this biofilm was "switched" to biocathodic conditions by feeding it with a medium containing nitrates and poising the electrode at -303mV vs. SHE. The electrochemical characterization indicated that both, acetate oxidation and nitrate reduction took place at a similar formal potential of -175±05 and -175±34mV vs. SHE, respectively. The biofilm was predominantly composed by Geobacter sp. at both experimental conditions. Taken together, the results indicated that both processes could be catalyzed by using the same electron conduit, and most likely by the same bacterial consortium. Hence, this study suggests that electroactive bacteria within biofilms could use the same electron transfer conduit for catalyzing anodic and cathodic reactions.

Extracellular electron transfer in yeast-based biofuel cells: A review

This paper reviews the state-of-the art of the yeast-based biofuel cell research and development. The established extracellular electron transfer (EET) mechanisms in the presence and absence of exogenous mediators are summarized and discussed. The approaches applied for improvement of mediator-less yeast-based biofuel cells performance are also presented. The overview of the literature shows that biofuel cells utilizing yeasts as biocatalysts generate power density in the range of 20 to 2440mW/m2, which values are comparable with the power achieved when bacteria are used instead. The electrons' origin and the contribution of the glycolysis, fermentation, aerobic respiration, and phosphorylation to the EET are commented. The reported enhanced current generation in aerobic conditions presumes reconsideration of some basic MFC principles. The challenges towards the practical application of the yeast-based biofuel cells are outlined.

Heterologous expression and purification of a multiheme cytochrome from a Gram-positive bacterium capable of performing extracellular respiration

Microbial electrochemical technologies are emerging as environmentally friendly biotechnological processes. Recently, a thermophilic Gram-positive bacterium capable of electricity production in a microbial fuel cell was isolated. Thermincola potens JR contains several multiheme c-type cytochromes that were implicated in the process of electricity production. In order to understand the molecular basis by which Gram-positive bacteria perform extracellular electron transfer, the relevant proteins need to be characterized in detail. Towards this end, a chimeric gene containing the signal peptide from Shewanella oneidensis MR-1 small tetraheme cytochrome c (STC) and the gene sequence of the target protein TherJR_0333 was constructed. This manuscript reports the successful expression of this chimeric gene in the Gram-negative bacterium Escherichia coli and its subsequent purification and characterization. This methodology opens the possibility to study other multiheme cytochromes from Gram-positive bacteria, allowing the extracellular electron transfer mechanisms of this class of organisms to be unraveled.