Microbial extracellular electron transfer and its relevance to iron corrosion

Extracellular electron transfer (EET) is a microbial metabolism that enables efficient electron transfer between microbial cells and extracellular solid materials. Microorganisms harbouring EET abilities have received considerable attention for their various biotechnological applications, including bioleaching and bioelectrochemical systems. On the other hand, recent research revealed that microbial EET potentially induces corrosion of iron structures. It has been well known that corrosion of iron occurring under anoxic conditions is mostly caused by microbial activities, which is termed as microbiologically influenced corrosion (MIC). Among diverse MIC mechanisms, microbial EET activity that enhances corrosion via direct uptake of electrons from metallic iron, specifically termed as electrical MIC (EMIC), has been regarded as one of the major causative factors. The EMIC‐inducing microorganisms initially identified were certain sulfate‐reducing bacteria and methanogenic archaea isolated from marine environments. Subsequently, abilities to induce EMIC were also demonstrated in diverse anaerobic microorganisms in freshwater environments and oil fields, including acetogenic bacteria and nitrate‐reducing bacteria. Abilities of EET and EMIC are now regarded as microbial traits more widespread among diverse microbial clades than was thought previously. In this review, basic understandings of microbial EET and recent progresses in the EMIC research are introduced.     

Microbial electrocatalysis: Redox mediators responsible for extracellular electron transfer

Redox mediator plays an important role in extracellular electron transfer (EET) in many environments wherein microbial electrocatalysis occurs actively. Because of the block of cell envelope and the low difference of redox potential between the intracellular and extracellular surroundings, the proceeding of EET depends mainly on the help of a variety of mediators that function as an electron carrier or bridge. In this Review, we will summarize a wide range of redox mediators and further discuss their functional mechanisms in EET that drives a series of microbial electrocatalytic reactions. Studying these mediators adds to our knowledge of how charge transport and electrochemical reactions occur at the microorganism-electrode interface. This understanding would promote the widespread applications of microbial electrocatalysis in microbial fuel cells, bioremediation, bioelectrosynthesis, biomining, nanomaterial productions, etc. These improved applications will greatly benefit the sustainable development of the environmental-friendly biochemical industries.     

A kinetic perspective on extracellular electron transfer by anode-respiring bacteria

In microbial fuel cells and electrolysis cells (MXCs), anode-respiring bacteria (ARB) oxidize organic substrates to produce electrical current. In order to develop an electrical current, ARB must transfer electrons to a solid anode through extracellular electron transfer (EET). ARB use various EET mechanisms to transfer electrons to the anode, including direct contact through outer-membrane proteins, diffusion of soluble electron shuttles, and electron transport through solid components of the extracellular biofilm matrix. In this review, we perform a novel kinetic analysis of each EET mechanism by analyzing the results available in the literature. Our goal is to evaluate how well each EET mechanism can produce a high current density (>10 A m⁻²) without a large anode potential loss (less than a few hundred millivolts), which are feasibility goals of MXCs. Direct contact of ARB to the anode cannot achieve high current densities due to the limited number of cells that can come in direct contact with the anode. Slow diffusive flux of electron shuttles at commonly observed concentrations limits current generation and results in high potential losses, as has been observed experimentally. Only electron transport through a solid conductive matrix can explain observations of high current densities and low anode potential losses. Thus, a study of the biological components that create a solid conductive matrix is of critical importance for understanding the function of ARB.     

生物工程强化微生物电合成转化CO_2的研究进展

微生物电合成(Microbial electrosynthesis,MES)可直接利用电能驱动微生物还原固定CO_2合成多碳化合物,为可再生新能源转化、精细化学品制备和生态环境保护提供新机遇。但是,微生物吸收胞外电极电子速率慢、产物合成效率低和产品品位不高,限制了MES实现工业化应用。在概述阴极电活性微生物吸收胞外电子的分子机制的基础上,重点综述近5年应用生物工程的理论和技术强化MES用于CO_2转化的策略与研究进展,包括改造和调控胞外电子传递通路和胞内代谢途径以及定向构建有限微生物混合培养菌群三方面,阐明了生物工程可有效突破MES中电子传递慢和可用代谢途径相对单一等瓶颈。针对目前生物工程在改进MES所面临的主要问题,从胞外电子传递机理研究、基因工具箱开发、组学技术与现代分析技术联用等角度展望了今后的研究方向。     

Extracellular Electron Transfer Between Birnessite and Electrochemically Active Bacteria Community from Red Soil in Hainan,China

The interplay between electrochemically active microorganisms (EAMs) and adjacent minerals universally occurs in natural environments, in which soil is an extremely typical and active one. We stimulated the extracellular electron transfer (EET) process between the bacterial community and birnessite in red soil (collected from Hainan, China) by constructing a microbial fuel cell equipped with synthetic birnessite cathode. Compared to graphite-cathode, the cell voltage of birnessite-cathode was increased by 22% when loading a 1000 Ω-resistance, indicating the EET between microbes and birnessite. Eleven genera of EAMs in red soil were confirmed through 16S rRNA analysis. Neither palpable novel mineral formation nor change of birnessite crystallinity was observed after reaction by Raman and SEM. As oxygen pumped into cathode chamber was the terminal electron acceptor, birnessite principally performed as an intermediate of holistic electron transfer process to favor the cathodic oxygen reduction.     

Rewiring the respiratory pathway of Lactococcus lactis to enhance extracellular electron transfer

Lactococcus lactis, a lactic acid bacterium with a typical fermentative metabolism, can also use oxygen as an extracellular electron acceptor. Here we demonstrate, for the first time, that L. lactis blocked in NAD⁺ regeneration can use the alternative electron acceptor ferricyanide to support growth. By electrochemical analysis and characterization of strains carrying mutations in the respiratory chain, we pinpoint the essential role of the NADH dehydrogenase and 2-amino-3-carboxy-1,4-naphtoquinone in extracellular electron transfer (EET) and uncover the underlying pathway systematically. Ferricyanide respiration has unexpected effects on L. lactis, e.g., we find that morphology is altered from the normal coccoid to a more rod shaped appearance, and that acid resistance is increased. Using adaptive laboratory evolution (ALE), we successfully enhance the capacity for EET. Whole-genome sequencing reveals the underlying reason for the observed enhanced EET capacity to be a late-stage blocking of menaquinone biosynthesis. The perspectives of the study are numerous, especially within food fermentation and microbiome engineering, where EET can help relieve oxidative stress, promote growth of oxygen sensitive microorganisms and play critical roles in shaping microbial communities.     

微生物种间直接电子传递机理及应用研究进展

微生物胞内产生的电子转移到其他电子受体而获得能量的过程称为微生物胞外电子传递,其中,另一微生物作为电子受体时发生的电子传递称为微生物种间电子传递。根据微生物种间电子传递机制,可分间接种间电子传递和种间直接电子传递。由于种间直接电子传递不需要其他物质介导,因此较间接种间电子传递效率更高、能量利用更高。本文系统阐述了微生物进行胞外电子传递的机理及应用,重点分析了种间直接电子传递机理,并概述种间直接电子传递应用领域,为寻找更多电连接的微生物群落以及应用微生物提供参考。     

微生物胞外电子传递效率的合成生物学强化

电活性微生物(产电微生物和亲电微生物)通过与外界环境进行双向电子和能量传递来实现多种微生物电催化过程(包括微生物燃料电池、微生物电解电池、微生物电催化等),从而实现在环境、能源领域的广泛应用,并为开发有效且可持续性生产新能源或大宗精细化学品的工艺提供了新机会。但是,电活性微生物的胞外电子传递效率比较低,这已经成为限制微生物电催化系统在工业应用中的主要瓶颈。以下综述了近年来利用合成生物学改造电活性微生物的相关研究成果,阐明了合成生物学如何用于打破电活性微生物胞外电子传递途径低效率的瓶颈,从而实现电活性微生物与环境的高效电子传递和能量交换,推动电活性微生物电催化系统的实用化进程。     

强化产电微生物与电极间电子传递速率的研究进展

产电微生物是微生物燃料电池、电解池和电合成等微生物电化学技术(Microbial electrochemical technologies,METs)的研究基础.产电微生物与电极界面间的胞外电子传递(Extracellular electron transfer,EET)效率低以及生物被膜形成能力弱限制了METs在有机...     

Initial development and structure of biofilms on microbial fuel cell anodes

Background  

Microbial fuel cells (MFCs) rely on electrochemically active bacteria to capture the chemical energy contained in organics and convert it to electrical energy. Bacteria develop biofilms on the MFC electrodes, allowing considerable conversion capacity and opportunities for extracellular electron transfer (EET). The present knowledge on EET is centred around two Gram-negative models, i.e. Shewanella and Geobacter species, as it is believed that Gram-positives cannot perform EET by themselves as the Gram-negatives can. To understand how bacteria form biofilms within MFCs and how their development, structure and viability affects electron transfer, we performed pure and co-culture experiments.     

Screening of natural phenazine producers for electroactivity in bioelectrochemical systems

Mediated extracellular electron transfer (EET) might be a great vehicle to connect microbial bioprocesses with electrochemical control in stirred-tank bioreactors. However, mediated electron transfer to date is not only much less efficient but also much less studied than microbial direct electron transfer to an anode. For example, despite the widespread capacity of pseudomonads to produce phenazine natural products, only Pseudomonas aeruginosa has been studied for its use of phenazines in bioelectrochemical applications. To provide a deeper understanding of the ecological potential for the bioelectrochemical exploitation of phenazines, we here investigated the potential electroactivity of over 100 putative diverse native phenazine producers and the performance within bioelectrochemical systems. Five species from the genera Pseudomonas, Streptomyces, Nocardiopsis, Brevibacterium and Burkholderia were identified as new electroactive bacteria. Electron discharge to the anode and electric current production correlated with the phenazine synthesis of Pseudomonas chlororaphis subsp. aurantiaca. Phenazine-1-carboxylic acid was the dominant molecule with a concentration of 86.1 μg/ml mediating an anodic current of 15.1 μA/cm². On the other hand, Nocardiopsis chromatogenes used a wider range of phenazines at low concentrations and likely yet-unknown redox compounds to mediate EET, achieving an anodic current of 9.5 μA/cm². Elucidating the energetic and metabolic usage of phenazines in these and other species might contribute to improving electron discharge and respiration. In the long run, this may enhance oxygen-limited bioproduction of value-added compounds based on mediated EET mechanisms.     

Mcc在脱色希瓦氏菌S12电极呼吸中的作用

【目的】研究脱色希瓦氏菌S12周质空间c型细胞色素Mcc的功能,进一步探索和补充微生物胞外电子传递过程的机制。【方法】借助自杀质粒敲除mcc基因,通过细胞浓度测定和激光共聚焦显微镜比较分析突变株和野生株之间的浮游细胞和生物膜的生长情况,并比较分析二者在微生物燃料电池电极还原、铁还原和胞外偶氮染料还原过程中的功能。【结果】Mcc缺失对铁还原和偶氮还原没有影响,但却造成电极呼吸活性下降34.1%;与野生株相比,mcc突变株的好氧生长和厌氧浮游细胞生长无明显影响,但却显著抑制了电极表面生物膜的形成。【结论】Mcc是希瓦氏菌S12电极呼吸过程中周质空间电子传递的重要组分之一,缺失会显著抑制其电极呼吸效率以及生物膜的形成。     

Bucking the current trend in bioelectrochemical systems: a case for bioelectroanalytics

Turning wastewater directly into electricity is alluring, widespread use of microbial fuel cells (MFCs) to achieve this at industrial scale appears increasingly unlikely despite intense research efforts lasting over a decade. Such endeavors have not been futile, however, and game-changing discoveries have resulted from these well-intentioned, scientifically rigorous but ultimately frustrated attempts to resolve the Waste-Energy dichotomy. The appeal of MFCs is largely of conceptual elegance rather than financial competitiveness, based on the green ideal that bacteria can be turned into cost effective bio-batteries. This notion is founded on the solid principles of extracellular electron transfer (EET), where microbes use electrodes interchangeably with other electron acceptors to generate current as a direct proxy for microbial metabolism. We contend that a nuanced understanding of EET has been restricted by focusing on device performance when in fact this information could be more beneficially channeled into addressing analytical questions pertaining to the presence and activity of microorganisms across systems of environmental and medical import, i.e. bioelectroanalytics. We discuss here relevant literature detailing bioelectrochemical systems and contrast energy-centric conclusions with observations geared towards bioelectroanalytics. We explore the expanding possibilities of bioelectroanalytics enabled by advances in genetic techniques and rooted in the concept that microbial interactions with an electrode extend to more than just cells seeking alternative electron acceptors. Our intention is to highlight alternative directions in the field and encourage researchers to harness bioelectroanalytics to address wider societal problems, in addition to addressing climate change.     

Enhancement of extracellular electron transfer and bioelectricity output by synthetic porin

The microbial fuel cell (MFC), is a promising environmental biotechnology for harvesting electricity energy from organic wastes. However, low bacterial membrane permeability of electron shuttles is a limiting factor that restricts the electron shuttle‐mediated extracellular electron transfer (EET) from bacteria to electrodes, thus the electricity power output of MFCs. To this end, we heterologously expressed a porin protein OprF from Pseudomonas aeruginosa PAO1 into Escherichia coli, which dramatically increased its membrane permeability, delivering a much higher current output in MFCs than its parental strain (BL21). We found that the oprF‐expression strain showed more efficient EET than its parental strain. More strikingly, the enhanced membrane permeability also rendered the oprF‐expression strain an efficient usage of riboflavin as the electron shuttle, whereas its parental strain was incapable of. Our results substantiated that membrane permeability is crucial for the efficient EET, and indicated that the expression of synthetic porins could be an efficient strategy to enhance bioelectricity generation by microorganisms (including electrogenic bacteria) in MFCs. Biotechnol. Bioeng. 2013; 110: 408–416. © 2012 Wiley Periodicals, Inc.     

分子生物学方法提高电活性微生物胞外电子传递效率的研究进展*

微生物细胞与电极之间的胞外电子传递效率是限制微生物电化学技术发展的关键因素,而分子生物学的发展为提高胞外电子传递效率带来了光明前景。从四种具有代表性的纯培养电活性微生物(奥奈达希瓦氏菌、铜绿假单胞菌、硫还原地杆菌和工程大肠杆菌)和混合培养电活性微生物出发,综述了利用分子生物学手段改造几种电活性微生物的研究成果,阐明了针对特异的电活性微生物,如何采取相应的分子生物学手段提高其胞外电子传递的效率,并展望了未来的研究方向。     

奥奈达希瓦氏菌CctA介导周质甲基橙还原的电子传递机理

电活性微生物奥奈达希瓦氏菌的胞外电子传递（extracellular electron transfer,EET）在污染物降解、环境修复、生物电化学传感、能源利用等方面具有广泛的应用潜力;四血红素细胞色素CctA （small tetraheme cytochrome）是希瓦氏菌周质空间中最丰富的蛋白质之一,能够参与多种氧化还原过程,但目前对CctA在EET中的行为和机理认识仍然有限。【目的】研究阐明CctA蛋白在希瓦氏菌模式菌株MR-1周质空间以偶氮染料作为电子受体的EET中的作用,补充和拓展希瓦氏菌的厌氧呼吸产能机制。【方法】以周质还原型偶氮染料甲基橙（methyl orange,MO）作为电子受体,在mteal reduction （Mtr）蛋白缺失菌株Δmtr中研究MO的周质还原特点,并通过基因敲除和回补表达研究CctA蛋白在周质电子传递中的作用。【结果】在缺失Mtr通道的情况下,细胞色素CctA可以介导周质空间的电子传递而还原MO。重组表达CctA在低水平时,MO在周质空间中的还原速率与其表达水平呈正相关,更高水平的CctA表达无助于进一步提高MO的还原速率。蛋白膜伏安结果展示了CctA与周质空间内其他高电位氧化还原蛋白的显著区别,可能参与构成一条低电位的MO还原通道。【结论】从分子动力学层面揭示了CctA在周质MO还原中的独特电子传递行为,为进一步推进对细菌周质电子传递机制的理解,以及通过合成生物学设计或改造胞外氧化还原系统、强化生物电化学在污染物降解中的应用提供了重要信息。     

Potential-dependent extracellular electron transfer pathways of exoelectrogens

Exoelectrogens are distinct from other bacteria owing to their unique extracellular electron transfer (EET) abilities that allow for anaerobic respiration with various external redox-active surfaces, including electrode and metal oxides. Although the EET process is known to trigger diverse extracellular redox reactions, the reverse impact has been long overlooked. Recent evidences show that exoelectrogens can sense the potential changes of external surfaces and alter their EET strategies accordingly, which imparts them remarkable abilities in adapting to diverse and redox-variable environment. This mini-review provides a condensed overview and critical analysis about the recent discoveries on redox-dependent EET pathways of exoelectrogens, with focus on Geobacter sulfurreducens and Shewanella oneidensis. We summarize the detailed responses of various EET components, analyze the drives and mechanisms of such responses, highlight the diversity of EET dynamics among different bacterial species and under integrated effects of redox potential and surface chemistry, and discusses the future research needs.