Simultaneous analysis of physiological and electrical output changes in an operating microbial fuel cell withShewanella oneidensis

Enhanced degradation and methane production of food waste anaerobic digestate using an integrated system of anaerobic digestion and microbial electrolysis cells for long-term operation

The integrated system of anaerobic digestion and microbial electrolysis cells (AD-MEC) was a novel approach to enhance the degradation of food waste anaerobic digestate and recover methane. Through long-term operation, the start-up method, organic loading, and methane production mechanism of the digestate have been investigated. At an organic loading rate of 4000 mg/L, AD-MEC increased methane production by 3-4 times and soluble chemical oxygen demand (SCOD) removal by 20.3% compared with anaerobic digestion (AD). The abundance of bacteria Fastidiosipila and Geobacter, which participated in the acid degradation and direct electron transfer in the AD-MEC, increased dramatically compared to that in the AD. The dominant methanogenic archaea in the AD-MEC and AD were Methanobacterium (44.4-56.3%) and Methanocalculus (70.05%), respectively. Geobacter and Methanobacterium were dominant in the AD-MEC by direct electron transfer of organic matter into synthetic methane intermediates. AD-MEC showed a perfect SCOD removal efficiency of the digestate, while methane as clean energy was obtained. Therefore, AD-MEC was a promising technology for deep energy transformation from digestate.

Sprayable biofilm – Agarose hydrogels as 3D matrix for enhanced productivity in bioelectrochemical systems

Bio-based energy production utilizing renewable resources can be realized by exoelectrogenic organisms and their application in bioelectrochemical systems (BES). These organisms catalyze the direct conversion of chemical into electrical energy and are already widely used in bioelectronics and biosensing. However, the biofilm-electrode interaction is a factor that limits sufficient space-time-yields for industrial applications. In this study, a hydrogel matrix consisting of agarose fibers was utilized as a scaffold for S. oneidensis cells to improve anodic processes in BES. This synthetic, scalable biofilm reached a higher current density in BES in comparison to naturally formed biofilms. Complemented with carbon nanofibers and riboflavin, the application of this functionalized hydrogel containing S. oneidensis cells led to an overall 9.1-fold increase in current density to 1324 mA m⁻² in comparison to 145 mA m⁻² for the planktonic control. In addition, the synthetic biofilm can be applied by spraying onto surfaces using a novel spray applicator. The latter allows to apply the biofilm effortless on large surfaces which will facilitate scalability and thus industrial application.

Microbial Fuel Cell United with Other Existing Technologies for Enhanced Power Generation and Efficient Wastewater Treatment

Nowadays, the world is experiencing an energy crisis due to extensive globalization and industrialization. Most of the sources of renewable energy are getting depleted, and thus, there is an urge to locate alternative routes to produce energy efficiently. Microbial fuel cell (MFC) is a favorable technology that utilizes electroactive microorganisms acting as a biocatalyst at the anode compartment converting organic matter present in sewage water for bioelectricity production and simultaneously treating wastewater. However, there are certain limitations with a typical stand-alone MFC for efficient energy recovery and its practical implementation, including low power output and high cost associated with treatment. There are various modifications carried out on MFC for eliminating the limitations of a stand-alone MFC. Examples of such modification include integration of microbial fuel cell with capacitive deionization technology, forward osmosis technology, anaerobic digester, and constructed wetland technology. This review describes various integrated MFC systems along with their potential application on an industrial scale for wastewater treatment, biofuel generation, and energy production. As a result, such integration of MFCs with existing systems is urgently needed to address the cost, fouling, durability, and sustainability-related issues of MFCs while also improving the grade of treatment received by effluent.

Electrical tension-triggered conversion of anaerobic to aerobic respiration of Shewanella putrefaciens CN32 cells while promoting biofilm growth in microbial fuel cells

A global gene expression analysis of Shewanella putrefaciens CN32 cells nearby a nanostructured microbial anode reveals an electrical tension-triggered conversion of anaerobic respiration to aerobic respiration with increased excretion of flavin electron shuttles and cytochrome C proteins, which sheds light on the role of electric tension in cell organisms.

Convenient non-invasive electrochemical techniques to monitor microbial processes: current state and perspectives

Real-time electrochemical monitoring in bioprocesses is an improvement over existing systems because it is versatile and provides more information to the user than periodic measurements of cell density or metabolic activity. Real-time electrochemical monitoring provides the ability to monitor the physiological status of actively growing cells related to electron transfer activity and potential changes in the proton gradient of the cells. Voltammetric and amperometric techniques offer opportunities to monitor electron transfer reactions when electrogenic microbes are used in microbial fuel cells or bioelectrochemical synthesis. Impedance techniques provide the ability to monitor the physiological status of a wide range of microorganisms in conventional bioprocesses. Impedance techniques involve scanning a range of frequencies to define physiological activity in terms of equivalent electrical circuits, thereby enabling the use of computer modeling to evaluate specific growth parameters. Electrochemical monitoring of microbial activity has applications throughout the biotechnology industry for generating real-time data and offers the potential for automated process controls for specific bioprocesses.

Biofilm systems as tools in biotechnological production

The literature provides more and more examples of research projects that develop novel production processes based on microorganisms organized in the form of biofilms. Biofilms are aggregates of microorganisms that are attached to interfaces. These viscoelastic aggregates of cells are held together and are embedded in a matrix consisting of multiple carbohydrate polymers as well as proteins. Biofilms are characterized by a very high cell density and by a natural retentostat behavior. Both factors can contribute to high productivities and a facilitated separation of the desired end-product from the catalytic biomass. Within the biofilm matrix, stable gradients of substrates and products form, which can lead to a differentiation and adaptation of the microorganisms' physiology to the specific process conditions. Moreover, growth in a biofilm state is often accompanied by a higher resistance and resilience towards toxic or growth inhibiting substances and factors. In this short review, we summarize how biofilms can be studied and what most promising niches for their application can be. Moreover, we highlight future research directions that will accelerate the advent of productive biofilms in biology-based production processes.

Recent developments and key barriers to advanced biofuels: A short review

Biofuels are regarded as one of the most viable options for reduction of CO₂ emissions in the transport sector. However, conventional plant-based biofuels (e.g., biodiesel, bioethanol)'s share of total transportation-fuel consumption in 2016 was very low, about 4%, due to several major limitations including shortage of raw materials, low CO₂ mitigation effect, blending wall, and poor cost competitiveness. Advanced biofuels such as drop-in, microalgal, and electro biofuels, especially from inedible biomass, are considered to be a promising solution to the problem of how to cope with the growing biofuel demand. In this paper, recent developments in oxy-free hydrocarbon conversion via catalytic deoxygenation reactions, the selection of and lipid-content enhancement of oleaginous microalgae, electrochemical biofuel conversion, and the diversification of valuable products from biomass and intermediates are reviewed. The challenges and prospects for future development of eco-friendly and economically advanced biofuel production processes also are outlined herein.

Anodic electro-fermentation of 3-hydroxypropionic acid from glycerol by recombinant Klebsiella pneumoniae L17 in a bioelectrochemical system

BACKGROUND

3-Hydroxypropionic acid (3-HP) is an important platform chemical which can be produced biologically from glycerol. Klebsiella pneumoniae is an ideal biocatalyst for 3-HP because it can grow well on glycerol and naturally synthesize the essential coenzyme B₁₂. On the other hand, if higher yields and titers of 3-HP are to be achieved, the sustained regeneration of NAD⁺ under anaerobic conditions, where coenzyme B₁₂ is synthesized sustainably, is required.

RESULTS

In this study, recombinant K. pneumoniae L17 overexpressing aldehyde dehydrogenase (AldH) was developed and cultured in a bioelectrochemical system (BES) with the application of an electrical potential to the anode using a chronoamperometric method (+0.5 V vs. Ag/AgCl). The BES operation resulted in 1.7-fold enhancement of 3-HP production compared to the control without the applied potential. The intracellular NADH/NAD⁺ ratio was significantly lower when the L17 cells were grown under an electric potential. The interaction between the electrode and overexpressed AldH was enhanced by electron shuttling mediated by HNQ (2-hydroxy-1,4-naphthoquinone).

CONCLUSIONS

Enhanced 3-HP production by the BES was achieved using recombinant K. pneumoniae L17. The quinone-based electron transference between the electrode and L17 was investigated by respiratory uncoupler experiments. This study provides a novel strategy to control the intracellular redox states to enhance the yield and titer of 3-HP production as well as other bioconversion processes.

Oxygen Tension and Riboflavin Gradients Cooperatively Regulate the Migration of Shewanella oneidensis MR-1 Revealed by a Hydrogel-Based Microfluidic Device

Shewanella oneidensis is a model bacterial strain for studies of bioelectrochemical systems (BESs). It has two extracellular electron transfer pathways: (1) shuttling electrons via an excreted mediator riboflavin; and (2) direct contact between the c-type cytochromes at the cell membrane and the electrode. Despite the extensive use of S. oneidensis in BESs such as microbial fuel cells and biosensors, many basic microbiology questions about S. oneidensis in the context of BES remain unanswered. Here, we present studies of motility and chemotaxis of S. oneidensis under well controlled concentration gradients of two electron acceptors, oxygen and oxidized form of riboflavin (flavin+), using a newly developed microfluidic platform. Experimental results demonstrate that either oxygen or flavin+ is a chemoattractant to S. oneidensis. The chemotactic tendency of S. oneidensis in a flavin+ concentration gradient is significantly enhanced in an anaerobic in contrast to an aerobic condition. Furthermore, either a low oxygen tension or a high flavin+ concentration considerably enhances the speed of S. oneidensis. This work presents a robust microfluidic platform for generating oxygen and/or flavin+ gradients in an aqueous environment, and demonstrates that two important electron acceptors, oxygen and oxidized riboflavin, cooperatively regulate S. oneidensis migration patterns. The microfluidic tools presented as well as the knowledge gained in this work can be used to guide the future design of BESs for efficient electron production.

Electron acceptor redox potential globally regulates transcriptomic profiling in Shewanella decolorationis S12

Electron acceptor redox potential (EARP) was presumed to be a determining factor for microbial metabolism in many natural and engineered processes. However, little is known about the potentially global effects of EARP on bacteria. In this study, we compared the physiological and transcriptomic properties of Shewanella decolorationis S12 respiring with different EARPs in microbial electrochemical systems to avoid the effects caused by the other physicochemical properties of real electron acceptor. Results showed that the metabolic activities of strain S12 were nonlinear responses to EARP. The tricarboxylic acid cycle for central carbon metabolism was down-regulated while glyoxylate shunt was up-regulated at 0.8 V compared to 0.2 and -0.2 V, which suggested that EARP is an important but not the only determinant for metabolic pathways of strain S12. Moreover, few cytochrome c genes were differentially expressed at different EARPs. The energy intensive flagella assembly and assimilatory sulfur metabolism pathways were significantly enriched at 0.8 V, which suggested strain S12 had stronger electrokinesis behavior and oxidative stress-response at high EARP. This study provides the first global information of EARP regulations on microbial metabolism, which will be helpful for understanding microorganism respiration.

Effect of oxygen on the per-cell extracellular electron transfer rate of Shewanella oneidensis MR-1 explored in bioelectrochemical systems

Extracellular electron transfer (EET) is a mechanism that enables microbes to respire solid‐phase electron acceptors. These EET reactions most often occur in the absence of oxygen, since oxygen can act as a competitive electron acceptor for many facultative microbes. However, for Shewanella oneidensis MR‐1, oxygen may increase biomass development, which could result in an overall increase in EET activity. Here, we studied the effect of oxygen on S. oneidensis MR‐1 EET rates using bioelectrochemical systems (BESs). We utilized optically accessible BESs to monitor real‐time biomass growth, and studied the per‐cell EET rate as a function of oxygen and riboflavin concentrations in BESs of different design and operational conditions. Our results show that oxygen exposure promotes biomass development on the electrode, but significantly impairs per‐cell EET rates even though current production does not always decrease with oxygen exposure. Additionally, our results indicated that oxygen can affect the role of riboflavin in EET. Under anaerobic conditions, both current density and per‐cell EET rate increase with the riboflavin concentration. However, as the dissolved oxygen (DO) value increased to 0.42 mg/L, riboflavin showed very limited enhancement on per‐cell EET rate and current generation. Since it is known that oxygen can promote flavins secretion in S. oneidensis, the role of riboflavin may change under anaerobic and aerobic conditions. Biotechnol. Bioeng. 2017;114: 96–105. © 2016 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Stimulation of oxygen to bioanode for energy recovery from recalcitrant organic matter aniline in microbial fuel cells (MFCs)

The challenge of energy generation from biodegradation of recalcitrant organics in microbial fuel cells (MFCs) is mainly attributed to their persistence to degradation under anaerobic condition in anode chamber of MFCs. In this work, we demonstrated that electricity generation from aniline, a typical recalcitrant organic matter under anaerobic condition was remarkably facilitated by employing oxygen into bioanode of MFCs. By exposing bioanode to air, electrons of 47.2 ± 6.9 C were recovered with aniline removal efficiency of 91.2 ± 2.2% in 144 h. Limited oxygen supply (the anodic headspace was initially filled with air and then closed) resulted in the decrease of electrons recovery and aniline removal efficiency by 52.5 ± 9.4% and 74.2 ± 2.1%, respectively, and further decline by respective 64.3 ± 4.5% and 82.7 ± 1.0% occurred under anaerobic condition. Community analysis showed that anode biofilm was predominated by several aerobic aniline degrading bacteria (AADB) and anode-respiration bacteria (ARB), which likely cooperated with each other and finally featured the energy recovery from aniline. Cyclic voltammetry indicated that anodic bacteria transferred electrons to anode mainly through electron shuttle. This study provided a new sight to acquaint us with the positive role of oxygen in biodegradation of recalcitrant organics on anode as well as electricity generation.

Catabolic and regulatory systems in Shewanella oneidensis MR-1 involved in electricity generation in microbial fuel cells

Shewanella oneidensis MR-1 is a facultative anaerobe that respires using a variety of inorganic and organic compounds. MR-1 is also capable of utilizing extracellular solid materials, including anodes in microbial fuel cells (MFCs), as electron acceptors, thereby enabling electricity generation. As MFCs have the potential to generate electricity from biomass waste and wastewater, MR-1 has been extensively studied to identify the molecular systems that are involved in electricity generation in MFCs. These studies have demonstrated the importance of extracellular electron-transfer (EET) pathways that electrically connect the quinone pool in the cytoplasmic membrane to extracellular electron acceptors. Electricity generation is also dependent on intracellular catabolic pathways that oxidize electron donors, such as lactate, and regulatory systems that control the expression of genes encoding the components of catabolic and electron-transfer pathways. In addition, recent findings suggest that cell-surface polymers, e.g., exopolysaccharides, and secreted chemicals, which function as electron shuttles, are also involved in electricity generation. Despite these advances in our knowledge on the EET processes in MR-1, further efforts are necessary to fully understand the underlying intra- and extracellular molecular systems for electricity generation in MFCs. We suggest that investigating how MR-1 coordinates these systems to efficiently transfer electrons to electrodes and conserve electrochemical energy for cell proliferation is important for establishing the biological basis for MFCs.

Enhanced Shewanella biofilm promotes bioelectricity generation

Electroactive biofilms play essential roles in determining the power output of microbial fuel cells (MFCs). To engineer the electroactive biofilm formation of Shewanella oneidensis MR-1, a model exoelectrogen, we herein heterologously overexpressed a c-di-GMP biosynthesis gene ydeH in S. oneidensis MR-1, constructing a mutant strain in which the expression of ydeH is under the control of IPTG-inducible promoter, and a strain in which ydeH is under the control of a constitutive promoter. Such engineered Shewanella strains had significantly enhanced biofilm formation and bioelectricity generation. The MFCs inoculated with these engineered strains accomplished a maximum power density of 167.6 ± 3.6 mW/m(2) , which was ∼ 2.8 times of that achieved by the wild-type MR-1 (61.0 ± 1.9 mW/m(2) ). In addition, the engineered strains in the bioelectrochemical system at poised potential of 0.2 V vs. saturated calomel electrode (SCE) generated a stable current density of 1100 mA/m(2) , ∼ 3.4 times of that by wild-type MR-1 (320 mA/m(2) ).

Innovative statistical interpretation of Shewanella oneidensis microbial fuel cells data

The last decade of research has made significant strides toward practical applications of Microbial Fuel Cells (MFCs); however, design improvements and operational optimization cannot be realized without equally considering engineering designs and biological interfacial reactions. In this study, the main factors contributing to MFCs' overall performance and their influence on MFC reproducibility are discussed. Two statistical approaches were used to create a map of MFC components and their expanded uncertainties, principal component analysis (PCA) and uncertainty of measurement results (UMR). PCA was used to identify the major factors influencing MFCs and to determine their ascendency over MFC operational characteristics statistically. UMR was applied to evaluate the factors' uncertainties and estimate their level of contribution to the final irreproducibility. In order to simplify the presentation and concentrate on the MFC components, only results from Shewanella spp. were included; however, a similar analysis could be applied for any DMRB or microbial community. The performed PCA/UMR analyses suggest that better reproducibility of MFC performance can be achieved through improved design parameters. This approach is exactly opposite to the MFC optimization and scale up approach, which should start with improving the bacteria-electrode interactions and applying these findings to well-designed systems.

Improving mediated electron transport in anodic bioelectrocatalysis

A microbial fuel cell loaded with bio-cocatalyst beads immobilized with recombinant riboflavin-secreting Escherichia coli shows significantly enhanced performance.

Characterization of anode and anolyte community growth and the impact of impedance in a microbial fuel cell

Background

A laboratory-scale two-chamber microbial fuel cell employing an aerated cathode with no catalyst was inoculated with mixed inoculum and acetate as the carbon source.

Electrochemical impedance spectroscopy (EIS) was used to study the behavior of the MFC during initial biofilm (week 1) and maximum power density (week 20). EIS were performed on the anode chamber, biofilm (without anolyte) and anolyte (without biofilm). Nyquist plots of the EIS data were fitted with two equivalent electrical circuits to estimate the contributions of intrinsic resistances to the overall internal MFC impedance at weeks 1 and 20, respectively.

Results

The results showed that the system tended to increase power density from 15 ± 3 (week 1) to 100 ± 15 mW/m² (week 20) and current density 211 ± 7 (week 1) to 347 ± 29 mA/m² (week 20). The Samples were identified by pyrosequencing of the 16S rRNA gene and showed that initial inoculum (week 1) was constituted by Proteobacteria (40%), Bacteroidetes (22%) and Firmicutes (18%). At week 20, Proteobacterial species were predominant (60%) for electricity generation in the anode biofilm, being 51% Rhodopseudomonas palustris. Meanwhile on anolyte, Firmicutes phylum was predominant with Bacillus sp.

This study proved that under the experimental conditions used there is an important contribution from the interaction of the biofilm and the anolyte on cell performance. Table 1 presents a summary of the specific influence of each element of the system under study.

Conclusions

The results showed certain members of the bacterial electrode community increased in relative abundance from the initial inoculum. For example, Proteobacterial species are important for electricity generation in the anode biofilms and Firmicutes phylum was predominant on anolyte to transfer electron.

R1 is the same in the three systems and no variation is observed over time.

The biofilm makes a significant contribution to the charge transfer processes at the electrode (R2 and Cdl) and, consequently, on the performance of the anode chamber.

The biofilm can act as a barrier which reduces diffusion of the anolyte towards the electrode, all the while behaving like a porous material.

The anolyte and its interaction with the biofilm exert a considerable influence on diffusion processes, given that it presents the highest values for Rd which increased at week 20.

Oxygen allows Shewanella oneidensis MR-1 to overcome mediator washout in a continuously fed bioelectrochemical system

Many bioelectrochemical systems (BESs) harness the ability of electrode-active microbes to catalyze reactions between electrodes and chemicals, often to perform useful functions such as wastewater treatment, fuel production, and biosensing. A microbial fuel cell (MFC) is one type of BES, which generates electric power through microbial respiration with an anode as the electron acceptor, and typically with oxygen reduction at the cathode to provide the terminal electron acceptor. Oxygen intrusion into MFCs is typically viewed as detrimental because it competes with anodes for electrons and lowers the coulombic efficiency. However, recent evidence suggests that it does not necessarily lead to lower performances—particularly for the model organism Shewanella oneidensis MR-1. Because flavin-mediated electron transfer is important for Shewanella species, which can produce this electron shuttle endogenuously, we investigated the role of flavins in the performance of pure-culture BESs with S. oneidensis MR-1 with and without oxygen. We found that oxygen increases current production more than twofold under continuously fed conditions, but only modestly increases current production under batch-fed conditions.We hypothesized that oxygen is more beneficial under continuously fed conditions because it allows S. oneidensis to grow and produce flavins at a faster rate, and thus lowers flavin washout. Our conclusions were supported by experiments with a flavin-secretion deficient mutant of S. oneidensis.

Theoretical exploration of optimal metabolic flux distributions for extracellular electron transfer by Shewanella oneidensis MR-1

BACKGROUND

Shewanella oneidensis MR-1 is one of the model microorganisms used for extracellular electron transfer. In this study, to elucidate the capability and the relevant metabolic processes of S. oneidensis MR-1 involved in an electron transferring environment, we employed genome-scale modelling to model the necessary metabolic states and flux adjustments for electricity generation in the cytochrome c-based direct electron transfer (DET) mode, the NADH-linked mediated electron transfer (MET) mode and a presumable mixed mode comprising DET and flavin secretion. These are difficult to develop experimentally.

RESULTS

The results showed that the microbe had the potential to achieve current outputs of up to 2.610 A/gDW in the DET mode, 2.189 A/gDW in the MET mode and 2.197 A/gDW in the mixed mode. Compared with the DET mode, which relied on cytochrome c oxidase (EC: 1.1.1.2) to mediate the electron transfer, the MET mode was mainly dependent on two routes, catalysed by isocitrate dehydrogenase (NAD) (EC: 1.1.1.4) and NAD transhydrogenase, for the computed high current density value. In the mixed mode, whereas the cytochrome c-based DET accounted for most of the computed maximum current output value, the two flavins combined, riboflavin and FMN, played a much less important role in the probed current value.

CONCLUSIONS

Shewanella oneidensis MR-1 has the potential to sustain a high extracellular electron transfer rate similarly to Geobacter sulfurreducens, but relies on different intracellular mechanisms. Various levels of electron transfer rates are achieved by different combinations of metabolic pathways. Flavins can significantly degenerate the maximum electricity generation capability of the cell and the biomass formation, and thus should be avoided in order to achieve a high coulombic efficiency.

Conductive carbon nanotube hydrogel as a bioanode for enhanced microbial electrocatalysis

Enhancing microbial electrocatalysis through new material design is essential to the efficient and stable operation of bio-electrochemical system (BES). In this work, a novel conductive carbon nanotube (CNT) hydrogel was fabricated by electrodepositing both CNT and chitosan onto a carbon paper electrode and used as a BES anode electrode. The microscopic, spectroscopic, and electrochemical analytical results show that the CNT hydrogel exhibited an excellent electrochemical activity. In the BES tests, the current generation and the maximum power density of the MFC with the CNT hydrogel increased by 23% and 65%, respectively, compared with the control. This demonstrates that the utilization of such a hydrogel offers an effective approach to enhance the current generation of BES. The great conductivity of CNT and the high content of oxygen-containing functional groups (C-OH, C═O, etc.) on their surface were found to be responsible for the improvements. Our work provides a facile way to prepare appropriate BES electrodes and offers a straightforward and effective route to enhance the BES performance.

Bio-electrocatalyzed electron efflux in Gram positive and Gram negative bacteria: an insight into disparity in electron transfer kinetics

Electron transfer (ET) behavior of bacteria varies significantly in a bio-electrocatalyzed environment based on the cell membrane.

Promotion of iron oxide reduction and extracellular electron transfer in Shewanella oneidensis by DMSO

The dissimilatory metal reducing bacterium Shewanella oneidensis MR-1, known for its capacity of reducing iron and manganese oxides, has great environmental impacts. The iron oxides reducing process is affected by the coexistence of alternative electron acceptors in the environment, while investigation into it is limited so far. In this work, the impact of dimethyl sulphoxide (DMSO), a ubiquitous chemical in marine environment, on the reduction of hydrous ferric oxide (HFO) by S. oneidensis MR-1 was investigated. Results show that DMSO promoted HFO reduction by both wild type and ΔdmsE, but had no effect on the HFO reduction by ΔdmsB, indicating that such a promotion was dependent on the DMSO respiration. With the DMSO dosing, the levels of extracellular flavins and omcA expression were significantly increased in WT and further increased in ΔdmsE. Bioelectrochemical analysis show that DMSO also promoted the extracellular electron transfer of WT and ΔdmsE. These results demonstrate that DMSO could stimulate the HFO reduction through metabolic and genetic regulation in S. oneidensis MR-1, rather than compete for electrons with HFO. This may provide a potential respiratory pathway to enhance the microbial electron flows for environmental and engineering applications.

Systematic screening of carbon-based anode materials for microbial fuel cells with Shewanella oneidensis MR-1

We present a systematic screening of carbon-based anode materials for microbial fuel cells with Shewanella oneidensis MR-1. Under anoxic conditions nanoporous activated carbon cloth is a superior anode material in terms of current density normalized to the projected anode area and anode volume (24.0±0.3 μA cm(-2) and 482±7 μA cm(-3) at -0.2 vs. SCE, respectively). The good performance can be attributed to the high specific surface area of the material, which is available for mediated electron transfer through self-secreted flavins. Under aerated conditions no influence of the specific surface area is observed, which we attribute to a shift from primary indirect electron transfer by mediators to direct electron transfer via adherent cells. Furthermore, we show that an aerated initial growth phase enhances the current density under subsequent anoxic conditions fivefold when compared to a similar experiment that was conducted under permanently anoxic conditions.

Immobilization of whole cells by chemical vapor deposition of silica

Effective entrapment of whole bacterial cells onto solid-phase materials can significantly improve bioprocessing and other biotechnology applications. Cell immobilization allows integration of biocatalysts in a manner that maintains long-term cell viability and typically enhances process output. A wide variety of functionalized materials have been explored for microbial cell immobilization, and specific advantages and limitations were identified. The method described here is a simple, versatile, and scalable one-step process for the chemical vapor deposition of silica to encapsulate and stabilize viable, whole bacterial cells. The immobilized bacterial population is prepared and captured at a predefined physiological state so as to affix bacteria with a selected metabolic or catalytic capability to compatible materials and surfaces. Immobilization of Shewanella oneidensis to carbon electrodes and immobilization of Acinetobacter venetianus to adsorbent mats are described as model systems.

Redox and pH microenvironments within Shewanella oneidensis MR-1 biofilms reveal an electron transfer mechanism

The goal of this research was to quantify the variations in redox potential and pH in Shewanella oneidensis MR-1 biofilms respiring on electrodes. We grew S. oneidensis MR-1 on a graphite electrode, which was used to accept electrons for microbial respiration. We modified well-known redox and pH microelectrodes with a built-in reference electrode so that they could operate near polarized surfaces and quantified the redox potential and pH profiles in these biofilms. In addition, we used a ferri-/ferrocyanide redox system in which electrons were only transferred by mediated electron transfer to explain the observed redox potential profiles in biofilms. We found that regardless of the polarization potential of the biofilm electrode, the redox potential decreased toward the bottom of the biofilm. In a fully redox-mediated control system (ferri-/ferrocyanide redox system), the redox potential increased toward the bottom when the electrode was the electron acceptor. The opposite behavior of redox profiles in biofilms and the redox-controlled system is explained by S. oneidensis MR-1 biofilms not being redox-controlled when they respire on electrodes. The lack of a significant variation in pH implies that there is no proton transfer limitation in S. oneidensis MR-1 biofilms and that redox potential profiles are not caused by pH.

Differential biofilms characteristics of Shewanella decolorationis microbial fuel cells under open and closed circuit conditions

Biofilms formation capacities of Shewanella species in microbial fuel cells (MFCs) and their roles in current generation have been documented to be species-dependent. Understandings of the biofilms growth and metabolism are essential to optimize the current generation of MFCs. Shewanella decolorationis S12 was used in both closed-circuit and open-circuit MFCs in this study. The anodic S. decolorationis S12 biofilms could generate fivefold more current than the planktonic cells, playing a dominant role in current generation. Anodic biofilms viability was sustained at 98 ± 1.2% in closed-circuit while biofilms viability in open-circuit decreased to 72 ± 7% within 96 h. The unviable domain in open-circuit MFCs biofilms majorly located at the inner layer of biofilm. The decreased biofilms viability in open-circuit MFCs could be recovered by switching into closed-circuit, indicating that the current-generating anode in MFCs could serve as a favorable electron acceptor and provide sufficient energy to support cell growth and metabolism inside biofilms.

Integration of a microbial fuel cell with activated sludge process for energy-saving wastewater treatment: taking a sequencing batch reactor as an example

In the research and application of microbial fuel cell (MFC), how to incorporate MFCs into current wastewater infrastructure is an importance issue. Here, we report a novel strategy of integrating an MFC into a sequencing batch reactor (SBR) to test the energy production and the chemical oxygen demand (COD) removal. The membrane-less biocathode MFC is integrated with the SBR to recover energy from the aeration in the form of electricity and thus reduce the SBR operation costs. In a lab-scale integrated SBR-MFC system, the maximum power production of the MFC was 2.34 W/m(3) for one typical cycle and the current density reached up to 14 A/m(3) . As a result, the MFC contributed to the 18.7% COD consumption of the integrated system and also recovered energy from the aeration tank with a volume fraction of only 12% of the SBR. Our strategy provides a feasible and effective energy-saving and -recovering solution to upgrade the existing activated sludge processes.

The utility of Shewanella japonica for microbial fuel cells

Shewanella-containing microbial fuel cells (MFCs) typically use the fresh water wild-type strain Shewanella oneidensis MR-1 due to its metabolic diversity and facultative oxidant tolerance. However, S. oneidensis MR-1 is not capable of metabolizing polysaccharides for extracellular electron transfer. The applicability of Shewanella japonica (an agar-lytic Shewanella strain) for power applications was analyzed using a diverse array of carbon sources for current generation from MFCs, cellular physiological responses at an electrode surface, biofilm formation, and the presence of soluble extracellular mediators for electron transfer to carbon electrodes. Critically, air-exposed S. japonica utilizes biosynthesized extracellular mediators for electron transfer to carbon electrodes with sucrose as the sole carbon source.

Probing electron transfer mechanisms in Shewanella oneidensis MR-1 using a nanoelectrode platform and single-cell imaging

Microbial fuel cells (MFCs) represent a promising approach for sustainable energy production as they generate electricity directly from metabolism of organic substrates without the need for catalysts. However, the mechanisms of electron transfer between microbes and electrodes, which could ultimately limit power extraction, remain controversial. Here we demonstrate optically transparent nanoelectrodes as a platform to investigate extracellular electron transfer in Shewanella oneidensis MR-1, where an array of nanoholes precludes or single window allows for direct microbe-electrode contacts. Following addition of cells, short-circuit current measurements showed similar amplitude and temporal response for both electrode configurations, while in situ optical imaging demonstrates that the measured currents were uncorrelated with the cell number on the electrodes. High-resolution imaging showed the presence of thin, 4- to 5-nm diameter filaments emanating from cell bodies, although these filaments do not appear correlated with current generation. Both types of electrodes yielded similar currents at longer times in dense cell layers and exhibited a rapid drop in current upon removal of diffusible mediators. Reintroduction of the original cell-free media yielded a rapid increase in current to ∼80% of original level, whereas imaging showed that the positions of > 70% of cells remained unchanged during solution exchange. Together, these measurements show that electron transfer occurs predominantly by mediated mechanism in this model system. Last, simultaneous measurements of current and cell positions showed that cell motility and electron transfer were inversely correlated. The ability to control and image cell/electrode interactions down to the single-cell level provide a powerful approach for advancing our fundamental understanding of MFCs.

Shewanella knowledgebase: integration of the experimental data and computational predictions suggests a biological role for transcription of intergenic regions

Shewanellae are facultative γ-proteobacteria whose remarkable respiratory versatility has resulted in interest in their utility for bioremediation of heavy metals and radionuclides and for energy generation in microbial fuel cells. Extensive experimental efforts over the last several years and the availability of 21 sequenced Shewanella genomes made it possible to collect and integrate a wealth of information on the genus into one public resource providing new avenues for making biological discoveries and for developing a system level understanding of the cellular processes. The Shewanella knowledgebase was established in 2005 to provide a framework for integrated genome-based studies on Shewanella ecophysiology. The present version of the knowledgebase provides access to a diverse set of experimental and genomic data along with tools for curation of genome annotations and visualization and integration of genomic data with experimental data. As a demonstration of the utility of this resource, we examined a single microarray data set from Shewanella oneidensis MR-1 for new insights into regulatory processes. The integrated analysis of the data predicted a new type of bacterial transcriptional regulation involving co-transcription of the intergenic region with the downstream gene and suggested a biological role for co-transcription that likely prevents the binding of a regulator of the upstream gene to the regulator binding site located in the intergenic region.

Database URL:http://shewanella-knowledgebase.org:8080/Shewanella/ or http://spruce.ornl.gov:8080/Shewanella/

Effects of oxygen on Shewanella decolorationis NTOU1 electron transfer to carbon-felt electrodes

To clarify the major factor caused by oxygen-enhancing charge production of Shewanella decolorationis NTOU1 towards a polarized anode, a series of experimental runs (i.e., with/without ambient air flushing and with/without ammonia addition as nitrogen source) were conducted in this study. Within 6-day of operation at +0.4 V vs. Ag|AgCl and starting with 35 mM of lactate, consistently the electrical charge production under the aerobic condition was higher than that under the anaerobic condition. In all the experimental runs, the values of nicotinamide adenine dinucleotide (NADH) production were found to be correlated positively and significantly with the charge production, but the highest Coulombic efficiency of 18% was observed under the anaerobic conditions without ammonia addition while the lowest charge production occurred. Those results indicate that NADH production enhanced by oxygen is the leading cause of the increase of the charge production, but the biomass production and the oxygen reduction would both consume NADH electrons and lead to lower electron recoveries. In addition, whether under constant aerobic or anaerobic, or alternating aerobic/anaerobic conditions, chronoamperometric results made it possible to rule out other factors, like lactate uptake rate or cell growth, which might increase the charge production under aerobic conditions. By using high performance liquid chromatography, some diffusive flavins (e.g., 0.5 microM of riboflavin) were found under the aerobic condition, but were not found under the anaerobic one. However, from results of cyclic voltammetry (CV), the signals of flavins were found to be approximately the same under both conditions. Although it is inferred that oxygen renders the flavins secreted extracellularly, that is not the major effect of oxygen for boosting the charge production. Furthermore, bound flavins under anaerobic condition were found to be effectively electrocatalytic according to sigmoidal CV result.

Aerated Shewanella oneidensis in continuously fed bioelectrochemical systems for power and hydrogen production

We studied the effects of aeration of Shewanella oneidensis on potentiostatic current production, hydrogen production in a microbial electrolysis cell, and electric power generation in a microbial fuel cell (MFC). The potentiostatic performance of aerated S. oneidensis was considerably enhanced to a maximum current density of 0.45 A/m(2) or 80.3 A/m(3) (mean: 0.34 A/m(2), 57.2 A/m(3)) compared to anaerobically grown cultures. Biocatalyzed hydrogen production rates with aerated S. oneidensis were studied within the applied potential range of 0.3-0.9 V and were highest at 0.9 V with 0.3 m(3) H(2)/m(3) day, which has been reported for mixed cultures, but is approximately 10 times higher than reported for an anaerobic culture of S. oneidensis. Aerated MFC experiments produced a maximum power density of 3.56 W/m(3) at a 200-Omega external resistor. The main reasons for enhanced electrochemical performance are higher levels of active biomass and more efficient substrate utilization under aerobic conditions. Coulombic efficiencies, however, were greatly reduced due to losses of reducing equivalents to aerobic respiration in the anode chamber. The next challenge will be to optimize the aeration rate of the bacterial culture to balance between maximization of bacterial activation and minimization of aerobic respiration in the culture.