Electrical Current Production Research Articles

Inhibitory effect of ethanol on the experimental electrical charge and hydrogen production in microbial electrolysis cells (MECs)

Fermentation effluents are exploited in microbial electrolysis cells (MECs) for hydrogen production, but they may include ethanol and other compounds that are detrimental to MEC performance. Ethanol can disrupt microbial viability, and lactate makes MEC feeding more complex. The present work aimed to assess the effects of lactate and ethanol on hydrogen and electric current production in MECs to determine the corresponding inhibitory concentrations. A two-chamber MEC was inoculated with compost leachate and fed with lactate-ethanol mixtures at four ratios (total 2.5g COD L−1), and a volatile fatty acid mixture was used as a base substrate to achieve anolyte at a final concentration of 3.5g COD L−1. The MEC operated at 1.0V for 21days, and its performance was evaluated for COD removal, cathodic and energy efficiency. The inhibitory ethanol concentration was 0.8g COD L−1 (0.38gL−1), and lactate reduced the current density lag phase. The start of hydrogen production required 218±35C in the MEC system utilized. This work shows that ethanol content exerts an inhibitory effect on current and hydrogen production; moreover, a relationship between accumulated charge and accumulated hydrogen volume was found suitable to characterize the process.

Electrode Colonization by the Feammox Bacterium Acidimicrobiaceae sp. Strain A6.

Acidimicrobiaceae sp. strain A6 (A6), from the Actinobacteria phylum, was recently identified as a microorganism that can carry out anaerobic ammonium (NH4+) oxidation coupled to iron reduction, a process also known as Feammox. Being an iron-reducing bacterium, A6 was studied as a potential electrode-reducing bacterium that may transfer electrons extracellularly onto electrodes while gaining energy from NH4+ oxidation. Actinobacteria species have been overlooked as electrogenic bacteria, and the importance of lithoautotrophic iron reducers as electrode-reducing bacteria at anodes has not been addressed. By installing electrodes in the soil of a forested riparian wetland where A6 thrives, in soil columns in the laboratory, and in A6-bioaugmented constructed wetland (CW) mesocosms and by operating microbial electrolysis cells (MECs) with pure A6 culture, the characteristics and performances of this organism as an electrode-reducing bacterium candidate were investigated. In this study, we show that Acidimicrobiaceae sp. strain A6, a lithoautotrophic bacterium, is capable of colonizing electrodes under controlled conditions. In addition, A6 appears to be an electrode-reducing bacterium, since current production was boosted shortly after the CWs were seeded with enrichment A6 culture and current production was detected in MECs operated with pure A6, with the anode as the sole electron acceptor and NH4+ as the sole electron donor.IMPORTANCE Most studies on electrogenic microorganisms have focused on the most abundant heterotrophs, while other microorganisms also commonly present in electrode microbial communities, such as Actinobacteria strains, have been overlooked. The novel Acidimicrobiaceae sp. strain A6 (Actinobacteria) is an iron-reducing bacterium that can colonize the surface of anodes in sediments and is linked to electrical current production, making it an electrode-reducing bacterium. Furthermore, A6 can carry out anaerobic ammonium oxidation coupled to iron reduction. Therefore, findings from this study open the possibility of using electrodes instead of iron as electron acceptors, as a means to promote A6 to treat NH4+-containing wastewater more efficiently. Altogether, this study expands our knowledge of electrogenic bacteria and opens the possibility of developing Feammox-based technologies coupled to bioelectric systems for the treatment of NH4+ and other contaminants in anoxic systems.

Boosting mediated electron transfer in bioelectrochemical systems with tailored defined microbial cocultures.

Bioelectrochemical systems (BES) hold great promise for sustainable energy generation via a microbial catalyst from organic matter, for example, from wastewater. To improve current generation in BES, understanding the underlying microbiology of the electrode community is essential. Electron mediator producing microorganism like Pseudomonas aeruginosa play an essential role in efficient electricity generation in BES. These microbes enable even nonelectroactive microorganism like Enterobacter aerogenes to contribute to current production. Together they form a synergistic coculture, where both contribute to community welfare. To use microbial co-operation in BES, the physical and chemical environments provided in the natural habitats of the coculture play a crucial role. Here, we show that synergistic effects in defined cocultures of P. aeruginosa and E. aerogenes can be strongly enhanced toward high current production by adapting process parameters, like pH, temperature, oxygen demand, and substrate requirements. Especially, oxygen was identified as a major factor influencing coculture behavior and optimization of its supply could enhance electric current production over 400%. Furthermore, operating the coculture in fed-batch mode enabled us to obtain very high current densities and to harvest electrical energy for 1 month. In this optimized condition, the coulombic efficiency of the process was boosted to 20%, which is outstanding for mediator-based electron transfer. This study lays the foundation for a rationally designed utilization of cocultures in BES for bioenergy generation from specific wastewaters or for bioprocess sensing and for benefiting from their synergistic effects under controlled bioprocess condition.

Electrochemical performance of biocathode microbial fuel cells using petroleum-contaminated soil and hot water spring

Microbial fuel cells is growing technology for energy production (electrical and gaseous) with potential electrochemically active bacteria from degradation of unwanted contaminants. Electrogenic [petroleum-contaminated soil (PCS) and hot spring water HSW)] and electrotrophic [activated sludge] bacterial communities were enriched and evaluated for electric current production in biocathode microbial fuel cells (MFC). Molecular phylogenetic (454 pyrosequencing) analysis of environmental samples revealed an overall change in bacterial density and diversity after second-stage enrichment. The predominant electrogenic bacteria grown at anodic biofilms belonged to phylum Proteobacteria (80–98%) in both MFC-1 (PCS) and MFC-2 (HSW) reactors. After enrichment, the major shift in the bacterial species on anodic surface was observed in case of Stenotrophomonas maltophilia (89%) and shewanella sp. (15%) in the respective reactors. Overall, among electrotrophic bacteria, the relative abundance (27–30%) of Pseudomonas aeruginosa was maximum on the cathodic biofilm in both fuel cells. Scanning electron and confocal laser scanning microscopies of biofilms revealed that anode and cathode surfaces were covered with different microcolonies and dispersed bacterial cells. Cyclic voltammetry (− 1 to 1 V vs. Ag/AgCl) further confirmed the presence of highly proficient electrogenic bacteria capable of generating high electricity ranging from ≥ 8 mA in MFC-1 and ≤ 0.37-Y in MFC-2. Maximum power density of 5500 mW m−2 at a current density of 100 mA m−2 (550 Ω)] was recorded in MFC-1 during enrichment stage 2; however, it (Pmax = 1201 mW m2) remained 78% lower in MFC-2. Fourier transform infrared spectroscopy and COD removal [86% (SD = 8.3 ± 2.0)] of anolyte (PCS) confirmed active degradation of petroleum contaminants during the operation of MFC-1.

Revealing metabolic storage processes in electrode respiring bacteria by differential electrochemical mass spectrometry

In this work we employ differential electrochemical mass spectrometry (DEMS) in combination with static and dynamic electrochemical techniques for the study of metabolic processes of electrochemically active bacteria. CO2 production during acetate oxidation by electrode respiring bacteria was measured, in-vivo and online with a sensitivity of 6.5 ⋅ 10−13 mol/s. The correlation of ion current and electrical current provides insight into the interaction of metabolic processes and extra-cellular electron transfer. In low-turnover CVs, two competing potential dependent electron transfer mechanisms were observed and formal potentials of two redox systems that are involved in complete oxidation of acetate to CO2 were determined. By balancing charge and carbon flows during dynamic measurements, two significant storage mechanisms in electrochemically active bacteria were identified: 1) a charge storage mechanism that allows substrate oxidation to proceed at a constant rate despite of external current flowing in cathodic direction. 2) a carbon storage mechanism that allows the biofilm to take up acetate at an unchanged rate at very low potentials even though the oxidation to CO2 stops. These storage capabilities allow a limited decoupling of electrical current and CO2 production rate.

Performance of electro-spun carbon nanofiber electrodes with conductive poly(3,4-ethylenedioxythiophene) coatings in bioelectrochemical systems

Abstract Bioelectrochemical systems (BESs) employ extracellular electron transfer from bacteria that grow at electrodes. Due to biofilm and electrode limitations, industrial-scale applications require large electrode areas, and thus inexpensive electrode materials. Here, electro-spun polyacrylonitrile (PAN) and carbon nanofiber (CNF) were manufactured. In addition, the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was applied as a coating to these materials and to carbon cloth (CC). We tested these materials as electrodes by using physicochemical measurements, cyclic voltammetry, and bioelectrochemical growth-studies with Geobacter sulfurreducens. PAN is a nonconductive material without capacitance, but with PEDOT coating the conductivity and capacitance became sufficient to support electric current production in our BES. CNF outperformed CC in capacitance, but behaved similarly in our BES when normalized to projected surface area. With the PEDOT coating, CNF increased electric current production by 38% in our BES, while this was 64% for CC. When applied to a gold microfluidic electrode, electric current with G. sulfurreducens increased almost three-fold. PEDOT added considerable specific surface area to electrodes possessing a low surface area, but not with a high surface area such as CNF. This work demonstrates that electro-spun electrodes and PEDOT coating are a promising electrode alternative that can be readily implemented into existing BESs.

Use of microbial fuel cells to monitor current production in Qi-deficient liver cells

ObjectiveTo monitor current production in Qi-deficient liver cells, and to study how cellular proton leakage might affect electric current production. MethodsCells were placed in an microbial fuel cells (MFC) anode and the electric current was measured. Mitochondrial-affecting chemicals, 2,4-dinitrophenol (DNP) and resveratrol (RVT), were used to induce proton leakage in cells and their effect on current production observed. MCF-7 breast cancer cells exhibited higher proton leakage relative to normal liver cells. A mouse model for Qi-deficiency was prepared according to the Methodology of Animal Experiment in Chinese medicine. The Qi-tonics Buzhongyiqi Tang (BZYQT), which is used to treat the Qi-deficiency condition, was applied to Qi-deficient liver cells to examine how current production was altered. ResultsAdding either DNP or RVT to normal liver cells increased current production. MCF-7 cells that possessed high proton leakage were also found to produce higher currents than normal liver cells. Higher current production, lower cellular glucose content, and lower adenosine triphosphate (ATP) production rate were found in Qi-deficient liver cells, in which the use of DNP or RVT further increased current production. The use of BZYQT to treat Qi-deficient liver cells decreased current production, counteracted the action of DNP, and also improved cellular glucose content. ConclusionHigh electric current production was found in liver cells with high cellular proton leakage. Positive current responses to both mitochondria-affecting chemicals, DNP and RVT, appeared to indicate proper mitochondrial function. The high proton leakage detected in Qi-deficient liver cells might have caused high energy losses, which served to explain the observed lower cellular glucose content and ATP production rate than in normal cells. These results might also explain the exhibited syndromes of low energy and fatigue in Qi-deficient patients. Proton leakage, induced by DNP or the Qi-deficient condition, was possibly caused by unusual uncoupling of oxidative phosphorylation and appeared to be inhibited by treatment with BZYQT, such that decreased current production was observed after BZYQT treatment.

First results of the MAVEN magnetic field investigation

AbstractTwo Mars Atmosphere and Volatile EvolutioN magnetic field sensors sample the ambient magnetic field at the outer edge of each solar array. We characterized relatively minor spacecraft‐generated magnetic fields using in‐flight subsystem tests and spacecraft maneuvers. Dynamic spacecraft fields associated with the power subsystem (≤1 nT) are compensated for using spacecraft engineering telemetry to identify active solar array circuits and monitor their electrical current production. Static spacecraft magnetic fields are monitored using spacecraft roll maneuvers. Accuracy of measurement of the environmental magnetic field is demonstrated by comparison with field directions deduced from the symmetry properties of the electron distribution function measured by the Solar Wind Electron Analyzer. We map the bow shock, magnetic pileup boundary, the V × B convection electric field and ubiquitous proton cyclotron, and 1 Hz waves in the ion foreshock region.

Neutral red as a mediator for the enhancement of electricity production using a domestic wastewater double chamber microbial fuel cell

Neutral red is an electron transport compound that is used in bacteria as a mediator for decolorization, degradation of different organic compounds and in electricity production using microbial fuel cells (MFCs). In this study, bacterial electron transport was followed by monitoring electricity production in a double chamber microbial fuel cell, and measured as current (mA). To enhance electron transfer and consequently increase electricity production, neutral red was added as a mediator. The optimal concentration that had minimal effect on bacterial growth was 15 mM. Spiking the culture with this concentration resulted in a maximal current of 1.36 mA and a power intensity that reached 467.6 mW/m2over 120 h, as compared to 0.74 mA and 209.5 mW/m2 over a period of just 24 h as a control. This was also confirmed by the increase in initial bacterial conductivity measured in the presence of neutral red as compared to low initial conductivity in the absence of a mediator. To prove that the process of electricity production is correlated to electron transfer, azole and azide were both used to inhibit membrane cytochrome complexes. The results showed a sharp decrease in electrical current production within less than 24 h, suggesting that the process takes place via membrane protein cytochromes. The predominant microorganism present under all previously mentioned conditions was Buttiauxella agrestis SW-1 (GenBank ID KP770022) as identified using 16S rRNA phylogenetic analysis, this bacterium was also studied morphologically to detect micropilli. Protein profiles of the membrane showed a distinct band close to that reported for cytochrome c; nevertheless, further investigation is required to highlight both its nature and its role.

UCP2- and non-UCP2-mediated electric current in eukaryotic cells exhibits different properties.

Using live eukaryotic cells, including cancer cells, MCF-7 and HCT-116, normal hepatocytes and red blood cells in anode and potassium ferricyanide in cathode of MFC could generate bio-based electric current. Electrons and protons generated from the metabolic reaction in both cytosol and mitochondria contributing to the leaking would mediate the generation of electric current. Both resveratrol (RVT) and 2,4-dinitrophenol (DNP) used to induce proton leak in mitochondria were found to promote electric current production in all cells except red blood cells without mitochondria. Proton leak might be important for electric current production by bringing the charge balance in cells to enhance the further electron leak. The induced electric current by RVT can be blocked by Genipin, an inhibitor of UCP2-mediated proton leak, while that induced by DNP cannot. RVT could reduce reactive oxygen species (ROS) level in cells better than that of DNP. In addition, RVT increased mitochondrial membrane potential (MMP), while DNP decreased it. Results highly suggested the existence of at least two types of electric current that showed different properties. They included UCP2-mediated and non-UCP2-mediated electric current. UCP2-mediated electric current exhibited higher reactive oxygen species (ROS) reduction effect per unit electric current production than that of non-UCP2-mediated electric current. Higher UCP2-mediated electric current observed in cancer cells might contribute to the mechanism of drug resistence. Correlation could not be established between electric current production with either ROS and MMP without distinguishing the types of electric current.

Rational engineering of Geobacter sulfurreducens electron transfer components: a foundation for building improved Geobacter-based bioelectrochemical technologies.

Multiheme cytochromes have been implicated in Geobacter sulfurreducens extracellular electron transfer (EET). These proteins are potential targets to improve EET and enhance bioremediation and electrical current production by G. sulfurreducens. However, the functional characterization of multiheme cytochromes is particularly complex due to the co-existence of several microstates in solution, connecting the fully reduced and fully oxidized states. Over the last decade, new strategies have been developed to characterize multiheme redox proteins functionally and structurally. These strategies were used to reveal the functional mechanism of G. sulfurreducens multiheme cytochromes and also to identify key residues in these proteins for EET. In previous studies, we set the foundations for enhancement of the EET abilities of G. sulfurreducens by characterizing a family of five triheme cytochromes (PpcA-E). These periplasmic cytochromes are implicated in electron transfer between the oxidative reactions of metabolism in the cytoplasm and the reduction of extracellular terminal electron acceptors at the cell's outer surface. The results obtained suggested that PpcA can couple e−/H+ transfer, a property that might contribute to the proton electrochemical gradient across the cytoplasmic membrane for metabolic energy production. The structural and functional properties of PpcA were characterized in detail and used for rational design of a family of 23 single site PpcA mutants. In this review, we summarize the functional characterization of the native and mutant proteins. Mutants that retain the mechanistic features of PpcA and adopt preferential e−/H+ transfer pathways at lower reduction potential values compared to the wild-type protein were selected for in vivo studies as the best candidates to increase the electron transfer rate of G. sulfurreducens. For the first time G. sulfurreducens strains have been manipulated by the introduction of mutant forms of essential proteins with the aim to develop and improve bioelectrochemical technologies.

Using live algae at the anode of a microbial fuel cell to generate electricity.

Live green microalgae Chlorella pyrenoidosa was introduced in the anode of a microbial fuel cell (MFC) to act as an electron donor. By controlling the oxygen content, light intensity, and algal cell density at the anode, microalgae would generate electricity without requiring externally added substrates. Two models of algal microbial fuel cells (MFCs) were constructed with graphite/carbon electrodes and no mediator. Model 1 algal MFC has live microalgae grown at the anode and potassium ferricyanide at the cathode, while model 2 algal MFC had live microalgae in both the anode and cathode in different growth conditions. Results indicated that a higher current produced in model 1 algal MFC was obtained at low light intensity of 2500 lx and algal cell density of 5 × 10(6) cells/ml, in which high algal density would limit the electricity generation, probably by increasing oxygen level and mass transfer problem. The maximum power density per unit anode volume obtained in model 1 algal MFC was relatively high at 6030 mW/m(2), while the maximum power density at 30.15 mW/m(2) was comparable with that of previous reported bacteria-driven MFC with graphite/carbon electrodes. A much smaller power density at 2.5 mW/m(2) was observed in model 2 algal MFC. Increasing the algal cell permeability by 4-nitroaniline would increase the open circuit voltage, while the mitochondrial acting and proton leak promoting agents resveratrol and 2,4-dinitrophenol would increase the electric current production in algal MFC.

Backbone, side chain and heme resonance assignments of cytochrome OmcF from Geobacter sulfurreducens.

Gene knockout studies on Geobacter sulfurreducens (Gs) cells showed that the outer membrane cytochrome OmcF is involved in respiratory pathways leading to the extracellular reduction of Fe(III) citrate and U(VI) oxide. In addition, microarray analysis of OmcF-deficient mutant versus the wild-type strain revealed that many of the genes with decreased transcript level were those whose expression is upregulated in cells grown with a graphite electrode as electron acceptor. This suggests that OmcF also regulates the electron transfer to electrode surfaces and the concomitant electrical current production by Gs in microbial fuel cells. Extracellular electron transfer processes (EET) constitute nowadays the foundations to develop biotechnological applications in biofuel production, bioremediation and bioenergy. Therefore, the structural characterization of OmcF is a fundamental step to understand the mechanisms underlying EET. Here, we report the complete assignment of the heme proton signals together with (1)H, (13)C and (15)N backbone and side chain assignments of the OmcF, excluding the hydrophobic residues of the N-terminal predicted lipid anchor.

Exoelectrogenic capacity of host microbiota predicts lymphocyte recruitment to the gut.

Electrotaxis, directional cell movement in response to an electric potential, has been demonstrated in a wide range of cell types including lymphocytes. Exoelectrogens, microorganisms capable of generating electrical currents, have been identified in microbial fuel cells. However, no studies have investigated exoelectrogenic microbes in fresh feces or the effects of an exoelectrogenic microbiota on the host organism. Here we show that commensal gut microbial populations differ in their capacity for electrical current production by exoelectrogens and that those differences are predictive of increased lymphocyte trafficking to the gut in vivo, despite the lack of increased production of canonical lymphocyte-specific chemokines. Additionally, we demonstrate that the difference in current production between mice purchased from different commercial sources correlates reproducibly with the presence or absence of segmented filamentous bacteria, and while our data do not support a direct role for segmented filamentous bacteria in ex vivo current production, an exoelectrogenic microbiota can be transferred in vivo via mucosa-associated bacteria present in the ileum. Moreover, we detect upregulation of microbial genes associated with extracellular electron transfer in feces of mice colonized with exoelectrogenic microbiota containing segmented filamentous bacteria. While still correlative, these results suggest a novel means by which the gut microbiota modulates the recruitment of cells of the immune system to the gut.

Metabolic modeling of spatial heterogeneity of biofilms in microbial fuel cells reveals substrate limitations in electrical current generation

Microbial fuel cells (MFCs) have been proposed as an alternative energy resource for the conversion of organic compounds to electricity. In an MFC, microorganisms such as Geobacter sulfurreducens form an anode-associated biofilm that can completely oxidize organic matter (electron donor) to carbon dioxide with direct electron transfer to the anode (electron acceptor). Mathematical models are useful in analyzing biofilm processes; however, existing models rely on Nernst-Monod type expressions, and evaluate extracellular processes separated from the intracellular metabolism of the microorganism. Thus, models that combine both extracellular and intracellular components, while addressing spatial heterogeneity, are essential for improved representation of biofilm processes. The goal of this work is to develop a model that integrates genome-scale metabolic models with the model of biofilm environment. This integrated model shows the variations of electrical current production and biofilm thickness under the presence/absence of NH4 in the bulk solution, and under varying maintenance energy demands. Further, sensitivity analysis suggested that conductivity is not limiting electrical current generation and that increasing cell density can lead to enhanced current generation. In addition, the modeling results also highlight instances such as the transformation into respiring cells, where the mechanism of electrical current generation during biofilm development is not yet clearly understood.

Exploration and comparison of inborn capacity of aerobic and anaerobic metabolisms of Saccharomyces cerevisiae for microbial electrical current production

Saccharomyces cerevisiae possesses numerous advantageous biological features, such as being robust, easily handled, mostly non-pathogenic and having high catabolic rates, etc., which can be considered as merits for being used as a promising biocatalyst in microbial fuel cells (MFCs) for electricity generation. Previous studies have developed efficient MFC configurations to convert metabolic electron shuttles, such as cytoplasmic NADH, into usable electric current. However, no studies have elucidated the maximum potential of S. cerevisiae for current output and the underlying metabolic pathways, resulting from the interaction of thousands of reactions inside the cell during MFC operation. To address these two key issues, this study used in silico metabolic engineering techniques, flux balance analysis (FBA), and flux variability analysis with target flux minimization (FATMIN), to model the metabolic perturbation of S. cerevisiae under the MFC-energy extraction. The FBA results showed that, in the cytoplasmic NADH-dependent mediated electron transfer (MET) mode, S. cerevisiae had a potential to produce currents at up to 5.781 A/gDW for the anaerobic and 6.193 A/gDW for the aerobic environments. The FATMIN results showed that the aerobic and anaerobic metabolisms are resilient, relying on six and five contributing reactions respectively for high current production. Two reactions, catalyzed by glutamate dehydrogenase (NAD) (EC 1.4.1.3) and methylene tetrahydrofolate dehydrogenase (NAD) (EC 1.5.1.5), were shared in both current-production modes and contributed to over 80% of the identified maximum current outputs. It is also shown that the NADH regeneration was much less energy costly than biomass production rate. Taken together, our finding suggests that S. cerevisiae should receive more research effort for MFC electricity production.

Simulation Biomass Effecting On Microbial Fuel Cell Electricity Properties and Substrate Degradation

Microbial fuel cell (MFC) mathematical model was established with suspended microorganisms, biomass on the electrode material, soluble chemical substrates and intermediary. By simulating the process of the substrate degradation, biomass growth and the electric current production process, With different initial biomass concentration, suspended microbial biomass and biomass attaching on electrode varing with time,current and charge varing with time,substrate concentration varing with time and mediator concenration varing with time were investigated. The results showed that initial suspended microbial is little, growth of biomass is main on electrode, biomass in solution grow slowly, producing electricity the main function biomass is on electrode, while initial suspended microbial concentration is high, growth of biomass is main in solution, biomass on electrode grow slowly, producing electricity the main function biomass is in solution; the more initial suspended microbial, the faster current reaching the maximum, the more charge, the faster substrate degradation, the faster mediator variation; initial add biomass benefit to MFC producing electricity. DOI: http://dx.doi.org/10.11591/telkomnika.v11i2.1977

Terminal oxidase mutants of the cyanobacterium Synechocystis sp. PCC 6803 show increased electrogenic activity in biological photo-voltaic systems

Biological photo-voltaic systems are a type of microbial fuel cell employing photosynthetic microbes at the anode, enabling the direct transduction of light energy to electrical power. Unlike the anaerobic bacteria found in conventional microbial fuel cells that use metals in the environment as terminal electron acceptors, oxygenic photosynthetic organisms are poorly adapted for electron transfer out of the cell. Mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 were created in which all combinations of the three respiratory terminal oxidase complexes had been inactivated. These strains were screened for the ability to reduce the membrane-impermeable soluble electron acceptor ferricyanide, and the results were compared to the performance of the mutants in a biological photo-voltaic system. Deletion of the two thylakoid-localised terminal oxidases, the bd-quinol oxidase and cytochrome c oxidase resulted in a 16-fold increase in ferricyanide reduction rate in the dark compared to the wild-type. A further improvement to a 24-fold increase was seen upon deletion of the remaining "alternative respiratory terminal oxidase". These increases were reflected in the peak power generated in the biological photo-voltaic systems. Inactivation of all three terminal oxidase complexes resulted in a substantial redirection of reducing power; in the dark the equivalent of 10% of the respiratory electron flux was channelled to ferricyanide, compared to less than 0.2% in the wild-type. Only minor improvements in ferricyanide reduction rates over the wild-type were seen in illuminated conditions, where carbon dioxide is preferentially used as an electron sink. This study demonstrates the potential for optimising photosynthetic microbes for direct electrical current production.

포도당을 주입한 수중퇴적물을 이용한 연료전지시스템에 있어서 미생물군집 분석

본 연구의 목적은 포도당을 자연의 저질기반 미생물연료전지에 주입할 경우 전기생산을 최적화하기 위한 것이며, 이 때 관련된 미생물의 군집을 분석하고 관련미생물의 역할을 검토하고자 하였다. 1,000 mg/L의 포도당이 주입되었을 때 생성되는 전류가 약 3배 가량 증가하였다. 이러한 증가는 주로 발효세균과 전기생성세균의 상호작용에 기인하는 것으로서, 이는 발효미생물에 의해 생성된 유기산이 전기생성 미생물에 의해서 분해되므로 유기산의 축적을 방지하여 되먹임저해(feedback inhibition) 현상을 감소 시키는데 그 원인이 있는 것으로 보인다. 반면, 더 높은 농도의 포도당이 주입되었을 시에는 전류가 떨어지거나 큰 증가가 일어나지 않았다. 만약 적절한 농도의 포도당이 주입될 시, 전기생성 미생물과 발효미생물이 동시에 포도당을 분해하면서 피드백을 제거하며 전류생성이 증가함을 알 수 있었다. 포도당을 토양에 주입하였을 시에 Clostridium sp.과 같은 발효미생물이 많이 나타났다. 포도당의 발효는 전기생성에 있어서 긍정적 영향과 부정적 영향을 미칠 수 있음이 밝혀졌다. 즉 발효산물이 전기생성미생물에 의해서 분해되어서 사용된다면 전기생성이 증가한다. 하지만, 발효산물이 전기생성미생물에 의해서 분해되지 못한다면 여러 전기생성을 억제하는 화학적반응(pH 저하, 메탄생성, 유기산 축적 등)이 일어나고 미생물연료전지와 관계없는 미생물들이 주입된 유기물을 대부분 분해하여 전기생성이 저하될 수 있음이 밝혀졌다. 적절한 농도의 포도당 주입을 통한 발효세균(Clostridium sp. 등)과 전기발생균(Geobacter sp. 등)의 적절한 조합은 자연상태에서의 혼합미생물존재 환경에서의 전기생산을 증가시킬 수 있을 것으로 기대된다. The purpose of this research was to optimize electric current production of sediment microbial fuel cells by injecting glucose and to investigate its impact on microbial communities involved. It was shown that injection of proper concentration of glucose could increase electric current generated from sediment microbial fuel cells. When 1,000 mg/L of glucose, as opposed to higher concentrations, was injected, electric current increased up to 3 times. This increase is mainly attributed to the mutual relationship between fermenting bacteria and exoelectrogenic bacteria. Here the organic acids generated by fermenting bacteria could be utilized by exoelectrogenic bacteria, removing feedback inhibition caused by the organic acids. When glucose was injected, the population of Clostridium increased as to ferment injected glucose. Glucose fermentation can have either a positive or negative effect on electric current generation. When exoelectrogenic bacteria may readily utilize the end-product, electric current could increase. However, when the end-product was not readily removed, then detrimental chemical reactions (pH decrease, methane generation, organic acids accumulation) occurred: exoelctrogenic bacteria population declined and non-microbial fuel cell related microorganisms prospered. By injecting a proper concentration of glucose, a mutual relationship between fermenting bacteria, such as Clostridium, and exoelectrogenic bacteria, such as Geobacter, should be fulfilled in order to increase electricity production in mixed cultures of microorganisms collected from the aquatic sediments.

Using multi-parameter flow cytometry as a novel approach for physiological characterization of bacteria in microbial fuel cells

This work demonstrates, for the first time, the potential of using multi-parameter flow cytometry to monitor changes in the microbial cytoplasmic membrane integrity and polarization during microbial fuel cells (MFC) operation. Such information is crucial to follow the dynamics of bacteria colonization of the electrodes and their viability maintenance during electrical current production. Interestingly, the results show that during voltage production, the electrostatic gradients of the bacteria cytoplasmic membrane are disturbed, leading to depolarization of a subpopulation (where less than 40% of the cells were polarized). Once the voltage dropped, due to substrate limitation, several cells in the anode supernatant restored their polarized state. This process was reversible and observed over more than 4 cycles of fresh substrate addition. Similar power outputs induced similar membrane polarization results, regardless of the substrate used. The percentage of non-viable cells was maintained constant during current production. This study opens new opportunities to monitor cell behavior, and thus increase the knowledge of dynamic mechanisms responsible for current production at the individual cell level. This technique could be of great interest for the development of new MFC configurations and optimization of MFC operation conditions toward increased performance.