Microbial fuel cell biosensor for in situ assessment of microbial activity

Microbial fuel cell (MFC)-based sensing was explored to provide useful information for the development of an approach to in situ monitoring of substrate concentration and microbial respiration rate. The ability of a MFC to provide meaningful information about in situ microbial respiration and analyte concentration was examined in column systems, where Geobacter sulfurreducens used an external electron acceptor (an electrode) to metabolize acetate. Column systems inoculated with G. sulfurreducens were operated with influent media at varying concentrations of acetate and monitored for current generation. Current generation was mirrored by bulk phase acetate concentration, and a correlation (R(2)=0.92) was developed between current values (0-0.30 mA) and acetate concentrations (0-2.3 mM). The MFC-system was also exposed to shock loading (pulses of oxygen), after which electricity production resumed immediately after media flow recommenced, underlining the resilience of the system and allowing for additional sensing capacity. Thus, the electrical signal produced by the MFC-system provided real-time data for electron donor availability and biological activity. These results have practical implications for development of a biosensor for inexpensive real-time monitoring of in situ bioremediation processes, where MFC technology provides information on the rate and nature of biodegradation processes.     

The effect of flavin electron shuttles in microbial fuel cells current production

The effect of electron shuttles on electron transfer to microbial fuel cell (MFC) anodes was studied in systems where direct contact with the anode was precluded. MFCs were inoculated with Shewanella cells, and flavins used as the electron shuttling compound. In MFCs with no added electron shuttles, flavin concentrations monitored in the MFCs' bulk liquid increased continuously with FMN as the predominant flavin. The maximum concentrations were 0.6 μM for flavin mononucleotide and 0.2 μM for riboflavin. In MFCs with added flavins, micro-molar concentrations were shown to increase current and power output. The peak current was at least four times higher in MFCs with high concentrations of flavins (4.5–5.5 μM) than in MFCs with low concentrations (0.2–0.6 μM). Although high power outputs (around 150 mW/m²) were achieved in MFCs with high concentrations of flavins, a Clostridium-like bacterium along with other reactor limitations affected overall coulombic efficiencies (CE) obtained, achieving a maximum CE of 13%. Electron shuttle compounds (flavins) permitted bacteria to utilise a remote electron acceptor (anode) that was not accessible to the cells allowing current production until the electron donor (lactate) was consumed.     

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.     

A lithotrophic microbial fuel cell operated with pseudomonads‐dominated iron‐oxidizing bacteria enriched at the anode

In this study, we attempted to enrich neutrophilic iron bacteria in a microbial fuel cell (MFC)‐type reactor in order to develop a lithotrophic MFC system that can utilize ferrous iron as an inorganic electron donor and operate at neutral pHs. Electrical currents were steadily generated at an average level of 0.6 mA (or 0.024 mA cm^–2 of membrane area) in reactors initially inoculated with microbial sources and operated with 20 mM Fe²⁺ as the sole electron donor and 10 ohm external resistance; whereas in an uninoculated reactor (the control), the average current level only reached 0.2 mA (or 0.008 mA cm^–2 of membrane area). In an inoculated MFC, the generation of electrical currents was correlated with increases in cell density of bacteria in the anode suspension and coupled with the oxidation of ferrous iron. Cultivation‐based and denaturing gradient gel electrophoresis analyses both show the dominance of some Pseudomonas species in the anode communities of the MFCs. Fluorescent in‐situ hybridization results revealed significant increases of neutrophilic iron‐oxidizing bacteria in the anode community of an inoculated MFC. The results, altogether, prove the successful development of a lithotrophic MFC system with iron bacteria enriched at its anode and suggest a chemolithotrophic anode reaction involving some Pseudomonas species as key players in such a system. The system potentially offers unique applications, such as accelerated bioremediation or on‐site biodetection of iron and/or manganese in water samples.     

Gaseous pollutant treatment and electricity generation in microbial fuel cells (MFCs) utilising redox mediators

Microbial fuel cell (MFC) is an emerging technology for sustainable energy generation and waste treatment. This paper reviews the potential of a gaseous substrate when it is combined with a mediator in an MFC to generate electricity and to treat toxic gaseous pollutants. Most MFCs for waste water treatment often cannot use mediator to enhance the electron transfer from the microbe to the anode because of the difficulty in recovering the expensive and potentially toxic compound. Combining gas feeds with mediators is possible since the soluble mediator would remain in the anode chamber as the gas passes through the reactor. In addition, this type of MFC is possible to be integrated into an anaerobic biofiltration system (BF-MFC), where the biofilter removes the gaseous contaminant and produces the reduced mediator and the MFC produces the electricity and recycles the reoxidised mediator. This paper also talks about the past research on gaseous feed MFCs, and reviews the mechanism and strategies of electron transfer in MFC using redox mediator. The advantages, process parameters and challenges of BF-MFC are discussed. This knowledge is very much required in the design and scale up of BF-MFC. This paper will be useful for those who work in the area of gaseous pollutant treatment and electricity generation.     

On the dynamic response of the anode in microbial fuel cells

A study of the dynamic response of a microbial fuel cell (MFC) using membrane electrode assemblies (MEAs) designed for air breathing cathode operation is reported. The MFC used four MEAs simultaneously and has a low internal resistance. An increased concentration of glucose produced a non-linear increase in the maximum current reached. The time to reach the maximum current increased with increasing glucose concentrations of 1-7 mM; varying from approximately 2.4 to 4.2h. The rate at which the current density increased with time was the same for all glucose concentrations up to current densities close to the maximum values. The peak power density varied approximately linearly with glucose concentrations from 2 to 77 mW/m(2) (1-7 mM) with a 1 kΩ resistance. The cell response appeared to be linked to a slow process of fuel transport to the bacteria and their metabolic processes. The dynamic response of the anode was analysed in terms of a substrate mass transport model. The application of different current ranges did not significantly change the dynamic response of either the anode community or the MFC polarization characteristics. Thus, it is likely that the bacterial communities that form under MFC operation contain sufficiently "dominant" electro-active species that are capable of producing high power for MFCs.     

Microbial fuel cells: novel biotechnology for energy generation

Microbial fuel cells (MFCs) provide new opportunities for the sustainable production of energy from biodegradable, reduced compounds. MFCs function on different carbohydrates but also on complex substrates present in wastewaters. As yet there is limited information available about the energy metabolism and nature of the bacteria using the anode as electron acceptor; few electron transfer mechanisms have been established unequivocally. To optimize and develop energy production by MFCs fully this knowledge is essential. Depending on the operational parameters of the MFC, different metabolic pathways are used by the bacteria. This determines the selection and performance of specific organisms. Here we discuss how bacteria use an anode as an electron acceptor and to what extent they generate electrical output. The MFC technology is evaluated relative to current alternatives for energy generation.     

产电微生物及微生物燃料电池最新研究进展

新型产电微生物(Electricigens)的发现,使得微生物燃料电池概念的内涵发生了根本性的变化,展现了广阔的应用前景。这种微生物能够以电极作为唯一电子受体,把氧化有机物获得的电子通过电子传递链传递到电极产生电流,同时微生物从中获得能量而生长。这种代谢被认为是一种新型微生物呼吸方式。以这种新型微生物呼吸方式为基础的微生物燃料电池可以同时进行废水处理和生物发电,有望可以把废水处理发展成一个有利可图的产业,是MFC最有发展前景的方向。     

Greenhouse gas emissions from rice microcosms amended with a plant microbial fuel cell

Methane (CH₄) release from wetlands is an important source of greenhouse gas emissions. Gas exchange occurs mainly through the aerenchyma of plants, and production of greenhouse gases is heavily dependent on rhizosphere biogeochemical conditions (i.e. substrate availability and redox potential). It is hypothesized that by introducing a biocatalyzed anode electrode in the rhizosphere of wetland plants, a competition for carbon and electrons can be invoked between electrical current-generating bacteria and methanogenic Archaea. The anode electrode is part of a bioelectrochemical system (BES) capable of harvesting electrical current from microbial metabolism. In this work, the anode of a BES was introduced in the rhizosphere of rice plants (Oryza sativa), and the impact on methane emissions was monitored. Microbial current generation was able to outcompete methanogenic processes when the bulk matrix contained low concentrations of organic carbon, provided that the electrical circuit with the effective electroactive microorganisms was in place. When interrupting the electrical circuit or supplying an excess of organic carbon, methanogenic metabolism was able to outcompete current generating metabolism. The qPCR results showed hydrogenotrophic methanogens were the most abundant methanogenic group present, while mixotrophic or acetoclastic methanogens were hardly detected in the bulk rhizosphere or on the electrodes. Competition for electron donor and acceptor were likely the main drivers to lower methane emissions. Overall, electrical current generation with BESs is an interesting option to control CH₄ emissions from wetlands but needs to be applied in combination with other mitigation strategies to be successful and feasible in practice.     

Glycerol-fed microbial fuel cell with a co-culture of <Emphasis Type="Italic">Shewanella oneidensis</Emphasis> MR-1 and <Emphasis Type="Italic">Klebsiella pneumonae</Emphasis> J2B

Glycerol is an attractive feedstock for bioenergy and bioconversion processes but its use in microbial fuel cells (MFCs) for electrical energy recovery has not been investigated extensively. This study compared the glycerol uptake and electricity generation of a co-culture of Shewanella oneidensis MR-1 and Klebsiella pneumonia J2B in a MFC with that of a single species inoculated counterpart. Glycerol was metabolized successfully in the co-culture MFC (MFC-J&M) with simultaneous electricity production but it was not utilized in the MR-1 only MFC (MFC-M). A current density of 10 mA/m² was obtained while acidic byproducts (lactate and acetate) were consumed in the co-culture MFC, whereas they are accumulated in the J2B-only MFC (MFC-J). MR-1 was distributed mainly on the electrode in MFC-J&M, whereas most of the J2B was observed in the suspension in the MFC-J reactor, indicating that the co-culture of both strains provides an ecological driving force for glycerol utilization using the electrode as an electron acceptor. This suggests that a co-culture MFC can be applied to electrical energy recovery from glycerol, which was previously known as a refractory substrate in a bioelectrochemical system.     

Enhanced wastewater treatment efficiency through microbially catalyzed oxidation and reduction: synergistic effect of biocathode microenvironment

Microbially catalyzed treatment of wastewater was evaluated in both the anode and cathode chambers in dual chambered microbial fuel cell (MFC) under varying biocathode microenvironment. MFC operation with aerobic biocathode showed significant increment in both TDS (cathode, 90.2±1%; anode, 39.7±0.5%) and substrate (cathode, 98.07±0.06%; anode, 96.2±0.3%) removal compared to anaerobic biocathode and abiotic cathode operations (COD, 80.25±0.3%; TDS, 30.5±1.2%). Microbially catalyzed reduction of protons and electrons at cathode will be higher during aerobic biocathode operation which leads to gradual substrate removal resulting in stable bio-potential for longer periods facilitating salts removal. Bio-electro catalytic behavior showed higher exchange current density during aerobic biocathode operation resulting in induced electrochemical oxidation which supports the enhanced treatment. Anaerobic biocathode operation depicted relatively less TDS removal (anode, 16.35%; cathode, 16.04%) in both the chambers in spite of good substrate degradation (anode, 84%; cathode, 87.39%). Both the chambers during anaerobic biocathode operation competed as electron donors resulting in negligible bio-potential development.     

Analysis of ammonia loss mechanisms in microbial fuel cells treating animal wastewater

Ammonia losses during swine wastewater treatment were examined using single- and two-chambered microbial fuel cells (MFCs). Ammonia removal was 60% over 5 days for a single-chamber MFC with the cathode exposed to air (air-cathode), versus 69% over 13 days from the anode chamber in a two-chamber MFC with a ferricyanide catholyte. In both types of systems, ammonia losses were accelerated with electricity generation. For the air-cathode system, our results suggest that nitrogen losses during electricity generation were increased due to ammonia volatilization with conversion of ammonium ion to the more volatile ammonia species as a result of an elevated pH near the cathode (where protons are consumed). This loss mechanism was supported by abiotic tests (applied voltage of 1.1 V). In a two-chamber MFC, nitrogen losses were primarily due to ammonium ion diffusion through the membrane connecting the anode and cathode chambers. This loss was higher with electricity generation as the rate of ammonium transport was increased by charge transfer across the membrane. Ammonia was not found to be used as a substrate for electricity generation, as intermittent ammonia injections did not produce power. The ammonia-oxidizing bacterium Nitrosomonas europaea was found on the cathode electrode of the single-chamber system, supporting evidence of biological nitrification, but anaerobic ammonia-oxidizing bacteria were not detected by molecular analyses. It is concluded that ammonia losses from the anode chamber were driven primarily by physical-chemical factors that are increased with electricity generation, although some losses may occur through biological nitrification and denitrification.     

Long-term operation of double chambered microbial fuel cell for bio-electro denitrification

The main aim of this study is to investigate the performance of organic oxidation and denitrification of the system under long-term operation. The MFC reactor was operated in continuous mode for 180 days. Nitrate was successfully demonstrated as terminal electron acceptor, where nitrate was reduced at the cathode using electron provided by acetate oxidation at the anode. The removal efficiencies of chemical oxygen demand (COD) and nitrate were higher in the closed circuit system than in open circuit system. Both COD and nitrate reduction improved with the increase of organic loading and subsequently contributed to higher power output. The maximum nitrate removal efficiency was 88 ± 4 % (influent of 141 ± 14 mg/L). The internal resistant was 50 Ω, which was found to be low for a double chambered MFC. The maximum power density was 669 mW/m³ with current density of 3487 mA/m³.     

Modeling the Impact of Interspecies Competition on Performance of a Microbial Fuel Cell

Previous models of biofilms growing in a microbial fuel cell (MFC) have primarily focused on modeling a single growth mechanism: growth via a conductive biofilm matrix, or growth utilizing diffusible electron shuttles or mediators. In this work, we implement both flavors of models in order to explore the competition for space and nutrients in a MFC biofilm populated by both species types. We find that the optimal growth conditions are for bacteria that utilize conductive EPS provided a minimal energy used to create the EPS matrix. Mediator-utilizing bacteria do have favorable niche regions, most notably close to the anode and where exposed to the bulk inflow, where oxidized mediator is readily available.     

Bioelectricity production from wastewater treatment in dual chambered microbial fuel cell (MFC) using selectively enriched mixed microflora: Effect of catholyte

The performance of aerated and ferricyanide catholytes on the bioelectricity production was evaluated in dual chambered microbial fuel cell (MFC) (mediatroless anode; graphite electrodes) employing selectively enriched H(2) producing mixed consortia as anodic inoculum. Two MFCs with aerated catholyte (MFC(AC)) and ferricyanide catholyte (MFC(FC)) were operated separately to elucidate the difference in power generation potential and carbon removal efficiency under similar operating conditions [ambient pressure; room temperature (28+/-2 degrees C); acidophilic microenvironment (pH 6)]. The experimental data demonstrated the feasibility of in situ bioelectricity generation along with wastewater treatment. Effective power generation and substrate removal efficiency was documented in the fuel cell operated with ferricyanide catholyte (586 mV; 2.37 mA; 0.559 kg COD/m(3) day) than aerated catholyte (572 mV; 1.68 mA; 0.464 kg COD/m(3) day). Maximum power yield (0.635 W/kg COD(R) and 0.440 W/kg COD(R)) and current density (222.59 mA/m(2) and 190.28 mA/m(2)) was observed at 100 Omega resistor with ferricyanide and aerated catholytes, respectively. The study documented both wastewater treatment and electricity production through direct conversion of H(2) in a single system.     

Anodes Stimulate Anaerobic Toluene Degradation via Sulfur Cycling in Marine Sediments

Hydrocarbons released during oil spills are persistent in marine sediments due to the absence of suitable electron acceptors below the oxic zone. Here, we investigated an alternative bioremediation strategy to remove toluene, a model monoaromatic hydrocarbon, using a bioanode. Bioelectrochemical reactors were inoculated with sediment collected from a hydrocarbon-contaminated marine site, and anodes were polarized at 0 mV and +300 mV (versus an Ag/AgCl [3 M KCl] reference electrode). The degradation of toluene was directly linked to current generation of up to 301 mA m⁻² and 431 mA m⁻² for the bioanodes polarized at 0 mV and +300 mV, respectively. Peak currents decreased over time even after periodic spiking with toluene. The monitoring of sulfate concentrations during bioelectrochemical experiments suggested that sulfur metabolism was involved in toluene degradation at bioanodes. 16S rRNA gene-based Illumina sequencing of the bulk anolyte and anode samples revealed enrichment with electrocatalytically active microorganisms, toluene degraders, and sulfate-reducing microorganisms. Quantitative PCR targeting the α-subunit of the dissimilatory sulfite reductase (encoded by dsrA) and the α-subunit of the benzylsuccinate synthase (encoded by bssA) confirmed these findings. In particular, members of the family Desulfobulbaceae were enriched concomitantly with current production and toluene degradation. Based on these observations, we propose two mechanisms for bioelectrochemical toluene degradation: (i) direct electron transfer to the anode and/or (ii) sulfide-mediated electron transfer.     

Development of Enterobacter aerogenes fuel cells: From in situ biohydrogen oxidization to direct electroactive biofilm

This study described an Enterobacter aerogenes-catalyzed microbial fuel cell (MFC) with a carbon-based anode that exhibited a maximum power density of 2.51 W/m³ in the absence of artificial electron mediators. The MFC was started up rapidly, within hours, and the current generation in the early stage was demonstrated to result from in situ oxidation of biohydrogen produced by E. aerogenes during glucose fermentation. Over periodic replacement of substrate, both planktonic biomass in the culture liquid and hydrogen productivity decreased, while increased power density and coulombic efficiency and decreased internal resistance were unexpectedly observed. Using scanning electron microscopy and cyclic voltammetry, it was found that the enhanced MFC performance was associated with the development of electroactive biofilm on the anodic surface, proposed to involve an acclimation and selection process of E. aerogenes cells under electrochemical tension. The significant advantage of rapid start-up and the ability to develop an electroactive biofilm identifies E. aerogenes as a suitable biocatalyst for MFC applications.     

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.     

Comparison of anode bacterial communities and performance in microbial fuel cells with different electron donors

Microbial fuel cells (MFCs) harness the electrochemical activity of certain microbes for the production of electricity from reduced compounds. Characterizations of MFC anode biofilms have collectively shown very diverse microbial communities, raising ecological questions about competition and community succession within these anode-reducing communities. Three sets of triplicate, two-chamber MFCs inoculated with anaerobic sludge and differing in energy sources (acetate, lactate, and glucose) were operated to explore these questions. Based on 16S rDNA-targeted denaturing gradient gel electrophoresis (DGGE), all anode communities contained sequences closely affiliated with Geobacter sulfurreducens (>99% similarity) and an uncultured bacterium clone in the Bacteroidetes class (99% similarity). Various other Geobacter-like sequences were also enriched in most of the anode biofilms. While the anode communities in replicate reactors for each substrate generally converged to a reproducible community, there were some variations in the relative distribution of these putative anode-reducing Geobacter-like strains. Firmicutes were found only in glucose-fed MFCs, presumably serving the roles of converting complex carbon into simple molecules and scavenging oxygen. The maximum current density in these systems was negatively correlated with internal resistance variations among replicate reactors and, likely, was only minimally affected by anode community differences in these two-chamber MFCs with high internal resistance.     

Effect of different acclimation methods on the performance of microbial fuel cells using phenol as substrate

Single-chamber microbial fuel cells (MFCs) with air-cathode were constructed. MFCs were fed different feedstocks during their inoculation, their role on phenol degradation and MFC performance were investigated. The results showed that the MFC inoculated using glucose exhibited the highest power density (31.3 mW m^?2) when phenol was used as the sole substrate for MFC. The corresponding biodegradation kinetic constant was obtained at 0.035 h^?1, at an initial phenol concentration of 600 mg L^?1. Moreover, the phenol degradation rates in this MFC with closed circuit were 9.8–16.5 % higher than those in MFC with opened circuit. The cyclic voltammograms revealed a different electrochemical activity of the anode biofilms in the MFC, and this led to differences in performance of the MFCs with phenol as sole substrate. These results demonstrated that phenol degradation and power production are affected by current generation and type of acclimation.