Bioelectrochemistry (v.78, #1)

Table of Contents (vii-viii).

Editorial by Alain Bergel; Damien Feron; Hans-Curt Flemming (1).

Is resistance futile? Changing external resistance does not improve microbial fuel cell performance by Delina Y. Lyon; Francois Buret; Timothy M. Vogel; Jean-Michel Monier (2-7).
Microbial fuel cells (MFCs) show promise as an alternative to conventional batteries for point source electricity generation. A better understanding of the relationship between the microbiological and electrical aspects of fuels cells is needed prior to successful MFC application. Here, we observed the effects of external resistance on power production and the anodic biofilm community structure. Large differences in the external resistance affected both power production and microbial community structure. After the establishment of the anodic microbial community, change in external resistance (from low to high and vice versa) changed the anodic microbial community structure, but the resulting community did not resemble the communities established at that same external resistance. Different microbial community structures, established under different external resistances, resulted in similar power production, demonstrating the flexibility of the MFC system.
Keywords: Microbial fuel cells; Biofilm community structure; Single-chamber; Wastewater;

Model based evaluation of the effect of pH and electrode geometry on microbial fuel cell performance by Cristian Picioreanu; Mark C.M. van Loosdrecht; Thomas P. Curtis; Keith Scott (8-24).
A mathematical model for microbial fuel cells (MFC) which integrates macro-scale time-dependent mass balances for solutes and biomass in the anodic liquid with a micro-scale individual-based two-dimensional biofilm model is developed. Computational fluid dynamics and Nernst–Plank mass and charge balances with diffusion, electromigration, convection and electroneutrality in the biofilm are combined to calculate spatial pH distribution and solutes speciation. Soluble redox mediators are the electron shuttle between microbial cells and the electrode. The model describes the generally observed variations of pH, solute concentrations and electrical current produced over time from electroactive biofilms. Numerical simulations also show the effect of bicarbonate buffer and mass transfer through the proton exchange membrane on the microbial population within a mixed anaerobic digestion sludge consortium of methanogenic and electrogenic microorganisms. In addition, the new modeling approach opens the way to study the influence of fluid flow and any two- or three-dimensional biofilm and electrode geometry on the MFC output parameters. Hydrodynamic calculations show that porous bio-electrodes with greater specific surface area do not necessarily produce more current, as long as convection through the pores is absent. An innovative model solution strategy combines in a very efficient and flexible way MATLAB, COMSOL finite element and Java codes.
Keywords: Microbial fuel cell; Model; Biofilm; Porous electrode; Redox mediator;

ATR-SEIRAs characterization of surface redox processes in G. sulfurreducens by Juan Pablo Busalmen; Abraham Esteve-Nuñez; Antonio Berná; Juan Miguel Feliu (25-29).
In this work we report on the occurrence of at least two different redox pairs on the cell surface of the electrogenic bacteria Geobacter sulfurreducens adsorbed on gold that are expressed in response to the polarization potential. As previously reported on graphite (Environ. Sci. Technol. 42 (2008) 2445) a typical low potential redox pair is found centered at around − 0.06 V when cells are polarized for a few hours at 0.2 V, while a new pair centered at around 0.38 V is expressed upon polarization at 0.6 V. Reversible changes in the IR band pattern of whole cells where obtained by Attenuated Total Reflection-Surface Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) upon potential cycling around both redox pairs. Changes clearly resemble the electrochemical turnover of oxidized/reduced states in c-type cytochromes, thus evidencing the nature of the involved molecules. The expression of external cytochromes in response to the potential of the electron acceptor suggests the existence of alternative pathways of electron transport with different energy yield, though it remains to be demonstrated.
Keywords: Electrode-reducing bacteria; Electron transport mechanism; Cytochromes;

Electrochemical activity and bacterial diversity of natural marine biofilm in laboratory closed-systems by Marco Faimali; Elisabetta Chelossi; Giovanni Pavanello; Alessandro Benedetti; Ilse Vandecandelaere; Paul De Vos; Peter Vandamme; Alfonso Mollica (30-38).
Even if a widely shared mechanism actually does not exist, it is now generally accepted that, in aerobic conditions, marine electrochemically active biofilms (MEABs) induce faster oxygen reduction on stainless steel immersed in seawater. This phenomenon has been widely studied, but nearly all the experiments found in literature have been conducted in open-systems (i.e. experimental environments where seawater is constantly renewed). In this work we tried to obtain, in open circuit and potentiostatic conditions, MEABs in different laboratory closed-systems without water renewal (mesocosms), in order to verify the relationship between electrochemical activity and biofilm composition. The diversity of the microbial populations of biofilms obtained by our new kind of approach was examined by the DGGE technique (denaturing gradient gel electrophoresis). MEABs were obtained in all the mesocosms from 2000 to 2 L, showing in some cases electrochemical performances comparable to those of open-systems, and a very high genetic variability. Our DGGE results underline the difficulty in finding a correlation between electrochemical activity and composition of microbial populations.
Keywords: Electrochemically active biofilms; Ennoblement; Cathodic potential; Cathodic depolarization; Bacterial diversity;

Microbial electrolysis cell with a microbial biocathode by Adriaan W. Jeremiasse; Hubertus V.M. Hamelers; Cees J.N. Buisman (39-43).
This study demonstrates, for the first time, the proof-of-principle of an MEC in which both the anodic and cathodic reaction are catalyzed by microorganisms. No expensive chemical catalysts, such as platinum, are needed. Two of these MECs were simultaneously operated and reached a maximum of 1.4 A/m2 at an applied cell voltage of 0.5 V. At a cathode potential of − 0.7 V, the biocathode in the MECs had a higher current density (MEC 1: 1.9 A/m2, MEC 2: 3.3 A/m2) than a control cathode (0.3 A/m2, graphite felt without biofilm) in an electrochemical half cell. This indicates that hydrogen production is catalyzed at the biocathode, likely by electrochemically active microorganisms. The cathodic hydrogen recovery was 17% for MEC 1 and 21% for MEC 2. Hydrogen losses were ascribed to diffusion through membrane and tubing, and methane formation. After 1600 h of operation, the current density of the MECs had decreased to 0.6 A/m2, probably caused by precipitation of calcium phosphate on the biocathode. The slow deteriorating effect of calcium phosphate, and the production of methane show the importance of studying the combination of bioanode and biocathode in one electrochemical cell, and of studying long term performance of such an MEC.
Keywords: MEC; Biocathode; Hydrogen; Microbial fuel cell; MFC; Scaling;

Improved energy output levels from small-scale Microbial Fuel Cells by I. Ieropoulos; J. Greenman; C. Melhuish (44-50).
This study reports on the findings from the investigation into small-scale (6.25 mL) MFCs, connected together as a network of multiple units. The MFCs contained unmodified (no catalyst) carbon fibre electrodes and for initial and later experiments, a standard ion-exchange membrane for the proton transfer from the anode to the cathode. The anode microbial culture was of the type commonly found in domestic wastewater fed with 5 mM acetate as the carbon-energy (C/E) source. The cultures were mature and acclimatised in the MFC environment for approximately 2 months before being re-inoculated in the experimental MFC units. The cathode was of the O2 diffusion open-to-air type, but for the purposes of the polarization experiments, the cathodic electrodes were moistened with ferricyanide. The main aim of this study was to investigate the effects of connecting multiples of MFC units together as a method of scale up by using stacks and comparison of the effects of different PEM and MFC structural materials on the performance. Impedance matching (maximum-power-transfer) was achieved through calculation of total internal impedance. Three different PEM materials were compared in otherwise identical MFCs in sets of three. For individual isolated MFCs, Hyflon E87-03 was shown to produce twice, whilst E87-10 produced approximately 1.5 times the power output of the control (standard) PEM. However, when MFCs containing the E87-03 and E87-10 membranes were connected in a stack, the system suffered from severe instability and cell reversal. To study the effects of the various polymeric MFC structural materials, four small-scale units were manufactured from three different types of RP material; acrylo-butadiene-styrene coated (ABS), ABS coated (ABS-MEK) and polycarbonate (polyC). The stack of four (4) units prototyped out of polyC produced the highest power density values in polarisation experiments (80 mW/m2).
Keywords: MFC stacks; Small-scale MFCs; PEM; Cell reversal; Rapid-prototype;

Marine aerobic biofilm as biocathode catalyst by Benjamin Erable; Ilse Vandecandelaere; Marco Faimali; Marie-Line Delia; Luc Etcheverry; Peter Vandamme; Alain Bergel (51-56).
Stainless steel electrodes were immersed in open seawater and polarized for some days at − 200 mV vs. Ag/AgCl. The current increase indicated the formation of biofilms that catalysed the electrochemical reduction of oxygen. These wild, electrochemically active (EA) biofilms were scraped, resuspended in seawater and used as the inoculum in closed 0.5 L electrochemical reactors. This procedure allowed marine biofilms that are able to catalyse oxygen reduction to be formed in small, closed small vessels for the first time. Potential polarisation during biofilm formation was required to obtain EA biofilms and the roughness of the surface favoured high current values. The low availability of nutrients was shown to be a main limitation. Using an open reactor continuously fed with filtered seawater multiplied the current density by a factor of around 20, up to 60 µA/cm2, which was higher than the current density provided in open seawater by the initial wild biofilm. These high values were attributed to continuous feeding with the nutrients contained in seawater and to suppression of the indigenous microbial species that compete with EA strains in natural open environments. Pure isolates were extracted from the wild biofilms and checked for EA properties. Of more than thirty different species tested, only Winogradskyella poriferorum and Acinetobacter johsonii gave current densities of respectively 7% and 3% of the current obtained with the wild biofilm used as inoculum. Current densities obtained with pure cultures were lower than those obtained with wild biofilms. It is suspected that synergetic effects occur in whole biofilms or/and that wild strains may be more efficient than the cultured isolates.
Keywords: Seawater; Stainless steel; Oxygen reduction; Biocathode; Microbial fuel cell;

Potential application of Candida melibiosica in biofuel cells by Yolina Hubenova; Mario Mitov (57-61).
Various prokaryote species have been widely studied for microbial fuel cell (MFC) application. However, the information about yeast utilization into biofuel cells is still scanty. The aim of this investigation is to verify if Candida melibiosica 2491, a yeast strain, possessing high phytase activity, could be applied as a biocatalyst in a yeast biofuel cell. The microbiological requirements were coupled with the electrochemical ones tracing main biochemical pathway metabolites such as different carbohydrate and inorganic phosphates and their assimilation with time. The obtained results show that from the three carbohydrates investigated — glucose, fructose and sucrose, fructose is the most suitable for the yeast cultivation. The presence of yeast extract and peptone improves the performance into the biofuel cell. The relationship between the yeast cell amount and the biofuel cell characteristics was determined. Analyses showed that electricity was generated by the yeast culture even in the absence of an artificial mediator. The addition of methylene blue at concentrations higher than 0.1 mM improves the current and power density output. The obtained experimental results proved that C. melibiosica 2491 belongs to the electrogenic strains.
Keywords: Biofuel cells; Yeast; Candida melibiosica; Biocatalyst; Electrogens;

Bacterial diversity of the cultivable fraction of a marine electroactive biofilm by Ilse Vandecandelaere; Olivier Nercessian; Marco Faimali; Eveline Segaert; Alfonso Mollica; Wafa Achouak; Paul De Vos; Peter Vandamme (62-66).
Stainless steel electrodes were cathodically polarized at − 200 mV versus an Ag/AgCl reference electrode in natural seawater in order to produce current. The current increased and stabilized at 0.5 A/m2 in less than 10 days. The cultivable fraction of the microbial biofilm population formed on the surface of the current harvesting cathodes was examined by culture dependent techniques.Three hundred fifty six isolates were obtained. They were primarily characterized by whole cell fatty acid methyl ester analysis followed by 16S rRNA gene sequencing. The results showed that the isolates represented different phylogenetic groups including members of the Alpha- and Gammaproteobacteria, the phylum Firmicutes, the family Flavobacteriacae and the phylum Actinobacteria.Denaturing gradient gel electrophoresis demonstrated that the microbial population of the biofilm formed on the cathode and of the surrounding seawater comprised the same dominant members.This study demonstrated that the cultivable microbial fraction of a marine electroactive biofilm is phylogenetically highly diverse.
Keywords: Electroactive biofilm; Polyphasic taxonomic analysis;

Towards implementation of a benthic microbial fuel cell in lake Furnas (Azores): Phylogenetic affiliation and electrochemical activity of sediment bacteria by Gilberto Martins; Luciana Peixoto; Daniel C. Ribeiro; Pier Parpot; António G. Brito; Regina Nogueira (67-71).
This work was conducted to examine the composition and electrochemical activity of the bacterial community inhabiting lake Furnas sediments (Azores). Fingerprinting analysis of the bacterial 16S rRNA gene fragment was done by denaturing gradient gel electrophoresis. The sequences retrieved from lake Furnas sediments were affiliated to Bacteroidetes/Chlorobi group, Chloroflexi, Alfa-, Delta-, and Gamma-subclasses of Proteobacteria, Cyanobacteria, and Gemmatimonadetes. A cyclic voltammetric study was carried out with an enriched sediment bacterial suspension in a standard two chamber electrochemical cell using a carbon paper anode. Cyclic voltammograms (scan rate of 50 mV/s) showed the occurrence of oxidation–reduction reactions at the carbon anode surface. The benthic microbial fuel cell operated with lake Furnas sediments presented a low power density (1 mW/m2) indicating that further work is required to optimise its power generation. These results suggested that sediment bacteria, probably from the Delta- and Gamma-subclasses of Proteobacteria, were electroactive under tested conditions.
Keywords: Sediment microbial community; Cyclic voltammetry; Benthic microbial fuel cell;

Directly applicable microbial fuel cells in aeration tank for wastewater treatment by Jaehwan Cha; Soojung Choi; Hana Yu; Hyosoo Kim; Changwon Kim (72-79).
The application of microbial fuel cell (MFC) for wastewater treatment is a promising strategy for the simultaneous treatment of pollutants and generation of electricity. However, for practical application, there are several limitations to the MFC that involve biological and engineering aspects. In this study, a single-chambered MFC able to submerge into the aeration tank of the activated sludge process was developed to optimize the cell configuration and electrode materials. Among four MFCs with different electrode materials, the MFC with a graphite felt (GF) anode and a GF cathode showed the highest power density of 16.7 W m− 3 and the lowest internal resistance of 17 Ω. When the blower was stopped to evaluate the effect of mixing intensity, the concentration of dissolved oxygen nevertheless remained at 8 mg O2 L− 1, and the cell voltage of MFCs dropped rapidly and reached 30 mV. However, the cell voltage immediately returned to around 200 mV after the blowing of air. The MFCs with a GF cathode were sensitive to mixing intensity. At the very low concentration of 0.2 mg O2 L− 1, the cell voltage remained at a high level of 200 mV when the oxygen close to the cathode remained and mixing was sufficient.
Keywords: Microbial fuel cell; Wastewater treatment; Electrode material; Mixing intensity; Bio-cathode;

A rapid methodology using fatty acid methyl esters to profile bacterial community structures in microbial fuel cells by Kristina Y. Nelson; Behrooz Razban; Dena W. McMartin; D. Roy Cullimore; Takaya Ono; Patrick D. Kiely (80-86).
A new methodology is presented here as an effective, preliminary technique for the identification of indigenous aerobic and facultatively anaerobic bacterial communities found within microbial fuel cells (MFCs). The dual-phased method, named Rapid Agitation Static Incubation–Microbial Identification, or RASI–MIDI, is comprised of rapidly agitating the sample within a SLYM-BART tester followed by stationary incubation which produces a biomass that is subjected to extraction of methyl ester fatty acids. These distinctive fatty acid profiles represent a bacterial community fingerprint unique to the MFC, and are stored in a library for analysis. A total of 84 samples were analyzed for bacterial community structures from seven different groups of MFCs, with each MFC group comprised of a different bacterial community. Results showed that comparisons of replicate MFCs comprising the same bacterial communities generated high similarity index (SI) numbers (SI values ranging from 0.77 to 0.97), indicating highly correlated fatty acid profiles. In contrast, comparisons of MFCs having known dissimilar community structures did not consistently generate SI values in the analysis considered to be a significant match. It was found that this protocol described herein uniquely and accurately produced MFC fatty acid profiles contained in bacterial communities and thus provides a potential method for routinely studying MFC bacterial community fingerprints.
Keywords: Microbial fuel cells; Bacterial community structure; Fatty acid; FAME analysis; Bacterial communities;

A MFC-based biosensor can act as online toxicity sensor. Electrical current is a direct linear measure for metabolic activity of electrochemically active microorganisms. Microorganisms gain energy from anodic overpotential and current strongly depends on anodic overpotential. Therefore control of anodic overpotential is necessary to detect toxic events and prevent false positive alarms. Anodic overpotential and thus current is influenced by anode potential, pH, substrate and bicarbonate concentrations. In terms of overpotential all factor showed a comparable effect, anode potential 1.2% change in current density per mV, pH 0.43%/mV, bicarbonate 0.75%/mV and acetate 0.8%/mV. At acetate saturation the maximum acetate conversion rate is reached and with that a constant bicarbonate concentration. Control of acetate and bicarbonate concentration can be less strict than control of anode potential and pH. Current density changes due to changing anode potential and pH are in the same order of magnitude as changes due to toxicity. Strict control of pH and anode potential in a small range is required.The importance of anodic overpotential control for detection of toxic compounds is shown. To reach a stable baseline current under nontoxic conditions a MFC-based biosensor should be operated at controlled anode potential, controlled pH and saturated substrate concentrations.
Keywords: Biosensor; MFC; Overpotential control;

It is known that electron transfer processes exist between microorganisms and electrodes. Many anaerobic bacteria, which can transfer electrons to solid electrodes, had been identified. However, little attention has been paid to the interactions between aerobic biofilms and electrodes. In this study, marine biofilms formation on graphite electrodes was characterized by open circuit potential and field emission scanning electron microscopy. Electron transfer between marine aerobic biofilms and graphite electrodes was investigated primarily by cyclic voltammograms and electrochemical impedance spectroscopy techniques. Herein, we suggest that marine biofilms are a kind of conductive biofilms that can transfer electrons to graphite electrodes under anaerobic and aerobic conditions. Some cytochrome species in bacterial biofilms may play a key role in the electron transfer process.
Keywords: Aerobic biofilms; Electron transfer; Graphite electrode;