Change in microbial communities in acetate- and glucose-fed microbial fuel cells in the presence of light

Power densities produced by microbial fuel cells (MFCs) in natural systems are changed by exposure to light through the enrichment of photosynthetic microorganisms. When MFCs with brush anodes were exposed to light (4000 lx), power densities increased by 8–10% for glucose-fed reactors, and 34% for acetate-fed reactors. Denaturing gradient gel electrophoresis (DGGE) profiles based on the 16S rRNA gene showed that exposure to high light levels changed the microbial communities on the anodes. Based on 16S rRNA gene clone libraries of light-exposed systems the anode communities using glucose were also significantly different than those fed acetate. Dominant bacteria that are known exoelectrogens were identified in the anode biofilm, including a purple nonsulfur (PNS) photosynthetic bacterium, Rhodopseudomonas palustris, and a dissimilatory iron-reducing bacterium, Geobacter sulfurreducens. Pure culture tests confirmed that PNS photosynthetic bacteria increased power production when exposed to high light intensities (4000 lx). These results demonstrate that power production and community composition are affected by light conditions as well as electron donors in single-chamber air-cathode MFCs.     

Influence of external resistance on electrogenesis, methanogenesis, and anode prokaryotic communities in microbial fuel cells

The external resistance (R(ext)) of microbial fuel cells (MFCs) regulates both the anode availability as an electron acceptor and the electron flux through the circuit. We evaluated the effects of R(ext) on MFCs using acetate or glucose. The average current densities (I) ranged from 40.5 mA/m(2) (9,800 Ω) to 284.5 mA/m(2) (150 Ω) for acetate-fed MFCs (acetate-fed reactors [ARs]), with a corresponding anode potential (E(an)) range of -188 to -4 mV (versus a standard hydrogen electrode [SHE]). For glucose-fed MFCs (glucose-fed reactors [GRs]), I ranged from 40.0 mA/m(2) (9,800 Ω) to 273.0 mA/m(2) (150 Ω), with a corresponding E(an) range of -189 to -7 mV. ARs produced higher Coulombic efficiencies and energy efficiencies than GRs over all tested R(ext) levels because of electron and potential losses from glucose fermentation. Biogas production accounted for 14 to 18% of electron flux in GRs but only 0 to 6% of that in ARs. GRs produced similar levels of methane, regardless of the R(ext). However, total methane production in ARs increased as R(ext) increased, suggesting that E(an) might influence the competition for substrates between exoelectrogens and methanogens in ARs. An increase of R(ext) to 9,800 Ω significantly changed the anode bacterial communities for both ARs and GRs, while operating at 970 Ω and 150 Ω had little effect. Deltaproteobacteria and Bacteroidetes were the major groups found in anode communities in ARs and GRs. Betaproteobacteria and Gammaproteobacteria were found only in ARs. Bacilli were abundant only in GRs. The anode-methanogenic communities were dominated by Methanosaetaceae, with significantly lower numbers of Methanomicrobiales. These results show that R(ext) affects not only the E(an) and current generation but also the anode biofilm community and methanogenesis.     

Microbial communities and electrochemical performance of titanium-based anodic electrodes in a microbial fuel cell

Four types of titanium (Ti)-based electrodes were tested in the same microbial fuel cell (MFC) anodic compartment. Their electrochemical performances and the dominant microbial communities of the electrode biofilms were compared. The electrodes were identical in shape, macroscopic surface area, and core material but differed in either surface coating (Pt- or Ta-coated metal composites) or surface texture (smooth or rough). The MFC was inoculated with electrochemically active, neutrophilic microorganisms that had been enriched in the anodic compartments of acetate-fed MFCs over a period of 4 years. The original inoculum consisted of bioreactor sludge samples amended with Geobacter sulfurreducens strain PCA. Overall, the Pt- and Ta-coated Ti bioanodes (electrode-biofilm association) showed higher current production than the uncoated Ti bioanodes. Analyses of extracted DNA of the anodic liquid and the Pt- and Ta-coated Ti electrode biofilms indicated differences in the dominant bacterial communities. Biofilm formation on the uncoated electrodes was poor and insufficient for further analyses. Bioanode samples from the Pt- and Ta-coated Ti electrodes incubated with Fe(III) and acetate showed several Fe(III)-reducing bacteria, of which selected species were dominant, on the surface of the electrodes. In contrast, nitrate-enriched samples showed less diversity, and the enriched strains were not dominant on the electrode surface. Isolated Fe(III)-reducing strains were phylogenetically related, but not all identical, to Geobacter sulfurreducens strain PCA. Other bacterial species were also detected in the system, such as a Propionicimonas-related species that was dominant in the anodic liquid and Pseudomonas-, Clostridium-, Desulfovibrio-, Azospira-, and Aeromonas-related species.     

Degradation of pentachlorophenol with the presence of fermentable and non-fermentable co-substrates in a microbial fuel cell

Pentachlorophenol (PCP) was more rapidly degraded in acetate and glucose-fed microbial fuel cells (MFCs) than in open circuit controls, with removal rates of 0.12 ± 0.01 mg/Lh (14.8 ± 1.0 mg/g-VSS-h) in acetate-fed, and 0.08 ± 0.01 mg/L h (6.9 ± 0.8 mg/g-VSS-h) in glucose-fed MFCs, at an initial PCP concentration of 15 mg/L. A PCP of 15 mg/L had no effect on power generation from acetate but power production was decreased with glucose. Coulombic balances indicate the predominant product was electricity (16.1 ± 0.3%) in PCP-acetate MFCs, and lactate (19.8 ± 3.3%) in PCP-glucose MFCs. Current generation accelerated the removal of PCP and co-substrates, as well as the degradation products in both PCP-acetate and PCP-glucose reactors. While 2,3,4,5-tetrachlorophenol was present in both reactors, tetrachlorohydroquinone was only found in PCP-acetate MFCs. These results demonstrate PCP degradation and power production were affected by current generation and the type of electron donor provided.     

Sampling location of the inoculum is crucial in designing anodes for microbial fuel cells

A Kraft pulp mill effluent was used as the inoculum to form microbial bioanodes under controlled potential at +0.4 V/SCE. Samples were collected at the inlet and outlet of the aerated lagoon of the treatment line. The outlet sample allowed efficient bioanodes to be designed (5.1 A/m²), which included Geobacter and Desulfuromonas sp. in their microbial community. In contrast, the bioanodes formed with the inlet sample did not contain directly connecting anode-respiring bacteria and led to lower currents. It was necessary to re-form this bioanode at lower applied potential (−0.2 V/SCE) to select more efficient electroactive species and increase the current density to 5 A/m².     

Evaluation of hydrolysis and fermentation rates in microbial fuel cells

This study determined the influence of substrate degradation on power generation in microbial fuel cells (MFCs) and microbial community selection on the anode. Air cathode MFCs were fed synthetic medium containing different substrates (acetate, glucose and starch) using primary clarifier sewage as source of electroactive bacteria. The complexity of the substrate affected the MFC performance both for power generation and COD removal. Power output decreased with an increase in substrate complexity from 99 ± 2 mW m⁻² for acetate to 4 ± 2 mW m⁻² for starch. The organic matter removal and coulombic efficiency (CE) of MFCs with acetate and glucose (82% of COD removal and 26% CE) were greater than MFCs using starch (60% of COD removal and 19% of CE). The combined hydrolysis–fermentation rate obtained (0.0024 h⁻¹) was considerably lower than the fermentation rate (0.018 h⁻¹), indicating that hydrolysis of complex compounds limits current output over fermentation. Statistical analysis of microbial community fingerprints, developed on the anode, showed that microbial communities were enriched according to the type of substrate used. Microbial communities producing high power outputs (fed acetate) clustered separately from bacterial communities producing low power outputs (fed complex compounds).     

Zooming on dynamics of marine microbial communities in the phycosphere of Akashiwo sanguinea (Dinophyta) blooms

Characterizing ecological relationships between viruses, bacteria and phytoplankton in the ocean is critical to understanding the ecosystem; however, these relationships are infrequently investigated together. To understand the dynamics of microbial communities and environmental factors in harmful algal blooms (HABs), we examined the environmental factors and microbial communities during Akashiwo sanguinea HABs in the Jangmok coastal waters of South Korea by metagenomics. Specific bacterial species showed complex synergistic and antagonistic relationships with the A. sanguinea bloom. The endoparasitic dinoflagellate Amoebophrya sp. 1 controlled the bloom dynamics and correlated with HAB decline. Among nucleocytoplasmic large DNA viruses (NCLDVs), two Pandoraviruses and six Phycodnaviruses were strongly and positively correlated with the HABs. Operational taxonomic units of microbial communities and environmental factors associated with A. sanguinea were visualized by network analysis: A. sanguinea–Amoebophrya sp. 1 (r = .59, time lag: 2 days) and A. sanguinea–Ectocarpus siliculosus virus 1 in Phycodnaviridae (0.50, 4 days) relationships showed close associations. The relationship between A. sanguinea and dissolved inorganic phosphorus relationship also showed a very close correlation (0.74, 0 day). Microbial communities and the environment changed dynamically during the A. sanguinea bloom, and the rapid turnover of microorganisms responded to ecological interactions. A. sanguinea bloom dramatically changes the environments by exuding dissolved carbohydrates via autotrophic processes, followed by changes in microbial communities involving host‐specific viruses, bacteria and parasitoids. Thus, the microbial communities in HAB are composed of various organisms that interact in a complex manner.     

Acetate oxidation by syntrophic association between Geobacter sulfurreducens and a hydrogen-utilizing exoelectrogen

Anodic microbial communities in acetate-fed microbial fuel cells (MFCs) were analyzed using stable-isotope probing of 16S rRNA genes followed by denaturing gradient gel electrophoresis. The results revealed that Geobacter sulfurreducens and Hydrogenophaga sp. predominated in the anodic biofilm. Although the predominance of Geobacter sp. as acetoclastic exoelectrogens in acetate-fed MFC systems has been often reported, the ecophysiological role of Hydrogenophaga sp. is unknown. Therefore, we isolated and characterized a bacterium closely related to Hydrogenophaga sp. (designated strain AR20). The newly isolated strain AR20 could use molecular hydrogen (H₂), but not acetate, with carbon electrode as the electron acceptor, indicating that the strain AR20 was a hydrogenotrophic exoelectrogen. This evidence raises a hypothesis that acetate was oxidized by G. sulfurreducens in syntrophic cooperation with the strain AR20 as a hydrogen-consuming partner in the acetate-fed MFC. To prove this hypothesis, G. sulfurreducens strain PCA was cocultivated with the strain AR20 in the acetate-fed MFC without any dissolved electron acceptors. In the coculture MFC of G. sulfurreducens and strain AR20, current generation and acetate degradation were the highest, and the growth of strain AR20 was observed. No current generation, acetate degradation and cell growth occurred in the strain AR20 pure culture MFC. These results show for the first time that G. sulfurreducens can oxidize acetate in syntrophic cooperation with the isolated Hydrogenophaga sp. strain AR20, with electrode as the electron acceptor.     

Microbial community differences between propionate-fed microbial fuel cell systems under open and closed circuit conditions

We report the electrochemical characterization and microbial community analysis of closed circuit microbial fuel cells (CC-MFCs) and open circuit (OC) cells continuously fed with propionate as substrate. Differences in power output between MFCs correlated with their polarization behavior, which is related to the maturation of the anodophilic communities. The microbial communities residing in the biofilm growing on the electrode, biofouled cation-exchange membrane and anodic chamber liquor of OC-and CC-MFCs were characterized by restriction fragment length polymorphism screening of 16S rRNA gene clone libraries. The results show that the CC-MFC anode was enriched in several microorganisms related to known electrochemically active and dissimilatory Fe(III) reducing bacteria, mostly from the Geobacter spp., to the detriment of Bacteroidetes abundant in the OC-MFC anode. The results also evidenced the lack of a specific pelagic community in the liquor sample. The biofilm growing on the cation-exchange membrane of the CC-MFC was found to be composed of a low-diversity community dominated by two microaerophilic species of the Achromobacter and Azovibrio genus.     

Convergent development of anodic bacterial communities in microbial fuel cells

Microbial fuel cells (MFCs) are often inoculated from a single wastewater source. The extent that the inoculum affects community development or power production is unknown. The stable anodic microbial communities in MFCs were examined using three inocula: a wastewater treatment plant sample known to produce consistent power densities, a second wastewater treatment plant sample, and an anaerobic bog sediment. The bog-inoculated MFCs initially produced higher power densities than the wastewater-inoculated MFCs, but after 20 cycles all MFCs on average converged to similar voltages (470±20 mV) and maximum power densities (590±170 mW m⁻²). The power output from replicate bog-inoculated MFCs was not significantly different, but one wastewater-inoculated MFC (UAJA3 (UAJA, University Area Joint Authority Wastewater Treatment Plant)) produced substantially less power. Denaturing gradient gel electrophoresis profiling showed a stable exoelectrogenic biofilm community in all samples after 11 cycles. After 16 cycles the predominance of Geobacter spp. in anode communities was identified using 16S rRNA gene clone libraries (58±10%), fluorescent in-situ hybridization (FISH) (63±6%) and pyrosequencing (81±4%). While the clone library analysis for the underperforming UAJA3 had a significantly lower percentage of Geobacter spp. sequences (36%), suggesting that a predominance of this microbe was needed for convergent power densities, the lower percentage of this species was not verified by FISH or pyrosequencing analyses. These results show that the predominance of Geobacter spp. in acetate-fed systems was consistent with good MFC performance and independent of the inoculum source.     

Establishing a core microbiome in acetate-fed microbial fuel cells

Establishing a core microbiome is the first step in understanding and subsequently optimizing microbial interactions in anodic biofilms of microbial fuel cells (MFCs) for increased power, efficiency, and decreased start-up times. In the present study, we used 454 pyrosequencing to demonstrate that a core anodic community would consistently emerge over a period of 4 years given similar conditions. The development and variation across reactor designs of these communities was also explored. The core members present in all high-power generating biofilms were Geobacter, Aminiphilus, Sedimentibacter, Acetoanaerobium, and Spirochaeta, accounting for 72?±?9 % of all genera. Aminiphilus spp., member of the Synergistetes phylum was present at higher abundances than previously reported in any other ecological studies. Results suggest a stable core microbiome in acetate-fed MFCs on both phylogenetic and functional levels.     

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.     

Continuous power generation and microbial community structure of the anode biofilms in a three-stage microbial fuel cell system

A mediator-less three-stage two-chamber microbial fuel cell (MFC) system was developed and operated continuously for more than 1.5 years to evaluate continuous power generation while treating artificial wastewater containing glucose (10 mM) concurrently. A stable power density of 28 W/m³ was attained with an anode hydraulic retention time of 4.5 h and phosphate buffer as the cathode electrolyte. An overall dissolved organic carbon removal ratio was about 85%, and coulombic efficiency was about 46% in this MFC system. We also analyzed the microbial community structure of anode biofilms in each MFC. Since the environment in each MFC was different due to passing on the products to the next MFC in series, the microbial community structure was different accordingly. The anode biofilm in the first MFC consisted mainly of bacteria belonging to the Gammaproteobacteria, identified as Aeromonas sp., while the Firmicutes dominated the anode biofilms in the second and third MFCs that were mainly fed with acetate. Cyclic voltammetric results supported the presence of a redox compound(s) associated with the anode biofilm matrix, rather than mobile (dissolved) forms, which could be responsible for the electron transfer to the anode. Scanning electron microscopy revealed that the anode biofilms were comprised of morphologically different cells that were firmly attached on the anode surface and interconnected each other with anchor-like filamentous appendages, which might support the results of cyclic voltammetry. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Controlling accumulation of fermentation inhibitors in biorefinery recycle water using microbial fuel cells

Background  

Microbial fuel cells (MFC) and microbial electrolysis cells are electrical devices that treat water using microorganisms and convert soluble organic matter into electricity and hydrogen, respectively. Emerging cellulosic biorefineries are expected to use large amounts of water during production of ethanol. Pretreatment of cellulosic biomass results in production of fermentation inhibitors which accumulate in process water and make the water recycle process difficult. Use of MFCs to remove the inhibitory sugar and lignin degradation products from recycle water is investigated in this study.     

Bioelectricity production from acidic food waste leachate using microbial fuel cells: Effect of microbial inocula

The effects of three different inocula (domestic wastewater, activated sludge, and anaerobic sludge) on the treatment of acidic food waste leachate in microbial fuel cells (MFCs) were evaluated. A food waste leachate (pH 4.76; 1000 mg chemical oxygen demand (COD)/L) was used as the substrate. The results indicate that the leachate itself can enable electricity production in an MFC, but the co-addition of different inocula significantly reduces the start-up time (approximately 7 days). High COD and volatile fatty acids removal (>87%) were obtained in all MFCs but with only low coulombic efficiencies (CEs) (14–20%). The highest power (432 mW/m³) and CE (20%) were obtained with anaerobic sludge as the co-inoculum. Microbial community analysis (PCR-DGGE) of the established biofilms suggested that the superior performance of the anaerobic sludge-MFC was associated with the enrichment of both fermentative (Clostridium sp. and Bacteroides sp.) and electrogenic bacteria (Magnetospirillum sp. and Geobacter sp.) at the anode.     

Electricity generation coupled to oxidation of propionate in a microbial fuel cell

Propionate was used as fuel to enrich an electrochemically-active microbial consortium in a microbial fuel cell, and the bacterial consortium was analyzed by culture-independent methods including denaturing gradient gel electrophoresis (DGGE) of the 16S rDNA, and by fluorescent in situ hybridization (FISH). MFCs fed with propionate produced a current of 4.88 ± 0.1 mA stably on 100 mg propionate/l as COD within 3 weeks of the enrichment. When the MFCs were fed with H₂-saturated fuel containing propionate, the current dropped to 3.82 ± 0.07 mA. The maximum current generated was up to 8.8 mA when MFCs were fed with 200 mg propionate/l as COD. The DGGE of 16S rDNA showed that propionate-enriched MFCs have a different bacterial population from that enriched with acetate and from the inoculum used for enrichment. The major member (42%) of the consortium was an unidentified bacterium followed by γ, β, and δ-proteobacteria.     

Enrichment of microbial electrolysis cell biocathodes from sediment microbial fuel cell bioanodes

Electron-accepting (electrotrophic) biocathodes were produced by first enriching graphite fiber brush electrodes as the anodes in sediment-type microbial fuel cells (sMFCs) using two different marine sediments and then electrically inverting the anodes to function as cathodes in two-chamber bioelectrochemical systems (BESs). Electron consumption occurred at set potentials of -439 mV and -539 mV (versus the potential of a standard hydrogen electrode) but not at -339 mV in minimal media lacking organic sources of energy. Results at these different potentials were consistent with separate linear sweep voltammetry (LSV) scans that indicated enhanced activity (current consumption) below only ca. -400 mV. MFC bioanodes not originally acclimated at a set potential produced electron-accepting (electrotrophic) biocathodes, but bioanodes operated at a set potential (+11 mV) did not. CO(2) was removed from cathode headspace, indicating that the electrotrophic biocathodes were autotrophic. Hydrogen gas generation, followed by loss of hydrogen gas and methane production in one sample, suggested hydrogenotrophic methanogenesis. There was abundant microbial growth in the biocathode chamber, as evidenced by an increase in turbidity and the presence of microorganisms on the cathode surface. Clone library analysis of 16S rRNA genes indicated prominent sequences most similar to those of Eubacterium limosum (Butyribacterium methylotrophicum), Desulfovibrio sp. A2, Rhodococcus opacus, and Gemmata obscuriglobus. Transfer of the suspension to sterile cathodes made of graphite plates, carbon rods, or carbon brushes in new BESs resulted in enhanced current after 4 days, demonstrating growth by these microbial communities on a variety of cathode substrates. This report provides a simple and effective method for enriching autotrophic electrotrophs by the use of sMFCs without the need for set potentials, followed by the use of potentials more negative than -400 mV.     

Effect of enrichment procedures on performance and microbial diversity of microbial fuel cell for Congo red decolorization and electricity generation

The purpose of this study was to determine the effect of enrichment procedure on the performance and microbial diversity of an air-cathode microbial fuel cell (MFC) which was explored for simultaneous azo dye decolorization and electricity generation. Two different enrichment procedures in which glucose and Congo red were added into the MFCs sequentially (EP1) or simultaneously (EP2) were tested by operating parallel MFCs independently for more than 6 months. The power density, electrode potential, Congo red decolorization, biofilm morphology, and bacterial diversity of the MFCs under the two enrichment procedures were compared and investigated. The results showed that the enrichment procedures have a negligible effect on the dye decolorization, but significantly affected the electricity generation. More than 90% decolorization at dye concentration of 300 mg/L was achieved within 170 h for the two tested enrichment procedures. However, the MFC with EP2 achieved a maximum power density of 192 mW/m², which was 75% higher than that of the MFC with EP1 (110 mW/m²). The depressed surfaces of the bacteria in the MFC with EP1 indicated the allergic response caused by the subsequent addition of Congo red. 16S rRNA sequencing analysis demonstrated a phylogenetic diversity in the communities of the anode biofilm and showed clear differences between the anode-attached populations in the MFCs with a different enrichment procedure. This study suggests that the enrichment procedure is important for the MFC explored for simultaneous dye decolorization and electricity generation.     

Anode microbial communities produced by changing from microbial fuel cell to microbial electrolysis cell operation using two different wastewaters

Conditions in microbial fuel cells (MFCs) differ from those in microbial electrolysis cells (MECs) due to the intrusion of oxygen through the cathode and the release of H₂ gas into solution. Based on 16S rRNA gene clone libraries, anode communities in reactors fed acetic acid decreased in species richness and diversity, and increased in numbers of Geobacter sulfurreducens, when reactors were shifted from MFCs to MECs. With a complex source of organic matter (potato wastewater), the proportion of Geobacteraceae remained constant when MFCs were converted into MECs, but the percentage of clones belonging to G. sulfurreducens decreased and the percentage of G. metallireducens clones increased. A dairy manure wastewater-fed MFC produced little power, and had more diverse microbial communities, but did not generate current in an MEC. These results show changes in Geobacter species in response to the MEC environment and that higher species diversity is not correlated with current.     

The impact of electron donors and anode potentials on the anode-respiring bacteria community

Both anode potentials and substrates can affect the process of biofilm formation in bioelectrochemical systems, but it is unclear who primarily determine the anode-respiring bacteria (ARB) community structure and composition. To address this issue, we divided microbial electrolysis cells (MECs) into groups, feeding them with different substrates and culturing them at various potentials. Non-turnover cyclic voltammetry indicated that the extracellular electron transfer components were uniform when feeding acetate, because the same oxidation peaks occurred at − 0.36 ± 0.01 and − 0.17 ± 0.01 V (vs. Ag/AgCl). Illumina MiSeq sequencing revealed that the dominating ARB was Geobacter, which did not change with different potentials. When the MECs were cultured with sucrose and mixed substrates, oxidation peak P3 (− 0.29 ± 0.015 V) occurred at potentials of − 0.29 and 0.01 V. This may be because of the appearance of Unclassified_AKYG597. In addition, oxidation peak P4 (− 0.99 ± 0.01 V) occurred at high and low potentials (0.61 and − 0.45 V, respectively), and the maximum current densities were far below those of the middle potentials. Illumina MiSeq sequencing showed that fermentation microorganisms (Lactococcus and Sphaerochaeta) dominated the biofilms. Consequently, substrate primarily determined the dominating ARB, and Geobacter invariably dominated the acetate-fed biofilms with potentials changed. Conversely, different potentials mainly affected fermentable substrate-fed biofilms, with dominating ARB turning into Unclassified_AKYG59.