Correlation between microbial community and granule conductivity in anaerobic bioreactors for brewery wastewater treatment

Geobacter Dominates the Inner Layers of a Stratified Biofilm on a Fluidized Anode During Brewery Wastewater Treatment

In this study, we designed a microbial electrochemical fluidized bed reactor (ME-FBR), with an electroconductive anodic bed made of activated carbon particles for treating a brewery wastewater. Under a batch operating mode, acetate and propionate consumption rates were 13-fold and 2.4-fold higher, respectively, when the fluidized anode was polarized (0.2 V) with respect to open circuit conditions. Operating in a continuous mode, this system could effectively treat the brewery effluent at organic loading rates (OLR) over 1.7 kg m^-3_NRV d^-1 and with removal efficiencies of 95 ± 1.4% (hydraulic retention time of 1 day and an influent of 1.7 g-COD L^-1). The coulombic efficiency values highly depended upon the OLR applied, and varied from a 56 ± 15% to 10 ± 1%. Fluorescence in situ hybridization (FISH) analysis revealed a relative high abundance of Geobacter species (ca. 20%), and clearly showed a natural microbial stratification. Interestingly, the Geobacter cluster was highly enriched in the innermost layers of the biofilm (thickness of 10 μm), which were in contact with the electroconductive particles of bed, whereas the rest of bacteria were located in the outermost layers. To our knowledge, this is the first time that such a clear microbial stratification has been observed on an anode-respiring biofilm. Our results revealed the relevant role of Geobacter in switching between the electrode and other microbial communities performing metabolic reactions in the outermost environment of the biofilm.

Granulation and microbial community dynamics in the chitosan-supplemented anaerobic treatment of wastewater polluted with organic solvents

The effect of chitosan on the development of granular sludge in upflow anaerobic sludge blanket reactors (UASB) when treating wastewater polluted with the organic solvents ethanol, ethyl acetate, and 1-ethoxy-2-propanol was evaluated. Three UASB reactors were operated for 219 days at ambient temperature with an organic loading rate (OLR) of between 0.3 kg COD m^-3 d^-1 and 20 kg COD m^-3 d^-1. One reactor was operated without the addition of chitosan, while the other two were operated with the addition of chitosan doses of 2.4 mg gVSS^-1 two times. The three reactors were all able to treat the OLR tested with COD removal efficiencies greater than 90%. However, the time required to reach stable operation was considerably reduced in the chitosan-assisted reactors. The development of granules in the reactors with chitosan was accelerated and granules larger than 2000 μm were only observed in these reactors. In addition, these granules exhibited better physicochemical characteristics: the mean particle diameter (540 and 613 μm) was approximately two times greater than in the control reactor (300 μm), and the settling velocities exceeded 35 m h^-1. The extracellular polymeric substances (EPS) in the reactors with the chitosan was found to be higher than in the control reactor. The protein-EPS content has been correlated with the granule size. The analyses of the microbial communities, performed through denaturing gradient gel electrophoresis and high-throughput sequencing, revealed that the syntrophic microorganisms belonging to genus Geobacter and the hydrogenotrophic methanogen Methanocorpusculum labreanum were predominant in the granules. Other methanogens like Methanosaeta species were found earlier in the chitosan-assisted reactors than in the control reactor.

Promoting electron transfer to enhance anaerobic treatment of azo dye wastewater with adding Fe(OH)3

In this study, Fe(OH)₃ was dosed into anaerobic reactor (R1) to stimulate the electron transfer process during the decoloration treatment. As results indicated, the chemical oxygen demand (COD) removal and decoloration efficiency were increased by 61.7% and 32.0% than that of the control reactor (R2), respectively. The cyclic-voltammetric analysis of the effluent demonstrated that the cycles of Fe(III)/Fe(II) and 1-imino-2-napthoquinone/1-amino-2-napthol (oxidative/reductive state of the decoloration intermediate product) had been established in R1. The concentration of cytochrome c and the conductivity of suspended sludge in R1 were also 3.2 and 2.1 times higher than that in R2. All experiments above indicated that the electron transfer process between substrates and azo bonds was accelerated efficiently. Furthermore, the abundance of iron reducing bacteria was higher than that of R2, indicating that dissimilatory iron reduction as a reason for the Fe(III)/Fe(II) cycle played an important role in the anaerobic decoloration treatment.

Advances towards understanding and engineering direct interspecies electron transfer in anaerobic digestion

Direct interspecies electron transfer (DIET) is a recently discovered microbial syntrophy where cell-to-cell electron transfer occurs between syntrophic microbial species. DIET between bacteria and methanogenic archaea in anaerobic digestion can accelerate the syntrophic conversion of various reduced organic compounds to methane. DIET-based syntrophy can naturally occur in some anaerobic digester via conductive pili, however, can be engineered via the addition of various non-biological conductive materials. In recent years, research into understanding and engineering DIET-based syntrophy has emerged with the aim of improving methanogenesis kinetics in anaerobic digestion. This article presents a state-of-art review focusing on the fundamental mechanisms, key microbial players, the role of electrical conductivity, the effectiveness of various conductive additives, the significance of substrate characteristics and organic loading rates in promoting DIET in anaerobic digestion.

Effects of carbon source on methanogenic activities and pathways incorporating metagenomic analysis of microbial community

In this study, the effects of four types of organic compounds (tryptone, acetate/propionate, glucose and ethanol) on methanogenesis, electron transfer processes and microbial community structure were examined. When tryptone and acetate/propionate were used, the dominant methanogenic pathway was aceticlastic methanogenesis and Methanosarcina was the most abundant methanogen. When glucose or ethanol were provided as the external carbon source, the aceticlastic and hydrogenotrophic pathways were utilised simultaneously, and Methanosarcina and Methanobacterium were enriched. However, the reactor fed with glucose was prone to acidification because volatile fatty acids accumulated in the medium, which inhibited methane synthesis. Geobacter was dominant in the reactor fed with ethanol and 45% of genes encoding pili synthesis were attributable to Geobacter, indicating that direct interspecies electron transfer may be a possible mechanism during syntrophic methanogenesis.

Bioelectrochemical enhancement of direct interspecies electron transfer in upflow anaerobic reactor with effluent recirculation for acidic distillery wastewater

Methane production in the upflow anaerobic bioelectrochemical reactor (UABE) treating acidic distillery wastewater was compared to the upflow anaerobic sludge blanket reactor (UASB), and the electron transfer pathways for methane production were also evaluated in the effluent recirculation. The methane productions from reactors were influenced by the low pH of influent wastewater. However, the methane production rate and yield of the UABE were 2.08L/L.d and 320mL/g COD_r, which were higher than the UASB. The effluent recirculation containing alkalinity neutralized the acidic influent and increased the upflow velocity in both reactors, and improved the direct interspecies electron transfer more in the UABE. When the effluent recirculation ratio was 3.0 in the UABE, the methane production rate and yield were reached up to 3.88L/L.d and 501.0mL/g COD_r, respectively. The UABE requires electrode installation and electrical energy for operation, but the benefits from increased methane production are much higher.

Methane production and characteristics of the microbial community in a two-stage fixed-bed anaerobic reactor using molasses

Molasses is a typical feedstock for fermentation, but the effluent is hard to treat. In this study, molasses containing a high concentration of organic matter was treated by a two-stage Fix-bed reactor system with an increased organic loading rate (OLR). The results indicated at high molasses loading rate, the two-stage system was more efficient (i.e. organic matter removal, the COD of effluent and biogas production) than the single-stage system. The relative abundance of Anaerolineaceae and W5_norank was higher in the first stage (R1), where these organisms digest carbohydrates, while the second stage (R2) had higher relative abundance of Synergistaceae and SB-1_norank, which digest VFAs and decomposition-resistant compounds to produce compounds used by hydrogen methanogens. The qPCR analysis demonstrated that the Methanosaetaceae dominated the archaeal community in the first stage (R1), while Methanomicrobiales and Methanobacteriales were predominant in the second stage (R2), where they were involved in hydrogen production.

Predicting Microbial Fuel Cell Biofilm Communities and Bioreactor Performance using Artificial Neural Networks

The complex interactions that occur in mixed-species bioelectrochemical reactors, like microbial fuel cells (MFCs), make accurate predictions of performance outcomes under untested conditions difficult. While direct correlations between any individual waste stream characteristic or microbial community structure and reactor performance have not been able to be directly established, the increase in sequencing data and readily available computational power enables the development of alternate approaches. In the current study, 33 MFCs were evaluated under a range of conditions including eight separate substrates and three different wastewaters. Artificial Neural Networks (ANNs) were used to establish mathematical relationships between wastewater/solution characteristics, biofilm communities, and reactor performance. ANN models that incorporated biotic interactions predicted reactor performance outcomes more accurately than those that did not. The average percent error of power density predictions was 16.01 ± 4.35%, while the average percent error of Coulombic efficiency and COD removal rate predictions were 1.77 ± 0.57% and 4.07 ± 1.06%, respectively. Predictions of power density improved to within 5.76 ± 3.16% percent error through classifying taxonomic data at the family versus class level. Results suggest that the microbial communities and performance of bioelectrochemical systems can be accurately predicted using data-mining, machine-learning techniques.

Electrically conductive pili from pilin genes of phylogenetically diverse microorganisms

The possibility that bacteria other than Geobacter species might contain genes for electrically conductive pili (e-pili) was investigated by heterologously expressing pilin genes of interest in Geobacter sulfurreducens. Strains of G. sulfurreducens producing high current densities, which are only possible with e-pili, were obtained with pilin genes from Flexistipes sinusarabici, Calditerrivibrio nitroreducens and Desulfurivibrio alkaliphilus. The conductance of pili from these strains was comparable to native G. sulfurreducens e-pili. The e-pili derived from C. nitroreducens, and D. alkaliphilus pilin genes are the first examples of relatively long (>100 amino acids) pilin monomers assembling into e-pili. The pilin gene from Candidatus Desulfofervidus auxilii did not yield e-pili, suggesting that the hypothesis that this sulfate reducer wires itself with e-pili to methane-oxidizing archaea to enable anaerobic methane oxidation should be reevaluated. A high density of aromatic amino acids and a lack of substantial aromatic-free gaps along the length of long pilins may be important characteristics leading to e-pili. This study demonstrates a simple method to screen pilin genes from difficult-to-culture microorganisms for their potential to yield e-pili; reveals new sources for biologically based electronic materials; and suggests that a wide phylogenetic diversity of microorganisms may use e-pili for extracellular electron exchange.

Boosting biomethane yield and production rate with graphene: The potential of direct interspecies electron transfer in anaerobic digestion

Interspecies electron transfer between bacteria and archaea plays a vital role in enhancing energy efficiency of anaerobic digestion (AD). Conductive carbon materials (i.e. graphene nanomaterial and activated charcoal) were assessed to enhance AD of ethanol (a key intermediate product after acidogenesis of algae). The addition of graphene (1.0g/L) resulted in the highest biomethane yield (695.0±9.1mL/g) and production rate (95.7±7.6mL/g/d), corresponding to an enhancement of 25.0% in biomethane yield and 19.5% in production rate. The ethanol degradation constant was accordingly improved by 29.1% in the presence of graphene. Microbial analyses revealed that electrogenic bacteria of Geobacter and Pseudomonas along with archaea Methanobacterium and Methanospirillum might participate in direct interspecies electron transfer (DIET). Theoretical calculations provided evidence that graphene-based DIET can sustained a much higher electron transfer flux than conventional hydrogen transfer.

Stay connected: Electrical conductivity of microbial aggregates

The discovery of direct extracellular electron transfer offers an alternative to the traditional understanding of diffusional electron exchange via small molecules. The establishment of electronic connections between electron donors and acceptors in microbial communities is critical to electron transfer via electrical currents. These connections are facilitated through conductivity associated with various microbial aggregates. However, examination of conductivity in microbial samples is still in its relative infancy and conceptual models in terms of conductive mechanisms are still being developed and debated. The present review summarizes the fundamental understanding of electrical conductivity in microbial aggregates (e.g. biofilms, granules, consortia, and multicellular filaments) highlighting recent findings and key discoveries. A greater understanding of electrical conductivity in microbial aggregates could facilitate the survey for additional microbial communities that rely on direct extracellular electron transfer for survival, inform rational design towards the aggregates-based production of bioenergy/bioproducts, and inspire the construction of new synthetic conductive polymers.

Control of VOCs from printing press air emissions by anaerobic bioscrubber: Performance and microbial community of an on-site pilot unit

A novel process consisted of an anaerobic bioscrubber was studied at the field scale for the removal of volatile organic compounds (VOCs) emitted from a printing press facility. The pilot unit worked under high fluctuating waste gas emissions containing ethanol, ethyl acetate, and 1-ethoxy-2-propanol as main pollutants, with airflows ranging between 184 and 1253 m³ h^-1 and an average concentration of 1126 ± 470 mg-C Nm^-3. Three scrubber configurations (cross-flow and vertical-flow packings and spray tower) were tested, and cross-flow packing was found to be the best one. For this packing, daily average values of VOC removal efficiency ranged between 83% and 93% for liquid to air volume ratios between 3.5·10^-3 and 9.1·10^-3. Biomass growth was prevented by periodical chemical cleaning; the average pressure drop was 165 Pa m^-1. Rapid initiation of anaerobic degradation was achieved by using granular sludge from a brewery wastewater treatment plant. Despite the intermittent and fluctuating organic load, the expanded granular sludge bed reactor showed an excellent level of performance, reaching removal efficiencies of 93 ± 5% at 25.1 ± 3.2 °C, with biogas methane content of 94 ± 3% in volume. Volatile fatty acid concentration was as low as 200 mg acetic acid L^-1 by treating daily average organic loads up to 3.0 kg COD h^-1, equivalent to 24 kg COD m^-3 bed d^-1. The denaturing gradient gel electrophoresis (DGGE) results revealed the initial shift of the domains Archaea and Bacteria associated with the limitation of the carbon source to a few organic solvents. The Archaea domain was more sensitive, resulting in a drop of the Shannon index from 1.07 to 0.41 in the first 123 days. Among Archaea, the predominance of Methanosaeta persisted throughout the experimental period. The increase in the proportion of Methanospirillum and Methanobacterium sp. was linked to the spontaneous variations of operating temperature and load, respectively. Among Bacteria, high levels of ethanol degraders (Geobacter and Pelobacter sp.) were observed during the trial.

Potentially direct interspecies electron transfer of methanogenesis for syntrophic metabolism under sulfate reducing conditions with stainless steel

Direct interspecies electron transfer (DIET) is an alternative to syntrophic metabolism in natural carbon cycle as well as in anaerobic digesters, but its function in anaerobic treatment of sulfate-containing wastewater have not yet to be described. Here, conductive stainless steel was added into anaerobic digesters for treating sulfate-containing wastewater to investigate the potential role of DIET in the response to the sulfate impact. Results showed that adding the conductive stainless steel made the anaerobic digestion less affected by the sulfate reduction than adding insulative plastic material. With adding stainless steel, methane production of the digesters increased by 7.5%-24.6%. Microbial analysis showed that the dissimilatory Fe (III) reducers like Geobacter species were enriched on the surface of the stainless steel. These results implied that the potential DIET of methanogenesis was established associating with stainless steel to outcompete the sulfate reduction.

Metatranscriptomic Evidence for Direct Interspecies Electron Transfer between Geobacter and Methanothrix Species in Methanogenic Rice Paddy Soils

The possibility that Methanothrix (formerly Methanosaeta) and Geobacter species cooperate via direct interspecies electron transfer (DIET) in terrestrial methanogenic environments was investigated in rice paddy soils. Genes with high sequence similarity to the gene for the PilA pilin monomer of the electrically conductive pili (e-pili) of Geobacter sulfurreducens accounted for over half of the PilA gene sequences in metagenomic libraries and 42% of the mRNA transcripts in RNA sequencing (RNA-seq) libraries. This abundance of e-pilin genes and transcripts is significant because e-pili can serve as conduits for DIET. Most of the e-pilin genes and transcripts were affiliated with Geobacter species, but sequences most closely related to putative e-pilin genes from genera such as Desulfobacterium, Deferribacter, Geoalkalibacter, and Desulfobacula, were also detected. Approximately 17% of all metagenomic and metatranscriptomic bacterial sequences clustered with Geobacter species, and the finding that Geobacter spp. were actively transcribing growth-related genes indicated that they were metabolically active in the soils. Genes coding for e-pilin were among the most highly transcribed Geobacter genes. In addition, homologs of genes encoding OmcS, a c-type cytochrome associated with the e-pili of G. sulfurreducens and required for DIET, were also highly expressed in the soils. Methanothrix species in the soils highly expressed genes for enzymes involved in the reduction of carbon dioxide to methane. DIET is the only electron donor known to support CO₂ reduction in Methanothrix Thus, these results are consistent with a model in which Geobacter species were providing electrons to Methanothrix species for methane production through electrical connections of e-pili.IMPORTANCEMethanothrix species are some of the most important microbial contributors to global methane production, but surprisingly little is known about their physiology and ecology. The possibility that DIET is a source of electrons for Methanothrix in methanogenic rice paddy soils is important because it demonstrates that the contribution that Methanothrix makes to methane production in terrestrial environments may extend beyond the conversion of acetate to methane. Furthermore, defined coculture studies have suggested that when Methanothrix species receive some of their energy from DIET, they grow faster than when acetate is their sole energy source. Thus, Methanothrix growth and metabolism in methanogenic soils may be faster and more robust than generally considered. The results also suggest that the reason that Geobacter species are repeatedly found to be among the most metabolically active microorganisms in methanogenic soils is that they grow syntrophically in cooperation with Methanothrix spp., and possibly other methanogens, via DIET.

A modeling approach to direct interspecies electron transfer process in anaerobic transformation of ethanol to methane

Recent studies have shown that direct interspecies electron transfer (DIET) plays an important part in contributing to methane production from anaerobic digestion. However, so far anaerobic digestion models that have been proposed only consider two pathways for methane production, namely, acetoclastic methanogenesis and hydrogenotrophic methanogenesis, via indirect interspecies hydrogen transfer, which lacks an effective way for incorporating DIET into this paradigm. In this work, a new mathematical model is specifically developed to describe DIET process in anaerobic digestion through introducing extracellular electron transfer as a new pathway for methane production, taking anaerobic transformation of ethanol to methane as an example. The developed model was able to successfully predict experimental data on methane dynamics under different experimental conditions, supporting the validity of the developed model. Modeling predictions clearly demonstrated that DIET plays an important role in contributing to overall methane production (up to 33 %) and conductive material (i.e., carbon cloth) addition would significantly promote DIET through increasing ethanol conversion rate and methane production rate. The model developed in this work will potentially enhance our current understanding on syntrophic metabolism via DIET.

Stimulation of methanogenesis in anaerobic digesters treating leachate from a municipal solid waste incineration plant with carbon cloth

Bio-methanogenic digestion of incineration leachate is hindered by high OLRs, which can lead to build-up of VFAs, drops in pH and ultimately in reactor souring. It was hypothesized that incorporation of carbon cloth into reactors treating leachate would promote DIET and enhance reactor performance. To examine this possibility, carbon cloth was added to laboratory-scale UASB reactors that were fed incineration leachate. As expected, the carbon-cloth amended reactor could operate stably with a 34.2% higher OLR than the control (49.4 vs 36.8kgCOD/(m³d)). Microbial community analysis showed that bacteria capable of extracellular electron transfer and methanogens known to participate in DIET were enriched on the carbon cloth surface, and conductivity of sludge from the carbon cloth amended reactor was almost twofold higher than sludge from the control (9.77 vs 5.47μS/cm), suggesting that microorganisms in the experimental reactor may have been expressing electrically conductive filaments.

Happy together: microbial communities that hook up to swap electrons

The discovery of direct interspecies electron transfer (DIET) and cable bacteria has demonstrated that microbial cells can exchange electrons over long distances (μm-cm) through electrical connections. For example, in the presence of cable bacteria electrons are rapidly transported over centimeter distances, coupling the oxidation of reduced sulfur compounds in anoxic sediments to oxygen reduction in overlying surficial sediments. Bacteria and archaea wired for DIET are found in anaerobic methane-producing and methane-consuming communities. Electrical connections between gut microbes and host cells have also been proposed. Iterative environmental and defined culture studies on methanogenic communities revealed the importance of electrically conductive pili and c-type cytochromes in natural electrical grids, and demonstrated that conductive carbon materials and magnetite can substitute for these biological connectors to facilitate DIET. This understanding has led to strategies to enhance and stabilize anaerobic digestion. Key unknowns warranting further investigation include elucidation of the archaeal electrical connections facilitating DIET-based methane production and consumption; and the mechanisms for long-range electron transfer through cable bacteria. A better understanding of mechanisms for cell-to-cell electron transfer could facilitate the hunt for additional electrically connected microbial communities with omics approaches and could advance spin-off applications such as the development of sustainable bioelectronics materials and bioelectrochemical technologies.

Communities stimulated with ethanol to perform direct interspecies electron transfer for syntrophic metabolism of propionate and butyrate

Direct interspecies electron transfer (DIET) has been considered as an alternative to interspecies H2 transfer (IHT) for syntrophic metabolism, but the microorganisms capable of metabolizing the key intermediates, such as propionate and butyrate, via DIET have yet to be described. A strategy of culturing the enrichments with ethanol as a DIET substrate to stimulate the communities for the syntrophic metabolism of propionate and/or butyrate was proposed in this study. The results showed that the syntrophic propionate and/or butyrate degradation was significantly improved in the ethanol-stimulated reactor when propionate/butyrate was the sole carbon source. The conductivity of the ethanol-stimulated enrichments was as 5 folds (for propionate)/76 folds (for butyrate) as that of the traditional enrichments (never ethanol fed). Microbial community analysis revealed that Geobacter species known to proceed DIET were only detected in the ethanol-stimulated enrichments. Together with the significant increase of Methanosaeta and Methanosarcina species in these enrichments, the potential DIET between Geobacter and Methanosaeta or Methanosarcina species might be established to improve the syntrophic propionate and/or butyrate degradation. Further experiments demonstrated that granular activated carbon (GAC) could improve the syntrophic metabolism of propionate and/or butyrate of the ethanol-stimulated enrichments, while almost no effects on the traditional enrichments. Also, the high H2 partial pressure could inhibit the syntrophic propionate and/or butyrate degradation of the traditional enrichments, but its effect on that of the ethanol-stimulated enrichments was negligible.

Hardwiring microbes via direct interspecies electron transfer: mechanisms and applications

Multicellular microbial communities are important catalysts in engineered systems designed to treat wastewater, remediate contaminated sediments, and produce energy from biomass. Understanding the interspecies interactions within them is therefore essential to design effective processes. The flow of electrons within these communities is especially important in the determination of reaction possibilities (thermodynamics) and rates (kinetics). Conventional models of electron transfer incorporate the diffusion of metabolites generated by one organism and consumed by a second, frequently referred to as mediated interspecies electron transfer (MIET). Evidence has emerged in the last decade that another method, called direct interspecies electron transfer (DIET), may occur between organisms or in conjunction with electrically conductive materials. Recent research has suggested that DIET can be stimulated in engineered systems to improve desired treatment goals and energy recovery in systems such as anaerobic digesters and microbial electrochemical technologies. In this review, we summarize the latest understanding of DIET mechanisms, the associated microorganisms, and the underlying thermodynamics. We also critically examine approaches to stimulate DIET in engineered systems and assess their effectiveness. We find that in most cases attempts to promote DIET in mixed culture systems do not yield the improvements expected based on defined culture studies. Uncertainties of other processes that may be co-occurring in real systems, such as contaminant sorption and biofilm promotion, need to be further investigated. We conclude by identifying areas of future research related to DIET and its application in biological treatment processes.

Microbial community structures and metabolic profiles response differently to physiochemical properties between three landfill cover soils

Landfills are always the most important part of solid waste management and bear diverse metabolic activities involved in element biogeochemical cycling. There is an increasing interest in understanding the microbial community and activities in landfill cover soils. To improve our knowledge of landfill ecosystems, we determined the microbial physiological profiles and communities in three landfill cover soils (Ninghai: NH, Xiangshan: XS, and Fenghua: FH) of different ages using the MicroResp(TM), phospholipid fatty acid (PLFA), and high-throughput sequencing techniques. Both total PLFAs and glucose-induced respiration suggested more active microorganisms occurred in intermediate cover soils. Microorganisms in all landfill cover soils favored L-malic acid, ketoglutarate, and citric acid. Gram-negative bacterial PLFAs predominated in all samples with the representation of 16:1ω7, 18:1ω7, and cy19:0 in XS and NH sites. Proteobacteria dominated soil microbial phyla across different sites, soil layers, and season samples. Canonical correspondence analysis showed soil pH, dissolved organic C (DOC), As, and total nitrogen (TN) contents significantly influenced the microbial community but TN affected the microbial physiological activities in both summer and winter landfill cover soils.

The Low Conductivity of Geobacter uraniireducens Pili Suggests a Diversity of Extracellular Electron Transfer Mechanisms in the Genus Geobacter

Studies on the mechanisms for extracellular electron transfer in Geobacter species have primarily focused on Geobacter sulfurreducens, but the poor conservation of genes for some electron transfer components within the Geobacter genus suggests that there may be a diversity of extracellular electron transport strategies among Geobacter species. Examination of the gene sequences for PilA, the type IV pilus monomer, in Geobacter species revealed that the PilA sequence of Geobacter uraniireducens was much longer than that of G. sulfurreducens. This is of interest because it has been proposed that the relatively short PilA sequence of G. sulfurreducens is an important feature conferring conductivity to G. sulfurreducens pili. In order to investigate the properties of the G. uraniireducens pili in more detail, a strain of G. sulfurreducens that expressed pili comprised the PilA of G. uraniireducens was constructed. This strain, designated strain GUP, produced abundant pili, but generated low current densities and reduced Fe(III) very poorly. At pH 7, the conductivity of the G. uraniireducens pili was 3 × 10^-4 S/cm, much lower than the previously reported 5 × 10^-2 S/cm conductivity of G. sulfurreducens pili at the same pH. Consideration of the likely voltage difference across pili during Fe(III) oxide reduction suggested that G. sulfurreducens pili can readily accommodate maximum reported rates of respiration, but that G. uraniireducens pili are not sufficiently conductive to be an effective mediator of long-range electron transfer. In contrast to G. sulfurreducens and G. metallireducens, which require direct contact with Fe(III) oxides in order to reduce them, G. uraniireducens reduced Fe(III) oxides occluded within microporous beads, demonstrating that G. uraniireducens produces a soluble electron shuttle to facilitate Fe(III) oxide reduction. The results demonstrate that Geobacter species may differ substantially in their mechanisms for long-range electron transport and that it is important to have information beyond a phylogenetic affiliation in order to make conclusions about the mechanisms by which Geobacter species are transferring electrons to extracellular electron acceptors.

Redox Conductivity of Current-Producing Mixed Species Biofilms

While most biological materials are insulating in nature, efficient extracellular electron transfer is a critical property of biofilms associated with microbial electrochemical systems and several microorganisms are capable of establishing conductive aggregates and biofilms. Though construction of these conductive microbial networks is an intriguing and important phenomenon in both natural and engineered systems, few studies have been published related to conductive biofilms/aggregates and their conduction mechanisms, especially in mixed-species environments. In the present study, current-producing mixed species biofilms exhibited high conductivity across non-conductive gaps. Biofilm growth observed on the inactive electrode contributed to overall power outputs, suggesting that an electrical connection was established throughout the biofilm assembly. Electrochemical gating analysis of the biofilms over a range of potentials (-600-200 mV, vs. Ag/AgCl) resulted in a peak-manner response with maximum conductance of 3437 ± 271 μS at a gate potential of -360 mV. Following removal of the electron donor (acetate), a 96.6% decrease in peak conductivity was observed. Differential responses observed in the absence of an electron donor and over varying potentials suggest a redox driven conductivity mechanism in mixed-species biofilms. These results demonstrated significant differences in biofilm development and conductivity compared to previous studies using pure cultures.

Expanding the Diet for DIET: Electron Donors Supporting Direct Interspecies Electron Transfer (DIET) in Defined Co-Cultures

Direct interspecies electron transfer (DIET) has been recognized as an alternative to interspecies H₂ transfer as a mechanism for syntrophic growth, but previous studies on DIET with defined co-cultures have only documented DIET with ethanol as the electron donor in the absence of conductive materials. Co-cultures of Geobacter metallireducens and Geobacter sulfurreducens metabolized propanol, butanol, propionate, and butyrate with the reduction of fumarate to succinate. G. metallireducens utilized each of these substrates whereas only electrons available from DIET supported G. sulfurreducens respiration. A co-culture of G. metallireducens and a strain of G. sulfurreducens that could not metabolize acetate oxidized acetate with fumarate as the electron acceptor, demonstrating that acetate can also be syntrophically metabolized via DIET. A co-culture of G. metallireducens and Methanosaeta harundinacea previously shown to syntrophically convert ethanol to methane via DIET metabolized propanol or butanol as the sole electron donor, but not propionate or butyrate. The stoichiometric accumulation of propionate or butyrate in the propanol- or butanol-fed cultures demonstrated that M. harundinaceae could conserve energy to support growth solely from electrons derived from DIET. Co-cultures of G. metallireducens and Methanosarcina barkeri could also incompletely metabolize propanol and butanol and did not metabolize propionate or butyrate as sole electron donors. These results expand the range of substrates that are known to be syntrophically metabolized through DIET, but suggest that claims of propionate and butyrate metabolism via DIET in mixed microbial communities warrant further validation.

Biogas Production: Microbiology and Technology

Biogas, containing energy-rich methane, is produced by microbial decomposition of organic material under anaerobic conditions. Under controlled conditions, this process can be used for the production of energy and a nutrient-rich residue suitable for use as a fertilising agent. The biogas can be used for production of heat, electricity or vehicle fuel. Different substrates can be used in the process and, depending on substrate character, various reactor technologies are available. The microbiological process leading to methane production is complex and involves many different types of microorganisms, often operating in close relationships because of the limited amount of energy available for growth. The microbial community structure is shaped by the incoming material, but also by operating parameters such as process temperature. Factors leading to an imbalance in the microbial community can result in process instability or even complete process failure. To ensure stable operation, different key parameters, such as levels of degradation intermediates and gas quality, are often monitored. Despite the fact that the anaerobic digestion process has long been used for industrial production of biogas, many questions need still to be resolved to achieve optimal management and gas yields and to exploit the great energy and nutrient potential available in waste material. This chapter discusses the different aspects that need to be taken into consideration to achieve optimal degradation and gas production, with particular focus on operation management and microbiology.

Biologically Produced Methane as a Renewable Energy Source

Methanogens are a unique group of strictly anaerobic archaea that are more metabolically diverse than previously thought. Traditionally, it was thought that methanogens could only generate methane by coupling the oxidation of products formed by fermentative bacteria with the reduction of CO₂. However, it has recently been observed that many methanogens can also use electrons extruded from metal-respiring bacteria, biocathodes, or insoluble electron shuttles as energy sources. Methanogens are found in both human-made and natural environments and are responsible for the production of ∼71% of the global atmospheric methane. Their habitats range from the human digestive tract to hydrothermal vents. Although biologically produced methane can negatively impact the environment if released into the atmosphere, when captured, it can serve as a potent fuel source. The anaerobic digestion of wastes such as animal manure, human sewage, or food waste produces biogas which is composed of ∼60% methane. Methane from biogas can be cleaned to yield purified methane (biomethane) that can be readily incorporated into natural gas pipelines making it a promising renewable energy source. Conventional anaerobic digestion is limited by long retention times, low organics removal efficiencies, and low biogas production rates. Therefore, many studies are being conducted to improve the anaerobic digestion process. Researchers have found that addition of conductive materials and/or electrically active cathodes to anaerobic digesters can stimulate the digestion process and increase methane content of biogas. It is hoped that optimization of anaerobic digesters will make biogas more readily accessible to the average person.

Comparing activated carbon of different particle sizes on enhancing methane generation in upflow anaerobic digester

Two sizes of conductive particles, i.e. 10-20 mesh granulated activated carbon (GAC) and 80-100 mesh powdered activated carbon (PAC) were added into lab-scale upflow anaerobic sludge blanket reactors, respectively, to testify their enhancement on the syntrophic metabolism of alcohols and volatile fatty acids (VFAs) in 95days operation. When OLR increased to more than 5.8gCOD/L/d, the differences between GAC/PAC supplemented reactors and the control reactor became more significant. The introduction of activated carbon could facilitate the enrichment of methanogens and accelerate the startup of methanogenesis, as indicated by enhanced methane yield and substrate degradation. High-throughput pyrosequencing analysis showed that syntrophic bacteria and Methanosarcina sp. with versatile metabolic capability increased in the tightly absorbed fraction on the PAC surface, leading to the promoted syntrophic associations. Thus PAC prevails over than GAC for methanogenic reactor with heavy load.

Enhancing syntrophic metabolism in up-flow anaerobic sludge blanket reactors with conductive carbon materials

Syntrophic metabolism of alcohols and fatty acids is a critical step in anaerobic digestion, which if enhanced can better stabilize the process and enable shorter retention times. Direct interspecies electron transfer (DIET) has recently been recognized as an alternative route to hydrogen interspecies transfer as a mechanism for interspecies syntrophic electron exchange. Therefore, the possibility of accelerating syntrophic metabolism of ethanol in up-flow anaerobic sludge blanket (UASB) reactors by incorporating conductive materials in reactor design was investigated. Graphite, biochar, and carbon cloth all immediately enhanced methane production and COD removal. As the hydraulic retention time was decreased the increased effectiveness of treatment in reactors with conductive materials increased versus the control reactor. When these conductive materials were removed from the reactors rates of syntrophic metabolism declined to rates comparable to the control reactor. These results suggest that incorporating conductive materials in the design of UASB reactors may enhance digester effectiveness.

Link between capacity for current production and syntrophic growth in Geobacter species

Electrodes are unnatural electron acceptors, and it is yet unknown how some Geobacter species evolved to use electrodes as terminal electron acceptors. Analysis of different Geobacter species revealed that they varied in their capacity for current production. Geobacter metallireducens and G. hydrogenophilus generated high current densities (ca. 0.2 mA/cm(2)), comparable to G. sulfurreducens. G. bremensis, G. chapellei, G. humireducens, and G. uraniireducens, produced much lower currents (ca. 0.05 mA/cm(2)) and G. bemidjiensis was previously found to not produce current. There was no correspondence between the effectiveness of current generation and Fe(III) oxide reduction rates. Some high-current-density strains (G. metallireducens and G. hydrogenophilus) reduced Fe(III)-oxides as fast as some low-current-density strains (G. bremensis, G. humireducens, and G. uraniireducens) whereas other low-current-density strains (G. bemidjiensis and G. chapellei) reduced Fe(III) oxide as slowly as G. sulfurreducens, a high-current-density strain. However, there was a correspondence between the ability to produce higher currents and the ability to grow syntrophically. G. hydrogenophilus was found to grow in co-culture with Methanosarcina barkeri, which is capable of direct interspecies electron transfer (DIET), but not with Methanospirillum hungatei capable only of H2 or formate transfer. Conductive granular activated carbon (GAC) stimulated metabolism of the G. hydrogenophilus - M. barkeri co-culture, consistent with electron exchange via DIET. These findings, coupled with the previous finding that G. metallireducens and G. sulfurreducens are also capable of DIET, suggest that evolution to optimize DIET has fortuitously conferred the capability for high-density current production to some Geobacter species.

Direct Interspecies Electron Transfer in Anaerobic Digestion: A Review

Direct interspecies electrons transfer (DIET) is a syntrophic metabolism in which free electrons flow from one cell to another without being shuttled by reduced molecules such as molecular hydrogen or formate. As more and more microorganisms show a capacity for electron exchange, either to export or import them, it becomes obvious that DIET is a syntrophic metabolism that is much more present in nature than previously thought. This article reviews literature related to DIET, specifically in reference to anaerobic digestion. Anaerobic granular sludge, a biofilm, is a specialized microenvironment where syntrophic bacterial and archaeal organisms grow together in close proximity. Exoelectrogenic bacteria degrading organic substrates or intermediates need an electron sink and electrotrophic methanogens represent perfect partners to assimilate those electrons and produce methane. The granule extracellular polymeric substances by making the biofilm matrix more conductive, play a role as electrons carrier in DIET.