Self-adaption of methane-producing communities to pH disturbance at different acetate concentrations by shifting pathways and population interaction

In-situ biological biogas upgrading using upflow anaerobic polyfoam bioreactor: Operational and biological aspects

A high rate upflow anaerobic polyfoam-based bioreactor (UAPB) was developed for lab-scale in-situ biogas upgrading by H2 injection. The reactor, with a volume of 440 mL, was fed with synthetic wastewater at an organic loading rate (OLR) of 3.5 g COD/L·day and a hydraulic retention time (HRT) of 7.33 h. The use of a porous diffuser, alongside high gas recirculation, led to a higher H2 liquid mass transfer, and subsequently to a better uptake for high CH4 content of 56% (starting from 26%). Our attempts to optimize both operational parameters (H2 flow rate and gas recirculation ratio, which is the total flow rate of recirculated gas over the total outlet of gas flow rate) were not initially successful, however, at a very high recirculation ratio (32) and flow rate (54 mL/h), a significant improvement of the hydrogen consumption was achieved. These operational conditions have in turn driven the methanogenic community toward the dominance of Methanosaetaceae, which out-competed Methanosarcinaceae. Nevertheless, highly stable methane production rates of 1.4-1.9 L CH4/Lreactor.day were observed despite the methanogenic turnover. During the different applied operational conditions, the bacterial community was especially impacted, resulting in substantial shifts of taxonomic groups. Notably, Aeromonadaceae was the only bacterial group positively correlated with increasing hydrogen consumption rates. The capacity of Aeromonadaceae to extracellularly donate electrons suggests that direct interspecies electron transfer (DIET) enhanced biogas upgrading. Overall, the proposed innovative biological in-situ biogas upgrading technology using the UAPB configuration shows promising results for stable, simple, and effective biological biogas upgrading.

Relative importance of aceticlastic methanogens and hydrogenotrophic methanogens on mercury methylation and methylmercury demethylation in paddy soils

The accumulation of methylmercury (MeHg) in paddy soil results from a subtle balance between inorganic mercury (e.g., HgII) methylation and MeHg demethylation. Methanogens not only act as Hg methylators but may also facilitate MeHg demethylation. However, the diverse methanogen flora (e.g., aceticlastic and hydrogenotrophic types) that exists under ambient conditions has not previously been considered. Accordingly, the roles of different types of methanogens in HgII methylation and MeHg degradation in paddy soils were studied using the Hg isotope tracing technique combined with the application of methanogen inhibitors/stimulants. It was found that the response of HgII methylation to methanogen inhibitors or stimulants was site-dependent. Specifically, aceticlastic methanogens were suggested as the potential HgII methylators at the low Hg level background site, whereas hydrogenotrophic methanogens were potentially involved in MeHg production as Hg levels increased. In contrast, both aceticlastic and hydrogenotrophic methanogens facilitated MeHg degradation across the sampling sites. Additionally, competition between hydrogenotrophic and aceticlastic methanogens was observed in Hg-polluted paddy soils, implying that net MeHg production could be alleviated by promoting aceticlastic methanogens or inhibiting hydrogenotrophic methanogens. The findings gained from this study improve the understanding of the role of methanogens in net MeHg formation and link carbon turnover to Hg biogeochemistry in rice paddy ecosystems.

Performance of anaerobic digestion of phenol using exogenous hydrogen and granular activated carbon and analysis of microbial community

Anaerobic conversion rate of phenol to methane was low due to its biological toxicity. In this study, the coupling of granular activated carbon (GAC) and exogenous hydrogen (EH) could enhance greatly methane production of phenol anaerobic digestion, and the metagenomic was firstly used to analyze its potential mechanism. The results indicated that a mass of syntrophic acetate-oxidizing bacteria and hydrogen-utilizing methanogens were enriched on the GAC surface, and SAO-HM pathway has become the dominant pathway. The energy transfer analysis implied that the abundance of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) oxidase increased. Furthermore, direct interspecies electron transfer (DIET) was formed by promoting type IV e-pili between Methanobacterium and Syntrophus, thereby improving the interspecies electron transfer efficiency. The dominant SAO-HM pathway was induced and DIET was formed, which was the internal mechanism of the coupling of GAC and EH to enhance anaerobic biotransformation of phenol.

Organic matter removal performance, pathway and microbial community succession during the construction of high-ammonia anaerobic biosystems treating anaerobic digestate food waste effluent

This study aimed to establish anaerobic biosystems which could tolerate high ammonia, and investigate the microbial community structure in these reactors. High-ammonia anaerobic biosystems that could tolerate 3600 mg L^-1 total ammonia nitrogen (TAN) and 1000 mg L^-1 free ammonia nitrogen (FAN) were successfully established. The removal efficiencies of COD and total volatile fatty acids (TVFAs) in R1 with dewatered sludge as inoculum were 68.8% and 69.2%, respectively. The maximum methane production rate reached 71.7 ± 1.0 mL CH₄ L^-1 d^-1 at a TAN concentration of 3600 mg L^-1. The three-dimension excitation-emission matrix analysis indicated that both easily degradable organics and refractory organics were removed from ADFE in R1 and R2. Functional microorganisms which could bear high ammonia were gradually enriched as TAN stress was elevated. Lysinibacillus, Coprothermobacter and Sporosarcina dominated the final bacterial community. Archaeal community transformed to hydrogenotrophic methanogen. The synergy of Coprothermobacter and Methanothermobacter undertook the organic matter degradation, and was enhanced by increasing TAN stress. This study offers new insights into anaerobic bioremediation of ammonia-rich wastewater.

Metabolic Regulation of Mesophilic Methanosarcina barkeri to Ammonium Inhibition

Undesirable ammonium concentrations can lead to unstable anaerobic digestion processes, and Methanosarcina spp. are the representative methanogens under inhibition. However, no known work seems to exist for directly exploring the detailed metabolic regulation of pure cultured representative Methanosarcina spp. to ammonium inhibition. We used transcriptomics and proteomics to profile the metabolic regulation of Methanosarcina barkeri to 1, 4, and 7 g N/L of total ammoniacal nitrogen (TAN), where free ammonia concentrations were between 1.5 and 36.1 mg N/L. At the initial stages of ammonium inhibition, the genes participating in the acquisition and assimilation of reduced nitrogen sources showed significant upregulation where the minimal fold change of gene transcription was about 2. Apart from nitrogen metabolism, the transcription of some genes in methanogenesis also significantly increased at the initial stages. For example, the genes encoding alternative heterodisulfide reductase subunits (HdrAB), energy-converting hydrogenase subunit (EchC), and methanophenazine-dependent hydrogenase subunits (VhtAC) were significantly upregulated by at least 2.05 times. For the element translocation at the initial stages, the genes participating in the uptake of ferrous iron, potassium ion, and molybdate were significantly upregulated with a minimal fold change of 2.10. As the cultivation proceeded, the gene encoding the cell division protein subunit (FtsH) was significantly upregulated by 13.0 times at 7 g N/L of TAN; meanwhile, an increment in OD₆₀₀ was observed at the terminal sampling point of 7 g N/L of TAN. The present study explored the metabolic regulation of M. barkeri in stress response, protein synthesis, signal transduction, nitrogen metabolism, methanogenesis, and element translocation. The results would contribute to the understanding of the metabolic effects of ammonium inhibition on methanogens and have significant practical implication in inhibited anaerobic digestion.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.

Effects of pH on ex-situ biomethanation with hydrogenotrophic methanogens under thermophilic and extreme-thermophilic conditions

Ex-situ biogas upgrading based on hydrogenotrophic methanogenic process has attracted much attention with the depletion of fossil fuels. Consumption of CO₂ leads to the pH increase in the mixed cultures of biogas upgrading system. The hydrogenotrophic methanogens were enriched at pH 5.5-6.0, 7.0-7.5, and 8.5-9.0 and at 55°C and 70°C. The methane production activity and microbial community structure were evaluated. Semi-continuous experimental results showed that stable and similar methane production was obtained at pH 7.0-7.5 and 8.5-9.0. In addition, pH 8.5-9.0 presented higher maximum methane production rate compared to pH 7.0-7.5. pH below 6 obtained the longest lag phase time of about 17.4 h, more than twice the values at pH 7.0-7.5 (8.8 h) and pH 8.5-9.0 (6.9 h) at 55°C. The predominant methanogen was the genus Methanothermobacter, a hydrogenotrophic methanogen at higher temperatures. Methanobacterium became predominant at pH 8.5-9.0 and the abundance increased to 83.6% at 55°C. Coprothermobacter and Caldanaerobacter were identified as the core functional bacteria under alkaline condition and were likely involved in syntrophic acetate oxidation with hydrogenotrophic methanogens.

Enhancement of syntrophic acetate oxidation pathway via single walled carbon nanotubes addition under high acetate concentration and thermophilic condition

The effect of single walled carbon nanotubes (SWCNT) on methane production under high acetate concentration and thermophilic condition was evaluated. An isotope labeling experiment verified that >85% of methane was generated from syntrophic acetate oxidation (SAO) at 50, 100 and 150 mM acetate and almost 100% at 200 mM. SWCNT addition had little effect on the methanogenesis pathway, whereas it accelerated methane production via decreasing lag phase times and increasing maximum methane production rates. Electrochemical impedance spectroscopy (EIS) results revealed the electrical resistivity of sludge in groups of SWCNT was distinctly smaller than CK groups, indicating higher sludge conductivity was achieved. Further, the results of communities described that Coprothermobacter and Thermacetogenium played the most important role in SAO under all conditions. Meanwhile, the enriched Thermacetogenium and direct interspecies electron transfer (DIET) pathway in SAO consortia contributed to the acceleration of methane production via SWCNT addition.

Microbial Consortiums of Hydrogenotrophic Methanogenic Mixed Cultures in Lab-Scale Ex-Situ Biogas Upgrading Systems under Different Conditions of Temperature, pH and CO

In this study, hydrogenotrophic methanogenic mixed cultures taken from 13 lab-scale ex-situ biogas upgrading systems under different temperature (20–70 °C), pH (6.0–8.5), and CO (0–10%, v/v) variables were systematically investigated. High-throughput 16S rRNA gene sequencing was used to identify the microbial consortia, and statistical analyses were conducted to reveal the microbial diversity, the core functional microbes, and their correlative relationships with tested variables. Overall, bacterial community was more complex than the archaea community in all mixed cultures. Hydrogenotrophic methanogens Methanothermobacter, Methanobacterium, and Methanomassiliicoccus, and putative syntrophic acetate-oxidizing bacterium Coprothermobacter and Caldanaerobacter were found to predominate, but the core functional microbes varied under different conditions. Multivariable sensitivity analysis indicated that temperature (p < 0.01) was the crucial variable to determine the microbial consortium structures in hydrogenotrophic methanogenic mixed cultures. pH (0.01 < p < 0.05) significantly interfered with the relative abundance of dominant archaea. Although CO did not affect community (p > 0.1), some potential CO-utilizing syntrophic metabolisms might be enhanced. Understanding of microbial consortia in the hydrogenotrophic methanogenic mixed cultures related to environmental variables was a great advance to reveal the microbial ecology in microbial biogas upgrading process.

Responses of Methanosarcina barkeri to acetate stress

BACKGROUND

Anaerobic digestion of easily degradable biowaste can lead to the accumulation of volatile fatty acids, which will cause environmental stress to the sensitive methanogens consequently. The metabolic characteristics of methanogens under acetate stress can affect the overall performance of mixed consortia. Nevertheless, there exist huge gaps in understanding the responses of the dominant methanogens to the stress, e.g., Methanosarcinaceae. Such methanogens are resistant to environmental deterioration and able to utilize multiple carbon sources. In this study, transcriptomic and proteomic analyses were conducted to explore the responses of Methanosarcina barkeri strain MS at different acetate concentrations of 10, 25, and 50 mM.

RESULTS

The trend of OD600 and the regulation of the specific genes in 50 mM acetate, indicated that high concentration of acetate promoted the acclimation of M. barkeri to acetate stress. Acetate stress hindered the regulation of quorum sensing and thereby eliminated the advantages of cell aggregation, which was beneficial to resist stress. Under acetate stress, M. barkeri allocated more resources to enhance the uptake of iron to maintain the integrities of electron-transport chains and other essential biological processes. Comparing with the initial stages of different acetate concentrations, most of the genes participating in acetoclastic methanogenesis did not show significantly different expressions except hdrB1C1, an electron-bifurcating heterodisulfide reductase participating in energy conversion and improving thermodynamic efficiency. Meanwhile, vnfDGHK and nifDHK participating in nitrogen fixation pathway were upregulated.

CONCLUSION

In this work, transcriptomic and proteomic analyses are combined to reveal the responses of M. barkeri to acetate stress in terms of central metabolic pathways, which provides basic clues for exploring the responses of other specific methanogens under high organics load. Moreover, the results can also be used to gain insights into the complex interactions and geochemical cycles among natural or engineered populations. Furthermore, these findings also provide the potential for designing effective and robust anaerobic digesters with high organic loads.

Pilot-scale biomethanation of cattle manure using dense membranes

This study aimed at studying the biomethanation process using a 100 L pilot-scale digester equipped with a dense membrane for hydrogen injection. Hydrogen mass transfer was characterized and the impact of hydrogen flowrate, agitation rate and of the co-injection of CO₂, on biogas production and composition, was precisely studied. A linear relationship between H₂ flowrate and the CO₂ and CH₄ rates in biogas was found but no impact on biogas flowrate was shown. It was also noticed that, without exogenous CO₂ injection, and for high H₂ injection flowrates, residual H₂ could be found at the digester outlet due to local CO₂ limitation. Thus, this study suggested that biogas production in biomethanation process at the pilot scale was probably rather limited by the dissolved CO₂ transport within the liquid phase than by the hydrogen mass transfer itself.

Growth Characteristics and Thermodynamics of Syntrophic Acetate Oxidizers

Syntrophic acetate oxidation (SAO) plays a pivotal role in biogas production processes when aceticlastic methanogens are inhibited. Despite the importance of SAO, the metabolic interactions and syntrophic growth of the organisms involved are still poorly understood. Therefore, we studied growth parameters and interactions within constructed defined cocultures comprising the methanogen Methanoculleus bourgensis and one, or several, of the syntrophic acetate oxidizers Syntrophaceticus schinkii, [ Clostridium] ultunense, and Tepidanaerobacter acetatoxydans and a novel, uncharacterized bacterium. Cultivation experiments in a design-of-experiment approach revealed positive effects on methane production rate of increased ammonium levels (up to 0.2 M), temperature (up to 45 °C), and acetate concentrations (0.15-0.30 M). Molecular analyses and thermodynamic calculations demonstrated close interlinkages between the microorganisms, with available energies of -10 kJ/mol for acetate oxidation and -20 kJ/mol for hydrogenotrophic methanogenesis. The estimated generation time varied between 3 and 20 days for all syntrophic microorganisms involved, and the acetate minimum threshold level was 0.40-0.45 mM. The rate of methanogenesis depended on the SAO bacteria present in the culture. These data are beneficial for interpretation of SAO prevalence and competiveness against aceticlastic methanogens in anaerobic environments.

Terminal restriction fragment length polymorphism is an "old school" reliable technique for swift microbial community screening in anaerobic digestion

The microbial community in anaerobic digestion has been analysed through microbial fingerprinting techniques, such as terminal restriction fragment length polymorphism (TRFLP), for decades. In the last decade, high-throughput 16S rRNA gene amplicon sequencing has replaced these techniques, but the time-consuming and complex nature of high-throughput techniques is a potential bottleneck for full-scale anaerobic digestion application, when monitoring community dynamics. Here, the bacterial and archaeal TRFLP profiles were compared with 16S rRNA gene amplicon profiles (Illumina platform) of 25 full-scale anaerobic digestion plants. The α-diversity analysis revealed a higher richness based on Illumina data, compared with the TRFLP data. This coincided with a clear difference in community organisation, Pareto distribution, and co-occurrence network statistics, i.e., betweenness centrality and normalised degree. The β-diversity analysis showed a similar clustering profile for the Illumina, bacterial TRFLP and archaeal TRFLP data, based on different distance measures and independent of phylogenetic identification, with pH and temperature as the two key operational parameters determining microbial community composition. The combined knowledge of temporal dynamics and projected clustering in the β-diversity profile, based on the TRFLP data, distinctly showed that TRFLP is a reliable technique for swift microbial community dynamics screening in full-scale anaerobic digestion plants.

In-situ biogas upgrading with pulse H2 additions: The relevance of methanogen adaption and inorganic carbon level

Surplus electricity from fluctuating renewable power sources may be converted to CH₄ via biomethanisation in anaerobic digesters. The reactor performance and response of methanogen population of mixed-culture reactors was assessed during pulsed H₂ injections. Initial H₂ uptake rates increased immediately and linearly during consecutive pulse H₂ injections for all tested injection rates (0.3 to 1.7L_H2/L_sludge/d), while novel high throughput mcrA sequencing revealed an increased abundance of specific hydrogenotrophic methanogens. These findings illustrate the adaptability of the methanogen population to H₂ injections and positively affects the implementation of biomethanisation. Acetate accumulated by a 10-fold following injections exceeding a 4:1 H₂:CO₂ ratio and may act as temporary storage prior to biomethanisation. Daily methane production decreased for headspace CO₂ concentrations below 12% and may indicate a high sensitivity of hydrogenotrophic methanogens to CO₂ limitation. This may ultimately decide the biogas upgrading potential which can be achieved by biomethanisation.

The full-scale anaerobic digestion microbiome is represented by specific marker populations

Anaerobic digestion is a well-established microbial-based technology for the treatment of organic waste streams and subsequent biogas recovery. A robust and versatile microbial community to ensure overall stability of the process is essential. Four full-scale anaerobic digestion plants were followed for one year to link operational characteristics with microbial community composition and structure. Similarities between digesters, community dynamics and co-occurrence between bacteria and archaea within each digester were analysed. Free ammonia concentration (>200 mg N L^-1) and conductivity (>30 mS cm^-1) hindered acetoclastic methanogenesis by Methanosaetaceae. Thus, methanogenesis was pushed to the hydrogenotrophic pathway carried out by Methanobacteriales and Methanomicrobiales. Firmicutes dominated the overall bacterial community in each of the digesters (>50%), however, principal coordinate analysis of Bray-Curtis indices showed that each of the four digesters hosted a unique microbial community. The uniqueness of this community was related to two phylotypes belonging to the Syntrophomonas genus (Phy32 and Phy34) and to one unclassified bacterium (Phy2), which could both be considered marker populations in the community. A clear differentiation in co-occurrence of methanogens with several bacteria was observed between the different digesters. Our results demonstrated that full-scale anaerobic digestion plants show constant dynamics and co-occurrence patterns in function of time, but are unique in terms of composition, related to the presence of marker populations.

High salinity in molasses wastewaters shifts anaerobic digestion to carboxylate production

Biorefinery wastewaters are often treated by means of anaerobic digestion to produce biogas. Alternatively, these wastewaters can be fermented, leading to the formation of carboxylates. Here, we investigated how lab-scale upflow anaerobic sludge blanket reactors could be shifted to fermentation by changing organic loading rate, hydraulic retention time, pH, and salinity. A strong increase in volatile fatty acid concentration up to 40 g COD L(-1) was achieved through increasing salinity above 30 mS cm(-1), as well as a decrease in methane production by more than 90%, which could not be obtained by adjusting the other parameters, thus, indicating a clear shift from methane to carboxylate production. Microbial community analysis revealed a shift in bacterial community to lower evenness and richness values, following the increased salinity and VFA concentration during the fermentation process. A selective enrichment of the hydrogenotrophic Methanomicrobiales took place upon the shift to fermentation, despite a severe decrease in methane production. Particle size distribution revealed a strong degranulation of the sludge in the reactor, related to the high salinity, which resulted in a wash-out of the biomass. This research shows that salinity is a key parameter enabling a shift from methane to carboxylate production in a stable fermentation process.

Potential for direct interspecies electron transfer in an electric-anaerobic system to increase methane production from sludge digestion

Direct interspecies electron transfer (DIET) between Geobacter species and Methanosaeta species is an alternative to interspecies hydrogen transfer (IHT) in anaerobic digester, which however has not been established in anaerobic sludge digestion as well as in bioelectrochemical systems yet. In this study, it was found that over 50% of methane production of an electric-anaerobic sludge digester was resulted from unknown pathway. Pyrosequencing analysis revealed that Geobacter species were significantly enriched with electrodes. Fluorescence in situ hybridization (FISH) further confirmed that the dominant Geobacter species enriched belonged to Geobacter metallireducens. Together with Methanosaeta species prevailing in the microbial communities, the direct electron exchange between Geobacter species and Methanosaeta species might be an important reason for the "unknown" increase of methane production. Conductivity of the sludge in this electric-anaerobic digester was about 30% higher than that of the sludge in a control digester without electrodes. This study not only revealed for the first time that DIET might be the important mechanism on the methanogenesis of bioelectrochemical system, but also provided a new method to enhance DIET by means of bioelectric enrichment of Geobacter species.

Ammonia and temperature determine potential clustering in the anaerobic digestion microbiome

Anaerobic digestion is regarded as a key environmental technology in the present and future bio-based economy. The microbial community completing the anaerobic digestion process is considered complex, and several attempts already have been carried out to determine the key microbial populations. However, the key differences in the anaerobic digestion microbiomes, and the environmental/process parameters that drive these differences, remain poorly understood. In this research, we hypothesized that differences in operational parameters lead to a particular composition and organization of microbial communities in full-scale installations. A total of 38 samples were collected from 29 different full-scale anaerobic digestion installations, showing constant biogas production in function of time. Microbial community analysis was carried out by means of amplicon sequencing and real-time PCR. The bacterial community in all samples was dominated by representatives of the Firmicutes, Bacteroidetes and Proteobacteria, covering 86.1 ± 10.7% of the total bacterial community. Acetoclastic methanogenesis was dominated by Methanosaetaceae, yet, only the hydrogenotrophic Methanobacteriales correlated with biogas production, confirming their importance in high-rate anaerobic digestion systems. In-depth analysis of operational and environmental parameters and bacterial community structure indicated the presence of three potential clusters in anaerobic digestion. These clusters were determined by total ammonia concentration, free ammonia concentration and temperature, and characterized by an increased relative abundance of Bacteroidales, Clostridiales and Lactobacillales, respectively. None of the methanogenic populations, however, could be significantly attributed to any of the three clusters. Nonetheless, further experimental research will be required to validate the existence of these different clusters, and to which extent the presence of these clusters relates to stable or sub-optimal anaerobic digestion.

Improved Anaerobic Fermentation of Wheat Straw by Alkaline Pre-Treatment and Addition of Alkali-Tolerant Microorganisms

The potential of two alkali-tolerant, lignocellulolytic environmental enrichment cultures to improve the anaerobic fermentation of Ca(OH)₂-pre-treated wheat straw was studied. The biomethane potential of pre-treated straw was 36% higher than that of untreated straw. The bioaugmentation of pre-treated straw with the enrichment cultures did not enhance the methane yield, but accelerated the methane production during the first week. In acidogenic leach-bed fermenters, a 61% higher volatile fatty acid (VFA) production and a 112% higher gas production, mainly CO₂, were observed when pre-treated instead of untreated straw was used. With one of the two enrichment cultures as the inoculum, instead of the standard inoculum, the VFA production increased by an additional 36% and the gas production by an additional 110%, again mainly CO₂. Analysis of the microbial communities in the leach-bed processes revealed similar bacterial compositions in the fermenters with pre-treated straw, which developed independently of the used inoculum. It was suggested that the positive metabolic effects with the enrichment cultures observed in both systems were due to initial activities of the alkali-tolerant microorganisms tackling the alkaline conditions better than the standard inocula, whereas the latter dominated in the long term.

The effects of elevated CO2 concentration on competitive interaction between aceticlastic and syntrophic methanogenesis in a model microbial consortium

Investigation of microbial interspecies interactions is essential for elucidating the function and stability of microbial ecosystems. However, community-based analyses including molecular-fingerprinting methods have limitations for precise understanding of interspecies interactions. Construction of model microbial consortia consisting of defined mixed cultures of isolated microorganisms is an excellent method for research on interspecies interactions. In this study, a model microbial consortium consisting of microorganisms that convert acetate into methane directly (Methanosaeta thermophila) and syntrophically (Thermacetogenium phaeum and Methanothermobacter thermautotrophicus) was constructed and the effects of elevated CO2 concentrations on intermicrobial competition were investigated. Analyses on the community dynamics by quantitative RT-PCR and fluorescent in situ hybridization targeting their 16S rRNAs revealed that high concentrations of CO2 have suppressive effects on the syntrophic microorganisms, but not on the aceticlastic methanogen. The pathways were further characterized by determining the Gibbs free energy changes (ΔG) of the metabolic reactions conducted by each microorganism under different CO2 concentrations. The ΔG value of the acetate oxidation reaction (T. phaeum) under high CO2 conditions became significantly higher than -20 kJ per mol of acetate, which is the borderline level for sustaining microbial growth. These results suggest that high concentrations of CO2 undermine energy acquisition of T. phaeum, resulting in dominance of the aceticlastic methanogen. This study demonstrates that investigation on model microbial consortia is useful for untangling microbial interspecies interactions, including competition among microorganisms occupying the same trophic niche in complex microbial ecosystems.

Bioelectrochemical enhancement of anaerobic methanogenesis for high organic load rate wastewater treatment in a up-flow anaerobic sludge blanket (UASB) reactor

A coupling process of anaerobic methanogenesis and electromethanogenesis was proposed to treat high organic load rate (OLR) wastewater. During the start-up stage, acetate removal efficiency of the electric-biological reactor (R1) reached the maximization about 19 percentage points higher than that of the control anaerobic reactor without electrodes (R2), and CH4 production rate of R1 also increased about 24.9% at the same time, while additional electric input was 1/1.17 of the extra obtained energy from methane. Coulombic efficiency and current recorded showed that anodic oxidation contributed a dominant part in degrading acetate when the metabolism of methanogens was low during the start-up stage. Along with prolonging operating time, aceticlastic methanogenesis gradually replaced anodic oxidation to become the main pathway of degrading acetate. When the methanogens were inhibited under the acidic conditions, anodic oxidation began to become the main pathway of acetate decomposition again, which ensured the reactor to maintain a stable performance. FISH analysis confirmed that the electric field imposed could enrich the H2/H(+)-utilizing methanogens around the cathode to help for reducing the acidity. This study demonstrated that an anaerobic digester with a pair of electrodes inserted to form a coupling system could enhance methanogenesis and reduce adverse impacts.

High concentrations of methyl fluoride affect the bacterial community in a thermophilic methanogenic sludge

To precisely control the application of methyl fluoride (CH₃F) for analysis of methanogenic pathways, the influence of 0–10% CH₃F on bacterial and archaeal communities in a thermophilic methanogenic sludge was investigated. The results suggested that CH₃F acts specifically on acetoclastic methanogenesis. The inhibitory effect stabilized at an initial concentration of 3–5%, with around 90% of the total methanogenic activity being suppressed, and a characteristic of hydrogenotrophic pathway in isotope fractionation was demonstrated under this condition. However, extended exposure (12 days) to high concentrations of CH₃F (>3%) altered the bacterial community structure significantly, resulting in increased diversity and decreased evenness, which can be related to acetate oxidation and CH₃F degradation. Bacterial clone library analysis showed that syntrophic acetate oxidizing bacteria Thermacetogenium phaeum were highly enriched under the suppression of 10% CH₃F. However, the methanogenic community did not change obviously. Thus, excessive usage of CH₃F over the long term can change the composition of the bacterial community. Therefore, data from studies involving the use of CH₃F as an acetoclast inhibitor should be interpreted with care. Conversely, CH₃F has been suggested as a factor to stimulate the enrichment of syntrophic acetate oxidizing bacteria.

Microbial succession during thermophilic digestion: the potential of Methanosarcina sp

A distinct succession from a hydrolytic to a hydrogeno- and acetotrophic community was well documented by DGGE (denaturing gradient gel electrophoresis) and dHPLC (denaturing high performance liquid chromatography), and confirmed by qPCR (quantitative PCR) measurements and DNA sequence analyses. We could prove that Methanosarcina thermophila has been the most important key player during the investigated anaerobic digestion process. This organism was able to terminate a stagnation phase, most probable caused by a decreased pH and accumulated acetic acid following an initial hydrolytic stage. The lack in Methanosarcina sp. could not be compensated by high numbers of Methanothermobacter sp. or Methanoculleus sp., which were predominant during the initial or during the stagnation phase of the fermentation, respectively.

Methanosarcina spp., the key to relieve the start-up of a thermophilic anaerobic digestion suffering from high acetic acid loads

This paper investigates if it is possible to produce inocula to counteract high acetic acid (CH3COO(-)) concentrations during thermophilic anaerobic digestion. To this end, fermenter sludge was exposed for different durations to either gradually increasing CH3COO(-) concentrations or directly exposed to a high concentration (150 mM). Altogether, these enrichments led to inocula with a distinct decrease of representatives of Methanobacteriales, while those of Methanoculleus spp. were hardly affected by any treatment. After the inoculation, good agreements of the abundance of Methanosarcinales and Methanoculleus spp. with total DNA content and methane production rate were apparent. In addition, a gradual adaptation of the inoculum for at least 4 weeks led to a significant increase of Methanosarcina spp. during the subsequent fermentation. These results demonstrate the potential of bioaugmentation to relieve the start-up of an anaerobic digestion suffering from high CH3COO(-) loads, especially pointing to the robust acetoclastic genus Methanosarcina.

Effective reduction of enteric methane production by a combination of nitrate and saponin without adverse effect on feed degradability, fermentation, or bacterial and archaeal communities of the rumen

This study evaluated the effects of Quillaja saponin (0.6 and 1.2g/L), propynoate (4 and 8mM), and nitrate (5 and 10mM), alone or in combinations, on methanogenesis, fermentation, bacterial and archaeal communities, and abundances of select ruminal microbial populations. All treatment decreased methane production, but combination of all three inhibitors at high dose achieved the greatest inhibition (85%). Propynoate, alone or in combination with nitrate or saponin, decreased feed degradability and total volatile fatty acid (TVFA) concentrations. However, saponin and nitrate alone at high dose and in combination at low dose inhibited methanogenesis substantially while increasing feed degradability and TVFA concentrations. The abundances of methanogens were lowered by all inhibitors except saponin alone. Fibrobacter succinogenes and Ruminococcus flavefaciens were increased by saponin, both alone and in combination with nitrate, but inhibited by propynoate. Combination of saponin and nitrate may have practical application in mitigating methane emission from ruminants.

Performance and microbial community analysis of the anaerobic reactor with coke oven gas biomethanation and in situ biogas upgrading

A new method for simultaneous coke oven gas (COG) biomethanation and in situ biogas upgrading in anaerobic reactor was developed in this study. The simulated coke oven gas (SCOG) (92% H2 and 8% CO) was injected directly into the anaerobic reactor treating sewage sludge through hollow fiber membrane (HFM). With pH control at 8.0, the added H2 and CO were fully consumed and no negative effects on the anaerobic degradation of sewage sludge were observed. The maximum CH4 content in the biogas was 99%. The addition of SCOG resulted in enrichment and dominance of homoacetogenetic genus Treponema and hydrogenotrophic genus Methanoculleus in the liquid, which indicated that H2 were converted to methane by both direct (hydrogenotrophic methanogenesis) and indirect (homoacetogenesis+aceticlastic methanogenesis) pathways in the liquid. However, the aceticlasitic genus Methanosaeta was dominant for archaea in the biofilm on the HFM, which indicated indirect (homoacetogenesis+aceticlastic methanogenesis) H2 conversion pathway on the biofilm.