Methanogenic Metabolism Research Articles

Enhancing proton-coupled electron transfer drives efficient methanogenesis in anaerobic digestion

The enhancement of electron or proton transfer between syntrophic microbes has been widely recognised as a means for improving methane generation. However, the uncoupled supplementation of electrons and protons in multiphase anaerobic environment hinders the balanced uptake of electrons and protons in the cytoplasm of methanogens, limiting methanogenesis efficiency. Herein, the cooperative effect of a proton-conductive material (PM) and an electron-conductive material (EM) in enhancing proton-coupled electron transfer (PCET) and driving efficient methanogenesis in anaerobic digestion was investigated. The cooperation of the PM and EM significantly increased methane production and the maximum methane generation rate by 78.9 % and 103.5 %, respectively, indicating enhanced methanogenesis efficiency. Analysis of the physicochemical properties, biochemical components, and microbial dynamics revealed that the cooperation of the PM and EM improved the metabolism of syntrophic microbes, which was critically dependent on electron and proton transfer. This enhancement was primarily due to the improvement in PCET, as mainly supported by hydrogen/deuterium kinetic isotope effect measurements, multi-omics integration analyses and reaction thermodynamics and kinetics analyses. Our findings suggest that the PCET enhancement stimulated efficient membrane-bound enzymatic reactions related to electron-driven proton translocation and facilitated electron and proton supply for CO2 reduction to realise highly efficient methane generation. These findings are expected to provide a new insight into effective electron and proton coupling transfer for methanogenic metabolism in multiphase anaerobic environments.

Synergistic effect of Ca(NO3)2 - CaO2 on the improvement of sediment microecology: Transformation of sulfur and ferrous iron, and improvement mechanism

The synergy of Ca(NO3)2 and CaO2 can effectively reduce the risks of secondary pollution caused by nitrate escape and release. However, the timing and suitable dosage of Ca(NO3)2 and CaO2 during the synergistic interaction have not been fully investigated. In this work, we separated acid volatile sulfides (AVS) oxidation and ferrous iron oxidation by controlling the dosage of Ca(NO3)2. Results indicated that at a low RN/1.6S2−ratio, there was an obvious competitive relationship between AVS oxidation and ferrous iron oxidation, and sulfide was oxidized prior to ferrous iron. More efficient AVS and ferrous iron oxidation was achieved using Ca(NO3)2 to oxidize AVS and CaO2 to oxidize ferrous iron. After the treatment with Ca(NO3)2 and CaO2, the anaerobic reduction environment of the sediment was modified, and the metabolic pathways dominated by sulfate respiration and methanogenic metabolism in the sediment were gradually converted to nitrate-reduction and dissimilatory iron reduction. The synergy of Ca(NO3)2 and CaO2 could reduce 99.61 % CH4 compared with control check and could reduce 81.66 % N2O compared with adding Ca(NO3)2 only. Furthermore, the biodegradability of organic matter was improved due to the transformation of stubborn organic matter. These results provide valuable guidance for applying Ca(NO3)2 and CaO2 in sediment remediation.

Extracellular-proton-transfer driving high energy-conserving methanogenesis in anaerobic digestion

Anaerobic digestion (AD) is a promising technology to realize the conversion from organic matters to methane, which is highly mediated by syntrophic microbial community via mutualistic interactions. However, small energy available in methanogenic conversion usually limits the metabolic activity. To adapt such energy-limited environment, efficient energy conservation is critical to support active physiological functions of anaerobic consortia for methanogenic metabolism. In this study, the contribution of extracellular proton transfer (EPT) enhancement to achieving energy-conserving methanogenesis in AD was explored. Proton-conductive medium (PCM) was applied to construct efficient proton transport pathway, and a large number of protons from extracellular water were found available to upregulate methanogenesis in AD, as indicated by the increase in the content of 2H (D) in methane molecules (over 40.7%), among which CO2-reduction-to-CH4 was effectively enhanced. The increases of adenosine triphosphate (ATP) concentration (+54.1%) and gene expression activities related to ATPase (+100.0%) and proton pump (+580.1%) revealed that enhanced EPT by PCM promoted transmembrane proton motive force generation to facilitate ATP synthesis. Based on genome-centric metatranscriptomic analyses, MAG14, MAG63 and MAG61 with high energy conservation activity displayed most pronounced positive response to the EPT enhancement. In these core MAGs, the metabolic pathway reconstruction and the key genes activity identification further proved that EPT enhancement-driven efficient ATP synthesis stimulated the cross-feeding of carbon and proton/electron to facilitate microbial mutualism, thereby resulting in the high energy-conserving methanogenesis. Overall, our work provides new insights into how EPT enhancement drives high energy-conserving methanogenesis, expanding our understanding of the ecological role of EPT in AD.

Biomethanisation of sewage sludge: Sonication pretreatment and monitoring of microbial communities

The improvement of mesophilic biomethanisation of recalcitrant sewage sludge derived from urban wastewater treatment through the application of a sonication pretreatment was evaluated in parallel in two pilot-scale anaerobic digesters (two biological replicates: reactors RA and RB). The valorisation process was monitored through a novel and holistic approach that related the biomethanisation yield, and its main batch operational parameters, with the abundance of archaeal and bacterial communities in the anaerobic inocula. Sonication allowed achieving a methane yield coefficient derived from sewage sludge of 240 ± 20 mLSTPCH4/g VS (volatile solids) at the load range of 0.8–4.0 g VS/L in both reactors. The process was more stable in reactor B, with a wider range of loads being allowed (up to 5.29 g VS/L). Monitoring the presence of Archaea in the mixing liquor revealed a variation in their abundance throughout the process which was directly related to the availability of organic matter and pH. Advanced metagenomic analysis showed the phylogenetic and functional diversity of the complex microbiome involved. While Bacteria were widely distributed in 35 phyla, Archaea fitted in only two. Euryarchaeota was the majoritarian archaeal phylum (99.5 %) and its more abundant families are linked to methanogenic metabolism. Functional analysis revealed several relevant metabolic pathways that followed similar trends in both reactors. “Methane metabolism” clearly diminished at the end of the process in concordance with the exhaust of methane generation, while “ABC transporters” or “two-component systems”, involved in bacterial survival to changing environments, followed the opposite pattern. This integrated approach could help to increase the methanogenic valorisation of sewage sludge.

Response of methanogenic metabolism to polystyrene microplastics at varying concentrations: The trade-off between inhibitory and protective effects in anaerobic digestion

Polystyrene (PS) is an important raw material of food packages, and its induced microplastics (MPs) inevitably enter into the food waste (FW) with mechanical or hydrothermal pretreatment, which may affect the FW anaerobic digestion (AD). However, the different microbial responses to PS MPs in AD system have rarely been reported at varying PS MPs levels. In this study, the trade-off between positive and negative effects of PS MPs on syntrophic methanogenesis in AD for FW treatment was investigated when the dosage varied from 10 to 200 mg/L. The results showed that methane (CH4) yield was upgraded by 4.72% at a lower PS MPs concentration (25 mg/L) compared with the control reactor without PS MPs dosing. In contrast, it would be inhibited by 10.13% when PS MPs concentration increased to 100 mg/L. More secretion of extracellular polymeric substances was the main reason for methanogenic metabolism enhancement at lower dosages of PS MPs due to its characteristics of cell-protection and electron transfer facilitation. Furthermore, the higher abundance of genes regarding superoxide dismutase and catalase in the reactor with PS MPs of 25 mg/L could impede reactive oxygen species formation and alleviate its toxicity. The findings of the study provide a comprehensive understanding of the promotion and inhibition mechamisms of syntrophic metabolism in the AD process by PS MPs and their released substances at varying concentrations.

Effect of Pretreatment of Anaerobic Acid Fermentation Sludge from Mixed Swine Wastewater and Food Waste Leachate on Biohydrogen Generation

Objectives:This study aims to investigate the effects of heat shock and 2-bromoethanesulfonate (2-BES) injection on hydrogen production and methanogens inhibition in anaerobic acid fermenter sludge and methane fermenter sludge using mixed swine wastewater and food waste leachate as substrates. Additionally, the analysis of biogas, fermentation products such as volatile fatty acids (VFAs) and alcohols, and microbial community changes were conducted to derive efficient pretreatment conditions for anaerobic hydrogen generation.Methods:Sludge samples were collected from two-phase anaerobic digesters treating swine wastewater and food waste leachate. Heat shock (90°C, 15 min) and 2-BES injection (1, 5, 10, 50, and 100 mmol/L) pretreatments were applied to the each sludge, along with control reactor without pretreatment before anaerobic digestion. Batch experiments were conducted at 30°C to assess biogas, organic compounds (COD, VS, VFAs, and alcohols), and microbial community composition.Results and Discussion:In methanogenic sludge, hydrogen gas was not produced under all conditions, but heat shock method inhibited methane production more effectively than 2-BES injection method. As the concentration of 2-BES increased, methane production decreased. Conversely, hydrogen was produced from the all acid fermentation sludges, mainly attributed to the butyric acid production by dominant Clostridium spp., but the pattern of biogas production varied over time. In the experimental group injected with 2-BES, even at a low concentration of 1 mmol/L, methane gas did not produce due to the inhibition of methanogens. However, in the control group, produced hydrogen was entirely converted to methane by hydrogenotrophic methanogens such as Methanobrevibacter and Methanosphaera. The heat shock group showed delayed hydrogen production due to prolonged lag phase of hydrogen-producing bacteria, with hydrogenotrophic methanogen metabolism starting after 12 days. Heat shock and 2-BES injection reduced bacterial diversity in the acid fermentation process compared to the initial stage.Conclusion:For efficient anaerobic hydrogen production from swine wastewater and food waste leachate, 2-BES injection into the acid fermentation sludge was found to be a preferable method over heat shock, considering hydrogen production efficiency and sustained methanogen inhibition. Further research is needed to find more economical and efficient pretreatment methods and operational strategies considering subsequent methane production processes for practical application in the field.

Mechanistic understanding of acclimation and energy metabolism of acetoclastic methanogens under different substrate to microorganism ratios

Mechanistic understanding of acetoclastic methanogenesis is pivotal for optimizing anaerobic digestion for efficient methane production. In this study, two different operational modes, continuous flow reactor (CFR) and sequencing batch reactor (SBR), accompanied with solids retention times (SRT) of 10 days (SBR10d and CFR10d) and 25 days (SBR25d and CFR25d) were implemented to elucidate their impacts on microbial communities and energy metabolism of methanogens in acetate-fed systems. Microbial community analysis revealed that the relative abundance of Methanosarcina (16.0%–46.0%) surpassed Methanothrix (3.7%–22.9%) in each reactor. SBRs had the potential to enrich both Methanothrix and Methanosarcina. Compared to SBRs, CFRs had lower total relative abundance of methanogens. Methanosarcina exhibited a superior enrichment in reactors with 10-day SRT, while Methanothrix preferred to be acclimated in reactors with 25-day SRT. The operational mode and SRT were also observed to affect the distribution of acetate-utilizing bacteria, including Pseudomonas, Desulfocurvus, Mesotoga, and Thauera. Regarding enzymes involved in energy metabolism, Ech and Vho/Vht demonstrated higher relative abundances at 10-day SRT compared to 25-day SRT, whereas Fpo and MtrA-H showed higher relative abundances in SBRs than those in CFRs. The relative abundance of genes encoding ATPase harbored by Methanothrix was higher than Methanosarcina at 25-day SRT. Additionally, the relative abundance of V/A-type ATPase (typically for methanogens) was observed higher in SBRs compared to CFRs, while the F-type ATPase (typically for bacteria) exhibited higher relative abundance in CFRs than that in SBRs.

Magnetic porous microspheres enhancing the anaerobic digestion of sewage sludge: Synergistic free and attached methanogenic consortia

The addition of exogenous materials is a commonly reported method for promoting the anaerobic digestion (AD) of sludge. However, most exogenous materials are nano-sized and their use encounters problems relating to a need for continuous replenishment, uncontrollability and non-recyclability. Here, magnetic porous microspheres (MPMs), which can be controlled by magnetic forces, were prepared and used to enhance the methanogenesis of sludge. It was observed that the MPMs were spherical particles with diameters of approximately 100 µm and had a stable macroporous hybrid structure of magnetic cores and polymeric shells. Furthermore, the MPMs had good magnetic properties and a strong solid–liquid interfacial electron transfer ability, suggesting that MPMs are excellent carriers for methanogenic consortia. Experimental results showed that the addition of MPMs increased methane production and the proportion of methane in biogas from AD by 100.0 % and 21.2 %, respectively, indicating the MPMs notably enhanced the methanogenesis of sludge. Analyses of variations in key enzyme activities and electron transfer in sludge samples with and without MPMs in AD revealed that the MPMs significantly enhanced the activities of key enzymes involved in hydrolysis, acidification and methanation. This was achieved mainly by enhancing the extracellular electron transfer to strengthen the proton motive force on the cell membrane, which provides more energy generation for methanogenic metabolism. A careful examination of the variations in the morphology, pore structure and magnetism of the MPMs before and after AD revealed that the MPMs increased the prevalence of many highly active anaerobes, and that this did not weaken the magnetic performance. The microbial community structure and metatranscriptomic analysis further indicated that the acetotrophic methanogens (i.e., Methanosaeta) were mainly in a free state and that CO2-reducing methanogens (i.e., Methanolinea and Methanobacterium) mainly adhered to the MPMs. The above synergistic metabolism led to efficient methanogenesis, which indicates that the MPMs optimised the spatial ecological niche of methanogenic consortia. These findings provide an important reference for the development of magnetic porous materials promoting AD.

Biomass Storage in Anoxic Marine Basins: Initial Estimates of Geochemical Impacts and CO2 Sequestration Capacity

AbstractIn combination with dramatic and immediate CO2 emissions reductions, net‐negative atmospheric CO2 removal (CDR) is necessary to maintain average global temperature increases below 2.0°C. Many proposed CDR pathways involve the placement of vast quantities of organic carbon (biomass) on the seafloor in some form, but little is known about their potential biogeochemical impacts, especially at scales relevant for global climate. Here, we evaluate the potential impacts and durability of organic carbon storage specifically within deep anoxic basins, where organic matter (OM) is remineralized through anaerobic processes that may enhance its storage efficiency. We present simple biogeochemical and mixing models to quantify the scale of potential impacts of large‐scale OM addition to the abyssal seafloor in the Black Sea, Cariaco Basin, and the hypersaline Orca Basin. These calculations reveal that the Black Sea in particular may have the potential to accept biomass storage at climatically relevant scales with moderate changes to the geochemical state of abyssal water and limited communication of that impact to surface water. Still, all of these systems would require extensive further evaluation prior to consideration of megatonne‐scale CO2 sequestration. Many key unknowns remain, including the partitioning of breakdown among sulfate‐reducing and methanogenic metabolisms and the fate of methane in the environment. Given the urgency of responsible CDR development and the potential for anoxic basins to reduce ecological risks to animal communities, efforts to address knowledge gaps related to microbial kinetics, benthic processes, and physical mixing in these systems are critically needed.

Metabolic mechanism of thermal pretreatment combined with ethanol pre-fermentation to enhance anaerobic digestion of food waste: A comprehensive analysis of key enzyme-encoding genes

Ethanol pre-fermentation (EP) can enhance methanogenic efficiency and facilitate food waste (FW) hydrolysis, while ensuring anaerobic digestion (AD) system stability. FW contains high-water content, and thermal pretreatment (TP) can significantly enhance solids and soluble organic matter content. The study focused on investigating the mechanism of TP combined with EP (TP-EP) on the AD of FW. The results showed that TP-EP enriched hydrolyzing and acid-producing bacteria, as well as the methanogens Methanosarcina, and Methanothrix. Consequently, the degradation rates of soluble chemical oxygen demand, volatile fatty acids, and cumulative methane yield exhibited an increase of 9.54%, 5.94%, and 20.42%, respectively, in comparison to the control. EP promoted carbohydrate metabolism, while TP promoted amino acid metabolism and enhanced energy metabolism by enriching F-type ATPase and NADHase. When combined as TP-EP, the processes integrated the advantages and promoted substrate hydrolysis and energy metabolism while enhancing methanogenic metabolism.

Electronic regulation to achieve efficient anaerobic digestion of organic fraction of municipal solid waste (OFMSW): strategies, challenges and potential solutions

Anaerobic digestion (AD) of organic fraction of municipal solid waste (OFMSW) is prone to system breakdown under high organic loading rates (OLRs) condition, which subsequently reduces the efficiency of digestion process and results in substantial economic losses. In this perspective paper, the substances metabolisms, electrons flow, as well as microbial interaction mechanisms within AD process are comprehensively discussed, and the underlying bottleneck that causes inefficient methane production is identified, which is “electrons surplus”. Systems encountering severe electron surplus are at risk of process failure, making it crucial to proactively prevent this phenomenon through appropriate approaches. On this basis, the present perspective proposes three potential electronic regulation strategies to prevent electrons surplus, namely, electron shunt, accelerating electron transfer and regulating methanogenic metabolism pathway, and presents specific methodologies for each strategy. Furthermore, the potential solutions to challenges that may occur during the electronic regulation process are also presented in this paper. This perspective aims to provide innovative approaches to achieve the efficient and stable operation of OFMSW anaerobic digestion, especially under high OLRs condition.

Iron-carbon micro-electrolysis material enhanced high-solid anaerobic digestion: Performance and microbial mechanism

Iron-carbon micro-electrolysis (ICME) materials are widely used as composite conductive materials for wastewater treatment to promote the degradation of volatile solids (VS), but there is virtually no research on their effect on high-solid anaerobic digestion (HSAD). This study aimed to investigate the effect of 10–50 g/L ICME dosages on HSAD and to reveal the microbial enhancement mechanism. The batch experiments (working volume of 400 mL) were conducted in a constant temperature water bath shaker. The results showed that the ICME materials promoted methane production by HSAD. The highest methane production rate was 297.79 mL/ g VS at a dosage of 50 g/L, which was 28% higher than that of the control. Scanning electron microscopy results showed that the ICME materials increased the attachment area of microorganisms. Further analyses of microbial community indicated that ICME materials facilitate symbiotic metabolism of syntrophic acetate-oxidizing bacteria (Synergistaceae) and hydrogenotrophic methanogens (Methanospirillum). This potentially promotes the conversion of the acetoacetic methanogenesis pathway to the syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathways. This study revealed ICME material enhancement in HSAD as well as new insights into the microbial mechanisms.

Heat-enhanced sulfite pretreatment improves the release of soluble substances and the stimulation of methanogenic pathways for anaerobic digestion of waste activated sludge

Waste activated sludge pretreatment is effective in improving sludge hydrolysis rate and methane production. Our previous study revealed that sulfite could be a viable method for sludge pretreatment. To further improve methane production, this study employed an integrated strategy by combining sulfite and heat pretreatment. WAS from a local full-scale plant was pretreated with heat-enhanced sulfite pretreatment (HESP) at 25, 35, 55 °C. Compared with the control (no sulfite, 25 °C), soluble substances (e.g., chemical oxygen demand, protein, polysaccharides, nitrogen and phosphorus) could be significantly enhanced by the absence of sulfite and the increasing temperature. A variation of − 8.32–19.90% in cumulative methane production was observed. Moreover, methane production could be significantly enhanced by 14.15% at sulfite of 300 mg S/L and temperature of 35 °C, while kinetic analysis showed that the highest methane production potential (1.43 times of the control) was obtained at this level. Besides, the microbial structure and pathways reveal that HESP promotes the enrichment of methanogenic bacteria and facilitates methanogenic metabolism. The findings of this study suggest that the HESP could be promising in enhancing methane production, particularly considering the reuse of sulfite-laden industrial wastes and the heat as the by-products of the anaerobic digestion plant with biogas power generation, but certain experimental conditions should be verified case by case.

Deeper insights into the synergy of material transformation, microbial network, and energy balance during pilot thermophilic and mesophilic dry anaerobic digestion systems

The present study investigated the synergistic characteristics between abiotic and biotic transformation with a view to improving the methane production efficiency of thermophilic and mesophilic sequencing batch dry anaerobic digestion (SBD-AD). The pilot scale experiment consisted of a lignocellulosic material based on a mixture of corn straw and cow dung. A leachate bed reactor was used for an AD cycle of 40 days. Several distinct differences are reflected in biogas (methane) production and VFA concentration and composition. A combination of first-order hydrolysis and a modified Gompertz model determined that the holocellulose (cellulose + hemicellulose) and maximum methanogenic efficiency at thermophilic temperatures were increased by 112.03 % and 90.09 %, respectively. Additionally, the methane production peak was extended by 3–5 days in comparison with that at mesophilic temperatures. The microbial community exhibited vastly different functional network relationships under the two temperature conditions (P < 0.05). The data indicate that Clostridales and Methanobacteria had preferable synergistic effects and that the metabolism of hydrophilic methanogens is necessary for the conversion of VFA to methane during thermophilic SBD-AD. The effect of mesophilic conditions on Clostridales was relative weakened, and acetophilic methanogens were mainly present. Moreover, simulation of the full-chain and operational strategy of SBD-AD engineering resulted in a decrease in heat energy consumption of 21.4–64.3 % at thermophilic temperatures and 30.0–90.0 % at mesophilic temperatures from winter to summer. Furthermore, the total net energy production of thermophilic SBD-AD was increased by 105.2 % in comparison with that at mesophilic temperatures, demonstrating strengthened energy recovery. Overall, raising the SBD-AD temperature to thermophilic levels has considerable application value for improving the treatment capacity of agricultural lignocellulosic waste.

Potential for homoacetogenesis via the Wood-Ljungdahl pathway in Korarchaeia lineages from marine hydrothermal vents.

The Wood-Ljungdahl pathway (WLP) is a key metabolic component of acetogenic bacteria where it acts as an electron sink. In Archaea, despite traditionally being linked to methanogenesis, the pathway has been found in several Thermoproteota and Asgardarchaeota lineages. In Bathyarchaeia and Lokiarchaeia, its presence has been linked to a homoacetogenic metabolism. Genomic evidence from marine hydrothermal genomes suggests that lineages of Korarchaeia could also encode the WLP. In this study, we reconstructed 50 Korarchaeia genomes from marine hydrothermal vents along the Arctic Mid-Ocean Ridge, substantially expanding the Korarchaeia class with several taxonomically novel genomes. We identified a complete WLP in several deep-branching lineages, showing that the presence of the WLP is conserved at the root of the Korarchaeia. No methyl-CoM reductases were encoded by genomes with the WLP, indicating that the WLP is not linked to methanogenesis. By assessing the distribution of hydrogenases and membrane complexes for energy conservation, we show that the WLP is likely used as an electron sink in a fermentative homoacetogenic metabolism. Our study confirms previous hypotheses that the WLP has evolved independently from the methanogenic metabolism in Archaea, perhaps due to its propensity to be combined with heterotrophic fermentative metabolisms.

Electromotive force induced by dynamic magnetic field electrically polarized sediment to aggravate methane emission

As a primary driving force of global methane production, methanogens like other living organisms are exposed to an environment filled with dynamic electromagnetic waves, which might induce electromotive force (EMF) to potentially influence the metabolism of methanogens. However, no reports have been found on the effects of the induced electromotive force on methane production. In this study, we found that exposure to a dynamic magnetic field enhanced bio-methanogenesis via the induced electromotive force. When exposed to a dynamic magnetic field with 0.20 to 0.40 mT of intensity, the methane emission of the sediments increased by 41.71%. The respiration of methanogens and bacteria was accelerated by the EMF, as the ratios of F420H2/F420 and NAD+/NADH of the sediment increased by 44.12% and 55.56%, respectively. The respiratory enzymes in respiration chains might be polarized with the EMF to accelerate the proton-coupled electron transfer to enhance microbial metabolism. Together with the enriched exoelectrogens and electrotrophic methanogens, as well as the increased sediment electro-activities, this study indicated that the EMF could enhance the electron exchange among extracellular respiratory microorganisms to increase the methane emission from sediments.

Effect of extreme pH conditions on methanogenesis: Methanogen metabolism and community structure

The control of pH is effective for inhibiting methanogenesis in the chain elongation fermentation (CEF) system. However, obscure conclusions exist especially with regard to the underlying mechanism. This study comprehensively explored the responses of methanogenesis in granular sludge at various pH levels, ranging from 4.0 to 10.0, from multiple aspects including methane production, methanogenesis pathway, microbial community structure, energy metabolism and electron transport. Results demonstrated that compared with that at pH 7.0, pH at 4.0, 5.5, 8.5 and 10.0 triggered a 100%, 71.7%, 23.8% and 92.1% suppression on methanogenesis by the end of 3 cycles lasting 21 days. This might be explained by the remarkably inhibited metabolic pathways and intracellular regulations. To be more specific, extreme pH conditions decreased the abundance of the acetoclastic methanogens. However, obligate hydrogenotrophic and facultative acetolactic/hydrogenotrophic methanogens were significantly enriched by 16.9%-19.5 fold. pH stress reduced the gene abundance and/or activity of most enzymes involved in methanogenesis such as acetate kinase (by 81.1%–93.1%), formylmethanofuran dehydrogenase (by 10.9%–54.0%) and tetrahydromethanopterin S-methyltransferase (by 9.3%–41.5%). Additionally, pH stress suppressed electron transport via improper electron carriers and decreased electron amount as evidenced by 46.3%–70.4% reduced coenzyme F420 content and diminished abundance of CO dehydrogenase (by 15.5%–70.5%) and NADH:ubiquinone reductase (by 20.2%–94.5%). pH stress also regulated energy metabolism with inhibited ATP synthesis (e.g., ATP citrate synthase level reduced by 20.1%–95.3%). Interestingly, the protein and carbohydrate content secreted in EPS failed to show consistent responses to acidic and alkaline conditions. Specifically, when compared with pH 7.0, the acidic condition remarkably reduced the levels of total EPS and EPS protein while both levels were enhanced in the alkaline condition. However, the EPS carbohydrate content at pH 4.0 and 10.0 both decreased. This study is expected to promote the understanding of the pH control-induced methanogenesis inhibition in the CEF system.

Thermophilic anaerobic digestion of kitchen waste driven by microbial electrolysis cell (MEC): Digestion performances, anodic microorganisms’ distribution and current utilization efficiency

The aims of present study were to investigate the performances of kitchen waste thermophilic anaerobic digestion driven by microbial electrolysis cell (MEC), and to clarify the methanogenic metabolism pathways on the electrodes (especially on the anodic biofilm). The results showed that both 0.6 V and 1.0 V voltages supplementation could enhance methane yields and production rates, while efficient methanogenesis failed to be sustained after power outages under high organic loading rate (OLR, 4.22 gVS/L/day) condition. Voltages supplementation could also enhance VS removal efficiencies from 83.24–83.35% to 87.01–87.46%, and strengthened the hydrolysis and acidogenesis of dissolved organic matters (DOMs) on the outer layer of anodic biofilm. Voltages supplementation accelerated the β-oxidation of volatile organic acids (VFAs) on the inner layer of anodic biofilm and facilitated acetoclastic methanogenesis on the outer layer of anodic biofilm. The generated electrons on the anode were transferred to the cathode for producing hydrogen as a substrate of hydrogenotrophic methanogenesis, rather than directly generating methane by reducing carbon dioxide through direct interspecies electron transfer (DIET) pathway. On the anodic biofilm, the syntrophic relationship between Syntrophomonas and Methanosarcina was confirmed as the main contributor of methanogenesis. Analysis of current utilization efficiency proved that a little amount of energy input could lead several folds of current utilization efficiencies in return, especially under high OLRs condition, which was anticipated to be applied into practical engineering of kitchen waste thermophilic digestion for higher economic benefits.

Microbial indicators along a metallic contamination gradient in tropical coastal sediments.

The structure and diversity of microbial community inhabiting coastal sediments reflect the exposition to contaminants. Aiming to assess the changes in the microbiota from Sepetiba Bay (SB, Brazil) sediments, correlations between the 16S rRNA gene data (V4-V5 region), metal contamination factors (CF), and the ecological risk classification provided by the Quality Ratio (QR) index were considered. The results show that microbial diversity differs significantly between the less (SB external sector) and the most (SB internal sector) polluted sectors. Also, differences in the microbial community structure regarding the ecological risk classifications validated the QR index as a reliable tool to report the SB chronic contamination. Microbial indicator genera resistant to metals (Desulfatiglans, SEEP-SRB1, Spirochaeta 2, among others) presented mainly anaerobic metabolisms. These genera are related to the sulfate reducing and methanogenic metabolisms probably participating in the natural attenuation processes but also associated with greenhouse gas emissions. In contrast, microbial indicator genera sensitive to metals (Rubripirellula, Blastopirellula, Aquibacter, among others) presented mainly aerobic metabolisms. It is suggested that future works should investigate the metabolic functions to evaluate the influence of metallic contaminants on microbial community inhabiting SB sediment.

Response of methanogenic system to long-term polycyclic aromatic hydrocarbon exposure: Adsorption and biodegradation, performance variation, and microbial function assessment

Phenanthrene (PHE) as a typical polycyclic aromatic hydrocarbon (PAH) is prevalent and harmful to organisms in petroleum-polluted sites. The effects of PHE concentration levels on performance, microbial community and functions in methanogenic system were comprehensively investigated by an operation of UASB reactor (198 days) and a series of batch tests. The results found that PHE was prone to accumulate in reactor by sludge adsorption (Final concentration = 12.53 mg/g TS Sludge), which posed significant influences on methanogenic system. The removal of chemical oxygen demand (COD), NH4+-N and volatile fatty acids (VFAs) in reactor were reduced with PHE accumulation. Meanwhile, microbes with higher ATPase secrete more EPS activity to self-protect against PHE toxicity. Sequencing analysis showed that PHE interfered significantly diversity and structure of microbial community. For bacteria, PHE was toxic to Bacteroidetes and Latescibacteria, while syntrophs (f_Syntrophaceae, Syntrophorhabdus, etc.) involved in VFAs oxidation and aromatic organics degradation were tolerant of PHE stress. For archaea, acetoclastic methanogens (Methanosaeta) abundance was continuously diminished by 45.1% under long-term PHE exposure. Further functions analysis suggested that microbial community accelerated amino acid metabolism, energy metabolism and xenobiotics biodegradation & metabolism to satisfy physiological demanding under PHE stress. Combining batch tests of methanogenic metabolism proved that acetoclastic methanogenesis was negatively affected by PHE due to inhibition of functional enzymes (acetate kinase, phosphate acetyltransferase, etc.) expression. These findings may provide the basis for enhancing bioremediation of PAH pollution in anaerobic environment.