Acetotrophic Methanogenesis Research Articles

Optimizing mass flow and extracellular electron transfer to promote energy recovery during treating municipal wastewater in a biochar enhanced anaerobic membrane bioreactor (AnMBR)

The anaerobic membrane bioreactor (AnMBR) is an alternative technology with energy neutrality potential when treating municipal wastewater (MWW). However, the poor bio-reaction kinetic conditions caused by low COD concentrations in MWW greatly limits the CH4 yield. In this study, the average methanogenesis rate was significantly increased from 164.8 to 275.5 mL/g COD with the two times biochar addition of 5 g/L. And following the second time addition of 5 g/L biochar, the CH4 component significantly increased to a high level of 89.0 ± 6.3 %. In addition, the rate for acetate converted into CH4 increased from 3.3 to 10.5 mmol/(g VS·d) with biochar addition of 10 g/L, indicating that acetotrophic methanogenesis route was significantly promoted by biochar. Bio-electrochemistry analysis showed that biochar accelerated 0.74 mol e-/g VS/min of electron transfer velocity in AD system. The addition of biochar enriched organic metabolism bacteria such as Ancinetobacter_lwoffii, Comamonas and Geobacter, and upregulated the expression of functional genes related to organics degradation such as lldD, sucD and fumC, facilitates the direct transfer of electrons and acetates produced by Comamonas and Geobacter to convert CO2 into CH4 through microbial mediation. Thus, the COD flow shifted significantly towards more CH4 production. The COD mass flow showed that biochar facilitated more bio-catabolism (up about 52.7 %) and less bio-anabolism (down about 92 %), and more COD in wastewater was converted to CH4, resulting a significant promoting of energy production. The energy balance analysis indicated that the net energy consumption was reduced by 15 % after two times addition of 5 g/L biochar, and the key to achieve the net energy output during treating MWW is the efficient operation of AnMBR at ambient temperature.

Mechanistic exploration of COVlD-19 antiviral drug ritonavir on anaerobic digestion through experimental validation coupled with metagenomics analysis

Aggregation of antiviral drugs (ATVs) in waste activated sludge (WAS) poses considerable environmental risk, so it is crucial to understand the behavior of these agents during WAS treatment. This study investigated the effects of ritonavir (RIT), an ATV used to treat human immunodeficiency virus infection and coronavirus disease 2019, on anaerobic digestion (AD) of WAS to reveal the mechanisms by which it interferes with anaerobic flora. The dosage influence results showed that methane production in AD of WAS decreased by 46.56 % when RIT concentration was increased to 1000 μg/kg total suspended solids (TSS). The AD staging test revealed that RIT mainly stimulated microbial synthesis of the extracellular polymeric substance (EPS), limiting organic matter solubilization. At 500 μg/kg TSS, RIT decreased CHO and CHON levels in dissolved organic matter by 23.12 % and 56.68 %, respectively, significantly reducing substrate availability to microorganisms. Metagenomic analysis of microbial functional gene sets revealed that RIT had greater inhibitory effects on protein and amino acid metabolism than on carbohydrate metabolism. Under RIT stress, methanogens switched from hydrogenotrophic and acetotrophic methanogenesis to methylotrophic and acetotrophic methanogenesis.

Effects of Fe-modified digestate hydrochar at different hydrothermal temperatures on anaerobic digestion of swine manure

Hydrothermal carbonization temperature is a key factor in controlling the physico-chemical properties of hydrochar and affecting its function. In this study, effects of hydrochar and Fe-modified hydrochar (Fe-HC) prepared at 180 °C (180C-Fe), 220 °C (220C-Fe) and 260 °C (260C-Fe) on anaerobic digestion (AD) performance of swine manure was investigated. Among the three Fe-HCs, 220C-Fe had the highest amount of Fe and Fe2+ on the surface. The relative methane production of control reached 174 %-189 % in the 180C-Fe and 220C-Fe treatments between days 11 and 12. The degradation efficiency of swine manure was highest in the 220C-Fe treatment (61.3 %), which was 14.8 % higher than in the control. Fe-HC could act as an electron shuttle, stimulate the coenzyme F420 formation, increase the relative abundance of Methanosarcina and promote electron transport for acetotrophic methanogenesis in the AD. These findings are helpful for designing an efficient process for treating swine manure and utilizing digestate.

Anaerobic methane oxidation is quantitatively important in deeper peat layers of boreal peatlands: Evidence from anaerobic incubations, in situ stable isotopes depth profiles, and microbial communities

Boreal peatlands store most of their carbon in layers deeper than 0.5 m under anaerobic conditions, where carbon dioxide and methane are produced as terminal products of organic matter degradation. Since the global warming potential of methane is much greater than that of carbon dioxide, the balance between the production rates of these gases is important for future climate predictions. Herein, we aimed to understand whether anaerobic methane oxidation (AMO) could explain the high CO2/CH4 anaerobic production ratios that are widely observed for the deeper peat layers of boreal peatlands. Furthermore, we quantified the metabolic pathways of methanogenesis to examine whether hydrogenotrophic methanogenesis is a dominant methane production pathway for the presumably recalcitrant deeper peat. To assess the CH4 cycling in deeper peat, we combined laboratory anaerobic incubations with a pathway-specific inhibitor, in situ depth patterns of stable isotopes in CH4, and 16S rRNA gene amplicon sequencing for three representative boreal peatlands in Western Siberia. We found up to a 69 % reduction in CH4 production due to AMO, which largely explained the high CO2/CH4 anaerobic production ratios and the in situ depth-related patterns of δ13C and δD in methane. The absence of acetate accumulation after inhibiting acetotrophic methanogenesis and the presence of sulfate- and nitrate-reducing anaerobic acetate oxidizers in the deeper peat indicated that these microorganisms use SO42− and NO3− as electron acceptors. Acetotrophic methanogenesis dominated net CH4 production in the deeper peat, accounting for 81 ± 13 %. Overall, anaerobic oxidation is quantitatively important for the methane cycle in the deeper layers of boreal peatlands, affecting both methane and its main precursor concentrations.

Selective syngas fermentation to acetate under acidic and psychrophilic conditions using mixed anaerobic culture

Syngas fermentation to acetate offers a promising solution for its valorisation, particularly when syngas contains a high N2 concentration, which otherwise impedes the utilisation of syngas biomethanation gaseous product in cogeneration or upgrading units. In this study, continuous lab-scale syngas fermentation assessing the effects of acidic pH and psychrophilic conditions (28 °C and 20 °C) on bioconversion efficiency and anaerobic consortium diversity was studied. The results showed that as temperature and pH decrease, acetate yield increases. The highest H2 and CO consumption rates were observed at 20 °C and pH 4.5, reaching 48.4 mmol/(L·d) and 31.5 mmol/(L·d), respectively, and methanogenic activity was not completely suppressed. The microbial community composition indicated an enhanced abundance of acetate-producing bacteria and hydrogenotrophic methanogens at 28 °C. The PICRUSt2 prediction of metabolic potential indicated that temperature and pH changes appear to have a more pronounced impact on acetotrophic methanogenesis genes than carbon dioxide-based methanogenesis genes.

Exogenous inoculants enhance anaerobic digestion of food and kitchen waste: Metabolic and microbial mechanisms

Anaerobic digestion (AD) of food and kitchen wastes requires breaking the restriction of hydrolysis and improving efficiency. Hydrolytic microbiomes are often used to enhance the AD process, while the interaction of exogenous inoculants with indigenous communities is uncertain. This study investigated Lactobacillus lactis and Bacillus velezensis inoculants introduced in the strategy of biopretreatment and bioaugmentation to reveal the interaction of microbiomes and metabolic mechanisms. Results showed that both bioaugmentation (+47.8–58.7 %) and biopretreatment (+36.9–57.1 %) significantly enhanced the removal rate of crude cellulose. Whereas, introducing strategies elevated AD in different domains. Based on representative sequences and taxonomy, functional analyses indicated that bioaugmentation of Lactobacillus lactis could multiply the methanogenic pathway and promote the methane yield by 22.7–33.6 %, owing to the enhancement of syntrophic metabolism and hydrogenotrophic methanogenesis. Bioaugmentation of Bacillus velezensis promoted the last key step of the methanogenesis process that related to heterodisulfide reductase and used COB-S-S-COM as the terminal electron acceptor. While biopretreatment groups were dominated by strict acetotrophic methanogenesis, Methanosaeta was above 76 %. Biopretreatment, which added inoculant for hydrolysis, was tactically superior in reducing the digestion retention time by enhancing the pathway of β-oxidation butyrate fermentation, shortening the lag time for 43.6–60.1 %. However, the methane yield decreased by 29.0–30.0 % due to the chemoheterotroph consumption. The combination of physicochemical pretreatment and bioaugmentation can achieve the highest methane production (+33.6 %). These results are expected to contribute to the rational application of exogenous inoculants and improve the AD efficiency of food and kitchen wastes.

Optimising organic composition of feedstock to improve microbial dynamics and symbiosis to advance solid-state anaerobic co-digestion of sewage sludge and organic waste

This study provided new insight to the underlying mechanisms, by which organic compositions, including protein, fat, degradable carbohydrates and lignocellulose, regulate the performance of solid-state anaerobic digestion (SSAD) for synergistic treatment of organic wastes. Results show that the feedstock with a balanced composition of protein, fat, degradable carbohydrates and lignocellulose could maintain SSAD homeostasis to enhance methane (CH4) production by 14–487%. On the other hand, organic waste with a high content of degradable carbohydrates and fat at 39 and 14%, respectively, showed an initially high CH4 production at the beginning but a lower overall CH4 production. This was because of the accumulation of volatile fatty acids (VFAs) and ammonium nitrogen (NH4+-N) at up to 17.3 and 7227.7 mg·L−1, respectively, leading to anaerobic activity inhibition. Microbial dynamic and modular network analyses indicated that the balanced feedstock secured stepwise biodegradation of different organic substances to reduce the relative abundance of hydrolytic bacteria (e.g. Rikenellaceae_RC9_gut_group and Tepidimicrobium), thus alleviating VFAs and NH4+-N stresses. In particular, fat in the balanced feedstock could not only enrich the phylum Firmicutes for macromolecular biodegradation and genes for VFAs production, but also inhibit relative oxidizers (e.g. Synergistes and Acinetobacter) to facilitate propionate and acetate production to strengthen acetotrophic methanogenesis for effective CH4 yield. Results in this study show that a balanced organic composition could regulate microbial dynamics and symbiosis to advance SSAD homeostasis and methanation in synergistic organic waste treatment.

Microbial Population Dynamics during Unstable Operation of a Semicontinuous Anaerobic Digester Fed with a Mild-Treated Olive Mill Solid Waste

This research evaluates process instability together with microbial population dynamics of the startup of an anaerobic digestion of a mild pretreated solid olive oil waste. The pretreatment consisted of a mild thermal treatment called thermo-malaxation and a subsequent dephenolized process of the olive mill solid waste. The anaerobic digestion process of the mild pretreated and partially dephenolized biomass was studied for three Hydraulic Retention Times (HRTs), with 21 days each HRT, with an organic load rate of 1 g VS/L d, carried out at mesophilic temperature (35 ± 1 °C). The average value of methane yield decreased from 204 ± 9 mL CH4/g VS d on day 21, the last day of the first HRT, to 87 ± 24 mL CH4/g VS d on day 60, the last day of the third HRT. The alkalinity decreased drastically, indicating instability of the anaerobic digestion process. Although phenolic compounds were partially extracted in the pretreatment, the observed increase in phenolic compounds during reactor operation might be contributed to the methane production decay. Interestingly, volatile fatty acids decreased with time, indicating that not only the methanogenic stage but also the hydrolysis stage was affected. Indeed, the microbial analysis showed that the abundance of hydrolytic bacteria decreased over time. It is also worth noticing that hydrogenotrophic methanogens, while present during the first two HRTs, were not observed at the end of the last HRT. This observation, together with the increase in the relative abundance of acetoclastic methanogens, showed a shift in the methane production pathway from hydrogenotrophic methanogenesis to acetotrophic methanogenesis.

Revisiting the biological pathway for methanogenesis in landfill from metagenomic perspective—A case study of county-level sanitary landfill of domestic waste in North China plain

Landfill is the third highest contributor to anthropogenic methane (CH4) emissions, produced primarily by the anaerobic decomposition of organic matter by microbes. However, how various microbial metabolic processes contribute to CH4 production in domestic waste landfill remains elusive. We addressed this problem by investigating the methanogenic communities, methanogenic functional genes, KEGG modules and KEGG pathways in a county-level MSW sanitary landfill in North China Plain, China. Results showed that Methanomicrobiales, Methanobacteriales, Methanosarcinales, Micrococcales, Corynebacteriales and Bacillales were the dominant methanogens. M00357, M00346, M00567 and M00563 were the four major methane metabolic modules. The most abundant genes were ACSS, ackA and fwd with the relative abundance of 19.26–54.54%, 6.14–25.78% and 6.76–16.51%, respectively. The two essential genes of methanogenesis were detected with the relative abundance of 2.66–9.58% (mtr) and 1.63–9.14% (mcr). These findings indicated that acetotrophic and hydrogenotrophic methanogenesis were the major pathways. Methanomicrobiales, Methanosarcinales and Clostridiales were the key microbes to these pathways identified by co-occurrence network. Analysis of relative contribution of species to function further showed that Micrococcales, Corynebacteriales and Bacillales were special contributors to acetotrophic methanogenesis pathway. Redundancy analysis revealed that above functional genes and microbes were mainly controlled by NH4+ and pH. Our results can help to provide develop the fine management strategies for methane utilization and emission reduction in landfill.

Simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater by a bioelectrocatalysis coupled two-phase anaerobic reactor

The microbial electrolysis cell coupled the two-phase anaerobic digestion (MEC-TPAD) was developed for simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater. In acidogenic phase, the produced sulfides were efficiently converted into bio-sulfur via anodic bio-oxidation, with a maximum recovery of 59 ± 5.5 %. The anode coupled acidogenesis produced more volatile fatty acids which were benefit for the subsequent methanogenesis. The cathode in methanogenic phase created a suitable pH condition and enhanced the methanogenesis. Correspondingly, the maximum bio-methane yield in MEC-TPAD was 2 times higher than that in TPAD. Microbial communities revealed that major functional consortia capable of sulfides oxidation (e.g. Alcaligenes) in anode biofilm, hydrogenotrophic methanogenesis (e.g. Methanobacterium) in cathode biofilm, and acetotrophic methanogenesis (e.g. Methanosaeta) in methanogenic sludge were enriched. Economic benefit could totally cover the cost of input electric energy. This work opens an appealing avenue for recovering nutrient and energy from wastewater.

Deep insights into the anaerobic co-digestion of waste activated sludge with concentrated leachate under different salinity stresses.

Treatment of high-salinity organic wastewater (e.g., concentrated leachate) is a major challenge. Anaerobic co-digestion can effectively treat high-salinity organic wastewater and recover energy. In this study, the concentrated landfill leachate and waste activated sludge (WAS) were anaerobic co-digested in the lab-scale continuous stirred tank reactors (CSTR) to understand their co-digestion performance under different salinity stresses. As revealed by the results, when the salinity was low (<10 g/L), the removal ratio of organic matter in the digester was kept at a high level (>91.3%), and the concentration of total volatile fatty acids (TVFAs) was low (<100 mg COD/L), indicating that the digester could operate efficiently and stably. However, when the salinity level was elevated from 10 g/L to 30 g/L, the removal ratio of organic matter in the digester decreased from ~91.3% to ~64.5%, the TVFAs continued to accumulate, the yields of biogas and methane also dropped sharply, and the performance of the digester decreased gradually. The results of microbial community and diversity analysis showed that there is limited adaptability of microbial community to high salinity in such process. Salinity could cause significant changes in the microbial community and diversity, thereby affecting the digestive performance. Metagenomic analysis showed that under high salinity conditions, the content of genes encoding hydrolase and methanogenic enzyme decreased, whereas the pathway of acetotrophic methanogenesis was weakened. Mechanism study showed that with the increase of salinity, the activity of microbial cells decreased, the structure of sludge flocs was damaged more significantly, and the extracellular polymeric substances (EPS) secreted by microbe increased continuously, which was used to resist the toxic effects of salinity stresses on microorganisms. The results of this study could provide certain theoretical guidance for anaerobic digestion under salinity stresses.

Chronic effects of benzalkonium chlorides on short chain fatty acids and methane production in semi-continuous anaerobic digestion of waste activated sludge

As an emerging pollutant, benzalkonium chlorides (BACs) potentially enriched in waste activated sludge (WAS). However, the microbial response mechanism under chronic effects of BACs on acidogenesis and methanogenesis in anaerobic digestion (AD) has not been clearly disclosed. This study investigated the AD (by-)products and microbial evolution under low to high BACs concentrations from bioreactor startup to steady running. It was found that BACs can lead to an increase of WAS hydrolysis and fermentation, but a disturbance to acidogenic bacteria also occurred at low BACs concentration. A noticeable inhibition to methanogenesis occurred when BAC concentration was up to 15 mg/g TSS. Metagenomic analysis revealed the key genes involved in acetic acid (HAc) biosynthesis (i.e. phosphate acetyltransferase, PTA), β-oxidation pathway (acetyl-CoA C-acetyltransferase) and propionic acid (HPr) conversion was slightly promoted compared with control. Furthermore, BACs inhibited the acetotrophic methanogenesis (i.e. acetyl-CoA synthetase), especially BAC concentration was up to 15 mg/g TSS, thereby enhanced short chain fatty acids (SCFAs) accumulation. Overall, chronic stimulation of functional microorganisms with increasing concentrations of BACs impact WAS fermentation.

Polycyclic aromatic hydrocarbons stimulate acidogenesis, acetogenesis and methanogenesis during anaerobic co-digestion of waste activated sludge and food waste

Polycyclic aromatic hydrocarbons (PAHs) have been reported to influence acetic acid production during anaerobic treatment. However, investigations of the impacts of PAHs on the anaerobic co-digestion of waste activated sludge and food waste are limited. Therefore, the effects of PAHs on anaerobic co-digestion were explored in this study. Four kinds of PAHs all exhibited positive contributions to methane production, especially phenanthrene. Mechanism exploration revealed that acidogenesis, acetogenesis, and methanogenesis were improved in the presence of phenanthrene, and acetotrophic methanogenesis had the greatest improvement with 69.4%. Dominant bacteria and archaea related to acetic acid and methane accumulation were changed by phenanthrene. Moreover, extracellular polymeric substances, coenzyme F420, and McrA gene copy number were promoted by phenanthrene, which was beneficial for the generation of acetic acid and methane. Overall, this study provides new insights into the role of organic pollutants in the anaerobic co-digestion of solid wastes.

Biochar assisted cellulose anaerobic digestion under the inhibition of dodecyl dimethyl benzyl ammonium chloride: Dose-response kinetic assays, performance variation, potential promotion mechanisms

This study evaluated the inhibitory effect and mitigation strategy of dodecyl dimethyl benzyl ammonium chloride (DDBAC) suppression on anaerobic digestion. With the 12 h-suppression, only 16.64% of anaerobes were alive, and acetotrophic methanogens were significantly inhibited. As for batch test, DDBAC suppression significantly prolonged the start-up of systems and decreased the biogas production. In cellulose semi-continuous digestion process, the DDBAC suppression induced volatile fatty acids accumulation and pH decrease. However, the biochar amended reactor effectively mitigated the DDBAC suppression and achieved 370.5 mL/d·g-chemical-oxygen-demand biogas production. Moreover, 17.8% more protein in extracellular polymeric substances was secreted as the bio-barrier to defense the DDBAC suppression. Furthermore, microbial analysis showed that biochar addition selectively enriched directed interspecies electron transfer (DIET) participant bacteria (Anaerolineaceae and Syntrophomonas) and methanogens (Methanosaeta and Methanobacterium). Meanwhile, the potential metabolic pathway analysis showed that the abundance of amino acids and energy metabolism were increased 28% and 8%, respectively. The abundance of encoding enzyme related to hydrogenotrophic and acetotrophic methanogenesis enriched 1.88 times and 1.48 times, respectively. These results showed the performance and mechanisms involved in DIET establishment with ethanol stimulation biochar addition.

Methanogenesis pathways of methanogens and their responses to substrates and temperature in sediments from the South Yellow Sea

Although coastal sediments are major contributors to the production of atmospheric methane, the effects of environmental conditions on methanogenesis and the community of methanogenic archaea are not well understood. Here, we investigated the methanogenesis pathways in nearshore and offshore sediments from the South Yellow Sea (SYS). Moreover, the effects of the supply of methanogenic substrates (H2/CO2, acetate, trimethylamine (TMA), and methanol) and temperature on methanogenesis and the community of methanogenic archaea were further determined. Methylotrophic, hydrogenotrophic and acetotrophic methanogenesis were found to be responsible for biogenic methane production in nearshore sediments. In the offshore sediments, methylotrophic methanogenesis was the predominant methanogenic pathway. The changes in methanogenic community structure under different substrate amendments were characterized. Lower diversities were detected in substrate-amended samples with methanogenic activity. Hydrogenotrophic Methanogenium, multitrophic Methanosarcina, methylotrophic Methanococcoide, Methanococcoide or methylotrophic Methanolobus were dominant in H2/CO2-, acetate-, TMA- and methanol-amended sediment slurries, respectively. PCoA showed that the methanogen community in H2/CO2 and acetate amendments exhibited greater differences than those in other treatments. Lower temperature (10 °C) limits hydrogenotrophic and acetoclastic methanogenesis, but methylotrophic methanogenesis is much less affected. The response of methanogen diversity to the incubation temperature varied among the different substrate-amended slurries. The multitrophic methanogen Methanosarcina became increasingly abundant in H2/CO2− and acetate-amended sediment slurries when the temperature increased from 10 to 30 °C.

Mechanistic insights into the effect of poly ferric sulfate on anaerobic digestion of waste activated sludge

Poly ferric sulfate (PFS), one of the typical inorganic flocculants widely used in wastewater management and waste activated sludge (WAS) dewatering, could be accumulated in WAS and inevitably entered in anaerobic digestion system at high levels. However, knowledge about its impact on methane production is virtually absent. This study therefore aims to fill this gap and provide insights into the mechanisms involved through both batch and long-term tests using either real WAS or synthetic wastewaters as the digestion substrates. Experimental results showed that the maximum methane potential and production rate of WAS was respectively retarded by 39.0% and 66.4%, whereas the lag phase was extended by 237.0% at PFS of 40 g per kg of total solids. Mechanism explorations exhibited that PFS induced the physical enmeshment and disrupted the enzyme activity involved in anaerobic digestion, resulting in an inhibitory state of the bioprocess of hydrolysis, acidogenesis, and methanogenesis. Furthermore, PFS's inhibition to hydrogenotrophic methanogenesis was much severer than that to acetotrophic methanogenesis, which could be supported by the elevated abundances of Methanosaeta sp and the dropped abundances of Methanobacterium sp in PFS-present digester, and probably due to the severe mass transfer resistance of hydrogen between the syntrophic bacteria and methanogens, as well as the higher hydrogen appetency of PFS-induced sulfate reducing bacteria. Among the derivatives of PFS, “multinucleate and multichain-hydroxyl polymers” and sulfate were unveiled to be the major contributors to the decreased methane potential, while the “multinucleate and multichain-hydroxyl polymers” were identified to be the chief buster to the slowed methane-producing rate and the extended lag time.

Understanding the fate and impact of capsaicin in anaerobic co-digestion of food waste and waste activated sludge

Anaerobic co-digestion is an attractive option to treat food waste and waste activated sludge, which is increasingly applied in real-world situations. As an active component in Capsicum species being substantially present in food waste in many areas, capsaicin has been recently demonstrated to inhibit the anaerobic co-digestion. However, the interaction between capsaicin and anaerobic co-digestion are still poorly understood. This work therefore aims to deeply understand the fate and impact of capsaicin in the anaerobic co-digestion. Experiment results showed that capsaicin was completely degraded in anaerobic co-digestion by hydroxylation, O-demethylation, dehydrogenation and doubly oxidization, respectively. Although methane was proven to be produced from capsaicin degradation, the increase in capsaicin concentration resulted in decrease in methane yield from the anaerobic co-digestion. With an increase of capsaicin from 2 ± 0.7 to 68 ± 4 mg/g volatile solids (VS), the maximal methane yield decreased from 274.6 ± 9.7 to 188.9 ± 8.4 mL/g VS. The mechanic investigations demonstrated that the presence of capsaicin induced apoptosis, probably by either altering key kinases or decreasing the intracellular NAD+/NADH ratio, which led to significant inhibitions to hydrolysis, acidogenesis, and methanogenesis, especially acetotrophic methanogenesis. Illumina Miseq sequencing analysis exhibited that capsaicin promoted the populations of complex organic degradation microbes such as Escherichia-Shigella and Fonticella but decreased the numbers of anaerobes relevant to hydrolysis, acidogenesis, and methanogenesis such as Bacteroide and Methanobacterium.

Calcium effect on microbial activity and biomass aggregation during anaerobic digestion at high salinity

The potential effect of different Ca2+ additions (150, 300, 450, 600 and 1000 mg/L) on microbial activity and aggregation, during anaerobic digestion at moderate (8 g/L Na+) and high salinity (20 g/L Na+) has been investigated. Batch tests were carried out in duplicate serum bottles and operated for 30 days at 37 °C. At 8 g/L Na+, methanogenic activity and protein degradation were comparable from 150 to 450 mg/L Ca2+, and a significant inhibition was only observed at a Ca2+concentration of 1000 mg/L. In contrast, at 20 g/L Na+, 150 to 300 mg/L were the only Ca2+ concentrations to maintain chemical oxygen demand (COD) removal, protein hydrolysis and methane production. Overall, increasing Ca2+ concentrations had a larger impact on acetotrophic methanogenesis at 20 g/L than at 8 g/L Na+. Increasing Ca2+ had a negative effect on the aggregation behaviour of the dominant methanogen Methanosaeta when working at 8 g/L Na+. At 20 g/L Na+ the aggregation of Methanosaeta was less affected by addition of Ca2+ than at 8 g/L Na+. The negative effect appeared to be connected with Ca2+ precipitation and its impact on cell-to cell communication. The results highlight the importance of ionic balance for microbial aggregation at high salinity, bringing to the forefront the effect on Methanosaeta cells, known to be important to obtain anaerobic granules.

The acetotrophic pathway dominates methane production in Zoige alpine wetland coexisting with hydrogenotrophic pathway

As a typical alpine wetland on the Tibetan Plateau, the Zoige wetland processes a large carbon stock and is a hotspot of methane emission. To date, many studies have investigated the methane flux in this wetland; however, the research on the source of methane in the soils of Zoige wetland is not clear enough. In this study, we determined the dynamic characteristics of the stable carbon isotopes during the methanogenesis of Zoige wetland soil and the corresponding microbial changes. The results showed that the δ13CH4 varied between −19.86‰ and −28.32‰ and the αC ranged from 1.0029 to 1.0104 in the methanogenesis process, which suggests the dominance of acetotrophic methanogenesis. And among the increased methanogens, acetotrophic methanogens multiplied more obviously than hydrogenotrophic menthanogens. In addition, the results of structural equation models showed that the variations in stable carbon isotopes during the process were mainly affected by acetotrophic methanogens. Although the acetotrophic pathway was dominate, the varied isotope characteristics, increased methanogens and ratio of carbon dioxide to methane all showed that hydrogenotrophic and acetotrophic methanogenesis coexisted in the Zoige wetland. Overall, our study provided a detailed and definitive information to the source of methane in the soil of the Zoige wetland and laid a foundation of mechanism to the research of greenhouse gas in this alpine wetland.

NanoFe3O4 as Solid Electron Shuttles to Accelerate Acetotrophic Methanogenesis by Methanosarcina barkeri.

Magnetite nanoparticles (nanoFe3O4) have been reported to facilitate direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens thereby improving syntrophic methanogenesis. However, whether or how nanoFe3O4 affects acetotrophic methanogenesis remain unknown. Herein, we demonstrate the unique role of nanoFe3O4 in accelerating methane production from direct acetotrophic methanogenesis in Methanosarcina-enriched cultures, which was further confirmed by pure cultures of Methanosarcina barkeri. Compared with other nanomaterials of higher electrical conductivity such as carbon nanotubes and graphite, nanoFe3O4 with mixed valence Fe(II) and Fe(III) had the most significant stimulatory effect on methane production, suggesting its redox activity rather than electrical conductivity led to enhanced methanogenesis by M. barkeri. Cell morphology and spectroscopy analysis revealed that nanoFe3O4 penetrated into the cell membrane and cytoplasm of M. barkeri. These results provide the unprecedented possibility that nanoFe3O4 in the cell membrane of methanogens serve as electron shuttles to facilitate intracellular electron transfer and thus enhance methane production. This work has important implications not only for understanding the mechanisms of mineral-methanogen interaction but also for optimizing engineered methanogenic processes.