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Conductive materials can facilitate syntrophic methane (CH) production by improving direct interspecies electron transfer. The effect of a conductive material, ferroferric oxide (FeO), on the anaerobic treatment of tryptone-based high-strength wastewater was investigated in two anaerobic sequencing batch reactors (ASBRs). One was the control reactor without amendment of FeO, and the other was the FeO-amended reactor. In an ASBR reaction cycle, the dosage of FeO increased the CH production rate by 12.2%, shortened the methanogenic lag phase by 32.3%, and improved the COD removal rate by 8.1%. During anaerobic treatment, the dosage of FeO not only enhanced the CH production from acetate but also facilitated the hydrolysis/acidification process fed by tryptone. Maximum electron transport activities were increased by 40.4, 137.3, and 64.6% in the processes of the ASBR reaction cycle, the CH production from acetate, and the hydrolysis/acidification of tryptone, respectively. Among all tested quorum sensing signal substances, N-octanoyl- l-homoserine lactone was dominant in both ASBRs, with the highest concentration in the biomass and the lowest concentration in the water phase. In addition, the concentration of signal substance was high in the ASBR cycle and low in the CH production from acetate and the hydrolysis/acidification of tryptone. Proteiniclasticum and vadinCA02 were dominant acidogens, and their relative abundances increased during the long-term operation with FeO dosing. Methanosarcina was the predominant methanogen, contributing to interspecies electron transfer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Contents of essential amino acids in microbial aggregates were determined. • The concentration of amino acids markedly varied in different samples. • Glutamic acid, lysine, and alanine was the top 3 most abundant amino acids. • Several syntrophic bacteria and methanogens cannot produce all amino acids. • Amino acids cross-feeding might exist between syntrophs and methanogens. Anaerobic digestion is a prevalent bioenergy production process relying on a complex network of symbiotic interactions, where the nutrient based cross-feeding is an essential microbial mechanism. Here, the cross-feeding function was assessed by analyzing extracellular polymeric substances-associated amino acids in microbial aggregates collected from 14 lab-scale anaerobic digesters, as well as deciphering their genetically biosynthetic potential by syntrophic bacteria and methanogens. The total concentration of essential amino acids ranged from 1.2 mg/g VSS to 174.0 mg/g VSS. The percentages of glutamic acid (8.5 ∼ 37.6%), lysine (2.7 ∼ 22.6%), alanine (5.6 ∼ 13.2%), and valine (3.0 ∼ 10.4%) to the total amount of detected amino acids were the highest in most samples. Through metagenomics analysis, several investigated syntrophs (i.e., Smithella , Syntrophobacter , Syntrophomonas , and Mesotoga) and methanogens (i.e., Methanothrix and Methanosarcina) were auxotrophies, but the genetic ability of syntrophs and methanogens to synthesize some essential amino acids could be complementary, implying potential cross-feeding partnership. [ABSTRACT FROM AUTHOR]

• Methanogenic inhibition mitigation by conductive materials was reviewed. • pH could be maintained by enhancing organic degradation with conductive materials. • Conductive materials ensured energy supply of methanogens for anti-ammonia inhibition. • Methanogens competition in sulfate-rich conditions was enhanced by conductive materials. • Extracellular polymeric substances were promoted for nano-cytotoxicity resistance. Methanogenesis can be promoted by the addition of conductive materials. Although stimulating effects of conductive materials on methane (CH 4) production has been extensively reported, the crucial roles on recovering methanogenic activities under inhibitory conditions have not been systematically discussed. This critical review presents the current findings on the effects of conductive materials in methanogenic systems under volatile fatty acids (VFAs), ammonia, sulfate, and nano-cytotoxicity stressed conditions. Conductive materials induce fast VFAs degradation, avoiding VFAs accumulation during anaerobic digestion. Under high ammonia concentrations, conductive materials may ensure sufficient energy conservation for methanogens to maintain intracellular pH and proton balance. When encountering the competition of sulfate-reducing bacteria, conductive materials can benefit electron competitive capability of methanogens, recovering CH 4 production activity. Conductive nanomaterials stimulate the excretion of extracellular polymeric substances, which can prevent cells from nano-cytotoxicity. Future perspectives about unraveling mitigation mechanisms induced by conductive materials in methanogenesis processes are further discussed. [ABSTRACT FROM AUTHOR]

• This study clarified the co-occurrence network of syntrophs and methanogens. • Diverse quorum sensing genes were detected in all anaerobic sludge samples. • Two quorum sensing signals are suggested to impact the regulation of type IV pili. • Environmental factors significantly affected the syntrophic patterns. Interspecies electron transfer plays an important role in syntrophic methanogenesis. Direct interspecies electron transfer (DIET) between syntrophic oxidizers and methanogens via conductive pili has been only confirmed in some specific co-cultures. This study examined potential syntrophic cooperation via type IV pili and quorum sensing between widespread syntrophic bacteria and methanogens through a metagenomic analysis of 12 anaerobic sludge samples. We found that Methanosaeta and Methanosarcina , which are reported to have DIET ability, were dominant in most methanogenic samples. Putative conductive pili genes were found in some typical syntrophic bacteria, which has rarely been reported previously. The existence of diverse quorum-sensing genes suggested that various quorum-sensing systems might participate in the communication of anaerobic microorganisms. Specifically, the diffusible signal factor and 3′-5′ cyclic diguanosine monophosphate related genes were mainly assigned to syntrophic bacteria. These results suggest that the combined regulation of these signals might be responsible for the biosynthesis of type IV pili and affect syntrophic interaction during methanogenesis. These novel results provide fresh evidence to support the widespread existence of DIET in anaerobic methanogenic systems; therefore, regulating the quorum-sensing system may promote syntrophic interaction. [ABSTRACT FROM AUTHOR]

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

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 (SBR 10d and CFR 10d) and 25 days (SBR 25d and CFR 25d) 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. [Display omitted] • Reduction in SMR shortened the lag phase in acetoclastic methanogenesis. • SBRs with 25-day SRT tend to enrich both Methanosarcina and Methanothrix. • SRT of 10 days benefited the enrichment of Methanosarcina , particularly in SBR. • The V/A-type and F-type ATPase were mainly affected by operational modes. [ABSTRACT FROM AUTHOR]

• Acetate enriched incomplete and complete SRBs while ethanol enriched incomplete SRB. • Complete SRB had higher competitiveness for acetate than methanogens. • Incomplete SRB could cooperate with methanogen to produce methane. • Methanogens and methanogenic pathways changed with the dosage of Fe 3 O 4. Effects of ferroferric oxide (Fe 3 O 4) and organic carbon on co-metabolism of sulfate and organic carbon were investigated. With Fe 3 O 4 , the degradation of acetate and sulfate was inhibited when fed with acetate, while the degradation of acetate and propionate produced from ethanol was promoted when fed with ethanol. The dominant sulfate reducing bacteria (SRB) of acetate-fed reactors were Desulfobacteraceae (complete oxidizing SRB, CO-SRB) and Desulfurmonas (incomplete oxidizing SRB, IO-SRB). IO-SRBs of Desulfobulbus and Desulfomicrobium were dominant in ethanol-fed reactors. CO-SRB had higher competitiveness than methanogens to utilize acetate, while IO-SRBs might cooperate with methanogens to produce methane when dosed with ethanol and Fe 3 O 4. The dosage of Fe 3 O 4 changed the dominant methanogen from Methanosarcina to Methanosaeta with acetate as the organic carbon, while increased the relative abundance of Methanosaeta with ethanol as the organic carbon. [ABSTRACT FROM AUTHOR]