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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., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Huanhuan Chang reports financial support was provided by China Scholarship Council. Guangxue Wu reports financial support was provided by Galway University Foundation CLG. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Anaerobic digestion (AD) is promising for treating high-strength wastewater. However, the effect of operational parameters on microbial communities of AD with sulfate is not yet fully understood. To explore this, four reactors were operated under rapid- and slow-filling modes with different organic carbons. Reactors in the rapid-filling mode generally exhibited a fast kinetic property. For example, the degradation of ethanol was 4.6 times faster in ASBR ER than in ASBR ES , and the degradation of acetate was 11.2 times faster in ASBR AR than in ASBR AS . Nevertheless, reactors in the slow-filling mode could mitigate propionate accumulation when using ethanol as organic carbon. Taxonomic and functional analysis further supported that rapid- and slow-filling modes were suitable for the growth of r-strategists (e.g., Desulfomicrobium) and K-strategists (e.g., Geobacter), respectively. Overall, this study provides valuable insights into microbial interactions of AD processes with sulfate through the application of the r/K selection theory., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

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., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Anaerobic digestion is achieved through cooperation among various types of microorganisms, and the regulation of microbial communities is key to achieving stable system operation. In this study, the r/K selection theory was adopted to examine the system performance and microbial characteristics in anaerobic reactors with different operating modes (continuous-flow reactors, CFRs; sequencing batch reactors, SBRs) and sludge retention times (25 and 10 days). Four lab-scale reactors (CFR 25d , CFR 10d , SBR 25d , and SBR 10d ) were operated. In the cycle reaction, CFR 25d achieved the highest methane yield (678.0 mL/L) and methane production rate (140.8 mL/(L·h)); while those in CFR 10d were the lowest, which could have been due to an accumulation of volatile fatty acids. CFR could wash out r-strategists efficiently, such as Methanosarcina. CFR 25d and CFR 10d significantly enriched the K-strategist Geobacter, with the relative abundances of 34.0% and 72.6%, respectively. In addition, the hydrogenotrophic methanogens of Methanolinea and Methanospirillum (K-strategists) dominated in CFR 25d and CFR 10d . Methanobacterium adapted to the diverse operational conditions, but the slow grower Methanosaeta only accounted for 0.9% in CFR 10d . Failure to enrich propionate oxidizers resulted in a functional absence of propionate degradation in the CFRs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Organic carbon can affect nitrogen removal in the anaerobic ammonia oxidation (anammox) process. Two continuous up-flow anaerobic sludge blanket (UASB) reactors were operated under autotrophic (UASB N , without organic carbon) and mixotrophic (UASB CN , with the intermittent addition of acetate and propionate) conditions. Stable operation of anammox systems was achieved, with the nitrogen removal rate and percentage of 2.12 g/(L·d) and 86.4% in UASB N , and 2.09 g/(L·d) and 85.0% in UASB CN , respectively. The network of Candidatus Kuenenia, Thauera, and Nitrosomanas contributed to both nitrogen and carbon metabolisms, and the intermittent addition of acetate and propionate strengthened Ca. Kuenenia's ability to utilize several types of carbon sources. Anammox bacteria showed activity in the presence of organic carbon and without inorganic carbon, confirming the mixotrophic characteristic of Ca. Kuenenia. Cross-feeding of amino acids and vitamins existed among functional microorganisms, with extracellular polymeric substances acting as the media for microbial interactions., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Wastewater management has experienced different stages, including pollutant removal, resource recovery, and water nexus. Within these stages, anaerobic ammonia oxidation-based biotechnology can be incorporated for nitrogen removal, which can help achieve sustainable wastewater management, such as reclamation and ecologization of wastewater. Here, the physiology, metabolism, reaction kinetics and microbial interactions of anammox bacteria are discussed, and strategies to start-up the anammox system are presented. Anammox bacteria are slow growers with a high doubling time and a low reaction rate. Although most anammox bacteria grow autotrophically, some types can grow mixotrophically. The reaction stoichiometric coefficients can be affected by loading rates and other biological reactions. Microbial interactions also contribute to enhanced biological nitrogen removal and promote activities of anammox bacteria. The start-up of the anammox process is the key aspect for its practical application, which can be realized through seed selection, system stimulation, and biomass concentration enhancement.

Long-term exposure of nitrifiers to high concentrations of free ammonia (FA) and free nitrous acid (FNA) may affect nitrifiers activity and nitrous oxide (N 2 O) emission. Two sequencing batch reactors (SBRs) were operated at influent ammonium nitrogen (NH 4 -N) concentrations of 800mg/L (SBR H ) and 335mg/L (SBR L ), respectively. The NH 4 -N removal rates in SBR H and SBR L were around 2.4 and 1.0g/L/day with the nitritation efficiencies of 99.3% and 95.7%, respectively. In the simulated SBR cycle, the N 2 O emission factors were 1.61% in SBR H and 2.30% in SBR L . N 2 O emission was affected slightly by FA with the emission factor of 0.22%-0.65%, while N 2 O emission increased with increasing FNA concentrations with the emission factor of 0.22%-0.96%. The dominant ammonia oxidizing bacteria (AOB) were Nitrosomonas spp. in both reactors, and their relative proportions were 38.89% in SBR H and 13.36% in SBR L . Within the AOB genus, a species (i.e., operational taxonomic unit [OTU] 76) that was phylogenetically identical to Nitrosomonas europaea accounted for 99.07% and 82.04% in SBR H and SBR L , respectively. Additionally, OTU 215, which was related to Nitrosomonas stercoris, accounted for 16.77% of the AOB in SBR L ., (Copyright © 2018. Published by Elsevier B.V.)

Effect of dosing ferroferric oxide (Fe 3 O 4 ) on the anaerobic treatment of azo dye Reactive Red 2 (RR2) was investigated in two anaerobic sequencing batch reactors (ASBRs). System performance, dye degradation pathways, and microbial activities and structure were examined. The addition of Fe 3 O 4 significantly improved treatment efficiency, with the removal efficiency of RR2 increased by 116%, the maximum methane (CH 4 ) yield potential and the peak CH 4 production rate improved by 7.7% and 22.3%, and the lag phase shortened by 39.6%, respectively. The activity of the electron transport system was significantly enhanced by dosing Fe 3 O 4 , with the maximum value increased by 77% and conductivity of the anaerobic sludge increased by 178%. According to the proposed pathway for the degradation of RR2, the degradation products from complete cleavage of the NN bond in RR2 were obtained at the presence of Fe 3 O 4 , while were absent without Fe 3 O 4 . At high initial dye concentrations, the dosage of Fe 3 O 4 alleviated the inhibition to microbes by RR2, and high degradation rate and removal efficiency were maintained. The microbial community structure changed during the long-term acclimation with the dosage of Fe 3 O 4 . Paludibacter, Trichococcus and Methanosarcina were predominant and their relative abundances increased with the addition of Fe 3 O 4 ., (Copyright © 2018 Elsevier B.V. All rights reserved.)