Syntrophic butyrate-oxidizing methanogenesis promoted by anthraquinone-2-sulfonate and cysteine: Distinct tendencies towards the enrichment of methanogens and syntrophic fatty-acid oxidizing bacteria

Gene abundance and microbial syntrophy as key drivers of anaerobic digestion revealed through 16S rRNA gene and metagenomic analysis

Genes in microorganisms influence the biological processes in anaerobic digestion (AD). However, key genes involved in the four metabolic steps (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) remain largely unexplored. This study investigated the abundance and distribution of key functional genes in full-scale anaerobic digesters processing food waste (FWDs) and municipal wastewater (MWDs) through 16S rRNA gene and shotgun metagenomic analysis. Our results revealed that FWDs exhibited a higher abundance of key genes in the metabolic steps, despite having significantly lower microbial diversity compared to MWDs. Pathways and genes associated with syntrophic oxidation of acetate (SAO) and butyrate (SBO) were more present in FWDs. SAO potentially used both the conventional reversed Wood-Ljungdahl pathway and its integration with the glycine cleavage system in FWDs, which complements pathways for acetate oxidation under ammonia stress conditions. Similarly, genes associated with SBO (atoB and croR) were notably more prevalent in FWDs compared to MWDs with an 8.4-fold and 108-fold increase, respectively, indicating the adaptation of SBO bacteria to convert butyrate into acetate. The higher abundance of key genes in FWDs was driven by microbes adapting to the feedstock compositions with higher levels of substrate content, volatile fatty acids, and ammonia. This study quantified the genes central to AD metabolism and uncovered the contributions of microbial diversity, gene abundance, syntrophy, and feedstock characteristics to the functionality of AD processes. These findings enhance understanding of the microbial ecology in AD and provide a foundation for developing innovative strategies to enhance biogas production and waste management.

Unlocking anaerobic digestion potential via extracellular electron transfer by exogenous materials: Current status and perspectives

The efficiency of energy transfer among microorganisms presents a substantial hurdle for the widespread implementation of anaerobic digestion techniques. Nonetheless, recent studies have demonstrated that enhancing the extracellular electron transfer (EET) can markedly enhance this efficiency. This review highlights recent advancements in EET for anaerobic digestion and examines the contribution of external additives to fostering enhanced efficiency within this context. Diverse mechanisms through which additives are employed to improve EET in anaerobic environments are delineated. Furthermore, specific strategies for effectively regulating EET are proposed, aiming to augment methane production from anaerobic digestion. This review thus offers a perspective on future research directions aimed at optimizing waste resources, enhancing methane production efficiency, and improving process predictability in anaerobic digestion.

Syntrophic methane production from volatile fatty acids: Focus on interspecies electron transfer

Methane is a renewable biomass energy source produced via anaerobic digestion (AD). Interspecies electron transfer (IET) between methanogens and syntrophic bacteria is crucial for mitigating energy barriers in this process. Understanding IET is essential for enhancing the efficiency of syntrophic methanogenesis in anaerobic digestion. Interspecies electron transfer mechanisms include interspecies H2/formate transfer, direct interspecies electron transfer (DIET), and electron-shuttle-mediated transfer. This review summarizes the mechanisms, developments, and research gaps in IET pathways. Interspecies H2/formate transfer requires strict control of low H2 partial pressure and involves complex enzymatic reactions. In contrast, DIET enhances the electron transfer efficiency and process stability. Conductive materials and key microorganisms can be modulated to stimulate the DIET. Electron shuttles (ES) allow microorganisms to interact with extracellular electron acceptors without direct contact; however, their efficiency depends on various factors. Future studies should elucidate the key functional groups, metabolic pathways, and regulatory mechanisms of IET to guide the optimization of AD processes for efficient renewable energy production.

Enhancing the Startup Rate of Microbial Methanogenic Systems through the Synergy of β-lactam Antibiotics and Electrolytic Cells

The slow startup and suboptimal efficiency of microbial carbon sequestration and methane-production systems have not been fully resolved despite their contribution to sustainable energy production and the reduction of greenhouse gas emissions. These systems often grapple with persistent hurdles, including interference from miscellaneous bacteria and the slow enrichment of methanogens. To address these issues, this paper examines the synergistic effect of coupling β-lactam antibiotics with an electrolytic cell on the methanogenic process. The results indicated that β-lactam antibiotics exhibited inhibitory effects on Campylobacteria and Alphaproteobacteria (two types of miscellaneous bacteria), reducing their relative abundance by 53.03% and 87.78%, respectively. Nevertheless, it also resulted in a decrease in hydrogenogens and hindered the CO2 reduction pathway. When coupled with an electrolytic cell, sufficient electrons were supplied for CO2 reduction to compensate for the hydrogen deficiency, effectively mitigating the side effects of antibiotics. Consequently, a substantial improvement in methane production was observed, reaching 0.57 mL·L-1·d-1, exemplifying a remarkable 6.3-fold increase over the control group. This discovery reinforces the efficiency of methanogen enrichment and enhances methane-production levels.

The influencing mechanism of AD-MEC domesticated sludge to alleviates propionate accumulation and enhances methanogenesis

Anaerobic digestion combined with microbial electrolysis cell (AD-MEC) could maintain stable reactor operation and alleviating the anaerobic digestion (AD) propionate accumulation. In this study, the addition of sludge to AD-MEC was examined as a way to enhance system performance and explore the microbial interaction mechanism after electric field domestication. The results showed that under 1000 and 4000 mg/L propionate, the methane production of the sludge from AD-MEC increased by 34.29 % and 9.70 %, respectively, as compared to the AD sludge. Gompertz fitting analysis showed that sludge after electric field domestication enhancing its continuous methanogenic capacity. Further analysis showed that sludge extracellular electron transfer capacity was enhanced in AD-MEC and that its domesticated granular sludge formed a microbial community function with acid-degrading synergistic methanogenesis. The results of the study may provide theoretical support and optimization strategies for the application of AD-MEC system.

Electron shuttles enhanced the removal of antibiotics and antibiotic resistance genes in anaerobic systems: A review

The environmental and epidemiological problems caused by antibiotics and antibiotic resistance genes have attracted a lot of attention. The use of electron shuttles based on enhanced extracellular electron transfer for anaerobic biological treatment to remove widespread antibiotics and antibiotic resistance genes efficiently from wastewater or organic solid waste is a promising technology. This paper reviewed the development of electron shuttles, described the mechanism of action of different electron shuttles and the application of enhanced anaerobic biotreatment with electron shuttles for the removal of antibiotics and related genes. Finally, we discussed the current issues and possible future directions of electron shuttle technology.

Effect of Pretreatment by Freeze Vacuum Drying on Solid-State Anaerobic Digestion of Corn Straw

As a common agricultural waste, corn straw (CS) has a refractory structure, which is not conducive to anaerobic digestion (AD). Appropriate pretreatment is crucial for addressing this problem. Thus, freeze vacuum drying (FVD) was proposed. In this study, fresh CS (F-CS) pretreated (5 h, −40 °C) by FVD and naturally dried CS (D-CS) were compared. Differences in substrate surface structure and nutrient composition were first investigated. Results show that a loose and porous structure, crystallinity, and broken chemical bonds, as well as higher proportions of VS, C, N, cellulose, hemicellulose, and crude proteins in F-CS show a potential for methane production. Besides, process performance and stability were also examined in both high (4, VS basis) and low (1, VS basis) S/I ratio AD. A higher degradation ratio of hemicellulose as well as richer dissolved microbial metabolites, coenzymes, tyrosine-like proteins, and hydrolysis rate of particulate organic matter in the F-CS system enhanced the efficiency of methane conversion. The cumulative methane yield increased from 169.66 (D-CS) to 209.97 (F-CS) mL/gVS in the high S/I ratio system (p = 0.02 < 0.05), and 156.97 to 171.89 mL/gVS in the low S/I ratio system. Additionally, 16S-rRNA-gene-based analysis was performed. Interestingly, the coordination of key bacteria (Clostridium_sensu_stricto_1, Bacillus, Terrisporobacter. Clostridium_sensu_stricto_7, Thermoclostrium, UCG-012, and HN-HF0106) was more active. Poorer Methanosarcina and Methanomassiliicoccus as well as richer Methanobrevibacter and Methanoculleus stimulated the co-relationship of key archaea with diverse methanogenesis pathways. This study aims to verify the positive effect of FVD pretreatment on AD of CS, so as to provide a reference for applications in waste management.

Enhanced bio-reductive degradation of fluoroglucocorticoids in the groundwater fluctuation zone by external electron donors: Performance, microbial community, and functional genes

This study evaluated the effectiveness of external electron donors on the bio-reductive degradation enhancement of fluoroglucocorticoids (FGCs) in the groundwater fluctuation zone during the wet season when reverse upward fluctuation of the groundwater table occurs and the dry season after the groundwater table declines. The results showed that the external electron donors, provided by the addition of nano zero-valent iron-modified biochar (nZVI@BC), inhibited the migration and enhanced the reductive defluorination of triamcinolone acetonide (TA), a representative FGC. The accumulation rate constant with temporal fluctuation depth and the attenuation rate constant with vertical fluctuation depth were -2.55 × 10^-3 and 4.20 × 10^-2, respectively, in the groundwater of the natural groundwater fluctuation zone (N-FZ). In contrast, the accumulation and attenuation rate constants were, respectively, 35.6% and 2.64 times higher in the groundwater fluctuation zone amended with nZVI@BC (nZVI@BC-FZ) as compared with those observed in the N-FZ. Furthermore, the decay rate constant of the TA residue in the dry season was 0.843 × 10^-2 μg/d in N-FZ and was 2.19 times higher in nZVI@BC-FZ. This enhancement effect, caused by the addition of external electrons, was positively correlated with the evolution of the microbial community and the expression of functional genes. The microbes evolved into functional genera with reductive dehalogenation (Xylophilus and Hydrogenophaga) and iron-oxidizing (Lysobacter, Pseudoxanthomonas, and Sphingomonas) abilities in the nZVI@BC-FZ system, which increased dehalogenation and iron oxide genes by a 4-5 order of magnitude. The utilization proportion of external electrons for TA metabolism was 50.04%, of which 30.82%, 10.26%, and 8.96% were utilized for defluorination, hydrogenation, and ring-opening, respectively. This study provides an effective method to reduce pollutant diffusion and enhance the bio-reductive degradation caused by groundwater table fluctuation.

16S rRNA genes- and metagenome-based confirmation of syntrophic butyrate-oxidizing methanogenesis enriched in high butyrate loading

The understanding and enrichment of consortia formed by syntrophic butyrate-oxidizing bacteria and methanogens in the complex environment are crucial for effectively degrading butyrate and preventing acid inhibition. In this study, the better butyrate-tolerated and highly efficient microbial consortia were domesticated and enriched through adding butyric acid ranging from 0.2 to 4.4 g/(L·d). The volumetric biogas production continuously increased to 1.65 L/(L·d). Microbial community diversity showed that a dramatic shift of bacterial structure occurred at BAL of 1.6 g/(L·d) and the structure presented better stability at high BAL. The syntrophic consortia and the main metabolic pathways were revealed through combination of the 16S rDNA and metagenome sequencing analyses. Syntrophomonas was the major butyrate-oxidizing bacterium and oxidized butyrate mainly through β-oxidaiton. Synergistaceae and Mesotoga acted as the main acetate-oxidizing bacteria. IHT and methanogenesis pathways were strongly enhanced by DMER64 and Methanosarcina as the main H₂ carrier and dominant methanogen, respectively.