Ensiling excessively wilted maize stover with biogas slurry: Effects on storage performance and subsequent biogas potential

Distribution Patterns of Antibiotic Resistance Genes and Their Bacterial Hosts in a Manure Lagoon of a Large-Scale Swine Finishing Facility

The spread of antibiotic resistance genes (ARGs) that are present in livestock manures, which are discharged into the environment, is a severe threat to human and animal health. Here, we used 16S rRNA gene profiling and metagenomic analysis to characterize microbial community composition and antibiotic resistance in a manure storage lagoon from a large-scale swine finishing facility. Manure samples were collected at intervals of two years. Both the prokaryotic community and the resistome were dominated by the Firmicutes, Proteobacteria and Bacteroidota. Metagenomic analysis of two samples revealed 726 and 641 ARGs classified into 59 and 46 AMR gene families. Besides multidrug efflux pumps, the predominating ARGs potentially encoded resistance to tetracyclines, macrolide-lincosamide-streptogramin, aminoglycosides, peptide antibiotics, rifamycin, chloramphenicol, and beta-lactams. Genes from all predominant AMR gene families were found in both samples indicating overall long-term stability of the resistome. Antibiotic efflux pumps were the primary type of ARGs in the Proteobacteria, while antibiotic target alteration or protection was the main mechanism of resistance in the Firmicutes, Actinobacteriota and Bacteroidota. Metagenome-assembled genomes (MAG) of four multidrug-resistant strains were assembled. The first MAG, assigned to Escherichia flexneri, contained 46 ARGs, including multidrug efflux pumps, modified porins, beta-lactamases, and genes conferring resistance to peptide antibiotics. The second MAG, assigned to the family Alcaligenaceae, contained 18 ARGs encoding resistance to macrolide-lincosamide-streptogramin, tetracyclines, aminoglycosides and diaminopyrimidins. Two other MAGs representing the genera Atopostipes and Prevotella, contained four and seven ARGs, respectively. All these MAGs represented minor community members and accounted for less than 0.3% of the whole metagenome. Overall, a few lineages originated from the gut but relatively rare in the manure storage lagoon, are the main source of ARGs and some of them carry multiple resistance determinants.

Insight into effects of polyethylene microplastics in anaerobic digestion systems of waste activated sludge: Interactions of digestion performance, microbial communities and antibiotic resistance genes

The environmental risks of microplastics (MPs) have raised an increasing concern. However, the effects of MPs in anaerobic digestion (AD) systems of waste activated sludge (WAS), especially on the fate of antibiotic resistance genes (ARGs), have not been clearly understood. Herein, the variation and interaction of digestion performance, microbial communities and ARGs during AD process of WAS in the presence of polyethylene (PE) MPs with two sizes, PE MPs-180μm and PE MPs-1mm, were investigated. The results showed that the presence of PE MPs, especially PE MPs-1mm, led to the increased hydrolysis of soluble polysaccharides and proteins and the accumulation of volatile fatty acids. The methane production decreased by 6.1% and 13.8% in the presence of PE MPs-180μm and PE MPs-1mm, respectively. Together with this process, hydrolytic bacteria and acidogens were enriched, and methanogens participating in acetoclastic methanogenesis were reduced. Meanwhile, ARGs were enriched obviously by the presence of PE MPs, the abundances of which in PE MPs-180μm and PE MPs-1mm groups were 1.2-3.0 times and 1.5-4.0 times higher than that in the control by the end of AD. That was associated with different co-occurrence patterns between ARGs and bacterial taxa and the enrichment of ARG-hosting bacteria caused by the presence of PE MPs. Together these results suggested the adverse effects of PE MPs on performance and ARGs removal during AD process of WAS through inducing the changes of microbial populations.

Effect of wheat straw pretreated with liquid fraction of digestate from different substrates on anaerobic digestion performance and microbial community characteristics

The effects of liquid fraction of digestate (LFD) pretreatment on anaerobic digestion (AD) performance and microbial community characteristics were estimated. Prior to AD, LFD (LFD_SM, LFD_FW, and LFD_WS) collected separately from three continuously stirred tank reactors (CSTRs) using swine manure (SM), food waste (FW), and wheat straw (WS) as the mono-substrate was applied to pretreat WS. The results showed that AD with LFD pretreatment resulted in biomethane yields of 240.2-277.9 mL·gVS^-1, a 33.57%-54.54% improvement over the yield of the control, and also produced a time saving of 32.26%-46.77%. The pretreatment parameters were optimized for LFD pretreatment. The enhancement effect was in the order of LFD_FW > LFD_SM > LFD_WS. Simultaneously, the cellulose, hemicellulose and lignin contents in the WS and their characteristics (surface properties, crystallinity index, etc.) varied accordingly. The function of the microbial community was strengthened during the pretreatment stage, but the structure of the microbial community had a clear response to the LFD source substrates. Bacteroidetes was the most dominant phyla and was positively correlated with the hydrolysis rate. Consequently, the LFD from the different substrates used as pretreat agents could improve the AD performance of WS.

Ensiling process for efficient biogas production from lignocellulosic substrates: Methods, mechanisms, and measures

Ensiling has been developed as mainstream technologies to preserve lignocellulose biomass for biogas production. However, the lack of general evaluation methods and process mechanism research hinders the understanding of its effectiveness. In this context, we reviewed existing studies and proposed some key considerations: (1) For assessing the ensiling process, determined dry matter contents should be corrected according to the volatilization loss in oven-drying method to obtain accurate storage loss and methane yield; (2) For comprehensive assessments, the trade-off between storage loss and enhanced biomethane yield should be evaluated from the entire-chain process; (3) The mechanism to enhance methane yield is primarily attributed to increased lignocellulosic biodigestibility through acid-based hydrolysis and biological degradation during ensiling; (4) Measures including co-storage, increasing buffering capacity, adjusting carbon/nitrogen ratio, and additives can be adopted to increase biogas production. The proposed methods, mechanisms, and measures (3Ms) could help initiate the specific quality criteria of biogas-oriented silages.

Urea-assisted ensiling process of wilted maize stover for profitable biomethane production

Low contents of water-soluble carbohydrates and/or low indigenous microbial activity in wilted maize stover (WMS) usually hinder the establishment of the ensiling process, thereby resulting in a low biogas production because of high loss of dry matter (DM). To enhance the biological activity and substrate biodegradability, this study applied the synergistic regulation of sucrose (carbon source) and increasing levels of urea (nitrogen source) during the ensiling process of WMS. Compared with the application of only sucrose, a higher organic acid content (lactic acid (85 g/kg-DM) and acetic acid (14 g/kg-DM)) and higher degradation ratios for lignocellulose (hemicellulose (28%), cellulose (22%), and lignin (17%)) were observed with urea applications of 1.7% (DM) and 3.9% (DM), respectively. This was caused by the enhanced activities of the hetero-fermenter (Weissella) and cellulolytic bacteria (Cellulosimicrobium). A simultaneous addition of urea and sucrose during the ensiling of WMS increased the specific methane yield by 11.2%-21.1% in comparison to raw WMS. Moreover, an economic cost estimation revealed that this approach could be an effective storage strategy for the efficient production of methane when employing a 1.7% (DM) urea application.