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

Study on the growth and decline patterns and environmental drivers of pathogens during the stabilization process of simulated landfilling municipal solid waste

Waste and leachate in landfills are substantial reservoirs of pathogens, however information about the risk of pathogen contamination during the stabilization process under different landfill conditions is very limited. In this study, dynamic changes of culturable pathogens, bacteria community, and human bacterial pathogens (HBPs) during the stabilization process under different landfill conditions were investigated, and the environmental drivers were explored. Results showed that total coliforms, Enterococcus, and Staphylococcus aureus were the dominant pathogens detected in waste and leachate samples. During the landfill stabilization process, the concentration of culturable pathogens peaked at the hydrolysis-acidification stage (3.6 × 105 CFU·g-1) in the anaerobic condition, fluctuated from 4.18 × 104 to 5.35 × 105 CFU·g-1 in the anaerobic leachate-recirculation condition, and kept rising (from 4.18 × 104 to 2.12 × 106 CFU·g-1) in the micro-aerobic condition. Moreover, HBPs abundance and diversity in the waste and leachate under micro-aerobic conditions were higher than those under the other two conditions, suggesting a higher risk of pathogen contamination. Sulfate and pH were significantly (p < 0.05) correlated with the composition of bacterial communities and HBPs, likely serving as the major environmental driving factors. Additionally, the interactions between HBPs and functional bacterial groups tended towards cooperative symbiotic relationships, with hydrolytic-acidogenic bacteria promoting the growth and proliferation of most pathogens. These findings will help to understand the changes and environmental drivers of pathogens during landfill stabilization, which will provide a theoretical basis for the risk prevention and control of pathogens in waste disposal.

Metagenomic insights into the functional potential of non-sanitary landfill microbiomes in the Indian Himalayan region, highlighting key plastic degrading genes

Solid waste management in the Indian Himalayan Region (IHR) is a growing challenge, intensified by increasing population and tourism, which strain non-sanitary landfills. This study investigates microbial diversity and functional capabilities within these landfills using a high-throughput shotgun metagenomic approach. Physicochemical analysis revealed that the Manali and Mandi landfill sites were under heavy metal contamination and thermal stress. Taxonomic annotation identified a dominance of bacterial phyla, including Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes, with genera like Pseudomonas and Bacillus prevalent. Squeezemeta analysis generated 9,216,983 open reading frames (ORFs) across the sampling sites, highlighting diverse metabolic potentials for heavy metal resistance and degrading organic, xenobiotics and plastic wastes. Hierarchical clustering and principal component analysis (PCA) identified distinct gene clusters in Manali and Mandi landfill sites, reflecting differences in pollution profiles. Functional redundancy of landfill microbiome was observed with notable xenobiotic and plastic degradation pathways. This is the first comprehensive metagenomic assessment of non-sanitary landfills in the IHR, providing valuable insights into the microbial roles in degrading persistent pollutants, plastic waste, and other contaminants in these stressed environments.

Accelerating electron transfer reduces CH4 and CO2 emissions in paddy soil

As an accelerated electron transfer device, the influence of microbial electrochemical snorkel (MES) on soil greenhouse gas production remains unclear. Electron transport is the key to methane production and denitrification. We found that the N2O amount of the MES treatment was comparable to the control however the cumulative CO2 and CH4 emissions were reduced by 50% and 41%, respectively. The content of Fe2+ in MES treatment increased by 31%, which promoted the electron competition of iron reduction to methanogenesis. Furthermore, the competition among iron-reducing, nitrifying and denitrifying bacteria reduced the abundance of methanogens by 19-20%. Additionally, the MES treatment decreased the abundance of genes associated with hydrogen methanogenesis pathway by 6-19%, and inhibited the further conversion of acetyl-CoA into CH4 for acetoclastic methanogenesis. This study reveals effects of accelerating electron transfer on greenhouse gas emission, and provides a novel strategy for reducing greenhouse gas emissions in paddy soil.

Contaminant-degrading bacteria are super carriers of antibiotic resistance genes in municipal landfills: A metagenomics-based study

Municipal landfills are hotspot sources of antimicrobial resistance (AMR) and are also important habitats of contaminant-degrading bacteria. However, high diversity of antibiotic resistance genes (ARGs) in landfills hinders assessing AMR risks in the affected environment. More concerned, whether there is co-selection or enrichment of antibiotic-resistant bacteria and contaminant-degrading bacteria in these extremely polluted environments is far less understood. Here, we collected metagenomic datasets of 32 raw leachate and 45 solid waste samples in 22 municipal landfills of China. The antibiotic resistome, antibiotic-resistant bacteria and contaminant-degrading bacteria were explored, and were then compared with other environmental types. Results showed that the antibiotic resistome in landfills contained 1,403 ARG subtypes, with the total abundance over the levels in natural environments and reaching the levels in human feces and sewage. Therein, 49 subtypes were listed as top priority ARGs for future surveillance based on the criteria of enrichment in landfills, mobilizable and present in pathogens. By comparing to those in less contaminated river environments, we elucidated an enrichment of antibiotic-resistant bacteria with contaminant-degrading potentials in landfills. Bacteria in Pseudomonadaceae, Moraxellaceae, Xanthomonadaceae and Enterobacteriaceae deserved the most concerns since 72.2 % of ARG hosts were classified to them. Klebsiella pneumoniae, Acinetobacter nosocomialis and Escherichia coli were abundant multidrug-resistant pathogenic species in raw leachate (∼10.2 % of total microbiomes), but they rarely carried contaminant-degradation genes. Notably, several bacterial genera belonging to Pseudomonadaceae had the most antibiotic-resistant, pathogenic, and contaminant-degrading potentials than other bacteria. Overall, the findings highlight environmental selection for contaminant-degrading antibiotic-resistant pathogens, and provide significant insights into AMR risks in municipal landfills.

Mitigating greenhouse gas emission and enhancing fermentation by phosphorus slag addition during sewage sludge composting

During the composting of sewage sludge (SS), a quantity of greenhouse gases has been produced. This study aimed to clarify the microbial mechanisms associated with the addition of industrial solid waste phosphorus slag (PS) to SS composting, specifically focusing on its impact on greenhouse gas emissions and the humification. The findings indicated that the introduction of PS increased the temperature and extended the high-temperature phase. Moreover, the incorporation of 10% and 15% PS resulted in a decrease of N2O emissions by 68.9% and 88.6%, respectively. Microbial diversity analysis indicated that PS improved waste porosity, ensuring the aerobic habitat. Therefore, the environmental factors of the system were altered, leading to the enrichment of various functional bacterial species, such as Firmicutes and Chloroflexi, and a reduction of pathogenic bacterium Dokdonella. Consequently, incorporating PS into SS composting represents an effective waste treatment strategy, exhibiting economic feasibility and promising application potential.

Food waste impact on dry anaerobic digestion of straw in a novel reactor: Biogas yield, stability, and hydrolysis-methanogenesis processes

Gradient anaerobic digestion reactor (GADR) can improve substrate utilization efficiency by solving the problem of the "short circuit" of materials. However, the substrate's composition significantly affects the reactor's performance. This study investigated the impact of food waste (FW) levels on corn straw's dry anaerobic digestion (AD) in a novel GADR. The results show that biomethane production can be improved by coupling urban and agricultural solid waste recycling. The mechanism is to increase the hydrolysis and acid production efficiency, and the abundance of enzymes related to methanogenesis. The maximum methane yield (494.2 mL CH4/g VS) and the highest anaerobic biodegradability (85.7 %) were obtained when the FW was added at 60 %. The co-digestion of FW and straw can improve the hydrolysis and acid production efficiency and methane yield, which improves the buffering capacity and stability of the system compared with the single digestion of FW.

Enhancement of hydrogenotrophic methanogenesis for methane production by nano zero-valent iron in soils

Nanoscale zero-valent iron (nZVI) is attracting increasing attention as the most commonly used environmental remediation material. However, given the high surface area and strong reducing capabilities of nZVI, there is a lack of understanding regarding its effects on the complex anaerobic methane production process in flooded soils. To elucidate the mechanism of CH4 production in soil exposed to nZVI, paddy soil was collected and subjected to anaerobic culture under continuous flooding conditions, with various dosages of nZVI applied. The results showed that the introduction of nZVI into anaerobic flooded rice paddy systems promoted microbial utilization of acetate and carbon dioxide as carbon sources for methane production, ultimately leading to increased methane production. Following the introduction of nZVI into the soil, there was a rapid increase in hydrogen levels in the headspace, surpassing that of the control group. The hydrogen levels in both the experimental and control groups were depleted by the 29th day of culture. These findings suggest that nZVI exposure facilitates the enrichment of hydrogenotrophic methanogens, providing them with a favorable environment for growth. Additionally, it affected soil physicochemical properties by increasing pH and electrical conductivity. The metagenomic analysis further indicates that under exposure to nZVI, hydrogenotrophic methanogens, particularly Methanobacteriaceae and Methanocellaceae, were enriched. The relative abundance of genes such as mcrA and mcrB associated with methane production was increased. This study provides important theoretical insights into the response of key microbes, functional genes, and methane production pathways to nZVI during anaerobic methane production in rice paddy soils, offering fundamental insights into the long-term fate and risks associated with the introduction of nZVI into soils.

Responses of methane production and methanogenic pathways to polystyrene nanoplastics exposure in paddy soil

Nanoplastics (NPs) have attracted increasing attention within terrestrial ecosystems. However, our understanding of their impacts on the intricate anaerobic methanogenesis processes occurring in paddy soils microbial communities remains limited with respect to nanoplastics shape, function, and metabolic effects. Herein, we explored the effects of polystyrene nanoplastics (PS-NPs) and microplastics (PS-MPs) on anaerobic methanogenesis in a typical paddy soil. The results show that PS-NPs delayed methane production and the time to reach peak acetate content in incubation process of paddy soils, and the methanogenic rate increased rapidly after 13 days, with a maximum increase of 87.97%. However, PS-MPs had no marked effect on CH4, CO2 and acetate production. In addition, PS-NPs affected soil physicochemical properties by reducing pH and increasing electrical conductivity. Acetoclastic methanogens were enriched and the relative abundance of the genes ackA, pta, ACSS, cdhC, cdhD and cdhE in the acetoclastic pathways were significantly increased under PS-NPs exposure. In addition, PS-MPs had significant effect on the microbial community structure but no effect on methanogenic pathways of the paddy soils. This study provides important insights into the response of key microorganisms, functional genes and methanogenesis pathways to NPs during anaerobic methanogenesis in paddy soils.

A comprehensive review on methane's dual role: effects in climate change and potential as a carbon-neutral energy source

The unprecedented population and anthropogenic activity rise have challenged the future look up for shifts in global temperature and climate patterns. Anthropogenic activities such as land fillings, building dams, wetlands converting to lands, combustion of biomass, deforestation, mining, and the gas and coal industries have directly or indirectly increased catastrophic methane (CH4) emissions at an alarming rate. Methane is 25 times more potent trapping heat when compared to carbon dioxide (CO2) in the atmosphere. A rise in atmospheric methane, on a 20-year time scale, has an impact of 80 times greater than that of CO2. With increased population growth, waste generation is rising and is predicted to reach 6 Mt by 2025. CH4 emitted from landfills is a significant source that accounts for 40% of overall global methane emissions. Various mitigation and emissions reduction strategies could significantly reduce the global CH4 burden at a cost comparable to the parallel and necessary CO2 reduction measures, reversing the CH4 burden to pathways that achieve the goals of the Paris Agreement. CH4 mitigation directly benefits climate change, has collateral impacts on the economy, human health, and agriculture, and considerably supports CO2 mitigation. Utilizing the CO2 from the environment, methanogens produce methane and lower their carbon footprint. NGOs and the general public should act on time to overcome atmospheric methane emissions by utilizing the raw source for producing carbon-neutral fuel. However, more research potential is required for green energy production and to consider investigating the untapped potential of methanogens for dependable energy generation.

Impact of incineration slag co-disposed with municipal solid waste on methane production and methanogens ecology in landfills

Co-landfill of incineration slag and municipal solid waste (MSW) is a main method for disposal of slag, and it has the potential of promoting methane (CH₄) production and accelerating landfill stabilization. Four simulated MSW landfill columns loaded with different amount of slag (A, 0%; B, 5%; C, 10%; D, 20%) were established, and the CH₄ production characteristics and methanogenic mechanisms were investigated. The maximum CH₄ concentration in columns A, B, C and D was 10.8%, 23.3%, 36.3% and 34.3%, respectively. Leachate pH and refuse pH were positively correlated with CH₄ concentration. Methanosarcina was the dominant genus with abundance of 35.1%∼75.2% and it was positively correlated with CH₄ concentration. CO₂-reducing and acetoclastic methanogenesis were the main types of methanogenesis pathway, and the methanogenesis functional abundance increased with slag proportion during stable methanogenesis process. This research can help understanding the impact of slag on CH₄ production characteristics and microbiological mechanisms in landfills.