Effect of hydrochar on antibiotic-resistance genes and relevant mechanisms during chicken manure composting

Deciphering the reduction of antibiotic resistance genes (ARGs) during medium-chain fatty acids production from waste activated sludge: Driven by inhibition of ARGs transmission and shift of microbial community

Medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS) by chain extension (CE) is a promising technology. However, the effects and mechanisms of CE process on the fate of antibiotic resistance genes (ARGs) remain unclear. In this study, the results showed that the removal efficiency of ARGs was 81.15 % in CE process, suggesting its efficacy in reducing environmental risks. Further, the observed decrease in mobile genetic elements (MGEs) indicated that CE process restricted the horizontal gene transfer (HGT). Complementing this, the increase in soluble organic matters and extracellular 16 S rDNA confirmed that MCFAs production caused bacterial damage. Decreased intracellular ARGs and increased extracellular ARGs further revealed that MCFAs production impaired ARGs hosts, thereby limiting the vertical gene transfer (VGT) of ARGs. Shift of microbial community combined with co-occurrence network analysis demonstrated that functional bacteria without host potential for ARGs were enriched, but potential ARGs and MGEs hosts decreased, showing the role of functional bacterial phylogeny and selection pressure of MCFAs in reducing ARGs. Finally, partial least squares path model was used to systematic verify the mechanism of ARGs removal in CE process, which was attributed to the inhibition of ARGs transmission (HGT and VGT) and shift of microbial community.

Synergistic removal of total petroleum hydrocarbons and antibiotic resistance genes in Yellow River Delta wetlands contaminated soil composting regulated by biogas slurry addition

The interactive effects between the emerging contaminant antibiotic resistance genes (ARGs) and the traditional pollutant total petroleum hydrocarbons (TPHs) in contaminated soils remain unclear. The synergistic removal of TPHs and ARGs from composted contaminated soil, along with the microbial mechanisms driven by the addition of biogas slurry, have not yet been investigated. This study explored the impact of biogas slurry on the synergistic degradation mechanisms and bacterial community dynamics of ARGs and TPHs in compost derived from contaminated soil. The addition of biogas slurry resulted in a reduction of targeted ARGs and mobile genetic elements (MGEs) by 9.96%-95.70% and 13.32%-97.66%, respectively. Biogas slurry changed the succession of bacterial communities during composting, thereby reducing the transmission risk of ARGs. Pseudomonas, Cellvibrio, and Devosia were identified as core microorganisms in the synergistic degradation of ARGs and TPHs. According to the partial least squares path model, temperature and NO3- indirectly influenced the removal of ARGs and TPHs by directly regulating the abundance and composition of host microbes and MGEs. In summary, the results of this study contribute to the high-value utilization of biogas slurry and provide methodological support for the low-cost remediation of contaminated soils.

Impacts of microplastic type on the fate of antibiotic resistance genes and horizontal gene transfer mechanism during anaerobic digestion

Microplastics (MPs) and antibiotic resistance genes (ARGs) are important pollutants in waste activated sludge (WAS), but their interactions during anaerobic digestion (AD) still need to be further explored. This study investigated variations in ARGs, mobile genetic elements (MGEs), and host bacteria during AD under the pressure of polyamide (PA), polyethylene (PE), and polypropylene (PP). The results showed that the MPs increased methane production by 11.7-35.5%, and decreased ARG abundance by 5.6-24.6%. Correlation analysis showed that the decrease of MGEs (plasmid, prophage, etc.) promoted the decrease of the abundance of multidrug, aminoglycoside and tetracycline resistance genes. Metagenomic annotation revealed that the reduction of key host bacteria (Arenimonas, Lautropia, etc.) reduced the abundance of major ARGs (rsmA, rpoB2, etc.). Moreover, PP MPs contributed to a reduction in the abundance of functional genes related to the production of reactive oxygen species, ATP synthesis, and cell membrane permeability, which was conducive to reducing the potential for horizontal gene transfer of ARGs. These findings provide insights into the treatment of organic waste containing MPs.

Deciphering and predicting changes in antibiotic resistance genes during pig manure aerobic composting via machine learning model

Livestock manure is one of the most important pools of antibiotic resistance genes (ARGs) in the environment. Aerobic composting can effectively reduce the spread of antibiotic resistance risk in livestock manure. Understanding the effect of aerobic composting process parameters on manure-sourced ARGs is important to control their spreading risk. In this study, the effects of process parameters on ARGs during aerobic composting of pig manure were explored through data mining based on 191 valid data collected from literature. Machine learning (ML) models (XGBoost and Random Forest) were utilized to predict the rate of ARGs changes during pig manure composting. The model evaluation index of the XGBoost model (R2 = 0.651) was higher than that of the Random Forest (R2 = 0.490), indicating that XGBoost had better prediction performance. Feature importance was further calculated for the XGBoost model, and the XGBoost black box model was interpreted by Shapley additive explanations analysis. Results indicated that the influencing factors on the ARGs variation in pig manure were sequentially divided into thermophilic period, total composting period, composting real time, and thermophilic stage average temperature. The findings gave an insight into the application of ML models to predict and decipher the ARG changes during manure composting and provided suggestions for better composting manipulation and optimization of process parameters.

Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution

This study presented the effects of hydrochar on humification, heavy metals (HMs) bioavailability and bacterial community succession during composting. Results indicated that hydrochar addition led to elevated composting temperature, 7.3% increase in humic acid (HA), and 52.9% increase in ratio of humic acid to fulvic acid. The diethylene triamine pentacetic acid extractable Zn, Cu, Pb, and Ni were reduced by 19.2%, 36.3%, 37.8%, and 27.1%, respectively, in hydrochar-involved composting system. Furthermore, main mechanisms driving the reduced HMs bioavailability by hydrochar addition were revealed. The addition of hydrochar significantly modified the microbial community structure. Correlation analysis and microbial analysis demonstrated that relative abundance of bacterial groups connected with humification and HMs passivation were increased. Consequently, the HA formation was promoted and the HMs bioavailability were reduced through bacterial bioremediation and HA complexation. This study demonstrates the addition of hydrochar as a promising strategy to mitigate the HMs bioavailability during composting.

Electron transfer and microbial mechanism of synergistic degradation of lignocellulose by hydrochar and aerobic fermentation

In response to the problem of asynchronous fermentation between lignocellulose and perishable materials in compost, the combined technology of low-temperature hydrochar and compost has been studied. Hydrochar was prepared through low-temperature hydrothermal reactions and applied to aerobic fermentation. The response relationship between lignocellulose content, electron transfer capability, and microbes was explored. The results showed that a pore structure with oxygen-containing functional groups was formed in hydrochar, promoting electron transfer during composting. With the rapid increase in composting temperature, the lignocellulose content decreased by 64.36 mg/g. Oceanobacillus, Cerasibacillus, Marinimicrobium, and Gracilibacillus promoted the degradation of lignocellulose and the carbon/nitrogen cycle during aerobic fermentation, and there was a significant response relationship between electron transfer capability and functional microbes. The combined application of hydrochar and aerobic fermentation accelerated the degradation of lignocellulose. This study provides technical support for the treatment of heterogeneous organic waste.

Macrogenomic analysis of the effects of aqueous-phase from hydrothermal carbonation of sewage sludge on nitrogen metabolism pathways and associated bacterial communities during composting

The effects of aqueous phases (AP) formed from hydrothermal carbonation of sewage sludge (with or without rice husk) as moisture regulators of nitrogen metabolism pathways during composting are currently unclear. Macrogenomic analyses revealed that both APs resulted in notably changes in bacterial communities during composting; increased levels of nitrogen assimilation, nitrification, and denitrification metabolic pathways; and decreased levels of nitrogen mineralization metabolic pathways. Genes associated with nitrogen assimilation and mineralization accounted for 34-41% and 32-40% of the annotated reads related to nitrogen cycling during composting, respectively, representing them as the most abundant nitrogen metabolism processes. The gudB and norB were identified as key genes for nitrogen mineralization and nitrous oxide emission, respectively. This research offers a better understanding of the effects of additional nitrogen sources on nitrogen metabolism pathways during composting.

Reduction of mobile genetic elements determines the removal of antibiotic resistance genes during pig manure composting after thermal pretreatment

Animal manure is a primary repository of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs). This work explored the efficiency of ARGs and MGEs removal during pig manure composting after thermal pretreatment (TPC) and the underlying mechanisms. TPC resulted in a decrease of 94.7% and 92.3% in the relative abundance of ARGs and MGEs which was 48.9% and 76.6% lower than control, respectively. Network analysis indicated that reductions of ARGs and MGEs in TPC were relevant to decrease in the amount and abundance of bacterial hosts. Furthermore, total ARGs abundance in TPC was correlated with that of intI1 and Tn916/1545 (P < 0.001). Redundancy analysis supported a leading role of MGEs in ARGs dynamics in TPC. Reduction of MGEs rather than bacterial hosts contributed mainly to ARGs removal in TPC, as revealed by structural equation modeling. In conclusion, TPC was an effective method to treat animal manure containing ARGs.