Entry pathways determined the effects of MPs on sludge anaerobic digestion system: The views of methane production and antibiotic resistance genes fates.

Sludge is one of the primary reservoirs of microplastics (MPs), and the effects of MPs on subsequent sludge treatment raised attention. Given the entry pathways, MPs would exhibit different properties, but the entry pathway-dependent effect of MPs on sludge treatment performance and the fates of antibiotic resistance genes (ARGs), another high-risk emerging contaminant, were seldom documented. Herein, MPs with two predominant entry pathways, including wastewater-derived (WW-derived) and anaerobic digestion-introduced (AD-introduced), were used to investigate the effects on AD performance and ARGs abundances. The results indicated that WW-derived MPs, namely the MPs accumulated in sludge during the wastewater treatment process, exhibited significant inhibition on methane production by 22.8%-71.6%, while the AD-introduced MPs, being introduced in the sludge AD process, slightly increased the methane yield by 4.7%-17.1%. Meanwhile, MPs were responsible for promoting transmission of target ARGs, and polyethylene terephthalate MPs (PET-MPs) showed a greater promotion effect (0.0154-0.0936) than polyamide MPs (PA-MPs) (0.0013-0.0724). Compared to size, entry pathways and types played more vital roles on MPs influences. Investigation on mechanisms based on microbial community structure revealed characteristics (aging degree and types) of MPs determined the differences of AD performance and ARGs fates. WW-derived MPs with longer aging period and higher aging degree would release toxics and decrease the activities of microorganisms, resulting in the negative impact on AD performance. However, AD-introduced MPs with short aging period exhibited marginal impacts on AD performance. Furthermore, the co-occurrent network analysis suggested that the variations of potential host bacteria induced by MPs with different types and aging degree attributed to the dissemination of ARGs. Distinctively from most previous studies, the MPs with different sizes did not show remarkable effects on AD performance and ARGs fates. Our findings benefited the understanding of realistic environmental behavior and effect of MPs with different sources.

Pathogenic Bacteria Are the Primary Determinants Shaping PM2.5-Borne Resistomes in the Municipal Food Waste Treatment System.

Bioaerosol pollution poses a substantial threat to human health during municipal food waste (FW) recycling. However, bioaerosol-borne antibiotic-resistant genes (ARGs) have received little attention. Herein, 48 metagenomic data were applied to study the prevalence of PM2.5-borne ARGs in and around full-scale food waste treatment plants (FWTPs). Overall, FWTP PM2.5 (2.82 ± 1.47 copies/16S rRNA gene) harbored comparable total abundance of ARGs to that of municipal wastewater treatment plant PM2.5 (WWTP), but was significantly enriched with the multidrug type (e.g., AdeC/I/J; p < 0.05), especially the abundant multidrug ARGs could serve as effective indicators to define resistome profiles of FWTPs (Random Forest accuracy >92%). FWTP PM2.5 exhibited a decreasing enrichment of total ARGs along the FWTP-downwind-boundary gradient, eventually reaching levels comparable to urban PM2.5 (1.46 ± 0.21 copies/16S rRNA gene, N = 12). The combined analysis of source-tracking, metagenome-assembled genomes (MAGs), and culture-based testing provides strong evidence that Acinetobacter johnsonii-dominated pathogens contributed significantly to shaping and disseminating multidrug ARGs, while abiotic factors (i.e., SO42-) indirectly participated in these processes, which deserves more attention in developing strategies to mitigate airborne ARGs. In addition, the exposure level of FWTP PM2.5-borne resistant pathogens was about 5-11 times higher than those in urban PM2.5, and could be more severe than hospital PM2.5 in certain scenarios (<41.53%). This work highlights the importance of FWTP in disseminating airborne multidrug ARGs and the need for re-evaluating the air pollution induced by municipal FWTP in public health terms.

Distinct effects of typical sludge pretreatment approaches on the antibiotic resistance genes variations, associated bacterial community dynamics and metabolic activities during anaerobic fermentation process.

Anaerobic fermentation is effective for waste activated sludge (WAS) disposal to realize resource generation and pollutants reduction, and various pretreatments were commonly applied to improve the performance. This work mainly investigated the effects of typical WAS pretreatment approaches on the antibiotic resistance genes (ARGs, as emerging contaminants) removal during anaerobic fermentation processes and unveiled the underlying mechanisms. The results indicated that all the pretreatment strategies exhibited evident effects on the overall ARGs removal with the order of Fe2+ activated persulfate (PS/Fe2+) > pH 10 > Ultrasonication > Heat, and showed selective removal tendency for the specific ARGs (namely easily removed (aadA1 and sul1) and persistent ARGs). Mechanistic analysis demonstrated that the pretreatments disrupted the extracellular polymeric substances (EPS) and rose the cell membrane permeability (particularly for PS/Fe2+ and Heat). Then the increased ARGs release benefitted the subsequent reduction of mobile genetic elements (MGEs) and extracellular ARGs (especially for PS/Fe2+ and pH10), resulting the ARGs attenuation. Pretreatments significantly shifted the microbial community structure and the abundances of potential ARGs hosts (i.e., Sulfuritalea, and Denitratisoma). Also, the different pretreatments exhibited distinct effects on the microbial metabolic traits related with ARGs proliferation (i.e., ABC transporters, two-component system and bacterial secretion systems), which also contributed to the ARGs attenuations during WAS fermentation. The partial least-squares path modeling (PLS-PM) analysis indicated that the bacterial community (total effects = 0.968) was key factor determining ARGs fates.

Reductive Stress Boosts the Horizontal Transfer of Plasmid-Borne Antibiotic Resistance Genes: The Neglected Side of the Intracellular Redox Spectrum.

The dissemination of plasmid-borne antibiotic resistance genes (ARGs) among bacteria is becoming a global challenge to the "One Health" concept. During conjugation, the donor/recipient usually encounter diverse stresses induced by the surrounding environment. Previous studies mainly focused on the effects of oxidative stress on plasmid conjugation, but ignored the potential contribution of reductive stress (RS), the other side of the intracellular redox spectrum. Herein, we demonstrated for the first time that RS induced by dithiothreitol could significantly boost the horizontal transfer of plasmid RP4 from Escherichia coli K12 to different recipients (E. coli HB101, Salmonella Typhimurium, and Pseudomonas putida KT2440). Phenotypic and genotypic tests confirmed that RS upregulated genes encoding the transfer apparatus of plasmid RP4, which was attributed to the promoted consumption of intracellular glutamine in the donor rather than the widely reported SOS response. Moreover, RS was verified to benefit ATP supply by activating glycolysis (e.g., GAPDH) and the respiratory chain (e.g., appBC), triggering the deficiency of intracellular free Mg2+ by promoting its binding, and reducing membrane permeability by stimulating cardiolipin biosynthesis, all of which were beneficial to the functioning of transfer apparatus. Overall, our findings uncovered the neglected risks of RS in ARG spreading and updated the regulatory mechanism of plasmid conjugation.

Acute toxicity evaluation of nanoparticles mixtures using luminescent bacteria.

As the application of nanoparticles (NPs) and their release to the environment has increased, it is important to verify their toxicity, with a special emphasis on particle solubilization and the interaction of NP mixtures. In the current study, a model luminescent bacteria, Vibrio fischeri, was employed to test the acute toxicity of individual NPs and their binary mixtures, including metal NPs (ZnNPs, CuNPs) and metal oxide NPs (ZnONPs, CuONPs). The independent action model was used to reflect the synergistic, additive, or antagonistic interactions of binary mixtures of these NPs. The results showed that the median effective concentration (EC50) inhibited the luminescence of V. fischeri were 20.5, 4.1, 11.6, and 118.7 mg L-1 for ZnNPs, CuNPs, ZnONPs, and CuONPs, respectively, suggesting that the toxicity of these NPs to V. fischeri were as the following order: CuNPs > ZnONPs > ZnNPs > CuONPs. The combined effect of NPs were found to be antagonistic for CuNPs-ZnONPs and CuNPs-CuONPs, synergistic for CuONPs-ZnNPs, CuNPs-ZnNPs, and ZnONPs-CuONPs, and additive for ZnNPs-ZnONPs, revealing a complex pattern of possible interactions. The differences of dissolved metal ions partly accounted for the different combined toxicity of binary mixtures of NPs. The findings have important implications for better understanding the true environmental risk of NP mixtures.

Effect of CO2 on Microbial Denitrification via Inhibiting Electron Transport and Consumption.

Increasing anthropogenic CO2 emissions have been reported to influence global biogeochemical processes; however, in the literature the effects of CO2 on denitrification have mainly been attributed to the changes it causes in environmental factors, while the direct effects of CO2 on denitrification remain unknown. In this study, increasing CO2 from 0 to 30 000 ppm under constant environmental conditions decreased total nitrogen removal efficiency from 97% to 54%, but increased N2O generation by 240 fold. A subsequent mechanistic study revealed that CO2 damaged the bacterial membrane and directly inhibited the transport and consumption of intracellular electrons by causing intracellular reactive nitrogen species (RNS) accumulation, suppressing the expression of key electron transfer proteins (flavoprotein, succinate dehydrogenase, and cytochrome c) and the synthesis and activity of key denitrifying enzymes. Further study indicated that the inhibitory effects of CO2 on the transport and consumption of electrons were caused by the decrease of intracellular iron due to key iron transporters (AfuA, FhuC, and FhuD) being down-regulated. Overall, this study suggests that the direct effect of CO2 on denitrifying microbes via inhibition of intracellular electron transport and consumption is an important reason for its negative influence on denitrification.

The effects of fulvic acid on microbial denitrification: promotion of NADH generation, electron transfer, and consumption.

The heterotrophic denitrification requires the participation of electrons which are derived from direct electron donor (usually nicotinamide adenine dinucleotide (NADH)), and the electrons are transferred via electron transport system in denitrifiers and then consumed by denitrifying enzymes. Despite the reported electron transfer ability of humic substances (HS), the influences of fulvic acid (FA), an ubiquitous major component of HS, on promoting NADH generation, electron transfer, and consumption in denitrification process have never been reported. The presence of FA, compared with the control, was found not only significantly improved the total nitrogen (TN) removal efficiency (99.9 % versus 74.8 %) but remarkably reduced the nitrite accumulation (0.2 against 43.8 mg/L) and N2O emission (0.003 against 0.240 mg nitrogen/mg TN removed). The mechanisms study showed that FA increased the metabolism of carbon source via glycolysis and tricarboxylic acid (TCA) cycle pathways to produce more available NADH. FA also facilitated the electron transfer activities from NADH to denitrifying enzymes via complex I and complex III in electron transport system, which improved the reduction of nitrate and accelerated the transformations of nitrite and N2O, and lower nitrite and N2O accumulations were therefore observed. In addition, the consumption of electrons in denitrification was enhanced due to FA stimulating the synthesis and the catalytic activity of key denitrifying enzymes, especially nitrite reductase and N2O reductase. It will provide an important new insight into the potential effect of FA on microbial denitrification metabolism process and even nitrogen cycle in nature niches.

Long-term effects of engineered nanoparticles on enzyme activity and functional bacteria in wastewater treatment plants.

The pervasive use of engineered nanoparticles (NPs) in a wide range of fields raises concerns about their potential environmental impacts. Previous studies confirmed that some NPs had already entered wastewater treatment plants (WWTPs). Wastewater nutrient removal depends on the metabolisms of activated sludge bacteria and their related key enzymes. Therefore, this study compared the possible influences of Al2O3, SiO2, TiO2, and ZnO NPs on the key enzymes activities and microbial community structures involved in wastewater treatment facilities. It was found that long-term exposure to these NPs significantly affected the microbial communities and changed the relative abundances of key functional bacteria, such as ammonia-oxidizing bacteria. Also, the gene expressions and catalytic activities of essential enzymes, such as ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase, and nitrite reductase, were decreased, which finally resulted in a lower efficiency of biological nitrogen removal.

Effects of titanium dioxide and zinc oxide nanoparticles on methane production from anaerobic co-digestion of primary and excess sludge.

Anaerobic co-digestion of primary and excess sludge is regarded as an efficient way to reuse sludge organic matter to produce methane. In this study, short-term and long-term exposure experiments were conducted to investigate the possible effects of titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NPs) on methane production from anaerobic co-digestion of primary and excess sludge. The data showed that TiO2 NPs had no measurable impact on methane production, even at a high concentration (150 mg/g total suspended solids (TSS)). However, short-term (8 days) exposure to 30 or 150 mg/g-TSS of ZnO NPs significantly decreased methane production. More importantly, these negative effects of ZnO NPs on anaerobic sludge co-digestion were not alleviated by increasing the adaptation time to 105 days. Further studies indicated that the presence of ZnO NPs substantially decreased the abundance of methanogenic archaea, which reduced methane production. Meanwhile, the activities of some key enzymes involved in methane production, such as protease, acetate kinase, and coenzyme F420, were remarkably inhibited by the presence of ZnO NPs, which was also an important reason for the decreased methane production. These results provide a better understanding of the potential risks of TiO2 and ZnO NPs to methane production from anaerobic sludge co-digestion.

Zinc oxide nanoparticles cause inhibition of microbial denitrification by affecting transcriptional regulation and enzyme activity.

Over the past few decades, human activities have accelerated the rates and extents of water eutrophication and global warming through increasing delivery of biologically available nitrogen such as nitrate and large emissions of anthropogenic greenhouse gases. In particular, nitrous oxide (N2O) is one of the most important greenhouse gases, because it has a 300-fold higher global warming potential than carbon dioxide. Microbial denitrification is a major pathway responsible for nitrate removal, and also a dominant source of N2O emissions from terrestrial or aquatic environments. However, whether the release of zinc oxide nanoparticles (ZnO NPs) into the environment affects microbial denitrification is largely unknown. Here we show that the presence of ZnO NPs lead to great increases in nitrate delivery (9.8-fold higher) and N2O emissions (350- and 174-fold higher in the gas and liquid phases, respectively). Our data further reveal that ZnO NPs significantly change the transcriptional regulations of glycolysis and polyhydroxybutyrate synthesis, which causes the decrease in reducing powers available for the reduction of nitrate and N2O. Moreover, ZnO NPs substantially inhibit the gene expressions and catalytic activities of key denitrifying enzymes. These negative effects of ZnO NPs on microbial denitrification finally cause lower nitrate removal and higher N2O emissions, which is likely to exacerbate water eutrophication and global warming.

Plasticizers inhibit food waste anaerobic digestion performance by affecting microbial succession and metabolism.

The widely existed plastic additives plasticizers in organic wastes possibly pose negative influences on anaerobic digestion (AD) performance, the direct evidence about the effects of plasticizers on AD performance is still lacking. This study evaluated the influencing mechanism of two typical plasticizers bisphenol A (BPA) and dioctyl phthalate on the whole AD process. Results indicated that plasticizers addition inhibited methane production, and the inhibiting effects were reinforced with the increase of concentration. By contrast, 50 mg/L BPA exhibited the strongest inhibition on methane production. Physicochemical analysis showed plasticizers inhibited the metabolism efficiency of soluble polysaccharide and volatile fatty acids. Microbial communities analyses suggested that plasticizers inhibited the direct interspecies electron transfer participators of methanogenic archaea (especially Methanosarcina) and syntrophic bacteria. Furthermore, plasticizers inhibited the methane metabolisms, key coenzymes (CoB, CoM, CoF420 and methanofuran) biosynthesis and the metabolisms of major organic matters. This study shed light on the effects of plasticizers on AD performance and provided new insights for assessing the influences of plasticizers or plastic additives on the disposal of organic wastes.

Mitigated dissemination of antibiotic resistance genes by nanoscale zero-valent iron and iron oxides during anaerobic digestion: Roles of microbial succession and regulation.

Nanoscale zero-valent iron (ZVI) and the oxides have been documented as an effective approach for mitigating the dissemination of antibiotic resistance genes (ARGs) during anaerobic digestion (AD). However, the mechanism of ARGs dissemination mitigated by nanoscale ZVI and iron oxides remain unclear. Here, we investigated the influencing mechanisms of nanoscale ZVI and iron oxides on ARGs dissemination during AD. qPCR results indicated that nanoscale ZVI and iron oxides significantly declined the total ARGs abundances, and the strongest inhibiting effect was observed by 10 g/L nanoscale ZVI. Mantel test showed ARGs distribution was positively correlated with physiochemical properties, integrons and microbial community, among which microbial community primarily contributed to ARGs dissemination (39.74%). Furthermore, redundancy and null model analyses suggested the dominant and potential ARGs host was Fastidiosipila, and homogeneous selection in the determinism factors was the largest factor for driving Fastidiosipila variation, confirming the inhibition of Fastidiosipila was primary reason for mitigating ARGs dissemination by nanoscale ZVI and iron oxides. These results were related to the inhibition of ARGs transfer related functions. This work provides novel evidence for mitigating ARGs dissemination through regulating microbial succession and regulation induced by ZVI and iron oxides.

Using machine learning to reveal drivers of soil microplastics and assess their stock: A national-scale study.

The issue of microplastic (MP) contamination in soil is a significant concern. However, due to limited large-scale studies and stock assessments, our understanding of the drivers of their distribution and fate remains incomplete. To address this, we conducted a comprehensive study in China, collected MP data from 621 sites, and utilized machine learning techniques for analysis. Our findings revealed 9 key factors influencing the distribution of soil MPs, highlighting their nonlinear influence processes. Among these factors, atmospheric deposition emerged as the most dominant driver, while wind and precipitation could lead to the transformation of soil from a sink to a source of MPs. MP concentrations in Chinese soils vary from 1.4 to 4333.1 particles/kg, with human activities significantly affecting their distribution, resulting in higher concentrations in the east and lower concentrations in the west. The estimated MP stock in Chinese soils is 1.92 × 1018 particles, equivalent to a mass of 2.11-8.64 million tonnes. This stock alone surpasses that found in global oceans, making global soil the largest reservoir of MPs. Overall, this study enhances our understanding of the environmental behavior of MPs and provides valuable data and theoretical support for the prevention, control, and management of this contamination.

Exposure modes determined the effects of nanomaterials on antibiotic resistance genes: The different roles of oxidative stress and quorum sensing.

The effects of co-occurrent pollutants on antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs) have raised attentions. However, how the different realistic exposure scenarios determining the effects of nanomaterials (NMs) on ARGs, was still unknown. Herein, the effects of NMs on ARGs under two realistic scenarios was investigated by short-term and long-term exposure modes. The presence of NMs with two different exposure modes could both promote the dissemination of ARGs, and the results were dose-, type- and duration-dependent. Compared to short-term exposure, the long-term exposure increased the abundances of ARGs with a greater extent except nano-ZnO. The long-term exposure increased the overall abundances of target ARGs by 2.9%-20.4%, while shot-term exposure caused the 3.4%-10.5% increment. The mechanisms of ARGs fates driven by NMs exposure were further investigated from the levels of microbial community shift, intracellular oxidative stress, and gene abundance. The variations of several potential bacterial hosts did not contribute to the difference in the ARGs transmission with different exposure modes because NMs types played more vital roles in the shift of microbial community compared to the exposure modes. For the short-term exposure, NMs were capable of triggering the QS by upregulating relevant genes, and further activated the production of surfactin and increased membrane permeability, resulting in the facilitation of ARGs transfer. However, NMs under long-term exposure scenario preferentially stimulated oxidative stress by generating more ROS, which then enhanced ARGs dissemination. Therefore, the exposure mode of NMs was one of the pivotal factors determining the ARGs fates by different triggering mechanisms. This study highlighted the importance of exposure scenario of co-occurrent pollutants on ARGs spread, which will benefit the comprehensive understanding of the actual environmental fates of ARGs.

Revealing mechanisms of NH3 and N2O emissions reduction in the rapid bio-drying of food waste: Insights from organic nitrogen composition and microbial activity.

Inevitably, aerobic biological treatment processes generate emissions of ammonia (NH3) and greenhouse gas (GHGs) emissions, especially nitrous oxide (N2O). The rapid bio-drying process (RBD) for food waste (FW) alleviates issues arising from its substantial growth. However, its emissions of NH3 and N2O remain unknown, and the correlation with nitrogen components in the substrate remains unclear, significantly impeding its widespread adoption. Here, the nitrogen loss and its mechanisms in RBD were investigated, and the results are as follows: The total emission of NH3 and N2O were1.42 and 1.16 mg/kg FW (fresh weight), respectively, achieving a 98 % reduction compared to prior studies. Structural equation modeling demonstrates that acid ammonium nitrogen (AN) decomposition chiefly generates NH3 in compost (p < 0.001). Strong correlation (p < 0.001) exists between amino acid nitrogen (AAN) and AN. In-depth analysis of microbial succession during the process reveals that the enrichment of Brevibacterium, Corynebacterium, Dietzia, Fastidiosipila, Lactobacillus, Mycobacterium, Peptoniphilus, and Truepera, are conducive to reducing the accumulation of AN and AAN in the substrate, minimizing NH3 emissions (p < 0.05). While Pseudomonas, Denitrobacterium, Nitrospira, and Bacillus are identified as key species contributing to N2O emissions during the process. Correlation analysis between physicochemical conditions and microbial succession in the system indicates that the moisture content and NO3- levels during the composting process provide suitable conditions for the growth of bacteria that contribute to NH3 and N2O emissions reduction, these enrichment in RBD process minimizing NH3 and N2O emissions. This study can offer crucial theoretical and data support for the resource utilization process of perishable organic solid waste, mitigating NH3 and GHGs emissions.

Advancing chronic toxicity risk assessment in freshwater ecology by molecular characterization-based machine learning.

The continuously increased production of various chemicals and their release into environments have raised potential negative effects on ecological health. However, traditional labor-intensive assessment methods cannot effectively and rapidly evaluate these hazards, especially for chronic risk. In this study, machine learning (ML) was employed to construct quantitative structure-activity relationship (QSAR) models, enabling the prediction of chronic toxicity to aquatic organisms by leveraging the molecular characteristics of pollutants, namely, the molecular descriptors, fingerprints, and graphs. The limited dataset size hindered the notable advantages of the graph attention network (GAT) model for the molecular graphs. Considering computational efficiency and performance (R2 = 0.78; RMSE = 0.77), XGBoost (XGB) was used for reliable QSAR-ML models predicting chronic toxicity using small- or medium-sized tabular data and the molecular descriptors. Further kernel density estimation analysis confirmed the high accuracy of the model for pollutant concentrations ranging from 10-3 to 102 mg/L, effectively aligning with most environmental scenarios. Model interpretation showed SlogP and exposure duration as the primary influential factors. SlogP, representing the distribution coefficient of a molecule between lipophilic and hydrophilic environments, had a negative effect on the toxicity outcomes. Additionally, the exposure duration played a crucial role in determining the chronic toxicity. Finally, the chronic toxicity data of bisphenol A validated the robustness and reliability of the model established in this research. Our study provided a robust and feasible methodology for chronic ecological risk evaluation of various types of pollutants and could facilitate and increase the use of ML applications in environmental fields.

Parachlorometaxylenol stress caused multidrug-type antibiotic resistance genes proliferation via simultaneously reshaping microbial community and interfering metabolic traits during wastewater treatment process.

Despite biological wastewater treatment processes (e.g., sequencing batch reactors (SBR)) being able to reduce the dissemination of antibiotic resistance genes (ARGs), the variation of ARGs under exogenous pollutant stress is an open question. This work investigated the impacts of para-chloro-meta-xylenol (PCMX, typical antibacterial contaminants) on ARGs spread in long-term SBR operation. Although the SBR process inherently decreased ARGs abundance, the presence of PCMX substantially amplified both the prevalence (mainly multidrug) and abundance of total ARGs (1.17-fold of the control). Further analysis demonstrated that PCMX disintegrated sludge structures as well as increased membrane permeability, facilitating the release of mobile genetic elements and subsequent horizontal transfer of ARGs. In addition, PCMX selectively enriched potential ARG hosts, notably Nitrospira and Candidatus Accumulibacter, which predominantly served as multidrug ARG hosts. Concurrently, the self-adaptive functions of ARGs hosts in the PCMX-exposed SBR system were activated via quorum sensing, two-component regulatory system, ATP-binding cassette transporters, and bacterial secretion system. The upregulation of these metabolic pathways also contributed to the dissemination of ARGs.

Disinfectant polyhexamethylene guanidine triggered simultaneous efflux pump antibiotic- and metal-resistance genes propagation during sludge anaerobic digestion.

The environmental transmission of antibiotic resistance genes (ARGs) and metal resistance genes (MRGs) exerted devastating threats to global public health, and their interactions with other emerging contaminants (ECs) have raised increasing concern. This study investigated that the abundances of ARGs and MRGs with the predominant type of efflux pump were simultaneously increased (8.4-59.1%) by disinfectant polyhexamethylene guanidine (PHMG) during waste activated sludge (WAS) anaerobic digestion. The aggregation of the same microorganisms (i.e., Hymenobacter and Comamonas) and different host bacteria (i.e., Azoarcus and Thauera) were occurred upon exposure to PHMG, thereby increasing the co-selection and propagation of MRGs and ARGs by vertical gene transfer. Moreover, PHMG enhanced the process of horizontal gene transfer (HGT), facilitating their co-transmission by the same mobile genetic elements (20.2-223.7%). Additionally, PHMG up-regulated the expression of critical genes (i.e., glnB, trpG and gspM) associated with the HGT of ARGs and MRGs (i.e., two-component regulatory system and quorum sensing) and exocytosis system (i.e., bacterial secretion system). Structural equation model analysis further verified that the key driver for the simultaneous enrichment of ARGs and MRGs under PHMG stress was microbial community structure. The study gives new insights into the aggravated environmental risks and mechanisms of ECs in sludge digestion system, providing guidance for subsequent regulation and control of ECs.

Pyrosequencing reveals the key microorganisms involved in sludge alkaline fermentation for efficient short-chain fatty acids production.

Short-chain fatty acids (SCFAs) have been regarded as the excellent carbon source of wastewater biological nutrient removal, and sludge alkaline (pH 10) fermentation has been reported to achieve highly efficient SCFAs production. In this study, the underlying mechanisms for the improved SCFAs production at pH 10 were investigated by using 454 pyrosequencing and fluorescent in situ hybridization (FISH) to analyze the microbial community structures in sludge fermentation reactors. It was found that sludge fermentation at pH 10 increased the abundances of Pseudomonas sp. and Alcaligenes sp., which were able to excrete extracellular proteases and depolymerases, and thus enhanced the hydrolysis of insoluble sludge protein and polyhydroxyalkanoates (PHA). Meanwhile, the abundance of acid-producing bacteria (such as Clostridium sp.) in the reactor of pH 10 was also higher than that of uncontrolled pH, which benefited the acidification of soluble organic substrates. Further study indicated that sludge fermentation at pH 10 significantly decreased the number of methanogenic archaea, resulting in lower SCFAs consumption and lower methane production. Therefore, anaerobic sludge fermentation under alkaline conditions increased the abundances of bacteria involved in sludge hydrolysis and acidification, and decreased the abundance of methanogenic archaea, which favored the competition of bacteria over methanogens and resulted in the efficient production of SCFAs.

Enhanced anaerobic digestion performance of food waste by zero-valent iron and iron oxides nanoparticles: Comparative analyses of microbial community and metabolism.

The effects of zero-valent iron (ZVI) and iron oxides nanoparticles on anaerobic digestion (AD) performance of food waste (FW) were comparably clarified in this study. Results indicated that the nanoparticles supplement effectively enhanced the methane yields. As observed, these nanoparticles accelerated organics transformation and alleviated acidification process. Also, the enriched total methanogens and functional bacteria (e.g., Proteiniphilum) were consistent with the promotion of oxidative phosphorylation, citrate cycle, coenzymes biosynthesis and the metabolisms of amino acid, carbohydrate, methane. Additionally, these nanoparticles stimulated electron transfer potential via enriching syntrophic genera (e.g., Geobacter, Syntrophomonas), primary acetate-dependent methanogens (Methanosaeta, Methanosarcina) and related functions (pilus assembly protein, ferredoxins). By comparison, ZVI nanoparticle presented the excellent performance on methanogenesis. This study provides comprehensive understanding of the methanogenesis facilitated by ZVI and iron oxides nanoparticles through the enhancement of key microbes and microbial metabolisms, while ZVI is an excellent option for promoting the methane production.