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Antibiotic resistance genes and correlations with microbial community and metal resistance genes in full-scale biogas reactors as revealed by metagenomic analysis Gang Luo, Bing Li, Li-Guan Li, Tong Zhang, and Irini Angelidaki Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05100 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017
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Antibiotic resistance genes and correlations with microbial community
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and metal resistance genes in full-scale biogas reactors as revealed by
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metagenomic analysis
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Gang Luo1#*, Bing Li2#, Li-Guan Li3, Tong Zhang3, Irini Angelidaki4∗
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1
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Department of Environmental Science and Engineering, Fudan University, 200433,
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Shanghai, China
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2
Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3),
Key Laboratory of Microorganism Application and Risk Control of Shenzhen,
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Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
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3
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Kong SAR, China
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4
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DK-2800, Kgs Lyngby, Denmark
Environmental Biotechnology Laboratory, The University of Hong Kong, Hong
Department of Environmental Engineering, Technical University of Denmark,
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∗
Corresponding author: email:
[email protected] (I. Angelidaki), tel: +45 45251429, fax:+45
45932850 Address: Department of Environmental Engineering, Technical University of Denmark, DK-2800, Kgs Lyngby, Denmark;
[email protected] (G. Luo), tel/fax: +86 65642297 Address: Department of Environmental Science and Engineering, Fudan University, 200433, Shanghai, China #
The authors contributed equally to the paper 1
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Abstract: :Digested residues from biogas plants are often used as biofertilisers for
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agricultural crops cultivation. The antibiotic resistance genes (ARGs) in digested
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residues pose a high risk to the public health due to their potential spread to the
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disease-causing microorganisms and thus reduce the susceptibility of disease-causing
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microorganisms to antibiotics in medical treatment. High-throughput sequencing
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(HTS)-based metagenomic approach was used in the present study to investigate the
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variations of ARGs in full-scale biogas reactors and the correlations of ARGs with
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microbial communities and metal resistance genes (MRGs). The total abundance of
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ARGs in all the samples varied from 7×10-3 to 1.08×10-1 copy of ARG/copy of
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16S-rRNA gene, and the samples obtained from thermophilic biogas reactors had
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lower total abundance of ARGs, indicating the superiority of thermophilic anaerobic
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digestion for ARGs removal. ARGs in all the samples were composed of 175 ARG
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subtypes, however, only 7 ARG subtypes were shared by all the samples. Principal
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component analysis and canonical correspondence analysis clustered the samples into
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three groups (samples from manure-based mesophilic reactors, manure-based
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thermophilic reactors and sludge-based mesophilic reactors), and substrate,
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temperature, hydraulic retention time (HRT) as well as volatile fatty acids (VFAs)
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were identified as crucial environmental variables affecting the ARGs compositions.
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Procrustes analysis revealed microbial community composition was the determinant
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of ARGs composition in biogas reactors, and there was also a significant correlation
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between ARGs composition and MRGs composition. Network analysis further
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revealed the co-occurrence of ARGs with specific microorganisms and MRGs. 2
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Abstract Graphic
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1. Introduction
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Anaerobic digestion (AD) has increasingly been used in the treatment of organic
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wastes and agricultural residues. AD has the advantages of low energy input and
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generation of renewable energy in the form of biogas. The digested residues are
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generally reused as fertilizer. In this way, nutrients in the organic wastes are recycled 1.
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The utilization of digested residues helps to increase crop production and reduce the
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use of mineral fertilizers 2. Nevertheless, the content of antibiotic resistance genes
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(ARGs) in the digested residues might increase the spread of antibiotic resistance 3, 4,
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and therefore the emergence and spreading of ARGs are currently urgent public health
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issues globally 5.
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Spreading of ARGs in the environment is a result of the extensive antibiotic use in
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humans and animals 6. It has been reported that farm antibiotic use is correlated with
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the rise and spread of ARGs in human bacterial pathogens
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were shown to not only select for metal resistance genes (MRGs), but also ARGs 9,10.
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Berg et al. demonstrated that Cu exposure co-selected for resistance to clinically
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important antibiotics (e.g. vancomycin)
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microbial populations via horizontal gene transfer 26. Thereby, bacteria with antibiotic
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resistance can be formed, which could easily infect humans by contact or
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consumption of raw vegetables
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integrons and insertion sequences, are crucial for horizontal gene transfer of ARGs in
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environments
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communities due to high mobility caused by horizontal gene transfer
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. In addition, metals
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. ARGs can be spread among different
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. Mobile genetic elements, including plasmids,
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. ARGs were speculated to be uncorrelated with microbial
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. However,
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it is possible that some of the mobile genetic elements, which are responsible for
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horizontal gene transfer, possess a narrow-host-range restricted to one or few certain
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species
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primary determinant of ARGs in soil samples
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ARGs and microbial communities as well as MRGs, and aims to provide new insights
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of the occurrence of ARGs in biogas reactors. Besides, the effects of operational
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conditions of biogas reactors on the removal of ARGs were previously investigated in
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lab-scale biogas reactors treating sewage sludge from wastewater treatment plants
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(WWTPs)
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The diversity and abundance of ARGs in full-scale biogas reactors treating various
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substrates are still unknown, and also the key environmental variables shaping the
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ARG composition remains to be elucidated.
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Various molecular tools (qPCR, DNA microarray) have been developed to detect
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selected ARGs 1, 20, 21. However, PCR bias and unspecific binding of primers limit the
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application of these methods, and it is hard to realize the broad-spectrum detection
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and quantification of ARGs in environmental samples 22. High-throughput sequencing
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(HTS)-based metagenomic approach has been successfully used in the analysis of
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functional genes and microbial community compositions of microbiomes obtained
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from biogas reactors, soil and WWTPs 23, 24, 25. With the high sequencing depth, ARGs
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can also be analyzed from the metagenomic data, and the analytical methods have
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been developed recently 22, 26.
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In Denmark, there are more than 40 centralized biogas plants, and they are running
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. For instance, a recent study found that microbial communities were the
2, 17
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. The present study is correlating
. However, only limited ARGs were quantified by qPCR approach 3, 19.
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with manure and industrial wastes as feedstock
. Moreover, most of the WWTPs
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have full-scale biogas reactors treating the primary and secondary sludge. The
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understanding of the presence of ARGs in the digested residues from full-scale biogas
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reactors is required to properly define the risks posed by land application. The present
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study made a detailed comparative analysis of ARGs in various full-scale biogas
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reactors via the HTS-based metagenomic approach to provide new insight of ARG
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profiles in biogas reactors. The objectives of the study were: (1) to reveal the diversity
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and abundance of ARGs; (2) to identify the key environmental variables determining
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the ARG contents; (3) to investigate the correlation between ARGs and microbial
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communities; and (4) to understand the co-occurrence of ARGs and MRGs in various
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full-scale biogas reactors.
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2. Materials and methods
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2.1 Sampling and DNA extraction
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The samples were obtained from Danish full-scale biogas reactors, which had been
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running for more than two years under similar operational conditions. The detailed
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information about operational condition and performance of each reactor is shown in
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Table 1. MT means the biogas reactors fed with manure and operated under
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thermophilic conditions. MM represents the biogas reactors fed with manure and
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operated under mesophilic conditions, and SM means the biogas reactors fed with
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sludge from WWTP and operated under mesophilic conditions. All the samples were
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collected in May of 2013. For the biogas reactors fed with manure, manure was a
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mixture mainly containing pig and cattle manure. For the biogas reactors fed with 6
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sludge, sludge was obtained from wastewater treatment plants. Samples MT2a and
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MT2b were collected from Blåhoj, and samples MT3a and MT3b were obtained from
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Lemvig biogas plants. Both Blåhoj and Lemvig biogas plants have two biogas
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reactors running in series, and thus samples were collected from both steps of the
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serial process. All the biogas plants were operated under normal conditions at the time
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of sampling, and no major changes had occurred prior to sampling, which could
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ensure the samples as representative as possible. Only one sample for each reactor
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was collected in our study since our previous study showed that HTS-based
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metagenomic approach was reproducible for ARG quantification22. The samples for
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the microbial analysis were collected in sterile tubes (15 mL) and frozen immediately
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in a cooler with dry ice. The samples for chemical analysis were collected in 0.5 L
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bottles and put in a cooler box with ice. All biogas reactors had sampling points in the
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effluent lines close to the reactors to ensure the samples as representative as possible.
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The sampling valve was opened for 5 min before sample acquisition to flush the
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sampling valve and pipeline. All the samples were transported to the laboratory within
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24 hours, and QIAamp DNA Stool Mini Kit (QIAGEN, 51504) was used to extract
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total genomic DNA of each sample. Thermo NanoDrop 1000 spectrophotometer was
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used to measure the DNA concentration and purity, and the related information can be
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found in Table S1.
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2.2 Metagenomic sequencing and quality filtering
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The genomic DNA was sent to Beijing Genomics Institute for library construction and
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sequencing using Illumina Hiseq 2000 platform by applying 101 bp paired-end 7
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strategy. Sequence reads with low quality or ambiguous were removed, and then the
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pair-end sequence reads were merged into tags to decrease the sequencing errors. The
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average length of tags was around 170 bp. The information of the raw sequenced data
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is shown in Table S1. All the metagenomic datasets were uploaded to MG-RAST
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(Rapid Annotation using Subsystems Technology for Metagenomes), and the
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accession number for each sample can be found in Table S1.
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2.3 ARGs analysis and MRGs analysis
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For the ARG annotation, the metagenomic tags of each sample were blasted against
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the structured non-redundant clean antibiotic resistance genes database (ARDB)
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(http://ardb.cbcb.umd.edu/) with the e-value at 1×10-5 by BLASTX 26. The lengths of
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antibiotic resistance genes in ARDB range widely from 186 to 4728 bp, and the
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average length is 1235 bp 22. The sequence was considered to be ARGs-like sequence
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when its best hit had similarity no less than 90 % to the reference sequences and had a
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query coverage no less than 25 amino acids
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automatically sorted into different types and subtypes of ARGs by a package of
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customized scripts, and the abundance of ARGs was normalized by the ARG
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reference sequence length and 16S rRNA gene sequence length according to our
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previous study
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commonly used database for the investigation of ARGs currently 30-36.
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For the MRGs analysis, MRGs annotation was conducted similarly by searching
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against the metal resistance genes database (http://bacmet.biomedicine.gu.se). The
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sequences were considered to be MRGs following the same cutoffs, i.e., e-value ≤
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. The ARG-like sequence were
22, 29
. The reference database used in our study is ARDB, which is
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1×10-5, similarity≥90% and query coverage ≥25 amino acids 37. The abundance of
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MRGs was also normalized by the MRG reference sequence length and 16S rRNA
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gene sequence length
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validated in our previous studies 26, 38.
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2.4 Microbial community analysis
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Microbial community analysis was conducted by MetaPhlAn, which mapped
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metagenomic tags against a catalogue of clade-specific marker sequences currently
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spanning the bacterial and archaeal phylogenies 39. The MetaPhlAn software and the
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database
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http://huttenhower.sph.harvard.edu/metaphlan/. All the parameters of MetaPhlAn
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utilized default settings except for the threshold of the e-value of 1×10−15.
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2.5 Data analysis
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Heatmaps of ARGs, microbial community and MRGs, principal components analysis
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(PCA),
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http://www.r-project.org/) with packages VEGAN, igraph, and Hmisc 18, 22. CANOCO
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5.0 were used for the canonical correspondence analysis (CCA) to correlate the ARGs
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compositions to environmental variables. All the environmental variables used for the
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CCA analysis were shown in Table S2. The co-occurrence of ARG subtypes with
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microbial community and MRGs were explored using network analysis based on
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strong (ρ>0.8) and significant (P-value2.0×10-4 copy of ARG per copy of 16S-rRNA gene in at least one sample) ARG subtypes in the anaerobic digestion samples.
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ARG
0.4 MT2a
SM5 SM1
PC 2 (27.4%)
MT4
SM3
MT2b SM2
MT3a MT3b
0.0
SM4
MT1
MM2
-0.4 MM1
-0.4
-0.2
MM3
0.0
0.2
0.4
PC 1 (49.9%)
1.0
(A) MM1 MM3
MM2
CCA2 (21.2%)
HRT
Sub
MT3b MT1 MT3a SM5
SM2 MT2b VFA Temp
SM4 SM1 SM3
-0.8
MT4 MT2a
-0.8
1.0 CCA1 (39.84%)
(B) Fig 3 PCA (A) and CCA (B) analysis of all the samples based on ARGs subtypes (Sub means substrate, and Temp means temperature)
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Fig 4 Venn diagram showing the number of shared and unique ARGs (at the subtype level) among MM, MT and SM.
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(B) Fig 5 Procrustes analyses of ARGs with microbial community (A) and MRGs (B)
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(B) Figure 6 The network analysis revealing the co-occurrence patterns of ARG subtypes with microbial taxa (A) and MRGs (B). The nodes in (A) were coloured according to genus/family and ARG types. The nodes in (B) were colored according to ARG types and MRGs. A connection represents a strong (Spearman’s correlation coefficient ρ>0.8) and significant (P-value