| Arsenic pollution is a worldwide environmental problem.Hundreds of millions of people worldwide are facing the threat of arsenic pollution in their ground water source.China is one of the countries suffering the most severe arsenic pollution,and the Yinchuan Plain in Ningxia is the region with serious arsenic pollution in China.Studies have shown that microorganisms can mediate the redox reactions of arsenic during geochemical changes and indirectly facilitating its release into ground water system.Although the fate of arsenic pollution in groundwater and sediments in the Yinchuan Plain has been extensively studied,the microbial mechanism underpinning the underground transformation of arsenic are still not very clear.Therefore,this thesis uses highthroughput metagenomic sequencing to analyze the composition and functional capacities of microbial communities in the sediments of high-arsenic aquifers and deep groundwater and sediments(230-600 meters)in the Yinchuan Plain,with the aim to reveal the effects of microbial metabolism on the migration and transformation of arsenic in the underground environment.Microbial community composition and arsenic resistance mechanism in high-arsenic aquifer sediments is studied.Six aquifer sediment samples with different arsenic levels were studied: one with extremely high arsenic concentration of 68.98 ?g/g,and two samples with high arsenic concentration of 12.27 ?g/g and 18.31 ?g/ g,and additionally three samples with low arsenic concentrations of 1.95 ?g/g,2.24 ?g/g and 4.01 ?g/g.Amplicons sequencing of 16 S r RNA gene delivered 418,656 high-quality sequences(at least 50,000 sequences per sample),and 249,324 operational taxonomic units(OTU)were directly denoised by non-clustering method.The alpha diversity of the high arsenic sediment microbial community(Shannon index = 7.21)is evidently lower than that of the low arsenic sample(Shannon index = 7.75),indicating arsenic content had exerted perceptible selection pressure on the microbial community.The dominant genera(abundance > 1%)in high arsenic sediment samples are: Acinetobacter(31.47%),Paracoccus(18.01%),Comamonas(11.03%),Delftia(7.32%),and so on Brevundimonas(1.07%),these dominant bacteria genera accounted for 62.04% of the total community.Principal component analysis shows that both total arsenic and total sulfur concentration significantly drives the community structure of sediment.Comamonas,Delftia,Acinetobacter,Brevundimonas,and Stenotrophomonas are the most discriminable genera that are inclined to be enriched in sediment with high arsenic and high sulfur content.Based on the binning analysis of metagenomic dataset,a total of 17 high-quality bacterial metagenomic assembled genome(MAG)were obtained(all with completeness above 85% and contamination below 5%).The results of phylogenetic tree analysis indicated that some MAGs belonged to the dominant genera including Delftia,Stenotrophomonas,Paracoccus and Microbacterium.The abundance of these genera in high arsenic sediment samples is significantly higher than that of low arsenic samples,and they generally carry arsenic resistance genes,including: arsenic reduction genes such as ars C1,ars C2,arsenic oxidation gene ars H and arsenic efflux genes ars A,ACR3.In addition to the arsenic resistance,the microbial community in this environment also carries resistance genes against a broad spectrum of heavy metals such as chromium,cobalt,cadmium,copper,mercury,zinc,nickel,etc.,indicating a plausible co-selection induced by arsenic.The microbiome of groundwater and sediment in the deep layer(230-600 meters)of Yinchuan was investigated.Amplicon sequencing was performed on 7 samples,and a total of 1,382,008 high-quality sequences and 1,101,675 OTUs were obtained.The alpha diversity of sediment microbial communities(Shannon index = 9.93)is higher than that of groundwater samples(Shannon index = 9.19).The dominant genera in groundwater and sediment samples are mainly Hydrogenophaga(13.8%),Acinetobacter(7.23%),Pseudomonas(6.17%),Thiobacillus(5.77%),Rheinheimera(3.86%),Rhodobacter(2.07%)and so on,these predominant genera accounted for 89.69% of the total community.A total of 24 MAGs were obtained based on binning analysis of metagenomic data.Part of MAGs belongs to Flavobacteriaceae,Microbacterium,Staphylococcus,and Geobacter.Most of the MAGs obtained in deep groundwater are genetically different from known genomes,showing unknown microorganisms are widely present in deep groundwater.The metabolic characteristics of novel MAGs revealed in this work may provide a valuable data basis to understand the microbial dark matter in underground environment.Metabolic pathway analysis results show that the microorganisms in the deep groundwater in Yinchuan are mainly autotrophic,and their carbon fixation was mainly enabled by the more primitive reducing tricarboxylic acid cycle rather than the Calvin cycle.While anaerobic methane oxidation(AMO)is the main energy source of microbial communities,the reduction process coupled with AMO in deep groundwater is nitrate reduction rather than sulfate reduction.In summary,in the first section of this thesis,microbiota of the high arsenic aquifer sediments of the Yinchuan Plain showed significant community variation along the gradient of total arsenic and total sulfur and the microbial mechanism of arsenic resistance is revealed in MAGs showing high prevalence within the community.In the second section,autotrophic lifestyle gaining energy via nitrate reduction coupled AMO was revealed in the MAGs dominating the community of the deep groundwater and sediments of the Yinchuan Plain.The genomic insight obtained by state-of-the-art metagenomic approach in this thesis,could serve as basis to understand the underground environment microbial world of Yinchuan Plain. |
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