Effect Mechanisms Of Microplastics And Plasticsphere Microbiome On Degradation Behavior Of Typical Polycyclic Aromatic Hydrocarbons In Freshwater Environment

Microplastics,as an emerging class of contaminants,have received widely attentions in recent years.Due to their stable nature,small size,larger specific surface area and stronger surface hydrophobicity,microplastics are important carriers for microbes and hydrophobic organic compounds（HOCs）.Microplastics could selectively enrich specific microbes from the surrounding environment,which could impact the degradation of HOCs.Urban rivers are important receptivity environment for microplastics,as well as"hot areas"for biogeochemical cycling.Thus,it is crucial to explore the carrier effects of microplastics on microbes and typical HOCs,and to evaluate the influence of urban riverine plastisphere on the degradation behavior of typical HOCs.In this thesis,we focus on the carrier effects of microplastics to investigate the community successions of plastispheres in Pearl River aquatic environment and to elucidate the distribution patterns,shaping mechanisms,and potential functions of abundant and rare taxa.We also examine the effect of microplastic adsorption on the bioavailability of phenanthrene（PHE）and explore the effect of humic acid（HA）on the degradation of PHE adsorbed on microplastics（PHE-MPs）.Besides,aimed at plastisphere microbiome in the Pearl River,the degradation potentials of PHE and pyrene（PYR）by plastisphere microbiome and the responses of microbial communities and functional genes are investigated.Moreover,the degradation characteristics of PHE and PYR by plastisphere microbiome enhanced by a soil functional bacterium across ecological niches and the interaction between the soil bacterium and plastisphere microbes were also discerned.The main conclusions in this thesis are as follows:（i）The exposure time and polymer type of microplastics significantly affected the diversity,richness and structure of plastisphere communities in the Pearl River.Some microbes involved in nitrogen and sulfur cycling and aromatic compound degradation were found to be signally enriched on microplastics.The temporal turnover rate of abundant taxa was similar to that of the whole community,but was signally higher than that of rare taxa,suggesting that abundant taxa might play an important role in driving community succession and turnover.The neutral model and null model implied that stochastic processes preponderantly governed the assembly processes of plastispheres.However,abundant and rare taxa exhibited distinct assembly mechanisms,in which the abundant taxa were dominated by the dispersal limitation and ecological drift,while the rare taxa were mainly manipulated by the dispersal limitation and heterogeneous selection.Molecular ecological network analysis demonstrated that the rare taxa played a crucial role in the formation and maintenance of plastisphere communities.Besides,the functional prediction results suggested that abundant taxa had stronger potentials in driving xenobiotics biodegradation and metabolism,membrane transport,cell motility and energy metabolism,as well as metabolic potentials associated with human diseases;while rare taxa exhibited higher metabolic potentials for amino acids,lipids,cofactors,and vitamins.Additionally,the Mantel test and variation partitioning analysis showed that the temperature,chemical oxygen demand,salinity,and nitrogen-related ions had significant effects on plastisphere communities in the Pearl River,with greater impacts on abundant taxa than those on rare taxa.This result indicated the higher stability of rare taxa and the higher community dynamics of abundant taxa in urban riverine plastispheres.（ii）The adsorption of microplastics to PHE significantly reduced the biodegradation efficiency of PHE by Consortium QY1 and induced lower microbial cytotoxicity,indicating that the adsorption of microplastics reduced the bioavailability of PHE in the aqueous environment.Notably,HA could enhance the bioavailability of PHE-MPs through promoting the desorption of PHE from microplastics.Moreover,HA could increase the community diversity and alter the community structure of the Consortium QY1,and some typical PAHs-degrading bacteria,such as Methylobacillus and Sphingomonas,significantly increased.HA could also increase the node number,link number and modularity of the microbial network of Consortium QY1,increase the network’s average-clustering coefficient and degree,and shorten the average geodesic distance between nodes,which were beneficial to the exchange of substances,energy and information among microorganisms,and thereby enhancing the functional stability of the Consortium QY1.Besides,HA significantly reduced the level of intracellular reactive oxygen species and apoptosis of QY1 cells,which suggested a possible role of HA in alleviating cytotoxicity of functional microbes.（iii）The plastisphere communities in the Pearl River exhibited promising potentials for the degradation of PHE and PYR.Specifically,the Consortium P5 could utilize PHE as sole carbon source,showing a degradation efficiency of 77.7%for 20 mg L^-1 PHE and 33.7%for 10 mg L^-1 PYR.During PHE degradation,the diversity and richness of the Consortium P5 were improved visibly.Notably,some potential PAHs-degrading bacteria（e.g.,Stenotrophomonas,Methylophilus and Novosphingobium）also signally increased,implying that they might be key bacteria involved in the biodegradation of PAHs by Consortium P5.Metagenomic sequencing analysis further verified a significant increase in the abundance of genes involved in PAHs-degrading functional enzymes,such as cytochrome P450,Naphthalene 1,2-dioxygenase,Salicylic acid 5-hydroxylase,Naphthol 2,3-dioxygenase,Gentianic acid 1,2-dioxygenase,and Protocatechuic acid 3,4-dioxygenase,which might play crucial roles in the degradation and transformation of PHE by the Consortium P5.The intermediate products of PHE degradation by Consortium P5 mainly included 1-hydroxy-2-naphthoic acid,1-naphthalenol,salicylic acid and phthalic acid.Thus,it was hypothesized that PHE could be transformed into 1-hydroxy-2-naphthol acid through aromatic ring hydroxylation reaction,and then further metabolized by salicylic acid pathway and phthalic acid pathway.Alternatively,1-naphthalenol was generated from 1-hydroxy-2-naphthoic acid through decarboxylation and then further metabolized by salicylic acid pathway and phthalic acid pathway towards achieving PHE biodegradation.（iv）Sphingobium sp.QY1-1,isolated from a soil microbial consortium,showed excellent degradability of high concentration PHE(500 mg L^-1)and significantly enhanced the degradation of PYR through PHE co-metabolism.Transcriptomic analysis revealed that the gene expression of Sphingobium sp.QY1-1 was markedly differentiated during PHE and PYR degradation.Some up-regulated expression genes related to membrane components and transmembrane transport,such as ABC transporters,Porins and Pore superfamily proteins,might be involved in the transmembrane transport of PHE and PYR.Various intracellular oxidoreductases,such as Aromatic ring-hydroxylating dioxygenase,Homogentisate 1,2-dioxygenase,FAD-dependent monooxygenase and Cytochrome P450,might be participated in the degradation and transformation of PHE and PYR.Inoculation with a suitable mass ratio of Sphingobium sp.QY1-1 could significantly enhance the co-degradation efficiencies of PHE and PYR by the Consortium P5,improving the community diversity and richness of the Consortium P5.Notably,molecular ecological network analysis indicated that Sphingobium sp.exhibited strong interactions with various microbes（including common“colonizers”in plastispheres and typical PAHs-degrading bacteria,such as,Delftia,Bosea and Romboutsia）in the Consortium P5 during PHE and PYR co-degradation,which was conducive to its adaptation and playing its specific functions in the plastisphere community.In summary,this thesis elucidated the formation and maintenance mechanisms of plastisphere microbial taxa and their potential ecological effects in the Pearl River.And it revealed the impact of microplastic adsorption on environmental behavior of typical PAHs.Moreover,the potentials,mechanisms and influencing factors of plastisphere microbiome of the Pearl River on the degradation of typical PAHs were studied,and the effects of plastisphere microbiome on the environmental behavior and fate of typical PAHs were systematically elucidated.Overall,this study provides valuable insights into the theoretical basis for assessing the pollution effects of microplastics and controlling and treating PAHs pollution.