Microbial Community Structures And Their Relationship To Water Quality In The Danjiangkou Reservoir,China

Lake microorganisms are important components of an aquatic ecosystem,affecting biogeochemical processes,such as material circulation and pollutant release through catabolic and anabolic processes.Investigations of the community composition and function of aquatic microorganisms have far-reaching significance for the ecological management and maintenance of aquatic environments.Danjiangkou Reservoir is the core water source area of the Middle Route of the South-to-North Water Diversion Project.The water quality of Danjiangkou Reservoir is directly related to drinking water safety for residents of the areas receiving water.Thus,it is of critical importance to carry out long-term monitoring of microbial community structures of this reservoir.In the present study,five typical ecological sites were selected in the Danjiang Reservoir area.High-throughput sequencing was used to analyze the bacterioplankton and planktonic fungal communities diversity in surface waters,sediment bacterial communities diversity in surface sediments and then to investigate the community composition and distribution characteristics.Redundancy analysis?RDA?and Spearman correlation analysis were used to investigate the relationship of Danjiangkou Reservoir microbial communities and physico-chemical parameters in the water.The aim of this study was to explore the ecological composition of the microbial community and its influencing factors in the Danjiangkou Reservoir and expect to provide the basis for using microbes as an indicator for water quality of reservoirs.The main results are summarized as follows:?1?The water quality monitoring results in May 2016 and May 2017 showed that the water quality of Danjiangkou Reservoir was generally good.However,agricultural nonpoint source pollution,and domestic sewage from villages and towns are present,resulting in TN values notably exceeding the standard limit.All indicators met the Grade I or?of the Environmental Quality Standards for Surface Water?GB38382-2002?,with the exception of TN and COD_Mn.The trophic status of the Danjiangkou Reservoir area was assessed by measuring the TN,TP,COD_Mn,Chl a,and SD 5 parameters according to the improved Carlson's trophic level index?TLI?.The TLI index of the Danjiangkou Reservoir area ranged from 40.10 to 43.29 in May 2016,with a mesotrophic status observed for Danjiangkou Reservoir;ranged from 28.16 to 35.41 in May 2017,with an oligotrophic status observed for Kuxin and Heijizui and a mesotrophic status observed for Qushou,Songgang,and Taizishan.?2?The composition of the planktonic fungal community in the surface water layer of the Danjiangkou Reservoir is investigated using Illumina MiSeq sequencing in May 2017.According to the results,the reservoir fungal community is of high diversity and primarily composed of 294 genera belonging to 5 phyla,including Ascomycota,Rozellomycota,Basidiomycota,Chytridiomycota,and Zygomycota.RDA of the planktonic fungal community and environmental factors showed DO?R=-0.6728,P=0.002?,COD?R=0.5792,P=0.011?,TN?R=0.6919,P=0.003?,Chl a?R=0.9217,P=0.003?,and COD_Mn?R=0.8899,P=0.001?to be important factors influencing the distribution of planktonic fungi.Spearman correlation analysis of the planktonic fungal community composition and diversity indices with physical and chemical water quality parameters showed that the impacts of TN,COD and DO were the most significant.?3?This study investigated the bacterioplankton community composition in surface water of the Danjiangkou Reservoir using the Illumina MiSeq sequencing platform.We found that the bacterioplankton community was mainly composed of 27 phyla and 336genera,including Actinobacteria,Proteobacteria,and Bacteroidetes,highlighting the richness of the community composition.Redundancy analysis?RDA?of the bacterioplankton communities and environmental variables showed that TN?R=-0.7522,P=0.001?,pH?R=-0.9851,P=0.027?,COD?R=0.8254,P=0.035?,and COD_Mn?R=0.7302,P=0.035?were important factors affecting the bacterioplankton distribution.Network analysis was constructed using the relative abundances of bacterioplankton based on the phylogenetic molecular ecological network?pMEN?method;the constructed networks met the characteristics of scale-free,small-world,modular,and hierarchical networks.The connectivity of node i within modules?Zi?,the connectivity of node i among modules?Pi?,and the number of key bacteria were higher at the Taizishan and Heijizui sites,which were associated with a higher TN content,than at the other sites.Among the water physico-chemical properties,TN,NH₄⁺-N,pH,COD,and DO might have great influences on the functional units of the bacterial communities in bacterioplankton molecular networks.?4?5 surface sediment samples were collected from the reservoir in May 2017.High-throughput sequencing was performed to analyze the distribution characteristics of the community structure and diversity of bacteria,and their relationship with environmental factors.Phylogenetic analysis based on 16S rDNA sequences showed that bacterial communities could be divided into 33 major phylogenetic groups.The dominant phylogenetic groups included Proteobacteria,Chloroflexi,Bacteroidetes,Verrucomicrobia,Nitrospirae,Acidobacteria,Planctomycetes and Omnitrophica.The results of redundancy analysis?RDA?for the relationship between the 33 bacterial phyla and 7 environmental factors showed that the pH,total phosphorus?TP?,sediments organic matter?SOM?,and ammonium nitrogen?NH₄⁺-N?were the main environmental factors affecting the distribution characteristics of the bacterial community.PICRUSt was used to understand the metabolic and functional abilities of the observed bacterial communities.The results obtained showed wide genetic diversity of organisms involved in various essential processes,such as energy production and conversion,signal transduction mechanisms,amino acid transport and metabolism,cell wall/membrane/envelope biogenesis,translation,ribosomal structure and biogenesis,replication,recombination and repair.The surface sediments of the Danjiangkou Reservoir can be ranked in the following order based on the16S rRNA gene copy number of the detected phylotype:Taizishan>Songgang>Kuxin>Qushou>Heijizui.