Winter And Summer Variations In The Composition Of Picocyanobacteria Community And Bacteriophage Isolates In The Yangtze River Estuary

Picocyanobacteria are important primary producers in aquatic ecosystems.Phages which infect picocyanobacteria,or cyanophage can influence the abundance,community structure and metabolic activities of picocyanobacteria.Compared to the coastal and open ocean,much less is known about picocyanobacteria and their viruses in the estuarine environment.Yangtze River estuary is the largest estuary in China with apperent spatiotemporal variation.The Yangtze River Estuary is an ideal estuarine ecosystem to study biogeographic distribution and genetic diversity of picocyanobacteria and cyanophage.In this study,we studied the abundance and community composition of picocyanobacteria and cyanophage in the Yangtze River Estuary on spatial and temporal scales in order to understand their potential ecological interactions in the estuary.The main results of this thesis are:1.Picocyanobacteria in the Yangtze River estuary are diverse.A total of 86 picocyanobacterial strains were isolated from the Yangtze River Estuary.These strains were identified(using???),and they belong to 11 phylogenetic clades,including Clade?,?,?,? of marine Synechococcus subcluster 5.1,CB4,CB5,CB8 of marine Syn.echococcus subcluster 5.2,CZ,Subalpine Cluster II,Bornholm sea of Cyanobium and a novel clade.At least 23 clades belonging to 5.1,5.2 subgroup or Cyanobium were identified when the picocyanobacterial community in the Yangtze River Estuary was analyzed.Different picocyanobacterial communities were found between winter and summer.The WPC2 group dominated the winter picocyaobacterial community and the offshore community.In contrast,clade II,Novel 3,and CB5 dominated the summer picocynobacterial community.There are a large number of unknown new groups in the freshwater area in winter and summer.In winter,the proportion of each branch is even,CB8,Subalpine Cluster I,Novel 1 and Novel 4 have advantages in different stations.In summer,Novel 1 and CB4 are dominant in different stations.Temperature,salinity,and nutrients are important factors that affect the branching season and spatial distribution of picocyanobacteria,and the community diversity.and salinity show a negative correlation.2.Cyanophages isolated from the Yangtze River Estuary included myoviruses and siphoviruses.The myoviruses were isolated in both winter and summer,and the siphoviruses were only isolated in summer.The host range of winter cyanophages was wider than those in summer,and the siphoviruses are highly specific.The myoviruses are all T4-like,and can be divided into four branches based on the phylogenetic analysis ofg20 gene.Most siphovirus isolates were similar in morphology,with elongated heads,and could be divided into three clades based on mcp,and showed close phylogenetic relationship with cyanosiphovirus S-CBS2.3.The genomic analysis of cyanosiphovirus phiA6-3-S showed that it had a close evolutionary relationship with S-CBS2.The phiA6-3-S genome encodes a photocyanobilin lyase gene related to photosynthetic pigment synthesis.In addition,six non-structural proteins were identified by proteomic analysis.These proteins may be encapsulated in virus particles or expressed in high quantity during the process of virus infection,which provides convenience for rapid replication of virus.