| Cyanobacteria are important primary producers in the marine environment.Cyanophages influence the population dynamics,community structure,metabolic activity and the evolution of their hosts.Although the isolation and culture of cyanophages has been studied for several decades,the knowledge about them is still limited.Therefore,more isolation are still needed to understand the cyanophage-host interactions.This thesis studied with three strains of cyanosiphovirus and one cyanomyovirus isolated from the Changjiang River Estuary to explore the genetic diversity,evolution and environmental distribution of their cyanophage clusters they belong to,and to predict the potential phage-host interactions and ecological importance of those phage clusters.The main research results of this thesis are:1.The newly isolated cyanosiphovirus from Changjiang River estuary,P-SS2,S-CBS2 and 9 homologous genome sequences of cyanosiphovirus which were assembled from freshwater,estuarine,coastal and offshore environments formed the S-CBS2-like phage cluster,which could be divided into 12 different genera.Based on the analysis of isolates and environmental homologous sequences,it was found that S-CBS2-like phages are diverse in evolutionary clades and environmental sources.In addition to conserved structure and DNA replication and metabolism genes,S-CBS2-like phage genomes also encode a variety of auxiliary metabolic genes(AMGs),including psbA and cpcVinvolved in photosynthesis,CP12 involved in carbon metabolism and five genes involved in host resistance.The psbA was first found in the homologous genome sequence of cyanosiphovirus.The S-CBS2-like phage genomes has AMGs which were common in the genomes of other cyanophages,and AMGs involved host resistance regulation which are specific in cyanophage retrieved from coastal environment.The former suggests that S-CBS2-like phages may regulate host metabolism in a similar way to other cyanophages,while the latter suggests that the coastal cyanophages may have their own unique ways to adapt to the eutrophic coastal environment.The AMGs in S-CBS2-like phage genomes have a distant evolutionary relationship with the host and other cyanophages,indicating that S-CBS2-like phages may have a different evolutionary pattern from their hosts and other cyanophages.S-CBS2-like phages are distributed in both freshwater and marine environment,and are most abundant in estuaries.2.Cyanophage S-SMPhi1 possesses the longest tail of the T4-like cyanophages identified so far.S-SMPhi1 and four cyanophages form a branch defined as T4-like cyanophage Cluster C.Most Cluster C cyanophage genomes lack the photosynthesis-related genes which are commonly found in Cluster A and Cluster B,and encode a phycobilin lyase of different type.There are two specific AMGs that are invol ved in host resistance in Cluster C genomes.suggesting that they may have an unknown interaction relationship with their hosts.The APH_ChoK_like family proteins and cytidylyltransferases detected in the proteome of S-SMPhiI were frequently detected in other cyanomyoviruses.suggesting that these proteins are likely carried by virions.During the phage infection,these proteins enter the cell and create an optimal cellular condition for the viral replication.Compared with Cluster A and B.Cluster C is more likely to be distributed in the estuarine environment.and its relative abundance is comparable to Cluster A and B in specific areas. |
