Diversities And Potential Ecological Impacts Of Viruses From Mangrove And Seamount Sediments

Viruses are the most abundant biological entities on earth;they are virtually present in all ecosystems.The abundance of viruses in marine ecosystems is about 10 times that of marine prokaryotes.Due to their high abundance and genetic diversity,viruses play a very important role in marine ecosystem.In this thesis,the diversity,geographical distribution and potential ecological functions of marine viruses were studied in terms of uncultivated virus(using the analysis method of metagenomic sequencing)and culturable virus(using the analysis method of bacteriophage isolation and culture)from the shallow-water habitat(mangrove)and deep-sea habitat(seamount).(1)Mangroves are among the most productive and ecologically important ecosystems on the planet.As one of the most carbon-rich biomes,mangroves account for 11% of the total input of terrestrial carbon into oceans.Previous studies mainly focused on the animal community,plant community and bacterial community in mangrove ecosystem,and little information is available regarding the community structure,genetic diversity and ecological roles of viruses in mangrove ecosystems.Here,we performed metagenomics to study viral communities in mangrove sediments sampled from different mangrove habitats(bay,river,and port).The comparison results show that the mangrove sediment viruses were found to be largely uncharacterised.Phylogenetic analyses of the major viral groups demonstrated extensive diversity and previously unknown viral clades,and suggested that mangrove viral communities possibly comprise evolutionarily close genotypes.Comparative analysis of viral genotypes revealed that mangrove sediment viromes are mainly affected by marine waters,with less influence coming from freshwaters.Notably,we identified abundant auxiliary carbohydrate-active enzyme(CAZyme)genes from mangrove viruses,most of which participate in biolysis of complex polysaccharides,which are abundant in mangrove soils and organism debris.Host prediction results showed that viral CAZyme genes are diverse and probably widespread in mangrove sediment viruses infecting diverse bacteria of different phyla.Our results showed that mangrove viruses are diverse and probably directly manipulate carbon cycling by participating in biomass recycling of complex polysaccharides,providing the knowledge essential in revealing the ecological roles of viruses in mangrove ecosystems.(2)As a unique ecological environment in the deep sea,seamounts often have high biomass,biodiversity and biological uniqueness.However,there is still a lack of knowledge about the diversity of viruses in seamount sediments and the impact of seamount geographic isolation on viral community structure community structure.In this thesis,we performed bulk-metagenomics to study viral communities in different seamount sediment samples.A total of 34 viral families and 245 viral genera were identified from seamount sediments,mainly Caudovirales.By comparing the viral community structure of different seamount sediment samples,we found that the viral community structure of seamount sediment had a high similarity in terms of the viral classification composition.But at the same time,there were specific virus groups in different seamount samples.The prediction results of viral gene function showed that the relative abundances of viral genes in different seamount sediment samples were quite different.In addition,we also briefly described the gene function of the seamount sediment virus,and found that there are a large number of viral auxiliary metabolic functions related to "amino acid transport and metabolism","lipid transport and metabolism","carbohydrate transport and metabolism","inorganic ion transport and metabolism","Secondary metabolites biosynthesis,transport and catabolism" in the seamount sediments.These auxiliary metabolic functions require further analysis to explore the virus–host interactions and the potential ecological function of seamount sediment viruses.(3)The majority of viruses in the marine environment are bacteriophages that infect bacteria,spirochete and other microorganisms.However,compared with the shallow sea,there are few pure culture phages obtained from the deep-sea environment.In this thesis,nine bacteriophages were isolated from deep-sea sediment samples.The morphological characteristics of these bacteriophages were observed by transmission electron microscopy.The results showed that the morphology of the cultured bacteriophage in deep-sea sediments was diverse.Bacillus phage Gxv1,a podophage isolated from the M2 seamount sidement,was further studied.It was found that Gxv1 is a new member of the genus Salasvirus and the family Podovirida,and has high homology with Bacillus phage phi29.The results of the one-step growth curve indicated that phage Gxv1 began to replicate approximately 3–3.5 h post infection and reached the growth plateau at 6.5 h post infection.A total of 34 genes are predicted from the Gxv1 genome,mainly encoding proteins related to DNA replication,transcriptional regulation,and cleavage functions.This study will help improve our understanding of viral diversity and virus–host interactions in deep-sea seamounts.