| Marine is one of the most important ecosystems on the Earth and is rich in microbial resources. It is of great interest to study the origin, evolution, environmental adaptation and ecological role of the marine microorganisms which also have great potential in industrial applications, such as in biomaterial, medicine and environment protection. With the rapid development of sequencing technology, genomics has become an important approach of microbiological research.After polyphasic taxonomic studies of seven strains isolated from the East China Sea, we analyzed the environmental adaptation mechanism of a type species of a novel genus based on whole genomic sequence data; and then a metagenomic library data was employed to study the physiological and metabolic features and the adaptation mechanisms of the microorganism community in a deep-sea sediment of seamount region.Seven halotolerant or slightly halophilic strains isolated from samples of the East China Sea were subjected to polyphasic taxonomic characterizations. One novel genus (Pelagibacterium gen. nov., Pelagibacterium halotolerans as type species Pelagibacterium luteolum as reference species) and five novel species(Marinobacter mobilis, Marinobacter zhejiangensis, Marinobacterium denitrificans, Marinobacterium sediminicola and Ruegeria marina) were proposed.Marine halotolerant bacterium Pelagibacterium halotolerans B2T was subjected to genome sequencing and analysis. The complete genome sequence of strain B2T consisted of one chromosome (3 944 837 bp; 61.4% G+C content) and one plasmid (4050 bp; 56.1% G+C content). Genomic data indicated that strain B2T may adapt to the oligotrophic and saline marine environment by various mechanisms. The genome encoded many carbohydrate and amino acid metabolic pathways and a large number of TRAP transporters, as well as PQQ-dependent alcohol dehydrogenases and enzymes involving in aromatic compounds degradation, which suggested that strain B2T adapted to the oligotrophic marine environment by diverse metabolic pathways. It can synthesize and transport compatible solutes, such as ectoine, hydroxyectoine and glycine betaine, to resist osmotic stress. Flagella and chemotaxis may play an important role in its marine planktonic lifestyle to acquire nutrients and avoid disadvantages. A large number of genes involving in heavy metal reduction and tolerance suggested that strain B2T may be a heavy metal resistant bacterium. In addition, genes involving in polysaccharide synthesis and degradation, aromatic compounds degradation and hydroxyectoine synthesis indicated this bacterium may have various application potentialities.We analyzed a metagenomic library from a deep-sea sediment sample of seamount region, which was characterized by low temperature (1.5℃), high hydrostatic pressure (5886 m-deep), high osmotic pressure (7.2% Na20) and rich in heavy metals (Mn, Ni, Co, Cu, Zn, Pb and Ba).2133 fosmid end-sequences were sequenced and analyzed. The library was screened based on 16S rRNA gene amplification and 9 microbial genome fragments were further sequenced and analyzed. The data suggested that microorganisms may play important roles in the biogeochemical cycles of nitrogen and carbon in this deep-sea sediment, especially the numerous marine group I Thaumarchaeota and some other bacteria which may participate through ammonia oxidization and carbon fixation. Genomic data also indicated that microorganisms adapt to this particular environment by various mechanisms, including genome evolution and horizontal gene transfer. In addition, analysis of predicted gene products encoded by an MBGA Thaumarchaeotal fosmid W5-61a and three bacterial fosmids of unknown taxa (fosmid W4-21b, W5-15b and W5-47b) revealed some potential metabolic and physiological features and adaptation mechanisms of the corresponding microorganisms.
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