Study Of Prochlorococcus In The Western Pacific Ocean And South China Sea

Prochlorococcus is the most abundant and smallest known free-living photosynthetic microorganism and is a key player in marine ecosystems and biogeochemical cycles.Prochlorococcus can account for nearly 50%of photosynthetic biomass and net primary productivity in oligotrophic tropical and subtropical sea areas.Prochlorococcus can be broadly divided into high-light-adapted(HL)and low-light-adapted(LL)clades,mainly distributed in the upper and the deep euphotic zone,respectively.The HLI and HLII clades,accounting for 93%of all Prochlorococcus,are adapted to low-temperature and high-temperature ecological niches,respectively.The surface layer of the western Pacific Ocean has the characteristics of low nutrients,low primary productivity,high temperature and high radiation.As a part of the warmest region in the world ocean,the western Pacific Ocean and its ecosystem and biogeochemical cycles have a global impact.Previous investigation found that the western Pacific Ocean has a high abundance of Prochlorococcus and Prochlorococcus also support a large portion of primary productivity in this region.However,at present,only a few Prochlorococcus isolates and genomes from Pacific Ocean have been reported.In this study,we successfully isolated 67 Prochlorococcus strains and obtained 15 genomes from the western Pacific Ocean and South China Sea.The study is the first large-scale isolation and culturing of Prochlorococcus in the Western Pacific and South China Sea,which helps to better understand the biogeographic distribution,diversity and evolution of Prochlorococcus.The paper has carried out research from the following aspects:1.In this study,we obtained 15 Prochlorococcus genomes including 13 HLII and 2 LLI from the western Pacific Ocean and the South China Sea using high throughput sequencing.The ecotype of 15 strains was confirmed by genomic DNA base similarity(average nucleotide identity,ANI),whole genome protein sequence CVTree phylogenetic and tetra-nucleotide signatures.We expanded the pan genome of HLII and LLI to 5,07M(?5500 genes)and 2.66 Mb(?3000 genes),respectively.2.We reconstructed Prochlorococcus evolution based on genome rearrangement.Our results showed that genome rearrangement might have played an important role in Prochlorococcus evolution.We also found that the Prochlorococcus clades with streamlined genomes maintained relatively high synteny throughout most of their genomes,and several regions served as rearrangement hotspots.Gene rearrangement analysis also found that the distribution of gene rearrangements may be related to gene islands,suggesting that genomic islands may be active regions that induce genomic rearrangement,thereby affecting the Prochlorococcus evolution.Backbone analysis showed that different clades shared a conserved backbone but also had clade specific regions,and the genes in these regions were associated with ecological adaptations.Three evolutionary directions of LLI,LLII/III and HL after clade differentiation,suggesting that the genomic streamline of Prochlorococcus may occur before the differentiation of LLI,LLII/III and HL.3.By using phylogenomic analysis of core genome SNPs,we divided the HLII clade into several sub-clades,and revealed several core genes with large SNP variations might be associated with subclade differentiation.We also investigated the accessory genome regions associated with subclade differentiation.In addition,we found a phage major capsid protein gene associated with a potential tRNA-Pro-based phage-binding site as well as multiple horizontal gene transfer events in several Prochlorococcus genomes.4.To better understand how dipoles shape microbial communities including Prochlorococcus,we examined depth-resolved distributions of microbial communities across a dipole in the South China Sea.Our data demonstrated that the dipole had a substantial influence on microbial distributions,community structure,and functional groups both vertically and horizontally.Large alpha and beta diversity differences were observed between anticyclonic and cyclonic eddies in surface and subsurface layers,consistent with distribution changes of major bacterial groups in the dipole.The dipole created uplift,downward transport,enrichment,depletion,and horizontal transport effects.We also found that the edge of the dipole might induce strong subduction,indicated by the presence of Prochlorococcus and Synechococcus in deep waters.Our findings suggest that dipoles,with their unique characteristics,might act as a driver for microbial community as well as Prochlorococcus dynamics.