Biogeographic Patterns And Assembly Mechanisms Of Microbiota In The Tropical And Subtropical North Pacific Ocean

The(sub)tropical North Pacific Ocean is one of the largest ecosystems on Earth,which phalys important roles in regulating regional and global climate.Bacteria and microeukaryotes are pivotal participators of global biogeochemical cycles and its turnover could have profound influences on ecosystem functions.The high-through sequencing(HTS)method has greatly promoted the development of marine microbial ecology,however,it could only provide two basic attributes(i.e.,genetic information and relative abundance)of a community.How would this influence our understanding of microbial ecology?Research on bacteria or microeukaryotes from open oceans is still limited.As fas as(sub)tropical North Pacific Ocean is concerned,previous studies have been conducted at only a few scattered stations or within only a few subdivisions of those regions,leaving few studies at a trans-basin scale.Moreover,most studies have focused on the surface and deep chlorophyll maximum layers,and few studies are conducted with plenty of samples from multiple layers.In the present study,we applied high-throughput quantification sequencing to compare the quantitative and relative bacterial abundances in revealing community structure,distribution patterns,and potentially influencing factors.Additionally,we simultaneously investigated the distribution patterns,major contributors and assembly mechanisms of bacteria and microeukaryotes in different regions and layers of the tropical North Pacific Ocean using high-throughput sequencing of 16S and 18S rRNA genes.Main results are as the following:(1)Total 16S rRNA gene abundance ranged from 1.86×108 to 1.14×109 copies/L.At the phylum level,the bacterial community was mainly composed of Proteobacteria and Cyanobacteria,while SAR11 and Synechococcales dominated at the order level.At the whole-community level,the relative and quantitative abundances of bacteria exhibited similar distribution patterns and potentially influencing factors,but innercommunity connections and correlations with variables differed at subgroup levels.(2)In the northwest and central tropical North Pacific Ocean,alpha-diversity indices such as species richness,phylogenetic diversity,and Shannon significantly differed between domains(i.e.,bacteria and microeukaryotes)and regions.Further,bacteria and microeukaryotes showed similar distribution patterns with samples clustered by regions,showing significant time-and distance-decay patterns.Generally,bacteria and microeukaryotes showed similar correlations patterns with temporal,spatial and environmental variables for alpha-and beta-diversity,respectively.The relative importance of main ecological processes such as selection,dispersal and ecological drift differed between bacteria and microeukaryotes:ecological drift was the principal mechanism that accounted for the structure of bacterial communities;heterogeneous selection,dispersal limitation,and ecological drift collectively explained much of the turnover of the microeukaryote communities.Moreover,we discovered that the potentially influencing factors of alpha-and beta-diversity,and relative contributions of ecological processes differed between domains and at different spatiotemporal scales of environmental heterogeneity.(3)In the western and central tropical North Pacific Ocean,the alpha diversity indices of bacteria and microeukaryotes increased firstly and then decreased with the increasing water depth,and the alpha diversity indices of microeukaryotes changed from significantly greater to less than or equal to that of bacterial communities from surface to bottom of seawater.Overall,the vertical composition variations of bacterial and microeukaryotic communities were greater than thier regional variations.Moreover,there were significant regional differences in bacterial and microeukaryotic communities,which showed significant distance-decay patterns in all layers.In deeper water,the regional variations of bacteria and microeukaryotes were greater than their vertical variations.In surface and subsurface layers,the turnover of bacterial communities was mainly structured by ecological drift,whereas,in deeper layers,dispersal limitation,ecological drift,and heterogeneous selection collectively explained much of the turnover of the bacterial communities.The tureover of microeukaryotic communities was determined primamily by dispersal limitation in the deep chlorophyll maximum,200-meter,and twilight-zone layers,and collectively shaped by dispersal limitation,heterogeneous selection,and ecological drift in other layers.