Microbial Diversity,Nitrogen Utilization Strategy And Environmental Change Response In The Northwest Pacific Ocean And The Basin Of The South China Sea

Marine microorganisms are an important component of marine ecosystem,which contribute largely to marine primary productivity and biomass.Nitrogen(N)is an essential macronutrient for microorganisms.In marine ecosystem,the species,concentration,distribution and bioavailability of N play important roles in regulating cell growth,distribution and community structure of marine microorganisms.It is known that marine microorganisms survive in the complex marine environments for a long time,and N is a limiting nutrient in many areas of the global ocean.Thus,understanding the microbial diversity,N utilization strategies and environmental change response in different habitats are of vital ecogical importance in marine ecological studies.Using the high-throughput sequencing and qPCR technologies,we studied the microbial structures,distributions and N utilization strategies in the Northwest Pacific Ocean and the basin of the South China Sea(SCS),and analyzed correlation between environmental factors and microbial distribution.We also investigated cell growth,physiology and quantitative proteomics of an oceanic Synechococcus strain WH8102 grown on two N sources(nitrate and urea)under present(250C)and predicted future(28?)temperature conditions,and discussed the underlying mechanisms influencing cell growth and physiology of Synechococcus.The main results of this study were as follows:(1)521 different planktonic bacterial operational taxonomic units(OTUs)and 204 different nitrogen-fixing microorganism OTUs were identified in the 16S rRNA and nifH libraries from the Northwest Pacific Ocean using the high-throughtput sequencing method.Proteobacteria,Cyanobacteria and Bacteroidetes dominated the bacterial communities.Prochlorococcus and Pseudoalteromonas were the most abundant at the genus level in the N-deficient regimes,while SAR86,Synechococcus and SAR92 were predominant in the Kuroshio-Oyashio confluence region.Cyanobacterium UCYN-A dominated the diazotrophic community in the N-deficient stations.Temperature was the main factor determining bacterial community structure and diversity while concentration of NOx-N was significantly correlated with structures and distributions of N2-fixing mircroorganisms.(2)1427 different planktonic bacterial OTUs and 749 different nitrogen-fixing microorganism OTUs were identified in the 16S rRNA and nifH libraries from the basin of the SCS using the high-throughput sequencing method.Cyanobacteria and a-Proteobacteria dominated the surface bacterial community,while a-Proteobacteria and y-Proteobacteria dominated bacterial community in the deeper waters.a-Proteobacteria dominated the surface diazotrophic community,and?-Proteobacteria and y-Proteobacteria increased their abundances in the diazotrophic community with the water depth.N and phosphorus were the main factors affecting the richness of bacterial species,while temperature,salinity,oxygen and nutrients,together,influenced both bacterial and diazotrophic community structures and distributions.(3)Gene expressions of microorganisms and dominant cyanobacterial groups in the Northwest Pacific Ocean and the basin of the SCS showed that both ammonium transporter and amino acid transporer were generally highly expressed in the surface seawater,and the highest abundance of urea transporter in Cyanobacteria was observed in the N-deficient regime of the Northwest Pacific Ocean as well as the station SEATs of the basin of the SCS,suggesting that ammonium,amino acid and urea were the important N sources for microorganisms.In the station A2 from the basin of the SCS,high expressions of both iron-deficiency induced protein idiA and urease in Prochlorococcs were observed,suggesting that iron deficiency might enhance urea utilization of Cyanobacteria in oligotrophic ocean.(4)Rising temperature significantly decreased growth rate,contents of chlorophyll a,protein and sugar,as well as photosynthetic efficiency in the nitrate-grown Synechococcus cells,but it only decreased protein content of the urea-grown cells.Carbonic anhydrase activity was depressed by rising temperature in both nitrate-and urea-grown cells but RuBisco carboxylase activity depressed only in the urea-grown cells.Proteomic results showed that rising temperature damaged carboxysome and ribosome,and increased oxidative stress in both nitrate-and urea-grown cells.Our results indicated that ocean warming impaired cell growth and physiology of Synechococcus,and this might reduce cell abundance and distribution of Synechococcus in a future ocean.