Study On Composing Characteristics Of Microbial Community And “Bacterial-Algal” Systems In Estuarine Reservoir

Due to the special locations,the aquatic ecosystem in estuary reservoirs are both affected by terrestrial and marine environments,which exhibited complicated variations.The“algal-bacterial”system as the most important interactive systems in the early evolution of life on the earth,playing important role in maintaining aquatic ecologic equilibrium.However,the excessive propagation of cyanobacterial species could destabilize the equilibrium of the“algal-bacterial”system,and further threatening the safety and stabilization of whole aquatic ecosystem.Also,considering the important functions of picophytoplanktons in aquatic ecosystems,especially in the microbial loops,the researches on freshwater picophytoplankton has been received wide attention recently.Therefore,through exploring the composing characteristics and variation rules of picophytoplanktons and“algal-bacterial”systems in estuary reservoirs,we will deeply understand the compositions of picophytoplankton compositions,the functions and effects of“algal-bacterial”system during the process of picophytoplankton compositions in estuarine aquatic ecosystems,which have scientific meanings and reference values for management of estuary reservoirs.In the support of SJTU-UNSW collabortive research funding and the international collaboration project E2S2 between Singapore and China（CS-B）,the Qingcaosha（QCS）Reservoir in Shanghai was selected as our research targets,also compared with the Marina Reservoir in Singapore,to explore the variations of water quality,cyanobacterial community compositions and picophytoplankton structures by a long term study by using flow cytometric analysis,high-throughput sequencing and Metagenomics Sequencing techniques.Besides,the molecular ecological network was using to explore the variations and composing characteristics of“algal-bacterial”systems,also we analyzed the correlations between key environmental factors and the composition of“algal-bacterial”system.The main conclusions of this study are listed as follows:（1）After stable operation of the QCS Reservoir,the water quality characteristics became stable gradually compared with the initial adjustment phases.The water quality has totally reached the second class of surface water’s environmental quality criterion.Whereas,the nutrients such as TP shown increasing trends inside the reservoir by years,and during this time,the diversity and richness of cyanobacterial community have obviously increased.These indicated that the enhanced effect of phosphorus released from sediments and rapid growth of phytoplankton during the stable operating phase.These implied that the potential risks of harmful cyanobacterial blooms have increased inside the reservoir.（2）The most dominant cyanobacterial taxa in QCS Reservoir was the oceanic type of Synechococcus,which cells contained large amount of phycoerythrin（PE）.Our study for the first time defined the dominant position of this kind of Synechococcus in QCS Reservoir.Based on the metagenomic functional gene analysis,the metagenomic functional gene analysis indicated that the dominant Synechococcus（PE-rich）in QCS Reservoir exhibited higher abundance in acid resistance,UVR resistance and triclosan resistance（detected in Yangtze estuary）.These implied that the Synechococcus（PE-rich）has competitive advantage compared with other algae.In addition,we also found functional genes related to terpenoids biosynthesis,which implied that the Synechococcus（PE-rich）was main potential producer for odor compounds in the water.（3）Although the water quality and environmental characteristics were significant differences（p<0.05）between QCS and Marina Reservoirs,the Synechococcus as the dominant cyanobacterial species in both reservoirs,and exhibited obviously higher abundance than other algae.In QCS Reservoir,the average abundance of Synechococcus maintained at 32%of total bacterial abundance during summer and autumn seasons.In contrast,the average abundance of Synechococcus maintained at47%of total bacterial abundance in Marina Reservoir during the whole year.The correlation analysis indicated that the values of TN/TP exhibited no significant influence（p>0.05）on the relative abundance of dominant Synechococcus in both reservoirs.Whereas,the differences of TN/TP obviously affected the diversity and composition of cyanobacterial community between two reservoirs.And the Marina Reservoir with lower TN/TP exhibited a higher diversity and composition of cyanobacterial community than in QCS Reservoir.（4）In end spring and early summer,the dominant algae has gradually changed from Diatom（Thalassiosira）to Cyanobacteria（Synechococcus）in QCS Reservoir.Based on metagenomic sequencing analysis,the dominant bacterial community composition characteristics and metabolic functions have significantly changed（p<0.05）during these periods.Also,the the activities of thymidylate synthase and ribonucleosidediphosphate reductase have significantly increased（p<0.05）during the process of advantaged algal succession.But the activities of NADHX epimerase,AMP conjugase and glycosyltransferase have significantly increased（p<0.05）after the period of Synechococcus became the advantaged algae in summer.（5）Although the“bacterial-algal”system of dominant Synechococcus exhibited highly dynamic variations in QCS Reservoir,the heterotrophic bacteria taxa such as CL500-29＿marine＿group（Actinobacteria）,hgc I＿clade（Actinobacteria）,Novosphingobium（α-Proteobacteria）,Terrimonas（γ-Proteobacteria）and Planctomyces（Planctomycetes）exhibited long-term and stable co-occurrence patterns with Synechococcus.In addition,the temperature,rainfall（30d）,ammonium nitrogen（NH₄-N⁺）and total organic carbon（TOC）exhibited significant positive effects（p<0.05）on“bacterial-algal”system,but total nitrogen（TN）and salt ions(including K⁺,Ca²⁺,Na⁺,Mg²⁺,Cl^-and SO₄^2-)exhibited significant negative effects（p<0.05）on“bacterial-algal”system in QCS Reservoir.Different with QCS Reservoir,the“bacterial-algal”system of dominant Synechococcus in Marina Reservoir was mainly composed by Verrucomicrobia（LD29）and Firmicutes（Clostridium＿sensu＿stricto＿1and Sarcina）.Besides,the environmental factors affected the compositions of“bacterial-algal”systems were much different between two estuary reservoirs.（6）Combined with the flow cytometry and high-throughput sequencing technologies,the composition and distribution of picophytoplanktons in QCS Reservoir were identified for the first time.Totally we have identified most picophytoplanktons from 14 groups of 6 phyla.The Spearman correlations suggested that the dissolve oxygen（DO）and UV254 caused significant growth inhibition of most picophytoplanktons.The variations of water temperature,COD＿Mn and p H were key factors that positively affected the growth of picocyanobacteria.In addition,total phosphorus（TP）and total organic carbon（TOC）caused significant growth promotion of most picoeukaryotes.