Bacterial Community Structures And Function Associated With Cyanobacterial Blooms In A Eutrophic Freshwater Lake (Lake Taihu)

Lake eutrophication has become an important environmental issues in China,and water bloom is one of the most obvious characteristics about eutrophication,which not only seriously hampers the lakes water resource availability,but also lead to a series of water environmental issues and poses a major threat to economic and social development.There has been a great deal of research on cyanobacterial blooms,but until now,we still find it difficult to control it effectively.One of the important reasons is that we have very little research on the microbial ecological processes in eutrophic lakes,especially the understanding of the relation between cyanobacteria and bacteria is still unclear.Therefore,it is very necessary to study the community structure and dynamics of cyanobacteria and bacteria in eutrophic lakes and find out the interaction between them,which will help to deepen the understanding of cyanobacterial blooms formation and provide a scientific basis for the prevention and control of cyanobacterial blooms and eutrophication.We selected a large shallow eutrophic lake,Lake Taihu,as the research object.Based on the long-term monitoring of spatiotemporal dynamics of the algal community in Lake Taihu from 2009 to 2011,bacterial community composition(BCC)and dynamics in Lake Taihu were analyzed by molecular ecology techniques such as 16S rRNA gene clone library and terminal restriction fragment length polymorphism.The relationship between algae,bacteria and environmental factors and their potential ecological links were studied by means of statistical analysis.To further understand the composition and function of bacterial communities during the formation of cyanobacterial blooms,the changes of bacterial community and their functional genes related to nitrogen transformation were analyzed by high-throughput sequencing and fluorescence quantitative PCR respectively.The main findings are as follows:1.The succession and influencing factors of algal community in Taihu.The algae community in Lake Taihu showed an obvious seasonal succession.Cyanobacteria,Bacillariophyta and Chlorophyta dominated together in the spring,and cyanobacteria gradually became the dominant population in the summer and fall.And then in the winter,the dominance of Bacillariophyta had been restored.Microcystis was always a dominant species in the process of seasonal succession,and had absolute predominance in the summer.The distribution of algal biomass has temporal and spatial differences.The biomass of cyanobacterial blooms in the northern lake area of Taihu Lake was the largest in the summer and higher than that in the southern lake area.Water temperature,algal diversity,algal biomass,latitude and TN/TP ratio were significantly correlated with algal community structure.In the low biomass,increased algal diversity promoted formation of higher biomass,and the highest diversity of algal community was observed in intermediate biomass,whereas high biomass caused by cyanobacterial blooms had reduced algal community diversity.2.The composition and dynamic changes of bacterial community in Taihu and the driving factors affecting the changes of algae biomass.Proteobacteria(included β-Proteobacteria,a-Proteobacteria and y-Proteobacteria),Planctomycetes,Actinobacteria and Cyanobacteria were principal member of the bacterial community in Lake Taihu,and they mostly belonged to the typical freshwater bacteria and were closely related to cyanobacteria blooms.Strong seasonal variations patterns of BCC were observed whilst the spatial variations of BCC were slight in the observation.BCC and environmental factors are closely related,β-Proteobacteria were dominant in autumn and winter in the libraries,while α-Proteobacteria and Planctomycetes are dominant in spring and summer in the libraries.The factors influencing their distribution mainly include water temperature,organic matter,nitrogen and phosphorus content and algal biomass.The results of T-RFLP confirm the dynamics of bacterial communities in Lake Taihu again,which has obvious seasonal variations patterns,and annually re-occurred.There were significant differences in BCC between summer/autumn and winter,and water temperature and cyanobacteria biomass had great influence on BCC.There were characteristic bacterial communities in the spring,summer/autumn and winter,and the dominant taxa are Rhodobacter,Microcystis and Methylophilus,respectively.However,core species bacteria persisted throughout the annual variations,which implied its important role in the maintenance of lake ecological fumction.Co-occurrence networks revealed that the correlations of bacteria-bacteria could be more critical than those between environment and bacteria in shaping the structure of microbial communities,and would be an important driving factor of BCC in Lake Taihu.The structural equation model shows that the algae diversity index is the most important direct influencing factor for algal blooms,followed by nutrient level(nitrogen content)and bacterial richness.The nutrient level,water temperature,suspended solids and organic matter content indirectly affect the algae biomass through the diversity of algae or bacteria.3.The relationship between the formation process of cyanobacteria blooms and the composition and nitrogen and phosphorus transformation ability of bacterial community.The results of high-throughput sequencing showed that the Microcystis aeruginosa had a competitive advantage in the formation of bloom,which resulted in reduction of bacterial community diversity.Lake water in bloom and non-bloom conditions harbored distinct lacustrine bacterial communities,and in particular,Actinobacteria and Bacteroidetes appeared to be depleted,whilst Firmicutes was enriched in parallel with the increasing density of Microcystis bloom.Moreover,Microcystis bloom enriched phosphate-accumulating organisms whilst inhibited the growth of nitrifiers and denitrifiers,especially in high density.The qPCR data also revealed that the abundance of the most the nitrification and denitrification genes was reduced in the case of heavy blooms,indicating some pathways of nitrogen transformation processes might be inhibited.