Effect Of Damming On Microbial Community In The Upper Reaches Of The Yangtze River

Microbes are the most diverse population on Earth and are found in almost every environment on the planet.Planktonic bacteria play an important role in the material and energy cycle of freshwater ecosystem.Planktonic eukaryotes are key components of the aquatic food web and are both a producer of organic matter and a major consumer of bacterial biomass.Sediment bacteria are the main driving force of energy and nutrient transport in water and sediment.Current studies mainly focus on the analysis of single species.There are relatively few studies on microorganisms in large rivers(planktonic bacteria and eukaryotes,sediment bacteria),and little is known about the biogeographic distribution of microorganisms and the mechanism of community assembly under the influence of cascade hydropower or large dams.In order to systematically understand the structure and characteristics of microbial communities in large rivers,the upper reaches of the Yangtze River,the largest river in China,were selected to carry out simultaneous monitoring of planktonic bacteria and planktonic eukaryotes.Moreover,we selected the mid-part of the Three Gorges Reservoir to implement simultaneous monitoring of planktonic bacteria and sediment bacteria.Based on high-throughput sequencing technology,we analyzed the factors influencing the spatial and temporal distribution of planktonic bacterial/planktonic eukaryotic and planktonic bacteria/sediment bacterial communities and their interaction mechanisms.The main research conclusions were divided into the following two parts:(1)Planktonic bacteria and eukaryotic communities play an important role in biogeochemical cycles,but their biogeographic patterns and driving processes in large rivers impacted by cascade hydropower development remain unclear.In this study,16S r RNA and 18S r RNA were used for sample sequencing analysis of planktonic bacteria and eukaryotic communities in the upper reaches of the Yangtze River,the largest river in China.Due to the influence of the cascade dams,the upper reaches of the Yangtze River can be divided into dam-affected zone and river zone.Results showed that there were significant differences in bacterial and eukaryotic communities between the two zones,and the dam would significantly reduce the?-diversity of bacterial communities.The co-occurrence network analysis indicated that the network in the river zone was denser than that in the dam-affected zone,and the relationship among the species of planktonic bacteria was more complex than that the planktonic eukaryotic network.The keystone species of bacteria in the dam-affected zone decreased significantly,and there were more keystone species in the dry season river zone,mainly Actinobacteria,Proteobacteria and Bacteroidetes.Dispersal limitation and ecological drift were the main processes for planktonic bacterial and eukaryotic communities in the dam-affected zone,respectively,while selection of planktonic bacterial and eukaryotic community increased in the river zone.The change of hydraulic conditions affected the suspended sediment and controls the microbial diversity in the river zone.These results suggested that dams were more sensitive to planktonic bacteria than planktonic eukaryotes,indicating that the distribution patterns and process of bacterial and eukaryotic communities were significantly different in large rivers.(2)Bacterial communities play an important role in biogeochemical cycle in reservoir ecosystems.However,the dynamic changes in both planktonic and sediment bacterial communities in a highly regulated dam reservoir remain unclear.This study investigated the temporal distribution patterns of bacterial communities in a transition section of the Three Gorges Reservoir(TGR)using Illumina Mi Seq sequencing.Results suggested that in comparison to the planktonic bacteria,sediment bacteria contributed more to the reservoir microbial communities,accounting for 97%of the 7,434 OTUs.The Shannon diversity index in the water(3.22?5.68)was generally lower than that in the sediment(6.72?7.56).In the high water level period(January and March),Proteobacteria,Actinobacteria,Cyanobacteria and Firmicutes were the most abundant phyla,whereas in the low water level period(May,July and September)the dominant phyla were Proteobacteria,Actinobacteria and Bacteroidetes.Sediment samples were dominated by Proteobacteria,Chloroflexi and Acidobacteria.Principal coordinate analysis of the bacterioplankton communities showed greater sensitivity to monthly changes than that of the sediment bacterial communities.Network analysis suggested that in comparison to planktonic bacterial communities,sediment bacterial communities were more complex and stable.The linear relationship between the CH₄/CO₂ ratio,water level and relative abundance of methanotrophs highlighted the potential methane-oxidizing process in the mid-part of the TGR.Moreover,the potential impact of dam regulation on the bacterial communities was revealed by the significant relationship between abundant phyla and the inflow of the TGR.