Response Of Microbial Communities In The Middle And Lower Reaches Of Lhasa River To Human Activities

Human activities,such as dam construction,irrigation and sewage disposal,change the flow pattern of river and the way water resources are used making adverse effects on river ecosystem.As one of the few pure lands on earth,the increased human activities may threaten the environmental security of rivers on Tibet Plateau.In recent years,the largest tributaryone of the most famous transboundary river,the Yarlung Zangbo River,on the Tibet Plateau─the Lhasa River,has been increasingly affected by human activities.However,there are few studies on the impact and development trend of human activities,including reservoir dam construction and sewage discharge,on its ecosystem.Through real-time fluorescent quantitative PCR and the second generation of high-throughput sequencing technologies,this research has analyzed the abundance of microorganisms associated with nitrogen transformation and community structures of bacteria,archaea and eukaryotic photosynthetic organisms in water and sediment of middle and lower reaches of Lhasa River in wet season,and discussed the relationship between microbial community structures and human activities to provide reference for exploitation and policy decision-making of the Lhasa River.Specific results are as follows:（1）The hydrology and water quality changes in the Lhasa River were studied.The pH,electrical conductivity and organic carbon content of the water in the reservoir area are all higher than those in the water under the dam.Among the sites under the dam,the contents of organic carbon and all forms of nitrogen are the highest in the water at M7.The contents of TOC,NH₄⁺-Nand TN in the sediments of the reservoir areas are significantly higher than those in the areas below dam.Dam construction and sewage discharge have led to the increase of nitrogen and phosphorus content in sediments.In addition,some of these physical–chemical properties and DNA contents gradually increased from upstream to downstream in the sewage discharge areas.The results showed that the Lhasa River had received industrial,agricultural and domestic sewage and gradually accumulated nutrients after flowing downstream through irrigation channels and the city.The hydrological situation of the original river was changed dramatically by the construction of dam-type hydropower station,resulting that the velocity of the river section in the reservoir area decreased to accumulate organic matter.（2）The change of microbial community in water body was studied.Due to enormous industrial,domestic sewage discharge and crop fertilization during flowing,the more downstream,the more abundant nutrients in the river,especially the concentration of ammonia nitrogen,leading to the increase of abundance of ammonia-oxidizing microorganisms in the downstream reach.The species of bacteria were abundant in the middle and lower reaches of the Lhasa River,with a total of 34phyla and the main dominant phyla being Proteobacteria,the main dominant genus being Acinetobacter.The community diversity of archaea in water was significantly lower than that of bacteria.The archaea community structures at different sites were relatively similar and the Crenarchaeota was dominant in all samples.The eukaryotic photosynthetic community in water contained 15 phylum and 182 genera,of which Chlorophyta and Cyanobium belonging to Cynobiaceae had the highest overall abundance.Compared to the archaea community in water,the bacterial community structure and eukaryotic photosynthetic community structure of the Lhasa River were impacted more dramatically by the changes of environmental factors caused by the dam construction and sewage discharge.Environmental factors,including water p H,total nitrogen and nitrate content,changed the microbial community structure and diversity by influencing the relative abundance of major microbial species.（3）The changes of microbial community and nitrogen cycling functional flora in the sediment of Lhasa River were studied.AOB abundance of the sediments in reservoir areas was significantly higher than that in the sediments below the dam,and abundances of denitrification genes in sewage discharge areas increased gradually from upstream to downstream,indicating that the dam construction,sewage from Lhasa City and Yambajan geothermal power plant resulted in the increase of nitrogen input which promoted the growth and breeding of the nitrifying and denitrifying microorganisms.According to the analysis of bacterial species composition on phylum level,it was found that the most dominant species in sediment of M3 was Cyanobacteria,and the most dominant species in sediment of M1 was Acidobacillus,and the most dominant species in sediment of sewage discharge area was Proteobacteria.These results showed that the high concentration of nutrients in the sediments of the reservoir and sewage discharge area may induce eutrophication conditions of the aquatic ecosystem and threaten the environmental security of the Lhasa River.The distribution of microbial community in sediment was affected by nutrient concentration of sediment.High contents of TN,NH₄⁺-N and organics increased the relative abundance of Rhodoplanes,Bacillus and Bradyrhizobium in sediment.The dominant bacteria that formed cyanobacteria─Leptolyngbya,Phormidium and Pseudanabaenaceae sp.,significantly had positive correlations with the concentration of NH₄⁺-N.These results confirmed that the variable environmental factors caused by dam construction and water pollution,including TOC,NH₄⁺-N,NO₃^--N,TN and AP,etc.,may affect the relative abundance of some sensitive species and thus change the overall microbial community structure in the sediment.（4）Differences in responses of microbial communities in water and sediment to human activities were studied.Because the major groups of microbial communities in water and sediment were quite different,and the response of microbes in different habitats to environmental factors were also different,the main environmental factors affecting the structure and diversity of microbial communities in water and sediment were different.T and NO₃^--N were the main factors affecting the diversity of bacterial community in water,while the contents of TOC、TN and NH₄⁺-N are the main factors affecting the diversity of bacterial community in sediment.Considering the response of physicochemical factors in water bodies and sediments to human activities,it can be seen that human activities had different impacts on the structure and diversity of microbial communities in different habitats.