The Spatial Distribution Pattern Of Environmental Microbial Diversity In The Mainstem Of Nu River And Its Driving Mechanism

Microorganisms are an indispensable component of ecosystems and play irreplaceable roles in maintaining river ecosystem functions,such as the carbon fixation and water purification.However,riverine microbial studies are mainly concentrated in western Europe,the United States,as well as the middle and eastern parts of China.The Nu River is an important international river in China.However,the spatial distribution pattern of microbial diversity and their driving mechanism in the Nu River was still far from well understood.In this thesis,water,sediment,and riparian soil samples were collected from 20sites with spatial intervals of approximately 30 km in the Yunnan section of Nu River.The 16S r DNA and ITS-based q PCR and amplicon high-throughput sequencing combined with the measurement of physicochemical properties and multivariate statistical analysis were used to reveal the spatial differentiation in microbial abundance and diversity and its drving mechanisms.The main findings are as follows:(1)The microbial abundance in the Yunnan section of the Nu River is significantly different among water,sediments and riparian soils.The prokaryotic abundances in one gram of river water,sediment and riparian soil are 4.86×10~6,2.74×10¹⁰,and 8.38×10~9,respectively.The average fungal abundances in one gram of river water,sediments and riparian soils are 5.19×10~3,7.25×10~7,and 3.65×10~7,respectively.In water,sediments,and riparian soils,microbial abundance shows an increasing trend along the decrease in latitude.Compared with other rives,the diversity of microorganisms in the Yunnan section of Nu River is relatively high.There are 31498 and 17484 operational taxonomic unites(OTUs)for prokaryotes and fungi,respectively.The micrbial OTU number shows an increasing trend from upstream to downstream.The function prediction based on FAPROTAX and FUNGuild showed that a large number of microbial groups have functional potentials such as pathogenicity,methanogenesis,nitrification,and denitrification.The composition of the microbial community also significantly differed among different environmental media and sampling sites.For example,the relative abundance of Bacteroides in water and sediments is significantly higher than that in soils,and the relative abundance of Acidobacteria in riparian soils is significantly higher than that in other environmtal media.Significant differences in the relative abundance of Bacteroides,Acidobacteria,and potential nitrifiers among different sampling sites were also observed.(2)The analysis based on MST model revealed that the community assembling of prokaryotes is controlled by both deterministic and stochastic process among different sites,and the modified stochastic rate is 44.29%.In contrast,the assembling of fungal community is dominatedly driven by deterministic process,and the the modified stochastic rate is only 18.05%.The further analysis based on i CAMP suggested that homogenous selection and dispersal limitation are the dominant community assembly processes,and the average contribution of those two processes are 67.67%.This thesis also found that geographical distance is an important environmental factor that drives the variation in microbial community composition among the study sites.Conductivity,turbidity,dissolved oxygen,and total suspended solids are also important factors influencing microbial community composition,and the average explanation of environmental factors reached 44.69%in three habitats.Source Tracker analysis showed that species exchange among sites and habitats,as well as within sites is also an important maintaining mechanism for the microbial diversity.(3)Rare taxa have the highest proportion of OTU richness and the relative abundance for prokaryotes and fungi are 69.93%and 57.07%,respectively.In contrast,the abundant taxa have the highest relative abundance(i.e.,53.77%for prokaryotes and60.90%for fungi).Both rare and abundant subcommunities show significant distance–decay relationships.The assemblies of abundant and rare subcommunities are mainly controlled by deterministic and stochastic process,respectively.The contribution of drift to the assembly of abundant,intermediate and rare subcommunities is 36.32%,22.06%,and 14.75%,respectively.These findings provide basics for the microbial resource developments and environmental health risk assessments in Nu River.They also provide references for the assembly mechanism studies of microbial communities along rivers.In addition,the Nu river is the last Asian river without hydroelectric developments,and thus the findings in this thesis provides critical reference data to assessing the environemtal effects of hydroelectric developments in other rivers.