The Shifts In The Structure And Diversity Of Bacterial Communities Along The Xiangjiang River

River is an important part of river basin ecosystem,and the water quality comprehensively reflects the impact of human activities and natural processes on the watershed environment.Pollution will lead to the changes in the biological characteristics and microbial community structure of the surface water and riverbed sediments.Microorganism is one of key players in maintaining and restoring the health and balance of ecosystems,regarding as a potential indicator of river pollution and health.Understanding how microbes change with environment changes is essential for water quality improvement and watershed management.However,the impact of human activities(especially urbanization)on river microbial diversity,structural composition and spatial distribution is still very limited.We selected Xiangjiang,China as the study object,taking 5 major cities(Yongzhou,Hengyang,Zhuzhou,Xiangtan and Changsha)as sampling sites along the Xiangjiang River.Using 16S rRNA Illumina-based sequencing approach characterized the shifts of bacterial diversity,structural composition,and spatial distribution of surface water and riverbed sediments.The structural equation model(SEM)was used to quantify the impact of environmental factors on bacterial communities.The main results are as follows:(1)High-throughput sequencing data showed that a total of 170 053 sequences,2 362 OTUs,and 11 bacterial phyla for the surface water.The "core bacterial communities"were Proteobacteria and Bacteroidetes,accounting for 61.36%and 44.42%of all the OTUs,respectively;a total of 127 833 sequences,9 0152 OTUs,and 14 bacterial phyla for the riverbed sediments.The "core bacterial communities" were Proteobacteria and Bacteroidetes,accounting for 23.83%and 14.04%of all the OTUs,respectively.(2)The abundances of the main functional genes were K8300(0.04834)and K02014(0.04166)for the surface water,and K02652(0.00327)and K07665(0.00222)for the riverbed sediments.According to the Kyoto Encyclopedia of Genes and Genomes(KEGG pathway),the abundances of the six metabolic pathways were significantly different(p?0.01).The primary metabolic pathways were metabolism(51.43%and 51.84%)and the information processing environment(15.60%and 14.53%)for surface water and riverbed sediments,respectively.(3)Structural equation model(SEM)was used to quantify the impacts of each environmental factor on the bacterial phyla.The results showed that pH,NO3-,turbidity,and antibiotics had the greatest influence on the bacterial structure of the surface water,while riverbed sediments were mainly affected by pH,SOM,heavy metals,and antibiotics.In summary,our results indicate that the shifts in bacterial community structure are related to changes in the relative abundance,rather than presence/absence of OTUs,suggesting the presence of "core bacterial community(i.e.,Proteobacteria and Bacteroidetes)" along the Xiangjiang River.The main reasons are that the heterogeneous input,enrichment effect,and "species sorting" in the Xiangjiang River upstream,which conform to the theory of river continue concept(RCC).Therefore,it is more essential to explore the shifts of the diversity and structure of bacterial communities and driving forces in Xiangjiang River.The results can provide a scientific theoretical basis for water quality monitoring and river management.