Characterizing Microbial Community Composition And Diversity Of Groundwater In A Salt-freshwater Transition Zone

Seawater intrusion is a global issue that is one of the major environmental disasters in coastal areas.The determination of identification index and monitoring method is directly related to the occurrence of seawater intrusion and the evaluation of the intrusion degree.This work takes Longkou City as the main research area and Dagu River as the auxiliary research area.Through field monitoring and regular sampling,high-throughput sequencing was used to explore the diversity,structure,interaction,and functional characteristics of groundwater microbial communities in the salt-freshwater mixing zone in terms of time?season?and space?latitude?.At the same time,combining physical and chemical parameters and laboratory cultivation experiments proves that microbial analysis is a method that can be used to explore seawater intrusion.The main conclusions based on the research results are as follows:1)Concentrations of Cl^-and TDS showed a decreasing trend from coast to inland in Longkou City.And a salt-freshwater interface was approximately between DZ2 and DZ3.The highest microbial diversity occurred in the irrigation water and lowest in the seawater.Groundwater microbial community was affected by many environmental parameters.DO was a main environmental factor influencing microbial community diversity and structure.The order Oceanospirillales and the family Alteromonadaceae could be used as indicators of seawater intrusion.2)Seasonal variation affected the microbial community in the salt-freshwater mixing zone of Longkou City.The genus Acidovorax dominated in the dry season,while the genus Hydrogenophaga dominated in the wet season,and the changes were related to NO₃^-.TOC was the main factor affecting the diversity and structure of groundwater microbial community in the dry season,while NO₃^-had a significant response in the wet season.DO and groundwater level had significant impact on microbial communities in both seasons.The interaction between microbial communities was significantly different in dry and wet seasons.Increased seawater intrusion led to a more compact microbial network and strengthening the groundwater microbial interactions.3)Groundwater microbial communities in salt-freshwater mixing zones of different coastal zones were significantly different.The study area of Dagu River was seriously affected by seawater intrusion,and Longkou City had obvious cation exchange.The denitrification bacteria in Longkou City were higher than that of Dagu River,and the latter had higher reduction capacity of sulfate ions.The microbial metabolic function of Longkou City was higher than that of Dagu River.4)In the laboratory cultivation experiments,the microbial community diversity was the highest and lowest in the environment with the seawater content of 2.5%and 75%.The increase of salt-freshwater displacement promoted the community diversity.The family Flavobacteriaceae and Rhodobacteraceae were positively correlated with seawater content,and they could be used as new indicators of seawater intrusion.Strong salt-freshwater displacement had more impact on the functional characteristics of microbial community.The lower of seawater content in the environment,the greater the impact,while the weak salt-freshwater displacement had less impact.This study is of major scientific significance for taking microorganism as a new seawater intrusion index and provides scientific basis for further understanding the structure and function of coastal aquifer ecosystem.