Relationship Between Bacterial Community Structure Around Roots Of Three Constructed Wetland Plants And Water Purification

With the rapid development of economy,theenvironmental problems in China such as water pollution are more prominent.Constructed wetland has the characteristics of low energy consumption,low investmentandlow operation cost and environmental protection.It is widely used to treat all kinds of urban wastewater.Plant species and their rhizosphere bacteria have an important impact on water purification capacityin constructed wetlands.At present,the ecological process of controlling the bacterial assembly in wetland plant rhizosphere（rhizosphere,rhizoplane to endosphere）,and the relationship between plant rhizosphere bacterial community diversity and water purification are not completely clear.In this study,the wetlands constructed by the common plants Acorus calamus,Canna indica and Hymenocallis littoralis respectively are taken as the research object.We used16S r RNA high-throughput sequencing to analyze the community structure and function of bacteria in the root compartments of three typical plant species in the constructed wetlands in South China,as well asthe relationship between bacterial composition and water purification in the dry and wet seasons,through sampling and testing of water quality indicators of different plant constructed wetlands,research on the characteristics of wetland plant water quality purification,and sequencing analysis of plant rhizosphere bacteriain dry season and wet season,in order to provide reference for the quality improvement of constructed wetlands.The results are as following:1.There were significant differences in the removal efficiency of pollutants in water bodiesamong the three plants in different seasons.In the dry season,the average removal rates of NO₂^--N,NO₃^--N and NH₄⁺-N in A.calamus wetland were the highest,65%,34%and 54%,while H.littoralis wetland system had the highest average removal rates of COD,TOC and TP,which are 26%,26%and 55%respectively.In the wet season,the average removal rates of NO₂^--N and COD in A.calamus wetland system were the highest,85%and 79%respectively.Overall,the purification effect of the three plant constructed wetland systems in wet season was greater than that in dry season and the nitrogen removal efficiency of A.calamus wetland was higher than that of C.indica and H.littoralis.2.The bacterial community assembly along the soil-plant in constructed wetlands was mainly determined by root compartments,plant species and season.The bacterial diversity in the rhizosphere ofthe same plantgraduallydecreased from the rhizosphere to the rhizoplane and then to the endosphere.Among the three plants,the bacterial diversity in different root compartments of H.littoralis was the highest in the dry season andthat of A. calamus was the highest in the wet season.3.In the dry season,Proteobacteriawas the dominant bacteria in the rhizosphere,rhizoplane and endosphere of the three wetland plants.In the wet season,the dominant bacteria in the rhizosphere and endosphere of the three wetland plants were Proteobacteria,while on rhizoplanewas Cyanobacteria.The main ways of removing pollutants betweenrhizosphere and rhizoplane,weremetabolism,and between rhizoplane and endosphere.4.Whether in dry season or wet season,the bacterial community structure in the rhizosphere,rhizoplane and endosphere of the three constructed wetland plantshada certain correlation with water purification.The correlation between the rhizosphere bacterial community and the removal of water pollutants was higher than the correlation between the rhizoplane bacteria and endosphere bacteria and the removal of water pollutants.In the dry season,the bacterial community in rhizosphere,rhizoplane and endosphere of the three wetlands plants were significantly correlated with the removal efficiency of TN,COD and NH4₄⁺-N in water.Among them,the main bacterial groups in the rhizosphere were Sphingomonas,Nitrospiraand Haliangium,the main bacterial groups in the rhizoplane were Novosphingobiumand Sphingomonas,and that in the endosphere were Piscinibacter,Nitrospiraand Sphingomonas.In the wet season,the bacterial communities in rhizosphere,rhizoplane and endosphere of three wetland plants were mainly correlated with the removal efficiency of TP,TOC and NO₃^--N in water.The rhizosphere bacteria were mainly Ellin6067,Nitrospira,Sphingomonas and Dongia,the rhizoplane bacteria were mainly Nitrospira,Piscinibacter,Terrimonas and Haliangium,and the endosphere bacteria were Sphingomonas,Piscinibacter and Nitrospira,which were significantly correlated with the removal efficiency of TP,TOC and NO₃^--N.Overall,our findings revealed that the removal of nitrogen in A.calamus constructed wetland was better than the other two plant wetlands in dry and wet seasons.The bacterial diversity of the same plant decreased gradually from the rhizosphere to the endosphere.The bacteria of Sphingomonas,Nitrospira,Piscinibacter and Haliangiumwere significantly related to the removal of water pollutants,and had a strong effect on water purification.The combination of A.calamus with above bacterial communitieswas more beneficial to water purification.Thepollutants were mainly removed by bacteria through metabolic pathways in the rhizosphere,rhizoplaneandendosphere.Ourfindings can provide a reference for the selection and combinationof the plants and bacteria for water purification in constructed wetlands in practice.