The Effect Organic Acids Excreted By The Root Of Submerged Macrophytes On The Abundance Of NirS-Type Denitrifiers And Anammox Bacteria In Rhizosphere Of Plants

Excessive nitrogen has been a global concern to cause lake eutrophication.The denitrification and anammox processes are considered to be effective biological pathways for nitrogen removal.Submerged macrophytes,as a vital ecological community in the aquatic ecosystem,also play an important role in the nitrogen cycle of lakes.However,the mechanism of submerged macrophytes on regulating biological nitrogen removal pathways has not been well quantified.Therefore,this study investigated the impacts of submerged macrophytes on the community structures and abundance of the nirS-type denitrifiers and anammox bacteria in the rhizospheres.The main research results are as follows:?1?Acetic acid,oxalic acid,succinic acid and malic acid were the main organic acids in the rhizospheres of Hydrilla verticillata,Vallisneria natans and Potamogeton maackianus,and the abundance of two genes was negatively correlated with the total organic acid content.This indicated that the organic acid inhibited the growth of both bacteria in the rhizospheres of three submerged macrophytes.The nirS sequences in rhizosphere sediments were Pseudomonas aeruginosa,Sulfuricaulis limicola,Herbaspirillum sp.,and Azospirillum brasilense.The anammox 16S rRNA sequences in rhizosphere sediments were Candidatus Jettenia,Candidatus Brocadia,Candidatus Kuenenia,Deferrisoma camini and Pelobacter acetylenicus.In addition,the highest diversity of nirS community was found in the rhizosphere sediment of H.verticillata,while the highest diversity of anammox bacteria community was found in the rhizosphere of P.maackianus.The results showed that different submerged macrophytes could affect the community diversity of denitrifying bacteria and anammox bacteria.The abundance of nirS gene in the rhizosphere sediments of three submerged macrophytes decreased from the mature stage to the decline stage,while the abundance of anammox 16S rRNA gene increased,indicating that anammox bacteria could better adapt to the decline of plants.The abundance of nirS and anammox 16S rRNA genes in the near rhizosphere of plants was lower than that in the root compartment and the non-rhizosphere.The distributions of two genes were significantly different among different sampling layers?P<0.001?.The RDA results illustrated that concentrations of NO₃^--N,NO₂^--N,citric acid,oxalic acid,malonic acid,malic acid and succinic acid were the key indicators which had the significant impact on the microbial community.By measuring the dissolved oxygen and redox potential in the vertical profile of the rhizosphere sediments,it was found that both the dissolved oxygen and redox potential tend to decrease as the depth of the sediments increases.The roots of submerged macrophytes could secrete oxygen.?2?The main organic acids in the rhizosphere of Potamogeton crispus were acetic acid,oxalic acid,succinic acid and malic acid,and contents of organic acids in the root compartment and non-rhizosphere were lower than that in the near rhizosphere.The abundance of both bacteria was positively correlated with the total organic acid content.The results showed that the organic acid in the rhizosphere of P.crispus could promote the growth of both bacteria.The nirS sequences in the rhizosphere sediments of P.crispus were Pseudomonas sp.,Sulfuricaulis limicola and Thauear sp.,and the anammox 16S rRNA sequences were Candidatus Jettenia and Candidatus Brocadia.The abundance of nirS and anammox 16S rRNA genes in root compartment and the non-rhizosphere of P.crispus was lower than that in the near rhizosphere.After the plants entered the stage of decline,the abundance of nirS and anammox 16S rRNA gene in the rhizosphere of higher damaged plants decreased significantly.The distributions of two genes were significantly different in the plant treatment group with different sampling periods?P<0.001?,The results of RDA analysis showed that the indexes which had a great influence on the abundance of the two genes were NH₄⁺-N,NO₂^--N,succinic acid and oxalic acid.By measuring the dissolved oxygen and redox potential in the vertical profile of the rhizosphere sediments,it was found that both the dissolved oxygen and redox potential tend to decrease as the depth of the sediments increases.The roots of submerged macrophytes could secrete oxygen.?3?The addition of organic acids would have an impact on the microbial diversity in the sediments.The microbial diversity in the sediments added organic acids was higher than that in the untreated group,and the diversity indexes of the experimental group added with organic acid concentration of 25 mmol/L were the highest.From the perspective of gene abundance,the abundance of the two genes decreased closely with the increase of the added organic acid concentration,indicating that the increase of organic acid concentration would inhibit the growth of denitrifying bacteria and anammox bacteria.With the increase of sampling time,the abundance of nirS gene increased,while that of anammox 16S rRNA gene decreased.In this study,different organic acid contents in the sediments had significant difference on abundance of nirS and anammox 16S rRNA genes?P<0.001?.The sampling periods also had a significant difference on the abundance of nirS and anammox 16S rRNA genes?P<0.05?.The results of RDA analysis showed that the indexes that had a great impact on nirS and anammox 16S rRNA abundance were NH₄⁺-N,NO₃^--N,organic matter and pH.