The role of floating macrophytes on modulating the microbial nitrogen removal is not well understood. In this study, the cultivation of Eichhornia crassipes in eutrophic water may affect the nitrogen (N) fate by modulating the denitrifying bacteria diversity and abundance. The gaseous N losses via denitrification were estimated by 15 N stable isotope tracing and the diversity and abundance of denitrifying genes (nirS, nirK, and nosZ) were investigated by molecular tools. The denaturing gradient gel electrophoresis (DGGE) profiles showed that the diversity of denitrifying genes in the treatments with E. crassipes was significantly higher than that in the treatment without E. crassipes. The real-time PCR (qPCR) results showed the trend of denitrifier abundance in the entire system was in the order of N-ER (nitrate with just root of E. crassipes) and A-ER (ammonia with just root of E. crassipes) > N-R (nitrate with E. crassipes) and A-R (ammonia with E. crassipes) > N-W (nitrate without plant) and A-W (ammonia without plant). The gaseous 15 N losses via denitrification were significantly and positively related to the abundance of nirK, nirS, and nosZ genes. The results indicated that cultivation of E. crassipes in eutrophic water could increase the diversity and abundance of denitrifying bacteria, resulting in more N being removed as gases via denitrification.
Although cyanobacterial blooms can change microbial communities, it is still unclear what impact such harmful blooms will have on denitrifying bacteria, the drivers of the removal of excessive nitrogen from water. In order to clarify the impact, populations of denitrifying bacteria, with periodic proliferation and dominance of cyanobacteria in a eutrophic shallow lake located in southeast China, were investigated using quantitative real-time polymerase chain reaction (qPCR) and 454-pyrosequencing based on the copper-containing nitrite reductase (nirK) gene, cytochrome cd1-containing nitrite reductase (nirS) gene and nitrous oxide reductase (nosZ) gene. Samples were collected periodically during a three-month period when the cyanobacterial density gradually increased. In the qPCR analyses, abundances of nirK, nirS and nosZ were intensely positively correlated with the biomass of cyanobacteria. Moreover, 454-pyrosequencing revealed that the community composition of denitrifying bacteria shifted with the increase in cyanobacterial density. These results indicated that the shifts of the community composition of denitrifying bacteria might be related to cyanobacterial blooms, which could potentially lead to alterations of denitrification in eutrophic water.
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