Nitrogen-fixing microorganisms play important roles in the structure and function of aquatic ecosystems. However, the diversity and distribution of diazotrophic bacteria in subtropical reservoirs are so far poorly understood, especially along the depth continuum. In this study, we describe the vertical pattern of diazotrophic community based on nitrogenase (nifH) gene in a subtropical deep reservoir during the strongly stratified period. Denaturing gradient gel electrophoresis (DGGE), clone libraries and quantitative real-time PCR were used together to facilitate an in-depth investigation of the diazotrophic community structure. Our DGGE analysis result illustrated that the diazotrophic community shifted between the oxic/anoxic interface and the nifH diversity increased along the water depth gradient. Our clone library result indicated that cyanobacteria, affiliated to the toxic bloom-forming Cylindrospermopsis raciborskii, were the dominant diazotrophic cluster in the surface waters, whereas diazotrophic Alphaproteobacteria were dominant in the bottom waters. Analysis of relationships between community and environmental factors clearly demonstrated that distinct differences in temperature, oxygen and total nitrogen concentration might have a great impact on the diazotrophic communities, thereby influencing the aquatic nitrogen cycle.

NifH gene;Cyanobacteria;Stratified reservoir;DGGE;Clone library;