Urban traffic changes the biodiversity, abundance, and activity of phyllospheric nitrogen-fixing bacteria

The phyllosphere provides appropriate conditions for colonization by microorganisms, including diazotrophic bacteria. However, a poor understanding of the effects of the atmospheric environment on the phyllospheric diazotrophic communities persists. We detected the biodiversity, abundance, and activity of nitrogen-fixing bacteria in the phyllospheres of two evergreen shrubs, Nerium indicum Mill. and Osmanthus sp., sampled from urban areas with heavy traffic, a college campus, and a forest. Quantitative PCR analysis indicated that the copy numbers of nifH sequences were highest in the phyllospheres of both plants in heavy-traffic urban areas and correlated with the recorded nitrogenase activities of the phyllospheres. Similarly, the phyllosphere from heavy-traffic urban areas also possessed the highest biodiversity indices of diazotrophic communities from both the two plants. Pyrosequencing analysis revealed a diversity of nifH sequences in phyllosphere that were mostly uniquely found in the phyllosphere, and many of these were proteobacteria-like and cyanobacteria-like. Members of the Proteobacteria, mostly of which were not closely related to unknown organisms, were detected exclusively in the phyllosphere and represented substantial fractions of their associated diazotrophic communities. Our study provides initial insight into the shifts in the biodiversity and community structure of N₂-fixing microorganisms in the phyllospheres of different atmospheric environments.