Abstract: To examine the characteristic of denitrification in Frankia, a symbiotic nitrogen-fixing microbe associated with non-leguminous plants, and its role as a N₂O source or sink, Casuarina root nodule endophyte Frankia was isolated using sectioning method, which was then purely cultured to investigate the denitrification process under NO₃^- addition. The results showed that after addition of NO₃^- to the medium under anaerobic condition, the concentration of NO₃^- decreased with time, while the concentrations of NO₂^- and N₂O initially increased and then decreased over time. Key denitrification genes and nitrogenase gene were detected at 26 h, 54 h and 98 h during incubation. Abundances of these genes significantly differed among each other, and their dynamics were asynchronous. Redundancy analysis of the effect of NO₃^-, NO₂^-, N₂O concentrations on abundances of denitrification genes and nitrogenase gene indicated that 81.9% of the total variation in gene abundances could be explained by the first two axes. Frankia had a denitrifying activity under anaerobic condition, with denitrification genes, including nitrous oxide reductase gene (nosZ), being identified. Our results suggested that Frankia possessed a complete denitrification pathway and the ability of N₂O reduction under anaerobic condition.

Key words: Casuarina, Frankia, pure culture, functional gene, denitrification