Isolation and characterization of 3-nitrophenol-degrading bacteria associated with rhizosphere of Spirodela polyrrhiza

The accelerated biodegradation of 3-nitrophenol (3-NP) in the rhizosphere of giant duckweed (Spirodela polyrrhiza) was investigated. Biodegradation of 3-nitrophenol in the rhizosphere of a floating aquatic plant, S. polyrrhiza, was investigated by using three river water samples supplemented with 10 mg l(-1) of 3-NP. Isolation and enrichment culture of 3-NP-degrading bacteria were performed in basal salts medium containing 3-NP (50 mg l(-1)). The isolated strains were physiologically and phylogenetically characterized by using an API20NE kit and 16S rRNA gene sequencing. Accelerated removal of 3-NP (100%) was observed in river water samples with S. polyrrhiza compared with their removal in plant-free river water. Also, 3-NP persisted in an autoclaved solution with aseptic plants, suggesting that the accelerated 3-NP removal resulted largely from degradation by bacteria inhabiting the plant rather than from adsorption and uptake by the plant. We successfully isolated six and four strains of 3-NP-degrading bacteria from the roots of S. polyrrhiza and plant-free river water, respectively. Phylogenetic analysis based on 16S rRNA gene divided the 3-NP-degrading bacteria into two taxonomic groups: the genera Pseudomonas and Cupriavidus. The strains belonging to the genus Cupriavidus were only isolated from the roots of duckweed. All strains isolated from the roots utilized 3-NP (0.5 mM) as a sole carbon and energy source, indicating that they could have contributed to the accelerated degradation of 3-NP in the rhizosphere of S. polyrrhiza. The rhizoremediation using S. polyrrhiza and its rhizosphere bacteria can be an effective strategy for cleaning up the 3-NP-contaminated surface waters.