Exposure to xenobiotics deeply affects the bacteriocenosis in the rhizosphere of helophytes

Abstract

Upon exposure to the aromatic xenobiotics 2,6-dimethylphenol and 4-nitrophenol, distinct changes occurred in the microbial structure of the rhizosphere of the helophytes Carex graclis, Phalaris arundinacea and Juncus effusus (total number, diversity, abundance). Generally speaking, exposure to xenobiotics resulted in an increase in the total numbers of xenobiotic-degrading bacteria on roots and sand particles. The number of bacteria per cm 2 was significantly higher on the root surface than on the surface of sand particles in the rhizosphere. PCR fingerprinting methods such as RAPD (Randomly Amplified Polymorphic DNA) and 16S-23S rDNA intergenic spacer amplification or alternatively rRNA-targeted gene probes for the α-, β-, γ-subclasses of Proteobacteria, the Xanthomonas branch of Proteobacteria and Actinobacteria, were used to determine the diversity of arbitrary samples of bacterial isolates. The diversity of bacteria isolated from the rhizosphere decreased depending on the exposure time. The degrading bacteria were dominated by GRAM-positives. In the case of exposure to 2,6-dimethylphenol of Carex gracilis, GRAM-positive isolates became more dominant as the exposure time increased. Thus it was that after more than six months exposure to 2,6-dimethylphenol, only the GRAM-positives were isolated from a sand-bed reactor planted with Carex gracilis. Some of these isolates show interesting properties in addition to a high 2,6-dimethylphenol degradation rate. The 2,6-dimethylphenol degrading Mycobacterium sp. DMP 20 represents the first fluorescing GRAM-positive bacterium so far described. The introduction of allochthonous, high-performance degraders resulted in Mycobacterium sp. DMP 20 being permanently established in the rhizosphere, whereas the naphthalene degrader Pseudomonas putida PpG7 did not remain in the system.