Pseudomonas, the dominant polycyclic aromatic hydrocarbon-degrading bacteria isolated from Antarctic soils and the role of large plasmids in horizontal gene transfer

Twenty-two polycyclic aromatic hydrocarbon (PAH)-degrading bacterial strains were isolated from Antarctic soils with naphthalene or phenanthrene as a sole carbon source, while no degrader was obtained from an unpolluted sampling site. Phylogenetic analysis showed that all belonged to the genus Pseudomonas except one that was identified as the genus of Rahnella. Some of them were closely related to previously reported cold-tolerant species, while some were separated in deeply rooted branches and represent new strains. All these strains showed a high efficiency to degrade naphthalene at 4 degrees C, and some additionally degraded phenanthrene. Using degenerate primers and polymerase chain reaction (PCR) amplification, ndo gene encoding naphthalene dioxygenase (NDO) was detected from all the isolates. Phylogenetic analysis grouped these genes into two clusters which shared 94% similarity to each other, and showed about 97% similarity within a cluster. However, no obvious difference was observed with mesophilic ndo genes; this indicates that the host cell is pivotal in cold adaptation. In addition, the mismatch between 16S rRNA and NDO phylogenetic trees strongly indicates horizontal gene transfer among these isolates and may have happened in situ. Further, Southern hybridization and plasmid curing confirmed that ndo genes were located on a large self-transmissible plasmid, which can be transferred to a mesophilic strains. The transconjugants acquired the ability to utilize naphthalene and phenanthrene. Results of this article imply that Pseudomonas plays an important role in PAH biodegradation in Antarctic soils, and the related genes might be originally transferred from outside Antarctica and spread among indigenous species.