Phenanthrene-degrading Sphingobium xenophagum are widely distributed in the western Pacific Ocean

Six phenanthrene-degrading bacteria were isolated from surface seawater sampled from the western Pacific Ocean. They were identified as Sphingobium xenophagum (formerly Sphingomonas xenophaga) based on morphological, biochemical, and chemical characteristics, and 16S rRNA sequences. The salinity ranges for the growth of these isolates were broader than those of the seven reported Sphingomonas strains isolated from soil, and the optimum NaCl concentration in the growth medium was higher than that for soil sphingomonads. These isolates also exhibited higher phenanthrene-degrading activity under briny conditions than that of a phenanthrene-degrading Sphingomonas strain isolated from soil. A DNA fragment carrying nah genes, which are encoded on the naphthalene-catabolic plasmid NAH of Pseudomonas putida PpG7, hybridised less strongly with the total DNA of all isolates. Certain genes involved in phenanthrene degradation were also preliminarily characterised in all isolates. This is the first demonstration that S. xenophagum strains, which are able to degrade phenanthrene, are widely distributed in marine environments, and that the growth and phenanthrene-degrading activity of these strains are adapted to briny conditions. The results also suggest that genes for phenanthrene degradation, which are dissimilar to nah genes, were also ubiquitously distributed in marine strains.