meta-Cleavage of hydroxynaphthoic acids in the degradation of phenanthrene by Sphingobium sp. strain PNB

Polycyclic aromatic hydrocarbons (PAHs) comprise a group of priority organic pollutants that are toxic and/or carcinogenic. Phenanthrene, the simplest PAH among recognized priority pollutants, is commonly used as a model compound for the study of PAH biodegradation. Sphingobium sp. strain PNB, capable of degrading phenanthrene as a sole carbon and energy source, was isolated from a municipal waste-contaminated soil sample. A combination of chromatographic and spectrometric analyses, together with oxygen uptake and enzyme activity studies, suggested the presence of phenanthrene degradation pathways in this strain. Identification of metabolites suggested that initial dioxygenation of phenanthrene took place at both 3,4- and 1,2-carbon positions; meta-cleavage of resultant diols led to the formation of 1-hydroxy-2-naphthoic acid and 2-hydroxy-1-naphthoic acid, respectively. The hydroxynaphthoic acids, in turn, were metabolized by a meta-cleavage pathway(s), leading to the formation of 2,2-dicarboxychromene and 2-hydroxychromene-2-glyoxylic acid, respectively. These metabolites were subsequently transformed to catechol via salicylic acid, which further proceeds towards the tricarboxylic acid cycle leading to complete mineralization of the compound phenanthrene. The present study establishes the metabolism of hydroxynaphthoic acids by a meta-cleavage pathway in the degradation of phenanthrene, expanding our current understanding of microbial degradation of PAHs.