Non-bioavailability of extracellular 1-hydroxy-2-naphthoic acid restricts the mineralization of phenanthrene by Rhodococcus sp. WB9

Rhodococcus sp. WB9, a strain isolated from polycyclic aromatic hydrocarbons contaminated soil, degraded phenanthrene (PHE, 100 mg L−1) completely within 4 days. 18 metabolites were identified during PHE degradation, including 5 different hydroxyphenanthrene compounds resulted from multiple routes of initial monooxygenase attack. Initial dioxygenation dominantly occurred on 3,4-C positions, followed by meta-cleavage to form 1-hydroxy-2-naphthoic acid (1H2N). More than 95.2% of 1H2N was transported to and kept in extracellular solution without further degradation. However, intracellular 1H2N was converted to 1,2-naphthalenediol that was branched to produce salicylate and phthalate. Furthermore, 131 genes in strain WB9 genome were related to aromatic hydrocarbons catabolism, including the gene coding for salicylate 1-monooxygenase that catalyzed the oxidation of 1H2N to 1,2-naphthalenediol, and complete gene sets for the transformation of salicylate and phthalate toward tricarboxylic acid (TCA) cycle. Metabolic and genomic analyses reveal that strain WB9 has the ability to metabolize intracellular 1H2N to TCA cycle intermediates, but the extracellular 1H2N can’t enter the cells, restricting 1H2N bioavailability and PHE mineralization.