Effects of pH on the degradation of phenanthrene and pyrene by Mycobacterium vanbaalenii PYR-1
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The effects of pH on the growth of Mycobacterium vanbaalenii PYR-1 and its degradation of phenanthrene and pyrene were compared at pH 6.5 and pH 7.5. Various degradation pathways were proposed in this study, based on the identification of metabolites from mass and NMR spectral analyses. In tryptic soy broth, M. vanbaalenii PYR-1 grew more rapidly at pH 7.5 (μ′=0.058 h−1) than at pH 6.5 (μ′=0.028 h−1). However, resting cells suspended in phosphate buffers with the same pH values displayed a shorter lag time for the degradation of phenanthrene and pyrene at pH 6.5 (6 h) than at pH 7.5 (48 h). The one-unit pH drop increased the degradation rates four-fold. Higher levels of both compounds were detected in the cytosol fractions obtained at pH 6.5. An acidic pH seemed to render the mycobacterial cells more permeable to hydrophobic substrates. The major pathways for the metabolism of phenanthrene and pyrene were initiated by oxidation at the K-regions. Phenanthrene-9,10- and pyrene-4,5-dihydrodiols were metabolized via transient catechols to the ring fission products, 2,2′-diphenic acid and 4,5-dicarboxyphenanthrene, respectively. The metabolic pathways converged to form phthalic acid. At pH 6.5, M. vanbaalenii PYR-1 produced higher levels of the O-methylated derivatives of non-K-region phenanthrene- and pyrene-diols. Other non-K-region products, such as cis-4-(1-hydroxynaphth-2-yl)-2-oxobut-3-enoic acid, 1,2-dicarboxynaphthalene and benzocoumarin-like compounds, were also detected in the culture fluids. The non-K-region polycyclic aromatic hydrocarbon oxidation might be a significant burden to the cell due to the accumulation of toxic metabolites.