Severe biodegradation of polycyclic aromatic hydrocarbons in reservoired crude oils from the Miaoxi Depression, Bohai Bay Basin

Aromatic hydrocarbons are major components in crude oil. Biodegradation of polycyclic aromatic hydrocarbons, such as biphenyls, naphthalenes, phenanthrenes and aromatic steroid hydrocarbons have been well studied. However, little is known about the relative susceptibilities to biodegradation of fluorene, dibenzofuran, dibenzothiophene, chrysene and pyrene, and their alkylated homologues. In this study, gas chromatography–mass spectrometry analyses of aromatic fractions of a suite of severely biodegraded crude oils from the Bohai Bay Basin have been performed to investigate the fate and relative susceptibilities of these polycyclic aromatic hydrocarbons during severe biodegradation under geological conditions.

Changes in concentrations of varying PAH classes with increasing biodegradation suggest their relative susceptibilities to microbial alteration are in the order biphenyls > naphthalenes > dibenzofurans > phenanthrenes ∼ fluorenes > dibenzothiophenes > chrysenes > pyrenes > C_26–28 triaromatic steroid hydrocarbons. The relative susceptibilities to biodegradation of individual isomers of chrysenes, dibenzothiophenes, dibenzofurans, fluorenes, pyrenes and triaromatic steroid hydrocarbons were determined based on their compositional changes. The susceptibility to biodegradation of PAHs is not directed related to the number of aromatic rings, but may be controlled by the shapes of the substrate molecules which control the ability to penetrate the cell membrane and to fit the active site of the microbial enzyme. Sulfur-containing compounds, such as dibenzothiphenes, are more resistant to biodegradation than those non-sulfur-containing components with similar molecular structure, which is likely due to their relatively greater biotoxicity. PAHs with more alkyl substituents are generally more resistant to biodegradation, but exceptions were also observed, and this observation may due to demethylation of a substituted compound. Undoubtedly, the positions of alkyl substituents control the susceptibilities to biodegradation of PAH isomers. However, the observation that PAH isomers with β-substituents are more readily biodegraded is not universal, suggesting that the susceptibility of PAH isomers is not controlled by thermodynamic stability, but may be related to their stereochemical structures.