Microbial Insights

Detect and quantify bacteria responsible for aerobic biodegradation of PAHs

Fuel oils including diesel are complex mixtures composed of varying fractions of n-alkanes, iso- and cycloalkanes, aromatic hydrocarbons, and sulfur, nitrogen, and oxygenated compounds.  As with gasoline, the aromatic hydrocarbon fraction typically raises concerns at diesel impacted sites.  Diesel fuel is usually described as an intermediate distillate of crude petroleum but is likely to contain both volatile monooaromatics (BTEX) and semi-volatile polycyclic aromatic hydrocarbons (PAHs).  Biodegradation of the monoaromatic compounds (BTEX) under both aerobic and anaerobic conditions is described in detail in the CENSUS Applications – Aerobic BTEX and Anaerobic BTEX.

As with BTEX, biodegradation of PAHs under aerobic conditions has been intensively studied over the past 30 years.  For most early studies aimed at elucidation of an aerobic catabolic pathway, naphthalene, the lowest molecular weight PAH, was used as the model compound.  Overall, the upper pathway for aerobic naphthalene biodegradation is analogous to that of monoaromatic hydrocarbons.  Biodegradation is initiated by a naphthalene dioxygenase enzyme producing a cis-dihydrodiol which is dehydrogenated to form dihydroxynaphthalene.  A second dioxygenase enzyme catalyzes cleavage of one of the aromatic rings eventually producing a monooaromatic compound salicylate which can be further metabolized.

CENSUS Targets for Aerobic PAHs

Naphthalene Dioxygenase (qNAH): As the name suggests, naphthalene dioxygenase catalyzes the incorporation of both atoms of molecular oxygen into naphthalene, however, the broad substrate specificity of naphthalene dioxygenase has been widely noted.  When expressed, naphthalene dioxygenase is capable of catalyzing the oxidation of larger PAHs (anthracene, phenanthrene, acenaphthylene, fluorene, and acenaphthene) and heterocyclic aromatic compounds (dibenzo-1,4-dioxin, dibenzothiophene, and dibenzofuran).

Phenol Hydroxylase (qPHE): Phenol hydroxylase catalyzes the continued oxidation of phenol and cresol intermediates.  In single compound microcosm studies, PHE was detected following amendment with naphthalene (in addition to microcosms containing benzene, toluene, p-xylene, and biphenyl) suggesting a role in the biodegradation of numerous aromatic compounds (or their metabolites).

Although originally viewed as recalcitrant under anaerobic conditions, naphthalene biodegradation under sulfate reducing conditions has been documented in recent years.  Currently however, biochemical pathways for anaerobic naphthalene biodegradation are not clear.  In fact, dehydrogenation, carboxylation, and methylation have all been proposed as the initial step suggesting multiple pathways may be used in the field.  Additional research into the pathway for anaerobic PAH biodegradation is needed before an appropriate CENSUS assay can be developed.

As mentioned previously, BTEX are also often present at diesel impacted sites.  For a complete discussion of CENSUS targets to evaluate BTEX biodegradation, please see CENSUS Applications – BTEX.