Biodegradation of Polycyclic Aromatic Hydrocarbons (PAH) from crude oil in sandy-beach microcosms

Though the lower n-alkanes are considered the most degradable components of crude oil, our experiments with microcosms simulating oiled beaches showed substantial depletion of fluorene, phenanthrene, dibenzothiophene, and other PAH in control treatments consisting of raw seawater cycled through the microcosms over a 30-day period. PAH was not detectable in pooled test system effluents. To resolve the issue of wash out versus degradation, we ran oiled-beach microcosms with sterile synthetic seawater. Triplicate treatments were: sterile control, 10 ppm of a rhamnolipid biosurfactant added to the seawater, biweekly inoculation of the microcosms with two marine bacteria that produce biosurfactants but degrade only n-alkanes. The systems inoculated with the alkanedegrading microbes showed depletion of the n-alkanes, but essentially all of the aromatic analytes were recoverable from the oiled sand. We recovered all of the analytes (PAH or alkanes) from the other two treatments. The results support that lower molecular weight PAH were substantially depleted through biodegradation by microorganisms indigenous to natural seawater under aerobic conditions. In contrast, the n-alkane components were not significantly depleted under the same conditions.