Repeated inoculation as a strategy for the remediation of low concentrations of phenanthrene in soil

Phenanthrene, a polycyclic aromatic hydrocarbon, becomes increasingly unava ilable to microorganisms for degradation as it ages in soil. Consequently, many bioaugmentation efforts to remediate polycyclic aromatic hydrocarbons in soil have failed. We studied the effect of repeatedly inoculating a soil with a phenanthrene-degrading Arthrobacter sp. on the mineralization kinet ics of low concentrations of phenanthrene. After the first inoculation, the initial mineralization rate of 50 ng/g phenanthrene declined in a biphasic exponential pattern. By three hundred hours after inoculation, there was n o difference in mineralization rates between the inoculated and uninoculate d treatments even though a large fraction of the phenanthrene had not yet b een mineralized. A second and third inoculation significantly increased the mineralization rate, suggesting that, though the mineralization rate decli ned, phenanthrene remained bioavailable. Restirring the soil, without inocu lation, did not produce similar increases in mineralization rates, suggesti ng absence of contact between cells and phenanthrene on a larger spatial sc ale (>mm) is not the cause of the mineralization decline. Bacteria inoculat ed into soil 280 hours before the phenanthrene was added could not maintain phenanthrene degradation activity. We suggest sorption lowered bioavailabi lity of phenanthrene below an induction threshold concentration for metabol ic activity of phenanthrene-degrading bacteria.