Microbial Transformation of Polycyclic Aromatic Hydrocarbons in Pristine and Petroleum-Contaminated Sediments

To determine rates of microbial transformation of polycyclic aromatic hydrocarbons (PAH) in freshwater sediments, ¹⁴C-labeled PAH were incubated with samples from both pristine and petroleum-contaminated streams. Evolved ¹⁴CO₂ was trapped in KOH, unaltered PAH and polar metabolic intermediate fractions were quantitated after sediment extraction and column chromatography, and bound cellular ¹⁴C was measured in sediment residues. Large fractions of ¹⁴C were incorporated into microbial cellular material; therefore, measurement of rates of ¹⁴CO₂ evolution alone would seriously underestimate transformation rates of ¹⁴C]naphthalene and ¹⁴C]anthracene. PAH compound turnover times in petroleum-contaminated sediment increased from 7.1 h for naphthalene to 400 h for anthracene, 10,000 h for benz(a)anthracene, and more than 30,000 h for benz(a)pyrene. Turnover times in uncontaminated stream sediment were 10 to 400 times greater than in contaminated samples, while absolute rates of PAH transformation (micrograms of PAH per gram of sediment per hour) were 3,000 to 125,000 times greater in contaminated sediment. The data indicate that four- and five-ring PAH compounds, several of which are carcinogenic, may persist even in sediments that have received chronic PAH inputs and that support microbial populations capable of transforming two- and three-ring PAH compounds.