PAH degradation capacity of soil microbial communities--does it depend on PAH exposure?

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants of the environment. But is their microbial degradation equally wide in distribution? We estimated the PAH degradation capacity of 13 soils ranging from pristine locations (total PAHs approximately 0.1 mg kg(-1)) to heavily polluted industrial sites (total PAHs approximately 400 mg kg(-1)). The size of the pyrene- and phenanthrene-degrading bacterial populations was determined by most probable number (MPN) enumeration. Densities of phenanthrene degraders reflected previous PAH exposure, whereas pyrene degraders were detected only in the most polluted soils. The potentials for phenanthrene and pyrene degradation were measured as the mineralization of (14)C-labeled spikes. The time to 10% mineralization of added (14)C phenanthrene and (14)C pyrene was inversely correlated with the PAH content of the soils. Substantial (14)C phenanthrene mineralization in all soils tested, including seven unpolluted soils, demonstrated that phenanthrene is not a suitable model compound for predicting PAH degradation in soils. (14)C pyrene was mineralized by all Danish soil samples tested, regardless of whether they were from contaminated sites or not, suggesting that in industrialized areas the background level of pyrene is sufficient to maintain pyrene degradation traits in the gene pool of soil microorganisms. In contrast, two pristine forest soils from northern Norway and Ghana mineralized little (14)C pyrene within the 140-day test period. Mineralization of phenanthrene and pyrene by all Danish soils suggests that soil microbial communities of inhabited areas possess a sufficiently high PAH degradation capacity to question the value of bioaugmentation with specific PAH degraders for bioremediation.