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The polycyclic aromatic hydrocarbon (PAH) contamination, bacterial community, and PAH-degrading bacteria were monitored in aged PAH-contaminated soil (Neuves-Maisons [NM] soil; with a mean of 1,915 mg of 16 PAHs ⅐ kg ؊1 of soil dry weight) and in the same soil previously treated by thermal desorption (TD soil; with a mean of 106 mg of 16 PAHs ⅐ kg ؊1 of soil dry weight). This study was conducted in situ for 2 years using experimental plots of the two soils. NM soil was colonized by spontaneous vegetation (NM-SV), planted with Medicago sativa (NM-Ms), or left as bare soil (NM-BS), and the TD soil was planted with Medicago sativa (TD-Ms). The bacterial community density, structure, and diversity were estimated by real-time PCR quantification of the 16S rRNA gene copy number, temporal thermal gradient gel electrophoresis fingerprinting, and band sequencing, respectively. The density of the bacterial community increased the first year during stabilization of the system and stayed constant in the NM soil, while it continued to increase in the TD soil during the second year. The bacterial community structure diverged among all the plot types after 2 years on site. In the NM-BS plots, the bacterial community was represented mainly by Betaproteobacteria and Gammaproteobacteria. Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants associated with a wide range of anthropogenic activities (gas plants, wood preservation plants, waste incineration, coke production, and petrochemical industries). The intensive industrial coal mining during the 19th and 20th centuries in northern France caused contamination of large areas where the soil now needs to be remediated.Microbiological degradation is the chief process for natural elimination of PAHs from contaminated soil (9). A wide range of bacteria are able to degrade low-molecular-weight PAHs, such as naphthalene, phenanthrene, and anthracene, while the high-molecular-weight PAHs (with four or more fused aromatic rings) are more recalcitrant, and relatively few microorganisms are able to use them as a sole carbon source (8). The first hydroxylation step of the PAH ring is crucial to initiate an efficient biodegradation. This step is performed mainly by aerobic bacteria possessing a PAH-ring hydroxylating dioxygenase (PAH-RHD) system. Homologous PAH-RHD enzymes are encoded by specific genes present in both gram-positive (GP) and gram-negative (GN) bacterial species (22). Recently, we developed real-time PCR assays to quantify the functional genes encoding the catalytic ␣ subunit of the PAH-RHD (PAH-RHD ␣ ) enzyme. The quantifications were performed on soil DNA samples, giving important information about the PAH-degrading bacterial population present in various PAH-contaminated soils (7).High PAH degradation rates have been observed in laboratory experiments with strains or consortia isolated from PAHcontaminated soils (6, 33). However, in situ degradation is often a slower process due to environmental constraints and low availability of PAHs in aged, polluted ...

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