Propylene glycol degrading bacteria and related functional biomarkers in Gardermoen airport contaminated soil

Gardermoen airport (Oslo) was built on the largest rain fed unconfined aquifer in Norway. The area is a glacial contact formation with sand and gravels dominating near the ground surface and the soil is highly leached, with low biological and biochemical activities. Every winter large amount of propylene glycol (PG) is used as de-icing compound for the removal of snow and ice on airplanes. Although this contaminant is easily degraded by biotic or abiotic factors along the soil profile, it may still threaten groundwater when the system is overloaded. The aim of the present study was the characterization of the indigenous bacterial communities affected by the PG spreading and the development of suitable genetic biomarkers for monitoring the biodegradation process. The selected functional probes were qgdA gene for PG alcohol dehydrogenase (ADH-IIG) and par gene for phenylacetaldehyde reductase (PAR), which react specifically with secondary alcohols. Primers for qgdA and par were designed and tested on bacterial isolates enriched from soils at different depths and which were able to use PG as sole carbon and energy source. Molecular identification through 16S rRNA gene sequence analyses revealed that microorganisms mostly belonged to different species of the genera Pseudomonas, and Rhodococcus, although representatives of the genera Streptomyces, Nocardia, Serratia and Ancylobacter were also found. The functional genes retrieved in Gammaproteobacteria and in Actinobacteria were highly homologous respectively to qgdA gene for ADH-IIG of P. putida strain HK5 (Toyama et al., 2005) and to par genes for PAR of Rhodococcus sp. strain ST-10 (Wang et al., 1999). Transcriptional analysis performed on representative strains highlighted the expression of qgdA and par genes when strains were grown in the presence of PG, thus confirming that both were specifically functional during in PG degradation. Our data allow to state that Gardermoen soil is characterized by a rich and well-established reservoir of bacteria potentially able to degrade PG in situ. This bacterial community is characterized by specific PG-degradative pathways based on the qdgA and par genes. These genes will be validated as biomarkers to quantitatively track the active PG-degrading bacterial populations in soil.