Degradative repression and co-metabolism, plant-microbe interactions affecting polycyclic aromatic hydrocarbon phytoremediation.

다국어 초록 (Multilingual Abstract)

Polycyclic aromatic hydrocarbons (PAH) are persistent organic pollutants that affect human health. Physical, chemical, and biological remediation is limited by contaminant hydrophobicity that promotes strong sorption to soil organic matter. Enhanced PAH degradation has been observed during phytoremediation, however, the processes responsible for increased degradation are not well understood. Hydrophobicity of PAHs prevents significant uptake/translocation within plants, and suggests that rhizodegradation (plant-microbe interactions in the root zone promoting degradation) is important for PAH phytoremediation.
Hybrid poplars were grown in hydrocarbon-contaminated soil and five passive methods of oxygen delivery were examined. When Oxygen Release Compound RTM (ORC) was placed in filters above hydrocarbon contaminated soil, a statistically significant increase of 145% was observed in poplar biomass growth, relative to controls. The ORC in filters also stimulated significant increases in root density. The positive response of poplars to oxygen amendments suggests the root zone may be an oxygen sink when soil contamination exerts a high biochemical oxygen demand, such as in former refinery sites.
Plant root exudates and extracts, model rhizosphere carbon sources, repressed the phenanthrene degrading activity of Pseudomonas putida ATCC 17484. Partial characterization of root extracts identified acetate, amino acids, and glucose indicating a complex mixture of substrates. Repression of enzymatic activity was also observed following exposure to root derived substrates including organic acids, glucose, and glutamate. This suggested that carbon source regulation (i.e. catabolite repression) was responsible for the observed repression of P. putida phenanthrene degrading activity by root exudates and extracts. Higher microbial concentrations in the rhizosphere could compensate for the observed repression of PAH catabolic genes and increase overall PAH degradation.
Co-metabolism of benzo[a]pyrene, a high molecular weight (HMW) PAH, by Sphingomonas yanoikuyae JAR02 was observed using plant root extracts as a primary substrate. These results provided evidence that co-metabolism of HMW PAH in the rhizosphere may be a feasible pathway for enhanced degradation. Mineralization of 14C labeled benzo[a]pyrene by S. yanoikuyae JAR02 (salicylate induced) yielded ∼4% 14CO 2 and ∼10% of a polar metabolite, tentatively identified as benzo[a]pyrene-8-hydroxy-7carboxylic acid using HPLC/MS.