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(325) Microbial mechanisms affecting the bioavailability of polycyclic aromatic hydrocarbons in soils. Ortega-Calvo, Jose-Julio1, Garcia-Junco, Marta1, Niqui-Arroyo, Jose-Luis1, Olmedo, Elvira1, 1 ABSTRACT- The microbial communities of soils contaminated by PAH are often presented, in terms of bioavailability, as passive elements of bioremediation schemes. The biodegradation of these pollutants becomes a slow process when they are sequestered into soil colloids and nonaqueous-phase liquids (NAPLs), what constitutes a hindrance to bioremediation technologies for achieving regulatory end-points. In this context, two different bioavailability-promoting mechanisms have been studied with bacteria isolated from PAH-contaminated soils: microbial adhesion to the chemical and production of biosurfactants. Both mechanisms affected bioavailability during the biodegradation of phenanthrene by Pseudomonas aeruginosa 19SJ. Growth on solid phenanthrene exhibited an initial phase not limited by dissolution rate, and a subsequent, carbon-limited phase. During the carbon-limited phase, the concentration of rhamnolipids in solution exceeded their critical micelle concentration. The biosurfactants promoted significantly the dissolution of phenanthrene, and increased the affinity of the bacterium for the water-dissolved compound. Curves of phenanthrene mineralization by this and other soil isolates belonging to Sphingomonas and Mycobacterium genera were S-shaped when the compound was present in NAPLs. The observed mineralization was attributed to bacterial populations growing at the NAPL-water interface, mineralizing the compound at higher rates than predicted by abiotic partitioning. The biodegradation of anthracene sorbed to soil humic acids was studied with the strain Mycobacterium sp. 501T, which exhibited a considerable adherence to humic acids. The bacterium was more efficient in mineralizing the substrate when it was sorbed than in the controls with all the substrate in solution. The explanation is a shift to maintenance metabolism due to anthracene sorption, which reduced extremely the concentration of substrate in solution Key words: bioavailability, PAH, adhesion, biosurfactant |
