Stimulation of pyrene mineralization in freshwater sediments by bacterial and plant bioaugmentation

As a means to study the fate of polycyclic aromatic hydrocarbons (PAHs) in freshwater sediments, pyrene mineralization was examined in microcosms spiked with [14C]pyrene. Some microcosms were planted with reeds (Phragmites australis) and/or inoculated with a pyrene-degrading strain, Mycobacterium sp. 6PY1. Mineralization rates recorded over a 61 d period showed that reeds promoted a significant enhancement of pyrene degradation, which possibly resulted from a root-mediated increase of oxygen diffusion into the sediment layer, as indicated by in situ redox measurements. In inoculated microcosms, mineralization reached a higher level in the absence (8.8%) than in the presence of plants (4.4%). Mineralization activity was accompanied by the release of water-soluble pyrene oxidation products, the most abundant of which was identified as 4,5-diphenanthroic acid. Pyrene was recovered from plant tissues, including stems and leaves, at concentrations ranging between 40 and 240 microg/g of dry mass. Plants also accumulated labeled oxidation products likely derived from microbial degradation. Pyrene-degrading strains were 35-70-fold more abundant in inoculated than in noninoculated microcosms. Most of the pyrene-degrading isolates selected from the indigenous microflora were identified as Mycobacterium austroafricanum strains. Taken together, the results of this study show that plants or PAH-degrading bacteria enhance pollutant removal, but their effects are not necessarily cumulative.