Betaproteobacteria dominance and diversity shifts in the bacterial community of a PAH-contaminated soil exposed to phenanthrene.

In this study, the PAH-degrading bacteria of a constructed wetland collecting road runoff has been studied through DNA stable isotope probing. Microcosms were spiked with (13)C-phenanthrene at 34 or 337 ppm, and bacterial diversity was monitored over a 14-day period. At 337 ppm, PAH degraders became dominated after 5 days by Betaproteobacteria, including novel Acidovorax, Rhodoferax and Hydrogenophaga members, and unknown bacteria related to Rhodocyclaceae. The prevalence of Betaproteobacteria was further demonstrated by phylum-specific quantitative PCR, and was correlated with a burst of phenanthrene mineralization. Striking shifts in the population of degraders were observed after most of the phenanthrene had been removed. Soil exposed to 34 ppm phenanthrene showed a similar population of degraders, albeit only after 14 days. Results demonstrate that specific Betaproteobacteria are involved in the main response to soil PAH contamination, and illustrate the potential of SIP approaches to investigate PAH biodegradation in soil.

Assessment of contaminated sediments with an indoor freshwater/sediment microcosm assay.

This study was conducted to assess the feasibility of using a 2-L, indoor microcosm assay to evaluate five contaminated sediments (A, B, C, D, and E). Toxic potential was deduced in the light of general contamination of sediments, pollutant partitioning in microcosms, and biological responses of species (Pseudokirchneriella subcapitata, Lemna minor, Daphnia magna, Hyalella azteca, Chironomus riparius): E > A > B > C > D. Sediments mainly were contaminated by metals (lead and zinc). Organic pollutant contents varied among the sediments. The major polycyclic aromatic hydrocarbon (PAH) compounds were pyrene, fluoranthene, and phenanthrene. Sediments A, B, and C highly stimulated duckweed growth (> 700%) and impaired daphnid (< 20%) and amphipod survival (< 30%). Sediment D had no significant effect on pelagic and benthic organisms. Finally, sediment E, the most toxic, limited duckweed growth (inhibition of 82%) and impaired daphnid survival (0% of survival). Amphipods were impaired dramatically by this sediment (0% of survival), in contrast with chironomids, for which no toxic effect was measured. The 2-L, indoor microcosm assay successfully was applied to the assessment of those five contaminated sediments. Sediments A, B, C, and E should not be deposited in gravel quarries, and new, more sensitive endpoint measurements should be developed.

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.