Three hydrocarbon-degrading Rhodococcus strains isolated from polluted Antarctic soils proved to be closely related despite their different origins. Strains had a similar hydrocarbon degradation pattern and optimum growth temperature ranged between 25ºC and 30ºC, showing that strains are psychrotolerant but not psychrophiles. Specific growth rate on rich media ranged between 0.12 and 0.21 h−1, higher than those observed on hydrocarbons as carbon source. Results suggest that in Antarctic contaminated soils, closely related Rhodococcus strains are present and could play an important role in decontamination. Microcosm systems showed that, although the natural microflora respond significantly to the pollutants, bioaugmentation with Rhodococcus strain (ADH), improved biodegradation either alone or mixed with a hydrocarbon-degrading Acinetobacter strain. In comparison with microcosm where only ADH was inoculated, a non-significant decrease in hydrocarbon concentration was observed when ADH was inoculated as mixed culture with a previously tested strain. Pollutants dramatically reduced bacterial groups in soils resulting in a dominance of Pseudomonas. Microcosms showed that when natural microflora has no previous history of exposure to the pollutants, bioaugmentation with autochthonous strains improves degradation of the contaminants. The positive response of the native bacteria to the pollutants leaves the question open as to whether bioaugmentation is necessary when soils have a long previous exposure to hydrocarbons.
Alfalfa (Medicago sativa L.) and other plants bearing an important root system have been shown to be effective in the removal of organic compounds, including polycyclic aromatic hydrocarbons (PAHs). Phenanthrene is one of the main contaminants arising from the petrochemical industry and is included in the USEPA's list of priority toxic pollutants. Hydroponic cultures of alfalfa were employed as a model system to evaluate their capability of removing phenanthrene and to study the plantpollutant interaction without the interference of a soil matrix. The removal of phenanthrene was followed over a period of 30 days. The half-life of phenanthrene in hydroponics (initial concentration 50 mg L -1 ) was reduced 2.7 times when plants were present. Growth index, chlorophyll content of leaves, and peroxidase activity of the roots of plants exposed to phenanthrene were lower than the corresponding values of nonexposed plants. Phenanthrene produced an acute negative effect on the total bacterial counts but also caused an increase in degraders/total bacteria ratio. The Ames Salmonella plate incorporation assay was employed to screen for potential genotoxic metabolites, which could be generated by metabolic activation of the parent compound. None of the samples exhibited a positive response. While lack of a positive response to this test is not a definitive evidence of the absence of genotoxic substances, these results suggest that the plant-assisted removal of phenanthrene merits further investigation.
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