Soils contaminated with mineral oil hydrocarbons are often cleaned in off-site bioremediation systems. In order to find out which bacteria are active during the degradation phase in such systems, the diversity of the active microflora in a degrading soil remediation system was investigated by small-subunit (SSU) rRNA analysis. Two sequential RNA extracts from one soil sample were generated by a procedure incorporating bead beating. Both extracts were analysed separately by generating individual SSU rDNA clone libraries from cDNA of the two extracts. The sequencing results showed moderate diversity. The two clone libraries were dominated by Gammaproteobacteria, especially Pseudomonas spp. Alphaproteobacteria and Betaproteobacteria were two other large groups in the clone libraries. Actinobacteria, Firmicutes, Bacteroidetes and Epsilonproteobacteria were detected in lower numbers. The obtained sequences were predominantly related to genera for which cultivated representatives have been described, but were often clustered together in the phylogenetic tree, and the sequences that were most similar were originally obtained from soils and not from pure cultures. Most of the dominant genera in the clone libraries, e.g. Pseudomonas, Acinetobacter, Sphingomonas, Acidovorax and Thiobacillus, had already been detected in (mineral oil hydrocarbon) contaminated environmental samples. The occurrence of the genera Zymomonas and Rhodoferax was novel in mineral oil hydrocarbon-contaminated soil.
INTRODUCTIONThe contamination of soils with mineral oil hydrocarbons is a widespread environmental problem. Strictly speaking, mineral oil hydrocarbons are not xenobiotic, but their largescale use and various applications lead in many cases to environmental contamination (Gallego et al., 2001). Such contamination may be a consequence of petroleum transport, storage and refining, or accidents (Juteau et al., 2003). Soil is an environmental compartment which is renewable only within long time periods, or even not renewable at all. Therefore, it is important to protect soil from such contamination or to clean contaminated soils. After treatment, the soil should be usable, at least for applications with a low purity requirement, such as road construction or the recultivation of industrial areas.One possibility for cleaning such soils is bioremediation (Juteau et al., 2003), which aims at the biological mineralization of organic compounds to CO 2 and water, or at least at transformation to less-toxic or innocuous forms (Plaza et al., 2003). The success of biodegradation depends on the predominant environmental conditions, on the chemical structure of the pollutants, on the bioavailability of the contaminating compounds, and thus on the interaction between pollutant, soil matrix and microorganisms (Del'Arco & de França, 2001;Volkering et al., 1998). Another problem of contaminated sites is the wide variety of mineral oil hydrocarbons (Li et al., 2004).There are many known consortia of micro-organisms which are able to degrade mineral oil hydro...
