Bacterial rhizosphere populations of black popular and herbal plants to be used for phytoremediation of diesel fuel

Phytoremediation -an environmentally sound method to clean up moderate and superficial site contamination- is based on the stimulation of rhizosphere microorganisms by plant roots. Tolerance of plants to the contamination is a basic criterion for successful phytoremediation. Degradation may be accelerated by inoculation with appropriate microorganisms that are able to break down pollutants and to compete with indigenous microorganisms. An outdoor pot experiment, in which 16 black poplar clones and four herbal plant varieties were cultivated in soil contaminated with diesel fuel, was performed in order to identify plants with high tolerance to the pollution. The effect of plant variety and diesel contamination an rhizosphere bacterial communities was tested using a 16S rRNA-based cultivation-independent approach to characterize the rhizoflora. To obtain potential inoculant strains, diesel-degrading microbes were isolated from the rhizosphere and analyzed for their ability to utilize n-alkanes as carbon sources, the presence of genes encoding alkane hydroxylase (alkB) and their abundance in the rhizosphere. All plant varieties were strongly affected by the diesel contamination, however, black poplar clones exhibited higher sensitivities than herbal plants. One black poplar clone (Brandaris) showed significantly higher tolerance to diesel fuel contamination, whereas the growth of the remaining clones were severely inhibited. Analysis of rhizosphere communities indicated that the diesel contamination had a more pronounced effect than the plant variety. Fourteen diesel degrading strains were isolated from the rhizosphere and identified by 16S rRNA gene sequencing. Taking into account the presence of alk B genes, the ability to utilize alkanes as well as the abundance in the rhizosphere, three promising plant/microbe combinations were identified for Future phytoremediation applications.