Potential of the rhizobacterium Ensifer meliloti for the phytoremediation of mixed PAH–heavy metal contamination

The potential of the rhizobial strain Ensifer meliloti P221 as a bacterium capable of degrading phenanthrene in the presence of a heavy metal (nickel) and exhibiting plant-growth promoting activity toward Sorghum bicolor under mixed contamination condition was studied. It was shown that nickel at a concentration of 0.1 mM inhibited the microbial growth and phenanthrene degradation by 43 and 34%, respectively. Analysis of the enzymes involved in the phenanthrene degradation by the rhizobium revealed the activity of the phenanthrenequinone reductase (PQR) and protocatechuate 2,3-dioxigenase (2,3-PCD). It was found that nickel at concentrations up to 0.2 mM did not affect significantly the PQR activity, but inhibited it by 30% when the metal concentration increased to 5 mM. Nickel at concentrations up to 5.0 mM did not affect significantly on the activity of 2,3-PCD. E. meliloti promoted elongation of sorghum seedling roots by 69% in medium co-contaminated with phenanthrene and nickel. The results obtained assume that the rhizobial strain studied can exhibit its PAH-degrading and plant-growth promoting activities in the environment co-contaminated with nickel and phenanthrene. This microorganism may be promising for bioremediation and phytoremediation of mixed PAH–heavy metal contamination.