Rhizosphere bacterial populations of metallophyte plants in heavy metal-contaminated soils from mining areas in semiarid climate

Rhizosphere bacterial populations associated with four metallophyte plants in one of major polymetallic (Pb–Zn–Cu) semiarid Moroccan Hercynian province (Draâ Sfar, Marrakech, Morocco) presenting long-term contamination mainly with Zn and Pb were analysed and compared to selected control soils. In the highly Zn-, Cu-, Pb- and Cd- contaminated soils, the total number of culturable heterotrophic bacteria were found in low proportions (< 2.6 × 102 – 1.6 × 104 g−1soil). This bacterial content was slightly similar to that found in moderately polluted and controls soils (6.7 × 104 – 5.8 × 106). However, the bacterial diversity and the rhizosphere/soil ratio, which compares the bacterial content (or bacterial charge) around the metallophyte plants with that in non-rhizosphere soil, were the bacteriological parameters mostly affected by heavy metal contamination. The chronic Zinc-stress results in an increase of tolerance to this metal of both the rhizosphere and non-rhizosphere bacterial communities. However, in general, the rhizosphere bacterial populations exhibited less tolerance to Zn toxicity than the bacterial population of non-rhizosphere soils. This result suggests that toxic effects of Zn decrease in the rhizosphere soils of the metallophyte plants.