Effect of hyperaccumulator plants and associated rhizobacteria onthe efficiency of nickel extraction

Many works have attempted to relate the association of different plants on the efficiency of inorganic pollutants extraction, with the hypothesis that these multi-species covers promoted the development and the activity of rhizosphere microorganisms, such as PGPR. Up to now, the focus has been on crop associations (maize, tabacco, Brassica species). Only few studies have concerned the effect of the combination of metal hyperaccumulator plants with other species non-hyperaccumulators. These experiments showed that co-cropping with nonhyperaccumulator plants could enhance the growth of the hyperaccumulator and metal accumulation. The objective of this work was to study the effect of species richness in vegetation cover (mono- vs co-cropping), which only consisted of four hyperaccumulator plant species (Brassicaceae), on the efficiency of Nickel (Ni) extraction from an ultramafic soil containing significant nickel concentrations (Ni = 1480 mg.kg-1). The effects on some soil physicochemical properties and on microbial communities colonizing the rhizosphere were also evaluated. An experiment was set up with four hyperaccumulator species (Leptoplax emarginata, Noccaea tymphaea, Alyssum murale and Bornmuellera tymphaea). Six treatments had been realised (one mixed cover, four monospecies covers and unplanted soil) with 7 replicates for each. After four months of culture in controlled conditions, the estimation of plant biomass and Ni concentrations in shoots and roots were evaluated. In the meanwhile, microbial biomass carbon, size of cultivable rhizosphere bacterial community (UFC), as well as the potential production of auxin compounds (AIA), were evaluated for each treatment. Bacterial communities were also characterized by genetic (SSCP) and metabolic (Biolog Ecoplate™) structures. Moreover, different microbial enzymes (ACCd - 1-Aminocyclopropane-1-carboxylate deaminase, urease, acid phosphatase, s-glucosidase, arylsulphatase and FDA-fluorescein diacetate hydrolysis) were measured in rhizosphere soil samples. The presence of a cover, whether single or multi-species, caused a reduction in the concentration of extractable Ni from the soil. In our case, this effect was most pronounced in the presence of mesocosms planted with B. tymphaea and N. tymphaea. Similarly, the Ni bioconcentration factor showed a good correlation with shoot biomass and Ni concentration in shoot, especially for B. tymphaea and N. tymphaea and in a less extend for cocropping species. Moreover, the treatment with N. tymphaea showed the lowest pH value – which favours Ni solubility in soil. A strong correlation between pH, microbial enzyme activities and the size of the bacterial community was also observed. No significant change in enzyme activities was observed with covers, except in the case of arylsulfatase. The characterization of bacterial communities from soil samples by genetic (SSCP) and metabolic (Biolog Ecoplate™) structures revealed differences between mesocosms. The co-cropping of the four hyperaccumulator species did not significantly improve the process of phytoextraction. However, the biomass produced by B. tymphaea was in the same range as that of N. tymphaea and L. emarginata but B. tymphaea bioconcentration factor was the highest among all four species (i.e. more than 1% Ni in its dried shoots). Therefore, B. tymphaea, and to a lesser extent N. tymphaea, were the two species with the greatest potential of phytoextraction of Ni in co-cropping systems. A combination of different hyperaccumulator plants appears promising in phytoremediation practices, but further research is needed to unravel the links between aboveground hyperaccumulating plants, the belowground rhizosphere microbial communities in metaliferous soils and the implication of these microbial communities in the survival of plants and their ability to extract Ni. In particular, associations of plants should be tested in pairs, to define the plant cover providing the best phytoextraction.