Ultramafic soils display high concentrations of nickel and a number of nutrient deficiencies. Nickel-hyperaccumulator plants, such as Alyssum morale, have evolved in these environments and developed specific metal homeostasis, showing concentrations of nickel (Ni) sometimes exceeding 1% in their aerial biomass. Rhizosphere bacterial communities associated with Ni-hyperaccumulator plants can differ from those of non-accumulating plants growing on the same site. Among the edaphic factors that could influence the phylogenetic structure of the bacterial communities, altitude and metal-bioavailability such as Ni in particular, could be significant. Our objectives were to understand the specific changes in the structure of the A. morale rhizosphere bacterial community that occurred across two gradients: elevation and Ni geochemistry, using a high-throughput sequencing technique (454-pyrosequencing). In this study, Chloroflexi was the major phylum present, with 53-77% of relative abundance. Moreover, we found that the higher the soil's chemically-available Ni contents, the higher was the relative abundance of Proteobacteria (particularly Alphaproteobacteria) and Actinobacteria. In contrast, the abundance of Chloroflexi decreased with increasing levels of available Ni. Our results demonstrate that the chemical availability of Ni in the studied soil drives the bacterial community diversity in the rhizosphere of A. murale, regardless of elevation gradient and other soil physicochemical parameters.Read less <