Nickel drives bacterial community diversity in the rhizosphere of the hyperaccumulator Alyssum murale

•First study using 454-pyrosequencing on bacterial diversity in Alyssum murale rhizosphere.•Nickel drives the bacterial community diversity, regardless of elevation gradient.•Chloroflexi was the major phylum present in A. murale rhizosphere.•The relative abundance of Proteobacteria and Actinobacteria was linked with available Ni contents.

Ultramafic soils display high concentrations of nickel and a number of nutrient deficiencies. Nickel-hyperaccumulator plants, such as Alyssum murale, 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. murale 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.