Rhizobacterial communities associated with the flora of three serpentine outcrops of the Iberian Peninsula

Aim: Plant-associated bacteria can improve phytoextraction by increasing plant growth and/or metal uptake. This study aimed to characterise the culturable rhizobacterial community associated with two Ni-hyperaccumulators and to obtain a collection of isolates for application in Ni phytomining. Methods: Non-vegetated and rhizosphere soil samples were collected from the Ni-hyperaccumulator Alyssum serpyllifolium ssp. lusitanicum (three populations) and Alyssum serpyllifolium ssp. malacitanum (one population), as well as from non-hyperaccumulating plants (Dactylis glomerata, Santolina semidentata and Alyssum serpyllifolium ssp. serpyllifolium). Rhizobacteria were isolated and characterised genotypically (BOX-PCR, 16S rDNA sequencing) and phenotypically (Ni tolerance, plant growth promoting (PGP) traits, biosurfactant production). Results: Hyperaccumulating Alyssum subspecies hosted higher densities of bacteria compared to either non-hyperaccumulators or non-vegetated soil. In some cases hyperaccumulators showed selective enrichment of Ni-tolerant bacteria. Most bacterial strains belonged to the Actinobacteria phylum and presented Ni resistance. Phosphorus-solubilisers were mostly associated with the hyperaccumulators, siderophore-producers with D. glomerata, and IAA-producers with both these species. Conclusion: Taxonomic diversity and phenotypic characteristics were soil-, plant species- and plant population-specific. Moreover, differences were observed between the two Ni-hyperaccumulating subspecies and amongst plant populations. Several strains presented PGP characteristics which could be useful when selecting microorganisms for bioaugmentation trials.