Pseudometallophytes colonising Pb/Zn mine tailings: a description of the plant-microorganism-rhizosphere soil system and isolation of metal-tolerant bacteria.

The plant-microorganism-soil system of three pseudometallophytes (Betula celtiberica, Cytisus scoparius and Festuca rubra) growing in a Pb/Zn mine was characterised. Plant metal accumulation, soil metal fractions (rhizosphere and non-vegetated) and bacterial densities were determined. Total Cd, Pb and Zn in non-vegetated soils was up to 50, 3000 and 20,000 mg kg(-1) dry weight, respectively. The residual fraction dominated non-vegetated soils, whereas plant-available fractions became important in rhizosphere soils. All plant species effectively excluded metals from the shoot. F. rubra presented a shoot:root transport factor of ≤0.2 and this population could be useful in future phytostabilisation trials. Culturable bacterial densities and diversity were low (predominantly Actinobacteria). Rhizosphere soils hosted higher total and metal-tolerant bacterial densities. Seventy-four metal-tolerant rhizobacteria were isolated, and characterised genotypically (BOX-PCR, 16S rDNA) and phenotypically [Cd/Zn tolerance, biosurfactant production and plant growth promoting (PGP) traits]. Several isolates resisted high concentrations of Cd and Zn, and only a few presented PGP traits. Fourteen isolates were evaluated for promoting plant growth of two species (Salix viminalis and Festuca pratensis). Thirteen inoculants enhanced growth of F. pratensis, while only three enhanced growth of S. viminalis. Growth enhancement could not always be related to isolate PGP traits. In conclusion, some isolates show potential application in phytostabilisation or phytoextraction techniques.

Nickel solubilizing capacity and characterization of rhizobacteria isolated from hyperaccumulating and non-hyperaccumulating subspecies of Alyssum serpyllifolium.

Bacterial strains were isolated from the rhizosphere of three populations of the Ni-hyperaccumulator Alyssum serpyllifolium subsp. lusitanicum (A. pintodasilvae; M, S, and L), one population of Ni-hyperaccumulator A. serpyllifolium subsp. malacitanum (A. malacitanum; SB), and one population of the non-hyperaccumulator A. serpyllifolium subsp. serpyllifolium (A. serpyllifolium; SN). Isolates were characterized genotypically by BOX-PCR genomic DNA fingerprinting and comparative sequence analysis of partial 16S rRNA gene, and phenotypically by their Ni tolerance (0-10 mM), presence of plant growth promoting traits (indoleacetic acid (IAA)-, siderophore-, or organic acid-production, and phosphate solubilization) or capacity to produce biosurfactants. Among the collection of rhizobacteria, 84 strains were selected (according to their BOX-PCR profiles and phenotypic characteristics) to assess their ability to modify Ni extractability from Ni-rich (serpentine) soils. Metabolites produced by 13 of the isolates mobilized soil Ni (originating from the rhizosphere of both Ni-hyperaccumulators and non-hyperaccumulator). In contrast, Ni extraction using culture medium filtrates which had supported the growth of 29 strains was significantly reduced. The remaining strains had no effect on Ni mobility. Bacterial induced Ni mobilization was not related to Ni resistance or the phenotypic traits tested. Isolates with potential use in phytoremediation techniques will be further studied in a plant-microorganism-soil system.

Rhizosphere microbial densities and trace metal tolerance of the nickel hyperaccumulator Alyssum serpyllifolium subsp. lusitanicum.

In this study we determine culturable microbial densities (total heterotrophs, ammonifiers, amylolytics and cellulolytics) and bacterial resistance to Co, Cr, and Ni in bulk and rhizosphere soils of three populations of the Ni-hyperaccumulator Alyssum serpyllifolium subsp. lusitanicum and the excluder Dactylis glomerata from ultramafic sites (two populations in Northeast (NE) Portugal (Samil (S), Morais (M)) and one population in Northwest (NW) Spain (Melide (L)). The relationship between bioavailable metal concentrations (H2O-soluble) and microbial densities were analysed. Significant differences in microbial densities and metal-resistance were observed between the two species and their three populations. The hyperaccumulator showed higher microbial densities (except cellulolytics) and a greater rhizosphere effect, but this was only observed in S and M populations. These populations of A. serpyllifolium also showed selective enrichment of Ni-tolerant bacteria at the rhizosphere where Ni solubility was enhanced (densities of Ni-resistant bacteria were positively correlated with H2O-soluble Ni). These rhizobacteria could solubilise Ni in the soil and potentially improve phytoextraction strategies.