Bacterial inoculants affecting nickel uptake by Alyssum murale from low, moderate and high Ni soils

Metal hyperaccumulator plants like Alyssum murale have a remarkable ability to hyperaccumulate Ni from soils containing mostly insoluble Ni. We have shown some rhizobacteria increase the phytoavailability of Ni in soils, thus enhancing Ni accumulation by A. murale. Nine bacterial strains, originally isolated from the rhizosphere of A. murale grown in serpentine Ni-rich soil, were examined for their ability to solubilize Ni in different soils and for their effect on Ni uptake into Alyssum. Microbacterium oxydans AY509223; Rhizobium galegae AY509213; Microbacterium oxydans AY509219; Clavibacter xyli AY509236; Acidovorax avenae AY512827; Microbacterium arabinogalactanolyticum AY509225; M. oxydans AY509222; M. arabinogalactanolyticum AY509226 and M. oxydans AY509221 were added to low, moderate and high Ni-contaminated soils. M. oxydans AY509223 significantly increased Ni extraction by 10mM Sr(NO₃)₂ from the high and medium soils and had no effect on Ni extraction from the low Ni soils. The other eight bacterial isolates significantly increased Ni extraction from all soils. There were no significant effects of bacterial inoculation on fresh and dry weight of A . murale shoots grown in the low and high Ni soils compared to an unamended control. M. oxydans AY509223 significantly increased Ni uptake of A. murale grown in the low, medium, and high soils by 36.1%, 39.3%, and 27.7%, respectively, compared with uninoculated seeds. M. oxydans AY509223 increased foliar Ni from the same soils from 82.9, 261.3 and 2829.3 mg kg^-1 to 129.7, 430.7, and 3914.3 mg kg ^-1, respectively, compared with uninoculated controls. These results show that bacteria are important for Ni hyperaccumulation and could potentially be developed as an inoculum for enhancing uptake during commercial phytoremediation or phytomining of Ni.