Accumulation of yttrium by Variovorax paradoxus

We screened oligotrophic microorganisms for those which were capable of reducing the concentration of yttrium (Y), a representative of the rate-earth elements, in culture medium. From 465 strains of oligotrophic microorganisms (grown on $\text{1}\text{100}$ diluted nutrient agar) isolated from soil and river water samples, 7 strains capable of reducing the concentration of Y in the diluted nutrient broth containing 5 ppm Y were selected. Three strains capable of reducing the concentration of Y to a great extent were identified as Variovorax paradoxus (strain Y-1) and Comamonas acidovorans (strains Y-2 and Y-3). Energy dispersive X-ray analyses revealed that V. paradoxus Y-1 incorporated Y into both the cell and excreted materials. The three strains tended to reduce the concentrations of mostly light rare-earth elements such as La, Ce, Pr and Nd, and intermediate elements such as Tb, Dy, Ho and Er to some extent, but did not reduce the concentrations of heavy elements such as Tm, Yb and Lu. Although V. paradoxus Y-1 could not reduce the concentration of trivalent metal ions such as Fe³⁺ and Cr³⁺ (5 ppm) which were added individually to the $\text{1}\text{100}$ diluted nutrient broth, when both Y and Fe³⁺ or Cr³⁺ were added to the broth, the concentration of Fe³⁺ or Cr³⁺ was reduced concomitantly with that of Y. In the case of divalent metal ions such as Mn²⁺, Cu²⁺ and Fe²⁺, such a phenomenon was not observed. Y induced the production of the extracellular materials by V. paradoxus Y-1, suggesting that Y might affect the physiological activity of this strain.