Chromium uptake, retention and reduction in photosynthetic Euglena gracilis

Photosynthetic Euglena gracilis grown with different K2CrO4 concentrations was analyzed for its ability to take up, retain and reduce Cr(VI). For comparison, cells were also exposed to CrCl3. Cellular Cr(VI) uptake at pH 7.2 showed a hyperbolic saturation pattern with K (m) of 1.1 mM, V (m) of 16 nmol (h x 10(7) cells)(-1), and K (i sulfate) of 0.4 mM. Kinetic parameters for sulfate uptake were similar, K (m) = 0.83 mM, V (m) = 15.9 nmol (h x 10(7)cells)(-1) and K (i chromate) = 0.3 mM. The capacity to accumulate chromium depended on the ionic species, external concentration and pH of the incubation medium. Cr(VI) or Cr(III) accumulation was negligible in the acidic (pH 3.5) culture medium, in which Cr(VI) was abiotically reduced to Cr(III). At pH 7.2 Cr(VI) was fully stable and high accumulation (> 170 nmol/1 x 10(7) cells at 1 mM K2CrO4) was achieved; surprisingly, Cr(III) accumulation was also significant (> 35 nmol/1 x 10(7) cells at 1 mM CrCl3). Cr(VI) was reduced by cells at pH 7.2, suggesting the presence of an external reductive activity. Cr(VI) induced an increased cysteine and glutathione content, but not in phytochelatins suggesting that chromium accumulation was mediated by monothiol compounds.