Effect of heavy metals on nitrate assimilation in the eukaryotic microalga Chlamydomonas reinhardtii

The effect of divalent metal cations (Cd2+, Cu2+, Zn2+, Fe2+, Co2+) on nitrate uptake and the effect of Cd2+ on nitrogen and sulphur assimilation enzymes were studied in the eukaryotic alga Chlamydomonas reinhardtii. Nitrate consumption was not inhibited by metal concentrations below 100 μM; however, concentrations exceeding 150 μM Cd2+, Cu2+ or Zn2+ induced 75 % inhibition, whereas Fe2+ or Co2+ did not significantly affect nitrate uptake. Among the enzymes of nitrogen and sulphur assimilatory pathways, 2-d exposure of C. reinhardtii cells to 100 μM Cd2+ did not affect ferredoxin-nitrite reductase (EC 1.7.7.1), ferredoxin-glutamate synthase (EC 1.4.7.1), NADH-glutamate synthase (EC 1.4.1.14) and O-acetyl-L-serine(thiol)lyase (EC 4.2.99.8) activities, but a 45 % inhibition of glutamine synthetase activity (EC 6.3.1.2) was observed. This inhibition increased with Cd2+ concentration and was reverted by adding Mn2+ to the reaction mixture, which can be interpreted as the result of a competitive inhibition of Mn2+ in glutamine synthetase by high cadmium concentration.