Effect of heavy metals on nitrate assimilation in the eukaryotic microalgaChlamydomonas reinhardtii

The effect of divalent metal cations (Cd2+: Cu2+, Zn2+, Fe2+, Co2+) On nitr ate uptake and the effect of Cd2+ on nitrogen and sulphur assimilation enzy mes were studied In the eukaryotic alga Chlamydomonas reinhardtii. Nitrate consumption was not inhibited by metal concentrations below 100 muM; howeve r, concentrations exceeding 150 muM Cd2+, Cu2+ Or Zn2+ induced 75 % inhibit ion, whereas Fe2+ or Co2+ did not significantly affect nitrate uptake. Amon g the enzymes of nitrogen and sulphur assimilatory pathways, 2-d exposure o f C. reinhardtii cells to 100 CIM Cd2+ did not affect ferredoxin-nitrite re ductase (EC 1.7.7.1), ferredoxin-glutamate synthase (EC 1.4.7.1), NADH-glut amate 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 interpret ed as the result of a competitive inhibition of Mn2+ in glutamine synthetas e by high cadmium concentration. (C) 2001 Editions scientifiques et medical es Elsevier SAS.