A medium modified by 4-fold (Zn) or 2.5-fold (Mn) decreased/increased content of these metals did not influence the growth of the green microalga Desmodesmus armatus cultured for 24 h at ambient and elevated (2%) CO₂, as compared with original BBM medium (Zn²⁺ = 2 mg/L, Mn²⁺ = 0.4 mg/L). The 4-fold increased content of Zn in the medium markedly enhanced the toxicity of CdCl₂ applied at a concentration (16.8 mg/L) corresponding to EC_50/24h. The 2.5-fold increase of Mn content reduced the inhibitory effect of Cd on the oxygen evolution rate and F_V/F_M. At low CO₂, Cd did not affect the oxygen evolution but markedly stimulated the activity of carbonic anhydrase (CA). At elevated CO₂, the rate of oxygen evolution was reduced by Cd, whereas CA activity remained unaffected.