Toxicity of inorganic cadmium salts to the microalga Scenedesmus armatus [Chlorophyta] with respect to medium composition, pH and CO2 concentration

Batch cultures of algae grown at low (0.1 %) and elevated (2.0 %) concentrations of CO2, as well as in original BBM (Bold Basal Medium) and BBM modified with phosphate, EDTA and a combination of both, were exposed to cadmium (Cd(NO3)2·4H2O, 3CdSO4·8H2O and CdCl2·H2O) for 24 h. Regardless of the salt applied, the concentration-dependent relationships of Cd toxicity were found to be biphasic, suggesting the different affinity of target sites to cadmium. Nominal values of EC50 obtained for algae grown in original BBM and at low CO2 were 18.0, 16.44 and 15.37 mg·dm−3 for cadmium nitrate, sulphate and chloride, respectively. However, it was estimated that 97 % of the free cadmium in the added salts were bound by components of original BBM such as EDTA, phosphates, chloride and sulphate. The effect of Cd-salts at concentrations corresponding to EC50 values on algae were tested in media with 10-fold reduced phosphates (BBM-P), BBM depleted of EDTA (BBM-EDTA) and of both phosphates and EDTA (BBM-P-EDTA). For algae grown at low CO2 and BBM-P, cadmium was about 25 % less toxic than those applied in original BBM. Cadmium greatly inhibited (about 85 % of the control) the growth of algae cultured in BBM-EDTA; this effect was only slightly dependent on the CO2 concentration. Deficits of both EDTA and P led to effects similar to those brought about by the absence of EDTA only. The toxicity of cadmium depends on CO2 concentration only when algae are grown in original BBM. The growth of algae under high CO2 conditions was reduced considerably less (about 80% of control) compared with low CO2 concentrations (about 50 % of control). A relationship was found between the toxicity of cadmium salts and final pH values only in variants of low-CO2 grown algae; with an increase of medium pH the toxicity decreased. The results suggest that both growth conditions and the binding ability of the medium markedly affect the toxicity of cadmium towards microalgae.