Toxicity of inorganic cadmium salts to the microalga Scenedesmus armatus (Chlorophyta) with respect to medium composition, pH and CO concentration

Batch cultures of algae grown at low (0.1 %) and elevated (2.0 %) concentrations of CO, 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(NO)·4HO, 3CdSO·8HO and CdCl·HO) 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 EC obtained for algae grown in original BBM and at low CO were 18.0, 16.44 and 15.37 mg·dm 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 EC 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 CO 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 CO 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 CO concentration only when algae are grown in original BBM. The growth of algae under high CO conditions was reduced considerably less (about 80 % of control) compared with low CO concentrations (about 50 % of control). A relationship was found between the toxicity of cadmium salts and final pH values only in variants of low-CO 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.