Medium composition affects the heavy metal tolerance of microalgae: a comparison

Tolerance of the three metals cadmium (Cd), copper (Cu), and zinc (Zn) by four microalgal species was investigated in three different culture media available in the literature together with a modified version in order to study the effect of growth media components in estimating the bioavailability of metals introduced into the medium. The free metal content of each medium was also determined using Visual MINTEQ version 3.1 to compare the bioassays. Four microalgal isolates were identified as Desmodesmus sp-I, Desmodesmus sp-II, Coelastrella sp., and Chlorella vulgaris. The present work demonstrated that the microalgal media components have a profound effect on the bioavailability of the metals in the media, so that the bioassay results may vary depending on the growth medium used in the experiments. Furthermore, the free metal contents in each media varied depending on the concentrations of metals added. The tolerance of microalgae evaluated as 50% effective concentration (EC50) of metals differed significantly (p < 0.05) depending on the growth medium used and also varied between the species of the same genus. Desmodesmus sp-I showed high sensitivity to Cd (EC50 0.220 ± 0.011 mg L−1) and Zn (EC50 0.464 ± 0.065 mg L−1), whereas Desmodesmus sp-II showed high sensitivity to Cu (EC50 0.098 ± 0.002 mg L−1) when grown in Test Medium 1 (TM1). The Chlorella vulgaris strain was found to be the most resistant microalga among the four isolates tested in this study. This study has significant implications for the risk assessment of these metals using algal bioassays.