Binding of cadmium by cyanobacterial growth media: Free ion concentration as a toxicity index to the cyanobacteriumNostoc UAM 208

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Abstract
A quantitative study of cadmium binding to three different growth media for nitrogen-fixing cyanobacteria was done with the aid of a solid state ion-specific electrode. Kratz and Myers modified medium and Arnon medium bound large amounts of Cd2+, BG110 medium had less binding capacity. Of the media components, phosphate ion showed the greatest ability to bind Cd2+. Different pHs, the size of cell inoculum and two buffers (Tricine and HEPES, 25 mM) also changed the availability of free cadmium ion in solution. The effect of free Cd2+ ion towards the cyanobacteriumNostoc UAM 208, isolated from a heavy metal polluted environment, also was tested. The effective concentration affecting 50% of population (EC50), at 120 h of exposure, was less for nitrogenase activity (0.26 μg/mL) than for growth (0.55 μg/mL), suggesting that this enzyme activity is more sensitive to cadmium than growth. Furthermore, cadmium toxicity was influenced by the addition of buffers to the growth medium. In the presence of buffer, Tricine (25 mM), growth and nitrogenase activity was reduced by 50% at a total cadmium concentration of about 115 μg/mL, although no free ion was detected in this case. These results suggest that although generally cadmium toxicity is a function of free metal ion concentration, this can also vary in the presence of complexing agents.