Heavy Metal Accumulation by the Green Alga Chlorella emersonii

Silver and cadmium uptake and interaction with the green alga Chlorella emersonii were examined under batch culture, during continuous culture and under non- growing conditions. It is suggested that both metals react exclusively with the cell envelope of the organism. There was a direct relationship between metal removal, the concentration of metal present and the amount of Chlorella biomass present. Metal uptake is shown to occur whether cells are living or dead, precluding active transport and facilitative diffusion as accumulation mechanisms. Comparisons of silver and cadmium uptake by Chlorella emersonii were made with two bacteria; Micrococcus luteus (Gram positive), Pseudomonas aeruginosa (Gram negative) and a yeast Saccharomyces cerevisiae. General uptake by the organisms showed similar profiles. Competition studies with the two metals and others (copper and lead) suggest that the metals do not share a common binding site. Electrophoresis studies on isolated cell wall envelope, showed that it possessed an overall negative charge and could accumulate large amounts of metal. The cell wall was more negatively charged under higher pH conditions. Binding of metal made the cell wall less negatively charged. Bound metal to a maximum of 40 % could be recovered using a strong acid (HNO,). Chlorella biomass could be reused up to three times for further metal removal but efficiency of binding was reduced. In toxicological studies, silver appeared to be more toxic than cadmium. An explanation for this is given. The findings in this study can be explained by a simple ion-exchange model.