Toxicity and bioaccumulation of cadmium in Chlorella pyrenoidosa

Chlorella pyrenoidosa cultures grown at pH 7 in the presence of 0, 0.25, 0.50, and 1.00 mg of Cd/liter had doubling times of 11, 21, 22, and 35 hours, respectively, whereas similarly exposed cultures grown at pH 8 had doubling times of 11, 16, 17, and 25 hours, respectively. C. pyrenoidosa is capable of concentrating cadmium, and the amount accumulated is directly proportional to the concentration of metal present initially and is dependent upon the pH of the medium. No accumulation occurs in the dark, at 4°C, or in dead cells. Cadmium accumulation is not affected by the concentration of calcium, magnesium, molybdenum, copper, zinc, or cobalt in the growth medium, whereas a level of manganese equal to 0.20 mg/liter completely blocks cadmium accumulation; iron may also play a role in regulating cadmium accumulation. Cells which had accumulated cadmium could still fix atmospheric CO 2, albeit at reduced rates; O 2 evolution was also inhibited, but to a lesser extent. The ability of C. pyrenoidosa to accumulate large concentrations of cadmium before showing adverse effects may be related to the presence of cadmium-sequestering agent(s) within the cell. The concentration of cadmium by C. pyrenoidosa could pose a hazard to the freshwater food chain.