Cadmium and zinc bioavailability to Selenastrum capricornutum (Chlorophyceae):: accidental metal uptake and toxicity in the presence of citrate

Toxicities of cadmium (Cd) and zinc (Zn) to the green alga Selenastrum capricormutum Printz were determined over 72 h in defined synthetic media buffered by citrate (FRAQ(CIT); [citrate] = 100 mu M or 5 mu M) or nitrilotriacetate (FRAQ(NTA); [NTA] = 5 mu M) Algal sensitivity to free Cd2+ or free Zn2+ in FRAQ(CIT) tvas much higher than in FRAQ(NTA). In parallel experiments, short-term Intracellular uptake of radiolabeled Cd-109 was measured as a function of time (0-30 min) in FRAQ(CIT) and FRAQ(NTA); for a given free Cd2+ concentration (8, 250, or 610 nM), intracellular accumulation of Cd tvas consistently higher in FRAQ(CIT) than in FRAQ(NTA). Under the same conditions, the alga accumulated C-14-labeled citrate almost linearly over a 2-h period, Loss of Cd-109 from algal cells that had been prelabeled with the radionuclide occurred slowly, and the loss rate tvas insensitive to the presence or absence of citrate, indicating that the overall permeability of the algal membrane to Cd was unaffected by citrate. The enhanced bioavailability of Cd in the presence of citrate could be explained by membrane transport of a charged Cd-citrate complex, presumably by accidental transport.