Toxicities of cadmium (Cd) and zinc (Zn) to the green alga Selenastrum capricornutum Printz were determined over 72 h in defined synthetic media buffered by citrate (FRAQCIT ; [citrate] = 100 μM or 5 μM) or nitrilotriacetate (FRAQNTA ; [NTA] = 5 μM). Algal sensitivity to free Cd2 + or free Zn2 + in FRAQCIT was much higher than in FRAQNTA . In parallel experiments, short-term intracellular uptake of radiolabeled 109 Cd was measured as a function of time (0-30 min) in FRAQCIT and FRAQNTA ; for a given free Cd2 + concentration (8, 250, or 610 nM), intracellular accumulation of Cd was consistently higher in FRAQCIT than in FRAQNTA . Under the same conditions, the alga accumulated 14 C-labeled citrate almost linearly over a 2-h period. Loss of 109 Cd from algal cells that had been prelabeled with the radionuclide occurred slowly, and the loss rate was 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.