Influence of a low molecular weight metabolite (citrate) on the toxicity of cadmium and zinc to the unicellular green alga Selenastrum Capricornutum: An exception to the free-ion model

Toxicities of Cd and Zn to the chlorophyte Selenastrum capricornutum were determined in synthetic media buffered by 100 μmol•l−1 of NTA (FRAQNTA or citrate (FRAQCIT), or in unbuffered FRAQUIL medium (FRAQ). Metal speciation in these exposure media was adjusted so as to maintain constant free concentrations of Ca2+, Mg2+ and Mez+ even while the free Zn2+ or Cd2+ concentrations were varied. The growth response of the algae to variations in free Zn2+ and free Cd2+ was similar in FRAQ and FRAQNTA media, but algal sensitivity to free Zn2+ or free Cd2+ increased markedly in FRAQCIT medium. Addition of Zn to FRAQ, FRAQNTA or FRAQCIT media (0.02–2 μmol•l−1 free Zn2+) caused a decrease in the algal growth rate, but the algae remained in exponential growth just as long as in the control (72 h). Similar results were obtained for exposures to Cd in FRAQ or FRAQNTA media (0.01–0.9 μmol•1−1) free Cd2+), but additions of Cd to FRAQCIT yielded anomalous growth curves (heightened sensitivity to Cd2+ at low Cd levels; shortened period of exponential growth at high Cd levels). Short-term (6 min) intracellular uptake of radiolabelled 109Cd was measured in the three media containing the same free Cd2+ concentration (0.25 μmol•l−1). Uptake of cadmium was more than 2-fold higher in FRAQCIT than in the other two media. Under the same conditions, the algae could be shown to accumulate citrate; the measured uptake rate (83 pmol citrate•m−2•s−1) was about 4 times greater than that of Cd in the presence of citrate (21 pmol Cd•m−2•s−1)—the citrate transporter would only have to be fooled once every four transport events to account for the enhanced cadmium uptake in the presence of citrate. Our results clearly show that the bioavailability of Cd and Zn diverges from the predictions of the Free-Ion Model in the presence of a low molecular weight metabolite such as citrate—accidental or piggy-back uptake of the metal-ligand complex across the biological membrane is a plausible explanation for this enhanced availability.