Within the biotic ligand model (BLM) construct, major cations are considered to be simple competitors for metal binding to uptake sites and may offer some protection against metal-induced toxicity, but the influence of essential trace elements and cell preconditioning to different micronutrient concentrations on metal uptake and toxicity is considered negligible. To test these underlying assumptions, we monitored Cd uptake and toxicity in a green alga (Chlamydomonas reinhardtii) after long-term exposures (60 h) to a range of environmentally realistic free Zn(2+) , Co(2+) , Fe(3+) , Mn(2+) , Ca(2+) , and Cu(2+) concentrations buffered with nitrilotriacetic acid. A 200-fold increase in free [Mn(2+) ] as well as a 100-fold increase in free [Fe(3+) ] did not affect Cd uptake or toxicity, whereas a 50-fold increase in free [Ca(2+) ] effectively offered some protection, as predicted by the BLM. However, a 10-fold increase in free [Cu(2+) ] significantly enhanced Cd toxicity by a factor of approximately 2, whereas a 100-fold increase in free [Zn(2+) ] and [Co(2+) ] from 10(-11) to 10(-9) M significantly decreased Cd uptake and toxicity by more than twofold. These effects did not change with prior algal acclimation to different essential micronutrient concentrations. Low essential trace metal concentrations may strongly affect the uptake and toxicity of Cd in freshwater algae and should be taken into consideration in future developments of the BLM.
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