Predicting Cadmium Accumulation and Toxicity in a Green Alga in the Presence of Varying Essential Element Concentrations Using a Biotic Ligand Model

摘要

This study refines the Biotic Ligand Model (BLM) approach by integrating the modulating effects of various essential elements on cadmium (Cd) uptake kinetics in the freshwater alga Chlamydomonas reinhardtii. The algae were first acclimated to a low (LM) or high trace metal (HM) medium as well as to low or high free Cd~(2+) and CO~(2+) concentrations. The short-term Cd transport capacity and affinity were then quantified in exposure media in which essential trace metals and calcium concentrations were manipulated. The results show that after acclimation to the LM medium, exposure to high free Ca~(2+) decreases the capacity of the Cd transport system. Also, acclimation to high (10~(-9) M free CO~(2+)) or low (10~(-11)M free CO~(2+)) did not significantly affect Cd uptake rates. When all essential trace metals were simultaneously increased in the acclimation (and exposure) medium, the capacity of the transport system decreased by ～60%, a decrease close to that due to high [Zn~(2+)] alone, suggesting that Zn is the main trace metal modulator of the Cd transporter capacity. Changes in Cd toxicity (growth inhibition) in the presence of different essential trace metal concentrations were strongly related to the steady-state concentration of intracellular cadmium, regardless of the cell's nutritional state. Our BLM incorporating the physiological effects of Ca~(2+) and other trace metals predicts steady-state Cd accumulation in the presence of varying concentrations of essential elements at 7 nM free Cd~(2+), but predictions over a wide range of free [Cd~(2+)] proved to be more difficult.