Accumulation of heavy metals in unicellular algae.

A model for metal uptake by microorganisms based on surface adsorption has been developed, and then applied to the uptake of cadmium by Chlorella vulgaris. A linear equilibrium relationship between metal in the solution and that adsorbed on the cell surface is assumed and confirmed at low cadmium concentrations by short-term uptake experiments. When it incorporates a description of cell growth, the model predicts an initial rapid uptake and a subsequent slow uptake. Such behavior has often been observed in experiments with growing microorganisms. This indicates that the slow uptake, sometimes thought to be active or metabolic, could be due to the simultaneous effects of growth and surface adsorption. The model shows that initial metal uptake is fast and approaches equilibrium within a few seconds. This prediction is in agreement with experimental results in a batch system: Equilibrium is reached before the first samples are taken (at 10 min) and there is then no measurable change until growth provides a significant increase in cell surface (after several hours). Thus the equilibrium constant can be calculated from experimental results of uptake at 10 min. The equilibrium is found to be affected by phosphate concentration; the amount of cadmium adsorbed on the cell decreases as the concentration of phosphate is increased. Long-term uptake experiments in growing cultures show a greater metal accumulation than predicted by the adsorption model, suggesting the involvement in the slow long-term uptake of some mechanism other than adsorption. This is confirmed by experiments in which uptake in cultures exposed to cadmium throughout the growth period is compared with short-term uptake in similar cultures grown in the absence of cadmium. The modeling approach to metal adsorption provides a basis for further development. A model combining description of adsorption and of intracellular accumulation is necessary to provide a more complete description. Such a model, with precise definitions of system parameters and means of evaluating these parameters from experimental results, will be a powerful tool in investigation of metal uptake by microorganisms.