Metal speciation dynamics and bioavailability: bulk depletion effects

Under conditions of bulk depletion, the speciation and bioavailability of trace metals must be considered at two different time scales: (i) the time scale of the biouptake flux, as determined by diffusion of the bioactive free metal, dissociation of the bioinactive complex species, and the internalization rate; and (ii) the time scale of depletion of the bulk medium. The implications of these two time scales for the speciation dynamics are discussed in terms of experimental conditions. The geometry of the system is taken into account via a spherical cellular model. It considers a spherical organism depleting a spherical volume in a nonstirred medium and assumes linear adsorption of the metal at the biointerface and first-order internalization kinetics. In cases where the rate of biouptake is fully controlled by the internalization step, concentration gradients in the medium are insignificant. Then the biouptake becomes independent of the geometry of the system, and the model has a much simpler solution. Examples of trace metal uptake by microorganisms are analyzed: (i) cobalt uptake by Prochlorococcus in the presence of NTA, under conditions where bulk depletion is the controlling process due to the large number of organisms and high internalization rates, (ii) silver uptake by Chlamydomonas reinhardtii with significant effects of bulk depletion, due to the high internalization rate; (iii) lead uptake by Chlorella vulgaris with pratically negligible bulk depletion due to the low internalization rate of the metal; and (iv) lead uptake by intestinal Caco-2 cells, illustrating the simplification of the bulk depletion model for a system with different geometry where internalization is the rate-controlling step