Zinc accumulation by Chlorella kesslerii (Chlorophycee) was studied for [Zn2+] ranging from 4 pM to 1 mM. A first-order uptake flux as predicted by the free ion activity model (FIAM) and the biotic ligand model (BLM) was not observed. Furthermore, when algae were preconditioned in slightly limiting (10 pM) versus optimal (1 nM) free zinc concentrations, the internalization flux increased and was nearly constant over the range of [Zn2+] examined. This observation was attributed to the synthesis of membrane-bound zinc transporters. Neither solution Zn chemistry nor surface-bound Zn was a good predictor of Zn uptake fluxes. Several hypotheses were examined to explain the failure of the steady-state uptake models. Although zinc excretion and a Zn diffusion limitation were observed, neither result could explain the majority of observed deviation from the models. Bioaccumulation experiments performed with inhibitors (vanadate and carbonyl cyanide m-chlorophenylhydrazone [CCCP]) demonstrated clearly that zinc transport is an energy-dependent process in Chlorella kesslerii. The presence of an active transport signifies that Zn uptake may function independently of the electrochemical Zn gradient and that, in some cases, both uptake fluxes and receptor-bound Zn may be independent of solution chemistry.