Metal complexation has long been implicated as a dominant control on the bioavailability of trace metals to aquatic organisms, and metal uptake is best predicted by the free metal ion concentration in the exposure medium. However, low-molecular-weight (LMW) weak ligands such as cysteine have been shown to enhance metal uptake in the presence of strong ligands by marine phytoplankton, but the effect is not observed in freshwater. We hypothesize that these contrasting results are due to the large differences in ambient Ca concentrations and metal-ligand exchange kinetics between marine and freshwaters; specifically, free metal ions newly liberated from LMW-metal complexes in the boundary layer surrounding algal cells (phycosphere) cannot be immediately sequestered in seawater by the metal buffer (which is largely bound to Ca or Mg), but can be rapidly complexed in fresh waters where free or protonated forms of the metal buffer are more available. In the present work, short-term Cd uptake experiments at different Ca concentrations with a model freshwater alga (Chlamydomonas reinhardtii) and a marine alga (Thalassiosira weissflogii) consistently support our proposed hypothesis. At constant concentrations of bulk Cd²⁺ buffered by a strong ligand: 1) in artificial freshwater with addition of seawater levels of Ca, Cd uptake by high-Ca pre-acclimated C. reinhardtii cultured with cysteine as the N source (to enhance local Cd²⁺ liberation via cysteine degradation) in the presence of cysteine was 2.2-fold higher than that in the absence of cysteine; 2) in the artificial freshwater but at a low Ca concentration, the enhancement was not observed in cysteine-acclimated C. reinhardtii; 3) at seawater levels of Ca, the enhancement disappeared as well when the high-Ca pre-acclimated C. reinhardtii were cultured with ammonium as the N source (to inhibit cysteine degradation and local Cd²⁺ liberation); and 4) in artificial seawater, when the total concentration of free/protonated strong ligand was higher than that of the Cd-cysteine complexes, the Cd uptake rate by T. weissflogii that had been cultured with cysteine did not change after addition of cysteine. These results suggest that the phycosphere effect could potentially increase metal toxicity or improve nutrition of essential trace metals such as Fe, Zn and Cu, neither of which can be predicted solely on the basis of the chemistry of the ambient bulk water.