Years of experimentation in well-defined media has demonstrated that trace metal bioavailability should be best predicted by the concentration (or activity) of the free cation (free ion activity model, FIAM). In the particular case of lead, uptake of Pb by Chlorella kessleri has been, however, shown to be higher in the presence of the Suwannee River fulvic acid and Suwannee River humic acid as compared to what was predicted by the free ion concentrations [Pb2+]. In order to estimate whether such exceptions of the FIAM can be extended to other freshwater phytoplankton, we measured the biouptake of Pb by C. reinhardtii another model phytoplankton, in solution containing these two model humic substances and for which free Pb2+ concentrations were evaluated using an ion-exchange technique. Our results showed that Pb internalization by C. reinhardtii can be directly related to the [Pb2+] but that the deviation from the FIAM when using C. kessleri was still observed under the present experimental conditions. Indeed, when using freshly isolated colloidal fraction (>3 kDa) of a marsh water and its photoaltered products, [Pb2+] measurements are shown to poorly predict Pb uptake by C. kessleri and surprisingly Pb uptake by C. reinhardtii.