The organic ligands naturally present in seawater, on the cell surface groups, and those released by the marine phytoplankton species, Thalassiosira weissflogii and Phaeodactylum tricornutum, and their physico-chemical interaction with copper ions were studied using differential pulse anodic stripping voltammetry (DPASV) as a function of pH, temperature, salinity and biomass. The acid–base properties were characterized from titration curves of diatom suspensions with proton. Three pKa values were determined for each diatom, all of which were similar. Titration curves with copper allowed us to determine the specific adsorption of copper in a heterogeneous adsorption model. An iterative method that combines both Scatchard and Van den Berg–Ruzic approaches was used in order to determine the complexing capacity and the binding constants. High-affinity surface groups of both algae have similar affinity for copper but the concentration of these groups is 45% per cell higher for P.tricornutum as compared to T. weissflogii. The low-affinity groups in T. weissflogii (9.37) have higher stability constants than those in the P.tricornutum (9.0). After 36 h equilibrium, a ligand concentration of 18.6×10−16 M cell−1P.tricornutum and 25.0×10−16 M cell−1T.weissflogii with a conditional stability constant of log K′=9.8 and log K′=10.0, respectively, was exuded into the original seawater. In the presence of lead, high specificity was observed for the lowest stability constant ligands for copper, while the highest stability constant ligands were affected more by the presence of lead in solution. The ligands of T.weissflogii were less affected.