Natural colloids (NC) play an utmost role in aquatic ecosystems by binding trace metals and influencing their speciation, bioavailability and detrimental effects. NCs are considered to protect the aquatic microorganisms from toxic metal stress by decreasing free metal ion concentration and thus decrease their bioavailability. Nonetheless, a fundamental understanding of the relationship between trace metal speciation, bioavailability and effects is still lacking for many environmental systems although the progress made in the recent years. Moreover, there is currently no consensus as to whether the existing discrepancies regarding the role of the colloids in metal bioavailability are due to difficulties in determining complexation in solution, the heterogeneity and polyfunctionality of the colloids or to other direct effects that could affect trace metal bioavailability.

The main objective of the present study is therefore to improve our mechanistic understanding on the role of natural colloids on the trace metal bioavailability to the aquatic phytoplankton with emphasis of the direct interaction between colloids and unicellular microorganisms. More specifically, the interaction of the freshwater microalgae Chlorella kesslerii and humic and extracellular polymeric substances, representing major classes of the colloidal organic matter in freshwater systems, was studied. The obtained results clearly demonstrated that NCs, adsorbed on the cell surface and equilibrium between the medium and algal surface was attained within less than 2 min. The adsorption of the NCs to algal cells was strongly affected by the nature, size and concentration of the NC, as well as by the medium pH and algal cell density. Measurements of the algal electrophoretic mobility revealed that the NC adsorbed on algal surface increased the overall negative charge and added supplementary metal binding sites on the cell surface. The consequences of the NCs adsorption on the algal surface for the bioavailability of priority pollutants, such as Cd(II), Cu(II) and Pb(II) were further addressed. The results demonstrated that Cd and Cu bioavailability in the presence of NC was consistent with that predicted from measured free metal concentrations, while Pb cellular concentrations were higher. This enhancement in the Pb cellular concentrations was related to the existence of additional metal binding sites on the cell surface, rather simple electrostatic consideration. The environmental implications of the results are discussed with respect to the development of site-specific water quality criteria.