Chemical Conditions in the Boundary Layer Surrounding Phytoplankton Cells Modify Cadmium Bioavailability

In this study we tested the hypothesis that metal uptake by unicellular algae may be affected by changes in metal speciation in the boundary layer surrounding the algal cells. The freshwater alga Chlamydomonas reinhardtii was preacclimated to different N nutrition regimes; changes in N nutrition are known to change the nature of extracellular metabolites (e.g., reactive oxygen species "ROS", and OH^-) and thus boundary layer chemical conditions. Specifically, at a constant bulk free Cd²⁺ concentration, Cd uptake by N-starved algae in cysteine-buffered solution was significantly higher than that in NTA-buffered solution. This enhancement was likely due to an increase of the free Cd²⁺ concentration in the boundary layer, resulting from localized cysteine oxidation by ROS released from these algae. On the other hand, Cd uptake was markedly lower when the free Cd²⁺ concentration near cell surface decreased as a result of an increase in the boundary layer pH of nitrate-acclimated algae or enhanced localized metal complexation. The results imply that redox, acid-base and metal complexation processes in the boundary layer differ from those in bulk water, even under chemically stable bulk conditions, and the boundary layer effect may well be of significance to phytoplankton acquisition of other trace metals.