The effect of pH on the uptake of zinc and cadmium in marine phytoplankton: Possible role of weak complexes

In natural samples from the New Jersey coast and the Gulf of Alaska, zinc (Zn) and cadmium (Cd) uptake rates by phytoplankton decreased on average about 30% as pH was decreased from 8.5 to 7.9 or 7.7, and the partial pressure of carbon dioxide (PCO₂) increased accordingly. The underlying mechanism was explored with the model species, Thalassiosira weissflogii and Emiliania huxleyi, using ethylenediaminetetraacetic acid (EDTA), desferrioxamine B, phytochelatin, and cysteine as complexing agents. Experiments with single complexing agents did not reproduce the effect of pH seen in field samples, ruling out two possible mechanisms: a direct effect on the uptake machinery or down-regulation of uptake at high PCO₂. Zn and Cd bioavailability must thus somehow decrease at low pH in natural seawater, which is counterintuitive since the protonation of complexing agents at low pH should increase the total free concentration of metals. However, in the presence of both a strong and a weak complexing agent, metal uptake rate may decrease at low pH if formation of the weak complex decreases and the metal in the weak complex is more "available" than in the strong complex. We obtained proof of concept for such a two-ligand mechanism for Zn uptake in the presence of EDTA + phytochelatin and EDTA + cysteine. Weak ligands that bind a small fraction of essential metals in surface seawater may thus be important in metal uptake by phytoplankton, and the dual effects of strong and weak complexing agents may control not just the magnitude but also the sign of the effect of pH-PCO₂ on metal uptake rates.