Uptake of neutral metal complexes by a green alga: Influence of pH and humic substances

We have examined the influence of pH and a natural humic acid on the short-term uptake (< 40 min) of a neutral, lipophilic metal complex by a unicellular freshwater alga, Pseudokirchneriella subcapitata. Cadmium diethyldithiocarbamate ([Cd(DDC)(2)](0)) was used as a model lipophilic metal complex and Suwannee River Humic Acid (SRHA) was chosen as a representative aquatic humic acid (6.5 mgC L-1). Under the experimental conditions virtually all the Cd was expected to be present as the lipophilic complex ([Cd](T) = 0.38 nM; [DDC] 1 mu M; [Cd2+]< 10(-15) M; pH 7.0, 6.0, or 5.5). Uptake of [Cd(DDC)(2)](0) proved to be sensitive to pH changes. It was lower at pH 6.0 and 5.5 than at pH 7.0. To our knowledge, this is the first demonstration of reduced uptake of a lipophilic metal complex at low pH. The presence of SRHA also affected uptake, either by binding the lipophilic complex in solution and reducing its bioavailability (pH 7.0) or by increasing the permeability of the algal membrane (pH 5.5).