UPTAKE OF LIPOPHILIC CADMIUM COMPLEXES BY THREE GREEN ALGAE: INFLUENCE OF HUMIC ACID AND ITS pH DEPENDENCE1

Cadmium forms neutral, lipophilic CdL₂⁰ complexes with diethyldithiocarbamate (L = DDC) and with ethylxanthate (L = XANT). In a synthetic solution and in the absence of natural dissolved organic matter (DOM), for a given total Cd concentration, uptake of these complexes by unicellular algae is much faster than the uptake of the free Cd²⁺ cation. The objective of the present study was to determine how this enhanced uptake of the lipophilic CdL₂⁰ complexes was affected by the presence of natural DOM (Suwannee River humic acid, SRHA). Experiments were performed with Cd(DDC)₂⁰ and Cd(XANT)₂⁰ at two pH values (7.0 and 5.5) and with the three chlorophytes Chlamydomonas reinhardtii P. A. Dang., Pseudokirchneriella subcapitata (Korshikov) Hindák, Chlorella fusca var. vacuolata Shihira et R. W. Krauss]. Short‐term uptake (30–40 min) of the CdL₂⁰ complexes was followed in the absence and presence of SRHA (6.5 mg C · L^?1). Acidification from pH 7.0 to 5.5 decreased CdL₂⁰ uptake by the three algae, in the presence or absence of humic acid (HA). The dominant effect of the HA was to decrease Cd uptake, due to its interaction with the CdL₂⁰ complexes in solution. However, if uptake of the free CdL₂⁰ complexes was compared in the presence and absence of HA, in four of eight cases initial uptake rate constants (k_i) were significantly higher (P < 0.05) in the presence of the HA, suggesting the operation of an interfacial effect of the HA at the algal cell membrane, favoring uptake of CdL₂⁰. Overall, the experimental results suggest that neutral metal complexes will be less bioavailable in natural waters than they are in synthetic laboratory media in the absence of natural DOM.