Can freshwater phytoplankton access cadmium bound to low-molecular-weight thiols?

Recent studies have suggested that low-molecular-weight (LMW) ligands can enhance trace metal uptake by marine diatoms. We have extended this work to freshwater phytoplankton and have investigated Cd bioavailability in the presence of LMW thiols, with two green algae ( Chlamydomonas reinhardtii and Pseudokirchneriella subcapitata) and a cyanobacterium ( Anabaena flos-aquae). Using nitrilotriacetic acid (NTA) to hold the free Cd²⁺ concentration constant, we progressively added cysteine to the exposure medium and determined short-term Cd uptake rates for each organism; the Cd uptake rates remained constant and were unaffected even when the concentrations of the Cd-cysteine complexes exceeded that of the free Cd²⁺. Similar results were obtained with algae pre-acclimated to different nitrogen sources or low zinc concentrations, even though such treatments affect their ability to utilize amino acids or take up Cd. A lack of effect was also observed for the two green algae with penicillamine, mercaptosuccinic acid, dithiothreitol and glutathione. These results suggest that the Cd-thiol complexes do not contribute to Cd uptake. However, when the experiments were repeated with cysteine in the absence of NTA, at the same Cd²⁺ concentration, a significantly higher Cd uptake rate was observed in the presence of the L- or D-cysteine stereoisomers than in the presence of NTA. This enhancement of Cd uptake in the presence of cysteine alone was sensitive to the nitrogen nutrition of these microorganisms, and we suggest that the enhancement likely results from biologically-induced chemical oxidation of cysteine in the boundary layer adjacent to the algal cell. [ABSTRACT FROM AUTHOR]