Uptake of Hydrophobic Metal Complexes by Three Freshwater Algae: Unexpected Influence of pH

Abstract

Cadmium forms neutral, lipophilic Cd(L)2(0) complexes with diethyldithiocarbamate (DDC) and with ethylxanthate (XANT). Uptake of these complexes bythree unicellularfreshwater green algae (Chlamydomonas reinhardtii, Chlorella fusca, and Pseudokirchneriella subcapitata) was determined at two pH values (7.0 and 5.5) and compared to uptake of the free, uncomplexed Cd2+ ion. Uptake of the lipophilic complexes over time, characterized by high initial uptake rates but tending toward a plateau after about 30 min, could be modeled successfully as the result of the following processes: first-order uptake from solution, partitioning of the accumulated Cd into two internal pools (labile and nonlabile), and first-order loss of Cd from the labile pool. At pH 7.0 initial uptake rates for both Cd(L)2(0) complexes were much higher than for Cd2+ alone (e.g., up to approximately 90 times higher for comparable dissolved Cd concentrations of approximately 0.4 nM). However, the initial uptake rates for the lipophilic complexes dropped dramatically when the pH was lowered from 7.0 to 5.5 (2- to 60-fold decrease, depending on the algal species and the nature of the neutral complex). Loss rates for the accumulated complexes also decreased atthe lower pH. The lipophilicity of the neutral complexes, as estimated from their octanol-water distribution coefficient (Dow), was not affected by the decrease in pH from 7.0 to 5.5. We thus conclude that the acidification of the external medium, i.e., the interaction of protons with the algal membrane, strongly affects algal membrane permeability.

Uptake and elimination kinetics of metals in soil invertebrates: A review

“…2e). A two-compartment kinetics model is then used for estimating kinetics parameters (e.g., Crossley et al, 1995;Vijver et al, 2005;Boullemant et al, 2009). The following equations are used for describing the twocompartment accumulation kinetics in the uptake phase:…”

When are metal complexes bioavailable?

“…The passive diffusion of metal complexes has been observed for complexes involving synthetic organic ligands, including 8-hydroxyquinoline (Ox), diethyldithiocarbamate (DDC) and ethylxanthate (XANT), [9][10][11] and for inorganic metal complexes such as HgCl 2 0 . [12] The passive diffusion of AgCl 0 remains questionable.…”

“…For example, whereas (CH 3 ) 2 their bioaccumulation by a marine alga (Thalassiosira weissflogii) is lower, presumably because elemental mercury and dimethylmercury do not readily form complexes with intracellular ligands. [12] Generally speaking, the observed uptake rates of lipophilic metal complexes are usually at least an order of magnitude higher than their hydrophilic metal counterparts, [10,11,16,17] which makes this mechanism potentially important for certain metals. Furthermore, uptake is sensitive to the pH of the medium, with a decreased bioaccumulation of the lipophilic metal complexes at low pH, potentially due to a reduction in the electrostatic repulsion between adjacent phospholipids, leading to their tighter packing and a decreased membrane permeability.…”

“…Furthermore, uptake is sensitive to the pH of the medium, with a decreased bioaccumulation of the lipophilic metal complexes at low pH, potentially due to a reduction in the electrostatic repulsion between adjacent phospholipids, leading to their tighter packing and a decreased membrane permeability. [11,18,19] With the exception of what is known about such hydrophobic metal species as the methylated forms of Hg, Sn, As and Sb [20][21][22] and the hydrophobic complexes described above, it is currently unclear to what extent lipophilic ligands are present in natural systems. Owing in large part to the analytical difficulties associated with measurements of (likely low concentrations of) hydrophobic complexes in aquatic systems, few data are available to estimate metal uptake fluxes for these species.…”

Importance de mieux connaître les mécanismes de transport des métaux pour la prédiction de l’accumulation et de la toxicité des métaux dissous chez le phytoplancton : récentes avancées et défis pour le développement du modèle du ligand biotique

“…En effet, quelques cas où le MLB ne pouvait prédire adéquatement la prise en charge et la toxicité des métaux chez les organismes aquatiques ont été retracés dans la littérature (CAMPBELL, 1995;CAMPBELL et al, 2002;SLAVEYKOVA et WILKINSON, 2005;ZHAO et al, 2016 (BOULLEMANT et al, 2009;FLORENCE et al, 1992;LAVOIE et al, 2012c;PHINNEY et BRULAND, 1994;STAUBER et FLORENCE, 1987 …”