Pb uptake by the freshwater alga Chlorella kesslerii in the presence of dissolved organic matter of variable composition

Abstract

Environmental context. Dissolved organic matter (DOM) is of utmost importance for a toxic metal's fate and ecotoxicity in the aquatic system, but the complex nature and variable composition of DOM makes the quantitative understanding of DOM's role in the environment very difficult. We have demonstrated that the assumption that the properties of a DOM mixture are the sum of the properties of its individual fractions can capture the main trends characterising the role of DOM in lead speciation and adsorption by freshwater microalgae. This was done by mixing the isolated, well-characterised fractions of DOM and measuring levels of free lead ion and Pb adsorbed and internalised by algae.Abstract. Dissolved organic matter (DOM) is a complex mixture of ill-defined components, which makes the quantitative understanding of DOM functions in aquatic systems a challenging task. The traditional approach for studying such complex mixtures involves their separation into groups of different components, while assuming minimal or no alteration of their properties. By mixing the pre-isolated and well-characterised individual fractions of the DOM, including humic, fulvic and alginic acids, we have demonstrated that the free Pb ion concentrations and the adsorbed Pb plus Pb internalised by the alga Chlorella kesslerii in the presence of DOM samples of different compositions can be predicted on the basis of the experiments performed in the presence of the individual DOM fractions. An additivity model assuming that the properties of the mixture can be considered as the sum of the properties of the individual components captured the Pb speciation and adsorption behaviour in the presence of DOM of variable compositions. Similarly to the results with the individual DOM fractions, internalised Pb concentrations in the presence of the reconstituted DOM were greater than that predicted by the corresponding free lead ion concentration. An improved fit between experimental observations and the model predictions of adsorbed plus internalised Pb in the presence of DOM of different compositions was observed by assuming that each individual component adsorbed by the algae gave access to additional binding sites for Pb. Furthermore, the contribution of the Pb-DOM complex to total cellular Pb was dominated by the humic and fulvic acids, whereas the contribution of alginate was minimal.

Microalgal‐driven pH changes in the boundary layer lead to apparent increases in Pb internalization by a unicellular alga in the presence of citrate

“…The authors proposed a mechanism that involved the formation of ternary surface complexes {L‐M‐R} (L being the organic ligand, M the metal ion and R the biotic ligand at the cell membrane) followed by internalization of the metal only. This mechanism was also previously proposed by Aristilde et al () to explain the enhanced Zn internalization by marine phytoplankton in the presence of LMW organic ligands, and it seems to be an evolution of the ternary complex hypothesis proposed earlier by Lamelas and coworkers (Lamelas et al ; Lamelas and Slaveykova ) to explain Pb bioavailability in the presence of humic substances. However, ternary complexes are only supposed to contribute to metal bioavailability in cases where either the whole complex (L‐M) is internalized or the ternary complex contributes to metal toxicity exerted at the binding site in the membrane (in cases where toxicity is being used to evaluate metal bioavailability).…”

Effect of Humic Acid on Cd(II), Cu(II), and Pb(II) Uptake by Freshwater Algae: Kinetic and Cell Wall Speciation Considerations

“…Hence, the adsorbed humic substances might affect the properties of the algal surface by enhancing membrane permeability of both lipophilic metal complexes (24) and hydrophilic substances (25), by increasing the negative surface charge (6,11), and by adding the additional binding sites (10,11,26).…”

Cu and Pb accumulation by the marine diatom Thalassiosira weissflogii in the presence of humic acids

“…An enhanced Pb uptake by freshwater microalgae has been shown to occur in the presence of several DOM types, when compared with the same free Pb concentrations in the absence of DOM. [11,12] To explain these observations, the possibility of formation of a ternary complex [18] between the Pb, humic substance and algal surface, as well as changes in speciation of metals adsorbed to the cell wall [19] were proposed. An increase in Pb uptake and toxicity to marine invertebrates was observed in the presence of humic acids (HA), when compared with the same total dissolved Pb concentrations in the absence of HA.…”

“…However, such an increase was not observed for Cu either in freshwater [20] or in seawater. [21][22][23] The humic and fulvic acids used in the cited studies [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] include soil-derived commercially available humic substances and those extracted from freshwater sources. Soil-derived HA present higher hydrophobicity and other distinct properties compared with waterborne HA [24] and their effects on biological membranes have been shown to be higher than that of aquatic DOM.…”