Lamelas, Pinheiro and Slaveykova (2009) showed that humic acid (HA) enhances Pb(II) uptake by the freshwater alga Chlorella kesslerii roughly tenfold relative to uptake at the same free Pb²⁺ concentration in the presence of a simple ligand (citric acid), and proposed — on the basis of chemodynamic modeling and cell-wall speciation calculations only — that a ternary complex HA–Pb–R_s between the Pb–humic complex and the membrane internalization sites R_s, followed by its dissociation, loads the transport sites and drives the enhanced internalization flux. Direct experimental identification of this interfacial ternary species and of its formation/dissociation kinetics remains unresolved; the in-situ stability constants of complexes inside the cell-wall layer (including whether HA adsorption modifies them and the role of electrostatics) and the generality of the mechanism beyond one alga, one HA sample and fixed water chemistry were left as acknowledged simplifications of that study's model. Later work (2010–2025) has reproduced and generalized the NOM-enhanced uptake anomaly (Pb, Cu, Zn, Al, La, Ga, Pt; freshwater and marine algae), and has substantiated a related surface phenomenon — multilayer adsorption via alga–Pb–(NOM–Pb) ternary complexes on the cell surface — but the internalization-site mechanism is still identified only through model inference, and 2024 reviews state that the mechanism(s) by which natural organic matter increases metal uptake fluxes remain to be elucidated.

Background

Trace-metal uptake by aquatic microorganisms has long been organized around the free ion activity model (FIAM) and its descendant, the biotic ligand model (BLM): metal accumulation and toxicity are predicted not by total dissolved metal but by the activity of the free metal ion, which competes with other cations for sensitive binding sites on the biological membrane. Natural waters, however, contain dissolved organic matter (DOM), of which humic substances — operationally divided into humic acids (HA) and fulvic acids — are the dominant metal-binding fraction. HA is a heterogeneous polyelectrolyte rich in carboxylic and phenolic groups that forms strong, partly labile complexes with metals such as Pb(II). Under the FIAM/BLM paradigm, complexation by DOM should reduce metal uptake, yet many experiments show the opposite for Pb: uptake and toxicity higher in the presence of humic or fulvic material than expected from the measured free ion concentration. To absorb these exceptions, extensions of FIAM proposed a ternary complex — a three-body surface complex metal–humic-substance–algal-surface — but those treatments assumed equilibrium at fixed exposure times and did not distinguish transport sites from adsorption sites. Lamelas, Pinheiro and Slaveykova (2009, Environ. Sci. Technol. 43:730–735) studied the ternary system {Pb(II) (also Cd, Cu), HA, Chlorella kesslerii} kinetically. The alga's cell wall layer (~100 nm) was treated as a micro-compartment containing two classes of sites: adsorption sites R_NS on the wall itself, and internalization sites R_s on the plasma membrane, which mediate transport into the cell; adsorbed and internalized metal were operationally separated by an EDTA wash, and uptake fluxes J_u were tracked over minutes. For Cd(II) and Cu(II), uptake in the presence of HA matched predictions from the measured free ion concentration. For Pb(II), both the internalization flux and the amount of Pb bound to internalization sites ({Pb–R_s}) were an order of magnitude larger with HA than with citric acid at equal free Pb²⁺. Speciation calculations for the cell-wall layer showed 80.5% of cell-wall Pb bound to HA (versus 18% on adsorption sites and 1.5% on internalization sites), leading the authors to propose that the Pb–HA complex interacts directly with R_s, forming a ternary complex that then dissociates to load the transport site: Pb–HA + R_s ⇌ HA–Pb–R_s ⇌ Pb–R_s + HA. Three modeling simplifications were explicitly flagged: electrostatic interactions in the charged cell wall were neglected "for simplicity", as having been shown to be of secondary importance for the increase of internalized Pb; the possibility that HA adsorption modifies the effective Pb–HA stability constant inside the wall (KPbHAK_{\mathrm{PbHA}}) was examined only by a sensitivity calculation, and a modification of the intrinsic affinity constant (KM-RsK_{\mathrm{M\text{-}R_s}}) only argued to be unlikely from the parallel shift of the uptake fluxes — neither was measured; and the conclusions were specific to one alga, one HA sample, and fixed pH and ionic strength. Subsequent work sharpened each edge of the problem: photoalteration of humic substances and DOM molecular-weight fractions change Pb (and Cd, Cu, Zn, Ni) bioavailability; the anomaly was extended to other algae (Chlorella vulgaris, Chlamydomonas reinhardtii, the marine diatom Thalassiosira weissflogii) and other metals (Al, La, Ga, Pt); and a related surface phenomenon — multilayer adsorption through ternary complexes of the form alga–Pb–(fulvic acid–Pb) on the cell surface — was demonstrated by bulk adsorption modeling (two-step adsorption model), electron microscopy, and, most recently, spectroscopic and machine-learning-assisted modeling showing that the ratio of the algae–Pb to Pb–NOM stability constants governs ternary-complex formation. Reviews through 2024 nevertheless classify the NOM-driven enhancement of metal uptake fluxes as an open knowledge gap: the free-ion-based models still cannot predict when or why complexes add to bioavailability, and no molecular-level identification of the metal–humic–transport-site species or its interfacial kinetics exists.

Problem Statement

Establish, at the molecular level, how humic acid enhances the uptake of Pb(II) by the freshwater alga Chlorella kesslerii beyond what the free Pb²⁺ concentration predicts — i.e., convert the chemodynamic-model inference of Lamelas et al. (2009) into a demonstrated mechanism. In the source system (C. kesslerii; 10 mg L⁻¹ standard Suwannee River humic acid compared with citric acid at the same free Pb²⁺ concentration; pH 6.0, I=5×10−3I = 5 \times 10^{-3} M), the internalization flux JuJ_u and the amount of Pb bound to the internalization sites {Pb–R_s} were about ten times larger with humic acid than with citrate (Ju=(8.7±0.2)×10−16J_u = (8.7 \pm 0.2) \times 10^{-16} mol cm⁻² s⁻¹ and {Pb–R_s} = 1.6×10−121.6 \times 10^{-12} mol cm⁻² with HA, versus Ju=(9.0±0.1)×10−17J_u = (9.0 \pm 0.1) \times 10^{-17} mol cm⁻² s⁻¹ and {Pb–R_s} = 1.8×10−131.8 \times 10^{-13} mol cm⁻² with citric acid). The authors could not attribute this enhancement to a larger total internalization-site density, to a modified intrinsic affinity KM-RsK_{\mathrm{M\text{-}R_s}}, to a higher free Pb²⁺ concentration at the wall/membrane interface produced by dissociation of Pb–HA in the wall, or to a weakened in-wall Pb–HA stability constant KPbHAK_{\mathrm{PbHA}}; they therefore assumed an interfacial ternary species, Pb–HA + R_s ⇌ HA–Pb–R_s ⇌ Pb–R_s + HA, supported only by chemodynamic modeling and cell-wall speciation calculations.

The question is to identify in situ, at the molecular level, the interfacial species that loads the internalization sites R_s in the cell-wall/membrane microenvironment the source describes — a ~100 nm wall layer containing wall adsorption sites R_NS and plasma-membrane internalization sites R_s, with adsorbed and internalized metal operationally separated by an EDTA wash. That is: determine whether the ternary species HA–Pb–R_s forms and dissociates at that interface, and characterize its formation and dissociation behaviour; or demonstrate that it does not exist and identify what instead increases the amount of Pb in contact with R_s. A proposed mechanism must be consistent with the source's own contrast that no comparable enhancement occurred for Cd(II) or Cu(II) under identical conditions.

Candidate contributing factors that the source treated as secondary or excluded by its own arguments — electrostatic partitioning in the charged, humic-coated wall, an in-wall modification of KPbHAK_{\mathrm{PbHA}} or of KM-RsK_{\mathrm{M\text{-}R_s}}, humic-induced changes in membrane permeability, and direct uptake of lipophilic complexes — are not to be adopted as assumptions: the proposed mechanism is required to state which of these factors it excludes and why; no separate quantitative result on any of them is required. The in-situ complexation equilibria inside the wall (including whether HA adsorption modifies KPbHAK_{\mathrm{PbHA}} and KM-RsK_{\mathrm{M\text{-}R_s}}, and the electrostatic contribution of the charged wall to metal partitioning) and the behaviour of the mechanism in other algae, with other DOM types and compositions, other water chemistry and other metals, remain open aspects of the same problem; they are context for the identification task, not separately required results.

References

  1. Lamelas, Cristina; Pinheiro, José Paulo; Slaveykova, Vera I. (2009). Effect of Humic Acid on Cd(II), Cu(II), and Pb(II) Uptake by Freshwater Algae: Kinetic and Cell Wall Speciation Considerations. Environmental Science & Technology 43(3), 730–735. DOI 10.1021/es802557r. https://doi.org/10.1021/es802557r
  2. Fang, Jingjing; Li, Feili; Shi, Wen; Wang, Zhaowen; Chen, Shiyu; Zhang, Gaoxiang (2025). Spectroscopic and modeling approaches to understanding formation of microalgae-Pb-NOM ternary complex. Journal of Environmental Management 376, 124431. DOI 10.1016/j.jenvman.2025.124431. https://doi.org/10.1016/j.jenvman.2025.124431
  3. Shi, Wen; Zhang, Gaoxiang; Li, Feili; Feng, Jianru; Chen, Xiujuan (2020). Two-step adsorption model for Pb ion accumulation at the algae-water interface in the presence of fulvic acid. Science of the Total Environment 742, 140606. DOI 10.1016/j.scitotenv.2020.140606. https://doi.org/10.1016/j.scitotenv.2020.140606
  4. Shi, Wen; Fang, Xiaoman; Wu, Xingfei; Zhang, Gaoxiang; Que, Weiyan; Li, Feili (2018). Alteration of bioaccumulation mechanisms of Cu by microalgae in the presence of natural fulvic acids. Chemosphere 211, 717–725. DOI 10.1016/j.chemosphere.2018.07.200. https://doi.org/10.1016/j.chemosphere.2018.07.200
  5. Que, Weiyan; Wang, Bihe; Li, Feili; Chen, Xiujuan; Jin, Hui; Jin, Zanfang (2020). Mechanism of lead bioaccumulation by freshwater algae in the presence of organic acids. Chemical Geology 540, 119565. DOI 10.1016/j.chemgeo.2020.119565. https://doi.org/10.1016/j.chemgeo.2020.119565
  6. Bai, Hongcheng; Wei, Shiqiang; Jiang, Zhenmao; He, Mingjing; Ye, Biying; Liu, Gaoyun (2019). Pb(II) bioavailability to algae (Chlorella pyrenoidosa) in relation to its complexation with humic acids of different molecular weight. Ecotoxicology and Environmental Safety 167, 1–9. DOI 10.1016/j.ecoenv.2018.09.114. https://doi.org/10.1016/j.ecoenv.2018.09.114
  7. Shi, Zhiqiang; Du, Haiyan; Wang, Changhui; Xu, Huacheng (2022). Quantifying the bioaccumulation of Pb to Chlorella vulgaris in the presence of dissolved organic matters with different molecular weights. Environmental Science and Pollution Research 29, 70921–70932. DOI 10.1007/s11356-022-19699-6. https://doi.org/10.1007/s11356-022-19699-6
  8. Zhang, Gaoxiang; Yang, Boxuan; Shao, Luze; Li, Feili; Leng, Yaling; Chen, Xiaoling (2022). Differences in bioaccumulation of Ni and Zn by microalgae in the presence of fulvic acid. Chemosphere 291, 132838. DOI 10.1016/j.chemosphere.2021.132838. https://doi.org/10.1016/j.chemosphere.2021.132838
  9. Worms, Isabelle A. M.; Slaveykova, Vera I.; Wilkinson, Kevin J. (2015). Lead Bioavailability to Freshwater Microalgae in the Presence of Dissolved Organic Matter: Contrasting Effect of Model Humic Substances and Marsh Water Fractions Obtained by Ultrafiltration. Aquatic Geochemistry 21, 217–230. DOI 10.1007/s10498-015-9256-0. https://doi.org/10.1007/s10498-015-9256-0
  10. Spierings, Julian; Worms, Isabelle A. M.; Miéville, Pascal; Slaveykova, Vera I. (2011). Effect of Humic Substance Photoalteration on Lead Bioavailability to Freshwater Microalgae. Environmental Science & Technology 45, 3452–3458. DOI 10.1021/es104288y. https://doi.org/10.1021/es104288y
  11. Worms, Isabelle A. M.; Adenmatten, David; Miéville, Pascal; Traber, Jacqueline; Slaveykova, Vera I. (2015). Photo-transformation of pedogenic humic acid and consequences for Cd(II), Cu(II) and Pb(II) speciation and bioavailability to green microalga. Chemosphere 138, 908–915. DOI 10.1016/j.chemosphere.2014.10.093. https://doi.org/10.1016/j.chemosphere.2014.10.093
  12. Zhao, Chun-Mei; Campbell, Peter G. C.; Wilkinson, Kevin J. (2016). When are metal complexes bioavailable? Environmental Chemistry 13, 425–433. DOI 10.1071/EN15205. https://doi.org/10.1071/EN15205
  13. Fortin, Claude (2024). Metal bioavailability in aquatic systems—beyond complexation and competition. Frontiers in Environmental Chemistry 5, 1345484. DOI 10.3389/fenvc.2024.1345484. https://doi.org/10.3389/fenvc.2024.1345484
  14. Hourtané, Océane; Smith, D. Scott; Fortin, Claude (2024). Natural organic matter (NOM) can increase the uptake fluxes of three critical metals (Ga, La, Pt) in a unicellular green alga. Chemosphere 365, 143311. DOI 10.1016/j.chemosphere.2024.143311. https://doi.org/10.1016/j.chemosphere.2024.143311
  15. Zilber, Louise; Parlanti, Edith; Fortin, Claude (2024). Impact of organic matter of different origins on lanthanum speciation, bioavailability and toxicity toward a green alga. Frontiers in Environmental Chemistry 5, 1342500. DOI 10.3389/fenvc.2024.1342500. https://doi.org/10.3389/fenvc.2024.1342500
  16. Sánchez-Marín, Paula; Slaveykova, Vera I.; Beiras, Ricardo (2010). Cu and Pb accumulation by the marine diatom Thalassiosira weissflogii in the presence of humic acids. Environmental Chemistry 7, 309–317. DOI 10.1071/EN10015. https://doi.org/10.1071/EN10015
  17. Stewart, Theodora J.; Szlachetko, Jakub; Sigg, Laura; Behra, Renata; Nachtegaal, Maarten (2015). Tracking the Temporal Dynamics of Intracellular Lead Speciation in a Green Alga. Environmental Science & Technology 49, 11176–11181. DOI 10.1021/acs.est.5b02603. https://doi.org/10.1021/acs.est.5b02603