Claude Fortin

The use of gadolinium (Gd)-based contrast agents for medical imaging analyses has led to extensive enrichment of Gd in aquatic systems, which is often quantified as positive Gd anomalies. This study aims to evaluate the bioavailability and toxicity of Gd3+ and 2 Gd-based contrast agents (Gd dihydrogen salt hydrate [Gd-DTPA] and gadobutrol monohydrate [Gd-BT-DO3A]) on the green alga Chlamydomonas reinhardtii in the presence and absence of dissolved organic matter (DOM). Growth inhibition tests were performed over 72 hr at pH 6. The chemical equilibrium software Visual MINTEQ was used to calculate the speciation of Gd in exposure media and verify the applicability of the biotic ligand model. Limited bioaccumulation of Gd was detected when algae were exposed to Gd-DTPA or Gd-BT-DO3A, both in the presence and absence of DOM. As for inorganic Gd, the presence of DOM decreased Gd toxicity. The 72 hr-EC50 values based on the total concentration of Gd were estimated at 3.7 [2.9–4.7] µM of Gd for the exposure to Gd without DOM and 6.6 [5.9–7.5] µM of Gd in the exposure with DOM. Internalized Gd by cells and estimated free ion concentrations were strongly linked to toxicity, which confirms the applicability of the biotic ligand model. Moreover, the 72 hr-EC50 estimations based on free Gd3+ ion concentrations were statistically indistinguishable at the 95% confidence level for the exposure conditions with and without DOM: 1.0 [0.6–2.0] µM and 2.1 [1.4–3.1] µM, respectively. These results highlight the central role of chemical speciation determination in assessing the ecotoxicity of metals and that the free Gd ion drives its toxicity to the green alga C. reinhardtii.

In a previous paper we developed Quantitative Ion Character-Activity Relationships (QICARs) to relate the intrinsic properties of a metal to its acute toxicity towards freshwater aquatic organisms. These predictive tools were developed for a set of data-rich training elements and then applied to a representative selection of Technology-Critical Elements (TCEs). The toxicity of the TCEs was reasonably well predicted, with most values located within the 95% prediction intervals. In the present work we have extended this approach to use the calculated metal speciation. Linear Free Energy Relationships were used to estimate some of the needed thermodynamic constants. Using this information, we expressed the concentration resulting in a 50% effect level (EC50) values as free metal activities and performed the regression analyses. For the training metals, the determination coefficients slightly increased compared to those obtained using the total dissolved metal. As before, the log transformed composite value of the covalent index (χ_m ²r) was the best predictor of their acute toxicity towards algae and daphnids (χ_m = metal’s electronegativity; r = ionic radius). However, for the TCEs the regressions were much poorer, particularly when the predicted free metal ion concentrations were very low (e.g., less than 10⁻¹⁸ M). We suggest that this result reflects the distinctive speciation of these metals, where (i) the free metal ion is present only at vanishingly low concentrations (the calculation of which is problematic), and (ii) in all but one case (Au(CN)₂ ^-), the metal’s calculated speciation is dominated by neutral polyhydroxo species (e.g., Au(OH)₃ ⁰, Ge(OH)₄ ⁰…). In our view, this result does not undermine the use of QICARs. Rather, the use of QICARs has revealed that the free-ion activity could be inadequate for predicting the toxicity of the studied data-poor metals.

Environmental context: Platinum is a metal of emerging concern in ecotoxicology, mainly due to its dissemination in the environment through the abrasion of car catalysts. However, its aqueous speciation in natural waters remains poorly known, and its binding to organic matter may modify its effects on organisms. Experimental measurements revealed the complex chemistry of Pt, low complexation and very slow kinetics. The use of PtII would be recommended for chronic exposure experiments. Rationale: It is a common consensus that the free ion concentration of a metal in an aqueous medium can be a good indicator of its toxicity. In this context, the ability to determine metal speciation is of paramount importance to evaluate possible effect on aquatic ecosystems. Although speciation can be predicted for well-studied metals, the task is difficult when little information on thermodynamic constants is available, as is the case for platinum. It is then necessary to turn to experimental methods. Methodology: For this purpose, a high-performance size exclusion liquid chromatography method coupled with inductively coupled plasma–mass spectrometry for online metal detection was used. In a synthetic freshwater medium, we first explored the inorganic speciation of platinum (added as PtIV) at pH 5 and 6, and using three equilibration periods (4 min, 48 h and 1 week). We then tested two different conditions with freshwater natural organic matter (NOM) from four different origins, both expected to provide different levels of complexation: low (pH = 5; [NOM] = 3 mg C L−1) and high (pH = 6; [NOM] = 10 mg C L−1). Results: Several inorganic forms of platinum (PtII and PtIV) were identified, with good separation and repeatability. In the absence of NOM, dissociation of [PtIVCl6]2− complexes was clearly more important at pH 5 than at pH 6. Also, [PtIVCl6]2−persisted over time, even after 1 week, even though this redox form is believed to be unstable in these conditions, suggesting that thermodynamic equilibrium was not reached. Discussion: Weak complexation by NOM was observed, and the initial form of [PtIVCl6]2− persisted. However, the presence of NOM resulted in the formation of additional Pt inorganic species. These unidentified peaks, which were relatively more abundant at high NOM levels, were interpreted as intermediate species between [PtIVCl6]2− and Pt–MON complexes.

Environmental context: The increasing global demand for platinum and palladium for the development of new technologies will likely enhance their biogeochemical mobility. However, our understanding of the environmental risks of these metals remains incomplete. To help fill this knowledge gap, we determined the toxicity of various forms of platinum and palladium on a green alga. We found that palladium was more toxic than platinum and that platinum toxicity depended on the form tested. Rationale: Chemical equilibrium-based models such as the biotic ligand model (BLM) can be used to predict metal toxicity towards aquatic organisms. However, some metals such as platinum (Pt) and to a lesser extent palladium (Pd) are known to be kinetically hindered, i.e. reactions between these elements and ligands in solution might be slow relative to the biota exposure time. The BLM applicability may thus be limited for these metals and their ecological risk assessment may require consideration of their initial form of entry in aquatic ecosystems. Methodology: This study investigates the relationship between inorganic Pt and Pd initial speciation and their bioavailability and toxicity toward the unicellular green alga Raphidocelis subcapitata. Chloro- or amino-coordinated Pt and Pd complexes such as PtIICl42−, PtIVCl62−, PtII(NH3)42+, PdIICl42− and PdII(NH3)42+ were examined in 96-h algae exposure experiments to determine growth inhibition and metal accumulation. Investigation of the complex stability over time of the tested complexes was carried out using spectrophotometry measurements and kinetic calculations. Results: Similar accumulation and no toxicity (half maximal effective concentration, EC50 > 150 µg L−1) were observed for both in our tested conditions. Nevertheless, evidence of PtIICl42− dissociation was observed whereas PtII(NH3)42+ was stable. Exposure to PtIVCl62− showed stronger algal growth inhibition (EC50 = 142 µg L−1) than exposure to PtIICl42− but similar accumulation. This phenomenon might be partly explained by the generation of free chloride radicals through PtIV reduction in the presence of light. Similar accumulation and toxicity were observed for both PdIICl42− (EC50 = 4.69 µg L−1) and PdII(NH3)42+ (EC50 = 2.40 µg L−1). Discussion: These results were coherent with the relatively rapid speciation changes of the PtII complexes as observed and modelled. We therefore conclude that the initial speciation of the studied chloro- and amino-coordinated PtII and PdII complexes is not a determining parameter of their accumulation and toxicity towards R. subcapitata.

Aluminum (Al) may play a role in the ocean's capacity for absorbing atmospheric CO₂ via influencing carbon fixation, export, and sequestration. Aluminum fertilization, especially in iron (Fe)‐limited high‐nutrient, low‐chlorophyll ocean regions, has been proposed as a potential CO₂ removal strategy to mitigate global warming. However, how Al addition would influence the solubility and bioavailability of Fe as well as the physiology of Fe‐limited phytoplankton has not yet been examined. Here, we show that Al addition (20 and 100 nM) had little influence on the Fe solubility in surface seawater and decreased the Fe bio‐uptake by 11–22% in Fe‐limited diatom Thalassiosira weissflogii in Fe‐buffered media. On the other hand, the Al addition significantly increased the rate of chlorophyll biosynthesis by 45–60% for Fe‐limited T. weissflogii and 81–102% for Fe‐limited Thalassiosira pseudonana, as well as their cell size, cellular chlorophyll content, photosynthetic quantum efficiency (F_v/F_m) and growth rate. Under Fe‐sufficient conditions, the Al addition still led to an increased growth rate, though the beneficial effects of Al addition on chlorophyll biosynthesis were no longer apparent. These results suggest that Al may facilitate chlorophyll biosynthesis and benefit the photosynthetic efficiency and growth of Fe‐limited diatoms. We speculate that Al addition may enhance intracellular Fe use efficiency for chlorophyll biosynthesis by facilitating the superoxide‐mediated intracellular reduction of Fe(III) to Fe(II). Our study provides new evidence and support for the iron–aluminum hypothesis.

Metal bioavailability in solution is mostly driven by two factors: complexation and competition. The first factor, complexation, contributes to decrease the overall reactivity of the metal by reducing the activity of the free metal ion, which is known as the common denominator of metal reactions involving either dissolved ligands or surface functional groups (abiotic or biotic). Ubiquitous in natural ecosystems, natural organic matter is, for several metals, the most important metal complexing ligand. The second factor, competition, contributes to decrease the availability of biotic ligands involved in the membrane transport of metals from the bulk solution to the intracellular medium. In freshwater systems, proton and hardness cation concentrations are the main parameters potentially modulating metal bioavailability. The above reflects the current accepted paradigm. In this paper, two knowledge gaps are identified: i) the role of natural organic matter other than metal complexation that may lead to an increase in metal bioavailability; and ii) the effects of multiple metals other than competition that may trigger biological feedback mechanisms which may, in turn, alter biotic ligand binding properties. More research efforts are needed to decipher the extent of these overlooked potential effects and to improve the predictability of metal bioavailability.

Natural dissolved organic matter (DOM) is a heterogeneous mixture of a variety of organic compounds, with a great importance for the environmental fate of metals and their ecotoxicity. However, its complex nature and variable composition make the understanding of its role a challenge. Lanthanum (La) has a strong affinity for DOM and is one of the rare earth elements that is widely used in many electronic and green technologies, and for which the demand may potentially increase for the foreseeable future. The present study examines the links between the optical and chemical properties of organic matter and its influence on the bioavailability and toxicity of La toward the green microalga Chlorella fusca. A total of four DOMs, two natural organic matter samples from Ontario (Luther Marsh and Bannister Lake) and two Suwannee River fulvic and humic acids, were characterized by absorbance and fluorescence spectroscopy as well as by asymmetrical flow field-flow fractionation. These suggest that Luther Marsh and the Suwannee River humic acid have higher molecular weight and are more aromatic, compared to the other two DOMs. The concentrations of free metal ion La³⁺ were measured by an ion exchange technique and a partial ultrafiltration method, and related to biological response. Toxicity tests over 96 h at pH = 5.0 were separately conducted with and without 3 mg C·L⁻¹ of each DOM. All DOM samples reduced La bioavailability but as opposed to what can be expected based on the Biotic Ligand Model, the presence of DOM exacerbated the toxicity of La and its accumulation in algal cells when only the free La³⁺ ion concentration was considered. These results indicate that the role of natural organic matter in modulating metal bioavailability remains to be deciphered.