Cadmium accumulation and toxicity in the unicellular alga Pseudokirchneriella subcapitata: Influence of metal‐binding exudates and exposure time

Abstract

Predicting metal availability and toxicity for chronic (several hours or days) metal exposure scenarios, even for unicellular algae, is a major challenge to existing toxicity models. This is because several factors affecting metal uptake and toxicity, such as the release of metal-binding exudates, changes in the kinetics of metal uptake and toxicity over time, and algal physiological acclimation to internalized metals, are still poorly understood. The present study assessed the influence of these factors on Cd uptake and toxicity in laboratory batch cultures of the freshwater alga Pseudokirchneriella subcapitata. To do so, changes in the free Cd(2+) concentrations caused by the release of metal-binding algal exudates were monitored, (109)Cd accumulation in algal cells was measured, and Cd-induced inhibition of algal growth as a function of exposure time (from 12 h to 96 h) was followed. Results indicate that metal-binding exudates may decrease the proportion of the free Cd(2+) ion in solution up to 2-fold, a decrease that affects Cd uptake and toxicity. Pseudokirchneriella subcapitata has the capacity to decrease net Cd uptake rate on short time scales (<24 h), but this reduction in the Cd uptake rate disappeared after 24 h, and Cd toxicity occurred at relatively high Cd concentrations in solution. These data illustrate some of the pitfalls of standard algal toxicity assays, which were designed for acute exposures, and suggest how robust chronic bioassays might be developed.

Dynamics of Metal Partitioning at the Cell–Solution Interface: Implications for Toxicity Assessment under Growth-Inhibiting Conditions

“…In addition, very few studies pay attention to the measurement of the free metal concentration in exposure medium, most of the studies focusing on the experimental evaluation of total metal content and on the use of thermodynamic speciation software for retrieving, indirectly, the content of free metal ions in solution. The difference between simulated (with MINEQL+) and experimentally measured free Cd 2+ concentration reported by Paquet al, 38 though not negligible, is further necessarily impacted by ill-defined experimental errors on both the metal titration and calibration method. For the lack of better, the data of Paquet et al 38 are used below to challenge the theory for differentiating the nature of the algistatic metal fraction over time.…”

“…Only at higher Cd 2+ concentrations (>62 nM) did Paquet et al 38 measure significant initial complexation of Cd in solution, but they did not, however, provide an explanation for this apparent initial Cd complexation at high Cd 2+ concentration. Despite this uncertainty, the set of data published by Paquet et al 38 constitutes a good start for confrontation with theory.…”

“…Only at higher Cd 2+ concentrations (>62 nM) did Paquet et al 38 measure significant initial complexation of Cd in solution, but they did not, however, provide an explanation for this apparent initial Cd complexation at high Cd 2+ concentration. Despite this uncertainty, the set of data published by Paquet et al 38 constitutes a good start for confrontation with theory. To the best of our knowledge, there is no such available complete set of data where c p (t), c M *(t), ϕ u (t), and Γ s (t) are systematically measured under different exposure conditions, which includes different initial metal concentrations and distinct exposure times.…”

“…We now consider the situation where P. subcapitata, a temperate microalgae species, is exposed over time (t exp = 12, 24, 48, 72, and 96 h) to different initial concentrations c M * 0 of cadmium (0−300 nM). Following the methodology detailed elsewhere, 38 the cell number density [c p (t), Figure 4A], the metal concentration in bulk solution [c M *(t), Figure 4B], the amounts of internalized and cell-surfacebound metals [Q u (t) and ∑ k=i,u,p Q s,k (t), respectively] were measured over time under the various exposure conditions tested. From these data, the relevant ϕ u (t) ( Figure 4C) and Γ s (t) ( Figure 4D), both defined in the preceding sections, were evaluated with the proper account of the algae size [a ∼ 2.5−3 μm for t ≤ 96 h, ⟨S a (t)/dt⟩ ∼ 0 or a ̅ (t) = 1, and the absence of significantly protuding surface structures from the membrane; i.e., d ≪ a].…”

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

“…On déduit donc intuitivement qu'il y aurait peu d'avantages à évoluer un système indépendant d'efflux du Cd chez C. reinhardtii en présence de [Cd 2+ ] < 7•10 -9 M. Nos résultats n'excluent cependant pas la possibilité qu'un tel système d'efflux existe chez C. reinhardtii et soit impliqué dans la détoxication du Cd à plus forte [Cd 2+ ]. En plus d'une modulation potentielle de l'efflux de Cd chez le phytoplancton marin, il a été démontré que l'algue verte d'eau douce P. subcapitata peut réduire grandement le taux d'accumulation du Cd sur une période de temps <24 h en régulant à la baisse le transport du Cd dans les cellules (ERRÉCALDE et CAMPBELL, 2000;PAQUET et al, 2015). Toutefois, après 24 h, les effets de l'accumulation et la toxicité du Cd sur le taux de croissance de l'algue pour une concentration dissoute de Cd 2+ donnée pouvaient s'accentuer, indiquant que l'effet toxique du Cd internalisé/adsorbé pouvait se faire ressentir à retardement.…”