Free Gd3+ concentration drives the accumulation of Gd and its subsequent toxicity to the green alga <i>Chlamydomonas reinhardtii</i>

The use of 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 at evaluating the bioavailability and toxicity of Gd3 + and two Gd-based contrast agents (GBCA: Gd-DTPA and 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 hours 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 h-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 h-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] 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 Chlamydomonas reinhardtii.