Not merely noxious? Time-dependent hormesis and differential toxic effects systematically induced by rare earth...

Progressive rare earth element (REE) enrichment in aquatic environments worldwide and their resulting anthropogenic anomalies have highlighted the need for a better understanding of their biological effects, with a special emphasis on microbial cells since they play a crucial role in good ecosystem functioning. Therefore, the primary aim of this work was to achieve simultaneous characterization of the 16 REE toxicity effects on the growth kinetics of the commonly found Gram-negative bacterium E. coli (BW25113 strain). Bacterial growth curve modelling showed hormetic effects in the presence of REEs, while EC₅₀ determination (in the mid-log phase) indicated that the four HREEs from Er to Lu in addition to Y were the most toxic metals (EC₅₀ in the range of 8.3 to 3 μM), just after Sc (EC₅₀ of 1.1 μM). Additional subcellular parameter assessment revealed cell membrane lipid peroxidation as well as enhanced membrane depolarization and permeability in the presence of La, Gd, or Yb as representatives of LREEs and HREEs. These subcellular effects appeared to be more intense with Gd and Yb compared with La-exposed cells, in relation to the overall higher toxicity potential reported for HREEs on bacterial growth. Also, the cellular ATP production decreased after REE exposure at their EC₅₀. Finally, these results emphasize the importance of growth kinetic consideration as well as the complexity of REE biological effect mechanisms towards bacteria.