Europium toxicity in Lemna minor cannot be explained by phosphorus deficiency alone

The rapid expansion of mining for rare earth elements, including europium (Eu), has raised concerns about potential contamination of aquatic ecosystems. The aquatic macrophyte Lemna minor was used to test the hypothesis that Eu toxicity is caused by phosphorus deficiency. Chemical equilibrium modeling predicted one-to-one binding of Eu and phosphate in the growth medium, suggesting no soluble Eu ions and a 20% reduction in phosphate ions. The concentration of Eu used was not environmentally relevant; it was chosen for its potential to bind enough phosphate to elicit a deficiency response in the plants. Despite the predicted lack of soluble Eu, plants grown for 15 days in media with 1 mM Eu and 4 or 5 mM total phosphate accumulated 19.3 ± 3.2 or 27.5 ± 8.0 µg/g Eu, respectively. Symptoms of Eu toxicity included 34% fewer fronds, 45% smaller frond area and 35-55% reduced biomass relative to control. In contrast, plants given 80% phosphate to mimic Eu-phosphate complexation were only 16-32% smaller than control plants. The activity of acid phosphatase in the growth medium increased by 44% relative to control for plants treated with Eu or 80% phosphorus, whereas activity was equal to control for plants treated with Eu and 120% phosphorus. This enzyme may have liberated both Eu and phosphate from the complex, which would explain the ability of Eu-treated plants to accumulate as much phosphorus as control plants. In conclusion, the impact of Eu toxicity on L. minor was greater than could be explained by Eu-induced phosphorus deficiency.