Sublethal and lethal toxicity assessment of lanthanum and gadolinium to Daphnia magna in a 7-day test method

The use of rare earth elements has increased in recent years, leading to a rise in environmental concentrations. Despite the growth in number of studies regarding toxicity, knowledge gaps remain. For Daphnia magna, standardized test methods involve exposure periods of either 48 h or 21 days to assess toxicological effects. In this study, the exposure period was adjusted to 7 days to evaluate sublethal endpoints not measurable in 48-h tests. Additionally, this approach enabled us to obtain results within a shorter time frame than that required for 21-day tests. This study focused on the individual toxicity of lanthanum (La) and gadolinium (Gd) to Daphnia magna. We assessed mortality, feeding rate, somatic growth, and maturity under static conditions, modifying the media by adding MOPS buffer to maintain an initial pH of 6.8 and providing Raphidocelis subcapitata as a daily food source. Results showed that the solubility of La decreased considerably, with the highest recovery rate dropping from 133.33% at the start to 32.73% by the end of the 7-day exposure period. In contrast, Gd solubility remained stable, with a recovery rate of 86.88% at the start and 81.30% at the end of the test. Daily lethal concentrations (LC_x) were calculated, revealing LC₁₀ values on the first day, LC₂₀ on the second day, and LC₅₀ by the third day for La and the second day for Gd. By the test's end, the LC₁₀, LC₂₀, and LC₅₀ values were 30.40, 78.96, and 403.67 µg L^-1 for La, and 10.67, 33.73, and 241.28 µg L^-1 for Gd. For the sublethal endpoints, maturity was the most sensitive endpoint with the EC₂₀ and EC₁₀ corresponding to 0.79 and 0.26 µg L^-1 for La and 0.39 and 0.14 µg L^-1 for Gd. Gd had a higher toxicity in all endpoints assessed. While a thorough comparison to existing literature remains challenging due to variations in endpoints assessed, the methodology employed in this study yielded a range of informative results. This approach provides a useful range-finding test for Daphnia magna toxicity assessments, particularly for preliminary screening, and may complement standardized methodologies.