Toxicity and biodistribution of lanthanum and gadolinium in Daphnia magna following chronic dietary and waterborne exposure

Daphnia magna has frequently been used to assess the toxicity of lanthanides (Ln) such as lanthanum (La) and gadolinium (Gd). However, most studies have focused on acute toxicity. Knowledge of chronic toxicity and the potential effects of dietary exposure on aquatic organisms is still scarce. The objective of this study was to investigate the effects of chronic waterborne and dietary route exposures of 0.5 mg L^{− 1} of La and Gd on the mortality, growth, and reproduction of D. magna. Four different exposure conditions were used: (i) control, (ii) dietary exposure, (iii) waterborne exposure, and (iv) a combination of dietary and waterborne exposure. The results showed that none of the Ln exposures affected the mortality or the growth of the organisms. Reproduction was identified as a more sensitive endpoint. For La, combined waterborne and dietary exposure delayed the release of the first brood by an average of approximately 1.1 days compared to the control group For Gd, dietary and combined exposures significantly decreased the total offspring of the organism by approximately 20 offspring per adult in 21 days. The biodistribution patterns differed for each metal, with La being uniformly localized in the intestine and Gd bioaccumulating differently depending on the exposure route. Dietary and combined Gd exposure led to an accumulation in the intestinal tract. However, waterborne exposure resulted in a more heterogeneous biodistribution within the individual D. magna. For both metals, dietary exposure led to the highest Ln body burden in the organisms, in contrast to the waterborne exposures. The effects observed confirm the importance of considering dietary exposure in long-term bioassays. A better understanding of the mode of action of Ln on D. magna is needed and could be achieved by exploring the effects of Ln accumulation in the gut and energy limitations for D. magna.