Evidence for low bioavailability of dietary nanoparticulate cerium in a freshwater food chain

Radioactive ¹⁴¹Ce in ionic (I-Ce), nano (N-Ce, 11 ± 9 nm mean primary particle size ± standard deviation) and micron-sized (M-Ce, 530 ± 440 µm) forms associated with natural and artificial diets in natural river water and synthetic freshwater were used to measure the real-time biokinetics of dietary ¹⁴¹Ce assimilation in a freshwater food chain. The model food chain consisted of microalgae (Raphidocelis subcapitata), snails (Potamopyrgus antipodarum) and prawns (Macrobrachium australiense). Pulse-chase experiments showed that 91-100 % of all forms of cerium associated with all diets and water types were eliminated from the digestive system of the snail and prawn within 24 h, with no detectable cerium assimilation. The prawn and snail median elimination times (ET50) and elimination rates (Ke) for all cerium forms ranged from 0.05 to 1.7 d, and 30 to >100 % per d, respectively. The pulse-chase results were supported by the autoradiographic evidence for N-Ce and M-Ce that confirmed no detectable assimilation and translocation within the tissue of the prawn over time. In contrast, the more soluble I-Ce was found to be associated in low quantities with the hepatopancreas in the prawn confirming that the lack of dissolution by N-Ce and M-Ce in the digestive environment of these organisms makes these forms less bioavailable. In addition, hetero-agglomeration of N-Ce and M-Ce resulted in particles that did not dissociate in digestive fluids and were too large to be assimilated thereby making them non-bioavailable. Based on the results from this study and from the literature review, the risk of N-Ce biomagnification and chronic dietary toxicity in freshwater ecosystems is no greater than the risk associated with M-Ce or I-Ce.