Toxicity of three rare earth elements, and their combinations to algae, microcrustaceans, and fungi

Rare earth elements (REEs) are naturally distributed in the environment, and are increasingly being used in agriculture and high technology materials worldwide, thereby increasing anthropogenic contamination and environmental risks. There exists scarce and contradictory toxicity information about REEs; hence, more studies are required, especially on their mixtures. Thus, this study aimed to assess the toxicities of La³⁺, Nd³⁺, Sm³⁺, and the combinations of these elements (binary 1:1 and ternary 1:1:1), to organisms from different trophic levels: producers (the microalgae Chlorella vulgaris and Raphidocelis subcapitata), primary consumers (the microcrustaceans Daphnia similis and Artemia salina), and decomposers (the fungi Penicillium simplicissimum and Aspergillus japonicus). Ecotoxicological bioassays were performed, and toxic concentrations were determined. Thereafter, toxicities of single and mixture REEs were classified as slightly to highly toxic according to their toxic units. Finally, a concentration addition (CA) model was used to estimate how REEs interact upon combining. Nd³⁺ was the most toxic element for all organisms, especially D. similis (48 h LC₅₀ 9.41 mg.L^-1), and was therefore classified as highly toxic. Sm³⁺ promoted cell agglomeration in Chlorella vulgaris and was the most toxic of the tested elements for this organism (72 h IC₅₀ 25.78 mg.L^-1). The CA model revealed synergistic responses for most of the combinations, principally Nd³⁺ + Sm³⁺, which was the most toxic combination for the tested organisms. Both fungi were the most resistant organisms, and A. japonicus produced exudate and sclerotia, which help in the detoxification of chemicals. Owing not only to the fact that fungi displayed a higher resistance to REEs, but also due to the absence of regulations for REEs released from the agricultural or industrial sector, and the lack of methods to treat effluents or to dispose of technological items containing REEs, these organisms should be considered as a model for the biosorption or bioremediation of REEs. Finally, the toxic effects of REEs, particularly Nd³⁺, on the biota and human health should be the focus of future studies due to their increased use in technology.