This review paper aims at understanding natural and anthropogenic sources of rare earth elements (REEs) in the environment, their mechanisms of action, individual toxicity of REEs and individual toxicity of their mixtures, including with other toxicants. We also summarized the scientific knowledge on REE bioaccumulation by plants, animals and fungi. The purpose of the work was to analyze published scientific information and identify unsolved scientific issues in the selected areas. At present, both natural positive geochemical anomalies of REEs and "spots" of anthropogenic origin are discovered. There is a relationship between natural and anthropogenic sources of REEs in the environment. Increasing REE mining leads to the exposure of rock layers rich in target elements, which are then leached from natural and waste rocks. As a result, the total content of REEs in soils of their extraction areas reaches 2.3 µg/kg, and in the snow of cities it is found from 0.411 µg/kg to 1.463 µg/kg, which is comparable with natural anomalies of elements. Most often such levels of REE accumulation are not dangerous for biota. REEs, like many trace elements, have an essential effect in low concentrations and have harmful effects only when the dose is increased. However, stimulation effects proven for agricultural plants and animals in the natural environment can influence the change in population and community structures. The effects of REE compounds have many uncertainties under the conditions of their joint action with each other and with other substances containing biogenic or potentially toxic elements. The article provides examples of additive, synergistic and antagonistic action of a mixture of elements. Further, it is shown that the bioaccumulation of REEs is comparable to the accumulation of heavy metals in living organisms. Data on the transfer of REEs along trophic chains are single and contradictory. Thus, REEs contribute to the toxic load on living organisms. The main “blank spots” in knowledge about the action of REEs that require further research are identification of the most sensitive and vulnerable species, establishment of long-term effects, and clarification of the joint action of REEs with other substances.

The study of joint effects of REE and HM is relevant, since they are often satellite deposits, their areas of application are similar, and an increase in concentrations of elements of these groups in the areas that are not places of their extraction and enrichment is proven. The purpose of this work was to find out the pre-lethal and lethal effects of La, Cu and their equimolar mixtures in tests for Daphnia magna Straus. Bioassays of artificially polluted natural waters, initially free of toxic elements, was carried out. In bioassays on the mortality of D. magna in the space of 96 hours it was found that acute toxicity of copper sulfate solutions is observed at the calculated concentration of Cu 2+ 0.1 mg/L (0.0016 mmol/L), and the acute toxicity of lanthanum sulfate is when the dose of La 3+ is equal to 50 mg/L (0.36 mmol/L). In the solutions comprising mixtures of Cu and La salts (1:1 calculated using metals), the concentrations of which are equimolar to the investigated solutions of copper sulfate, the mortality of D. magna begins in the solution containing 10 times less toxic elements. It was found that 25% of individuals died in the variant "0.00016 mmol/L", the mortality of 100% of individuals was at the total metal concentration of 0.0008 mmol/L. The solutions containing La (0.072-0.72 mmol/L) and Cu (0.00016-0.0016 mmol/L) naturally inhibit the motor activity of D. magna by 1.3-5.3 times and 1.2-1.9 times in 1 hour and 1.7-2.8 and 1.4-2.2 times in 24 hours, respectively. The solutions containing mixtures of Cu and La salts inhibited the motor activity of D. magna in the same way as copper sulfate solutions with the Cu 2+ concentrations equimolar "Cu 2+ + La 3+ ". Therefore, when testing the solutions with the same molar concentrations of Cu 2+ and the mixture of "Cu 2+ + La 3+ " it was shown that La potentiates the pre-lethal effect of Cu to the level of individual effects of Cu. The additions of La salt to the solutions containing pre-lethal doses of Cu lead to lethal effects of such mixtures for D. magna.

The purpose of the work was to compare the sensitivity of autotrophic and heterotrophic organisms used in bioassay to lanthanum and cerium under the conditions of a model experiment with aqueous media. Using bioassay methods, the pre-lethal effects of La and Ce in heterotrophic Paramecium caudatum and Escherichia coli, as well as autotrophic Chlorella vulgaris and Nostoc linckia were determined. Model solutions of La 2 (SO 4 ) 3 •8H 2 O and Ce 2 (SO 4 ) 3 •8H 2 O were tested in the concentration range of 0.1-200 mg/l. As a result, it was shown that heterotrophic organisms are more sensitive to water pollution with La and Ce than autotrophic ones. According to the totality of experiments, cerium turned out to be more toxic than lanthanum. When planning the environmental studies of wastewater or reservoirs polluted with REE, it is recommended to focus on comparative sensitivity of bioassay methods, taking into account the test-functions used: bioassay for chemotaxis of P. caudatum > bioassay for changes in bioluminescence of E. coli (strain M-17) > bioassay for the content of chlorophyll а and intensity of lipid peroxidation in N. linckia > bioassay on the increase in the number of Cl. vulgaris.

The available data on the combined action of rare earth elements and heavy metals are contradictory. Therefore, the goal of the presented work is relevant – to determine the effects of solutions of copper, lanthanum salts and their equimolar mixtures under controlled conditions for Daphnia magna Straus. It has been established that the death of D. magna over 50% in solutions containing Cu2+ is observed at a calculated concentration of 0.05 mg/L (0.0008 mmol/L), and a similar effect of solutions with La3+ occurs at a dose of 50 mg/L (0.36 mmol/L). The potentiation of the lethal effect of copper in the presence of lanthanum is shown. E.g., in solutions of mixtures of Cu and La salts, where the sum of metals is equimolar to non-lethal concentrations of Cu2+ (0.00016 and 0.0008 mmol/L), 100% death of D. magna is observed after 96 and 24 hours, respectively. Further, the potentiation effect was confirmed in bioassays evaluating the locomotor activity of D. magna. Solutions containing a mixture of “Cu2+ and La3+” (1:1) inhibit the motor activity of daphnia similarly to equimolar solutions containing only Cu2+. Thus, despite the significant difference between the effective lethal and non-lethal concentrations of Cu2+ and La3+ (by 1,000), it was found that under the combined action of equimolar doses of the metals, an increase in the toxicity of copper in the presence of lanthanum is observed.