| Summary: | The low-dose toxicity of chloride and nitrate salts of three lanthanides (La, Ce and Nd) was tested on six microfungal species. Five of them (<i>Geomyces vinaceus</i>, <i>Aspergillus niveoglaucus</i>, <i>Pseudogymnoascus pannorum</i>, <i>Penicillium simplicissimum</i> and <i>Umbelopsis isabellina</i>) were isolated from the loparite ore tailings on the Kola Peninsula, northwestern Russia. <i>Sydowia polyspora</i> was a control strain. In the case of nitrate salts, the toxicity of REEs to four of six microorganisms was significantly (<i>p</i> < 0.5) lower compared to chloride salts. In this case, nitrates can play the role of exogenous nutrients, compensating for the toxic effect of REEs. Interestingly, <i>U. isabellina</i> only showed an opposite response, indicating the highest toxicity of nitrate (IC<sub>5</sub> = 9–20 mg/L) REEs’ salts compared to chlorides (IC<sub>5</sub> = 80–195 mg/L) at low concentration levels. In addition, treatment with lanthanides showed a “hormesis effect” on fungal growth with stimulation at low doses and inhibition at high doses. However, <i>U. isabellina</i> and <i>S. polyspora</i> demonstrated the absence of hormetic response under the treatment of REEs’ nitrate salt. Taking into account the specific hormetic responses and high tolerance of <i>P. simplicissimum</i> and <i>U. isabellina</i> to lanthanides, our findings may be useful in the assessment of the potential application of the selected fungi to bioremediation and REE bioleaching.
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