The development of microbial chassis strains with high rare earth element (REE) tolerance is critical for the advancement of new metal biomining and bioprocessing technologies. In this study, we present a mechanistic understanding of how hyperacidophilic bioleaching organism Acidithiobacillus ferrooxidans resists REE-mediated damage at concentrations of REEs as high as 100 mM, while mesophilic Escherichia coli BL21 is significantly inhibited by far lower concentrations of REEs (IC50 between similar to 5 mu M and similar to 140 mu M depending on the element). Using light microscopy to document physiological changes and fluorescent probes to quantify membrane quality, we prove that cell surface interactions explain REE toxicity and demonstrate its reversibility through the addition of chelators. Removal of the A. ferrooxidans outer membrane and cell wall confers REE sensitivity comparable to that of E. coli, corroborating the importance of the outer membrane surface. To conclude, we present a model of differential REE sensitivity in the two strains tested, with implications for industrial metal bioprocessing. IMPORTANCE Demand for rare earth elements (REEs), a technologically critical group of metals, is rapidly increasing (US Geological Survey, 2024. Mineral commodity summaries. Reston, VA). To expand the supply chain without creating environmentally hazardous conditions, there is growing interest in the application of bioprocessing and bioextraction techniques to REE mining and separation. While REE toxicity has been demonstrated in Escherichia coli and other mesophilic neutrophiles, the effect of REEs on organisms currently used in metal bioleaching has been less studied. We present physiological evidence suggesting that REEs damage the outer membrane of E. coli, resulting in growth inhibition that is reversible by chelation. In contrast, Acidithiobacillus ferrooxidans tolerates saturating REE concentrations without apparent inhibition. This study fills gaps in the rapidly expanding body of literature surrounding REE's impact on microbial physiology. Furthermore, A. ferrooxidans resistance to REEs at saturating concentrations (50-100 mM at pH 1.6) is unprecedented in the literature and demonstrates the potential utility of this organism in REE biotechnology.

Plant growth requires a complex network of arbuscular mycorrhizal (AM) fungi and bacteria to supply organic compounds and major (C, N, P, etc.) and trace nutrients to the roots. Hyperaccumulation by certain plant species is based on the threshold 'maxima' a plant can safely ingest/absorb an element from soils without tissue damage. The latter criteria for hyperaccumulation vary between elements. The amount of an element a plant can absorb depends both on the ability of the species to uptake the element and on the element concentrations and bioavailability in the substrate. A plant growing directly on a mineral-rich substrate or a short height above the soil should be able to access inorganic matter via the roots. In contrast, a plant capable of accumulating inorganic elements, but growing on a peat without direct root contact with the inorganic soil fraction, would suffer a dearth of mineral nutrients. Partitioning of elements occurs within hyperaccumulators. For example, the preferential binding of heavy rare earth elements (HREE) to organic ligands leads to the relative enrichment of HREE in aerial plant structures. The presence of hyperaccumulators in a wide range of present-day plant species suggests that this mechanism was present among peat-forming plants in the fossil record. Examples from peats through low-rank coals to high volatile A bituminous coals show that hyperaccumulation provides a viable hypothesis for the consequent enrichment of certain elements. Complications from the depositional history and diagenetic alteration of the peat; metamorphism and mineralization through the history of the coal; and, not the least, the problems implicit in sampling suitable intervals in working mines, cores, natural exposures, etc., present problems in the extrapolation of the modern plant mechanisms to coals. Coal represents natural settings and, apart from Miocene and younger coals produced from vegetation with known relatives in modern setting, we cannot experiment on the ancient plants. The analogies between the geochemical appearance of coals and the element uptake and partitioning behavior of modern plants, however, does offer hope that hyperaccumulation might have been a mechanism, potentially one of many mechanisms, for the organic associations of inorganic elements in coals.

Artisanal mining is intensely carried out in developing countries, including Brazil and especially in the Amazon. This method of mineral exploration generally does not employ mitigation techniques for potential damages and can lead to various environmental problems and risks to human health. The objectives of this study were to quantify the concentrations of rare earth elements (REEs) and estimate the environmental and human health risks in cassiterite and monazite artisanal mining areas in the southeastern Amazon, as well as to understand the dynamics of this risk over time after exploitation. A total of 35 samples of wastes classified as overburden and tailings in active areas, as well as in areas deactivated for one and ten years were collected. Samples were also collected in a forest area considered as a reference site. The concentrations of REEs were quantified using alkaline fusion and ICP-MS. The results were used to calculate pollution indices and environmental and human health risks. REEs showed higher concentrations in anthropized areas. Pollution and environmental risk levels were higher in areas deactivated for one year, with considerable contamination factors for Gd and Sm and significant to extreme enrichment factors for Sc. Human health risks were low (< 1) in all studied areas. The results indicate that artisanal mining of cassiterite and monazite has the potential to promote contamination and enrichment by REEs.