The outer membrane in <i>Acidithiobacillus ferrooxidans</i> enables high tolerance to rare earth elements

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 <i>Acidithiobacillus ferrooxidans</i> resists REE-mediated damage at concentrations of REEs as high as 100 mM, while mesophilic <i>Escherichia coli</i> BL21 is significantly inhibited by far lower concentrations of REEs (IC₅₀ between ~5 µM and ~140 µM 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 <i>A. ferrooxidans</i> outer membrane and cell wall confers REE sensitivity comparable to that of <i>E. coli</i>, 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.IMPORTANCEDemand for rare earth elements (REEs), a technologically critical group of metals, is rapidly increasing (US Geological Survey, 2024. <i>Mineral commodity summaries</i>. 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 <i>Escherichia coli</i> 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 <i>E. coli</i>, resulting in growth inhibition that is reversible by chelation. In contrast, <i>Acidithiobacillus ferrooxidans</i> 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, <i>A. ferrooxidans</i> 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.