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.

Microbes can cause or accelerate metal corrosion, leading to huge losses in corrosion damages each year. Geobacter sulfurreducens is a representative electroactive bacterium in many soils, sediments, and wastewater systems. It has been confirmed to directly extract electrons from elemental metals. However, little is known about the effect of electron shuttles in G. sulfurreducens corrosion on stainless steel. In this study, we report that exogenous flavins promote iron-to-microbe electron transfer, accelerating microbial corrosion. G. sulfurreducens caused 1.3 times deeper pits and increased electron uptake (with 2 times increase of i corr ) from stainless steel when riboflavin was added to the culture medium. OmcS -deficient mutant data suggest that G. sulfurreducens utilizes riboflavin as a bound-cofactor in outer membrane c type cytochromes. The finding that, in the presence of microbes, riboflavin can substantially accelerate corrosion highlights the role of flavin redox cycling for enhanced iron-to-microbe electron transfer by G. sulfurreducens and provides new insights in microbial corrosion. (c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.