Genetic Modification of Acidithiobacillus ferrooxidans for Rare-Earth Element Recovery under Acidic Conditions

Biomolecular Actuators for Soft Robots

Biomolecules present promising stimuli-responsive mechanisms to revolutionize soft actuators. Proteins, peptides, and nucleic acids foster specific intermolecular interactions, and their boundless sequence design spaces encode precise actuation capabilities. Drawing inspiration from nature, biomolecular actuators harness existing stimuli-responsive properties to meet the needs of diverse applications. This review features biomolecular actuators that respond to a wide variety of stimuli to drive both user-directed and autonomous actuation. We discuss how advances in biomaterial fabrication accelerate prototyping of precise, custom actuators, and we identify biomolecules with untapped actuation potential. Finally, we highlight opportunities for multifunctional and reconfigurable biomolecules to improve the versatility and sustainability of next-generation soft actuators.

The outer membrane in Acidithiobacillus ferrooxidans 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 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 ~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 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.IMPORTANCEDemand 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.

Lanmodulin-Decorated Microbes for Efficient Lanthanide Recovery

Rare earth elements (REEs) are essential for many clean energy technologies. Yet, they are a limited resource currently obtained through carbon-intensive mining. Here, bio-scaffolded proteins serve as simple, effective materials for the recovery of REEs. Surface expression of the protein lanmodulin (LanM) on E. coli, followed by freeze-drying of the microbes, yields a displayed protein material for REE recovery. Four REE cations (Y3+, La3+, Gd3+, and Tb3+) are captured efficiently, with over 80% recovery even in the presence of competitive ions at one-hundred-fold excess. Moreover, these materials are readily integrated into a filter with high capture capacity (12 mg g-1 dry cell weight) for the selective isolation and recovery of REEs from complex matrices. Further, the proteins in the filter remain stable over ten bind-and-release cycles and a week of storage. To improve the deployability of this filter material, a simple colorimetric assay with the dye alizarin-3-methyliminodiacetic acid is incorporated. The assay can be performed in under 5 min, enabling rapid monitoring of REE recovery and filter efficiency. Overall, this low-cost, robust material will enable environmentally friendly recycling and recovery of critical elements.

Site Directed Mutagenesis of the Cyc2 Outer Membrane Protein from Acidithiobacillus ferrooxidans Reveals a Critical Role for Bound Iron Atoms in Extracellular Electron Transfer

Extracellular electron transfer (EET) processes by metal respiratory bacteria rely on outer membrane proteins (OMPs) to exchange electrons across the insulating cell membrane. The most studied OMPs from metal reducing bacteria contain multiple sequential heme groups. However, many iron-oxidizing bacteria, including the industrial bioleaching microbe Acidithiobacillus ferrooxidans, contain monoheme OMPs and the mechanism of electron transfer through these smaller structures has not been elucidated. Computational modeling was previously used to predict two iron ion binding sites in the Cyc2 protein structure from A. ferrooxidans. To determine if these binding sites are critical for protein function, the monoheme Cyc2 OMP from A. ferrooxidans is recombinantly expressed in E. coli outer membrane vesicles (OMVs) which are then incorporated into biomimetic cell-membrane supported lipid bilayers (SLB) on electrodes to measure electron transfer. Site-directed mutagenesis is used to disrupt the putative ion binding sites predicted from modeling to elucidate the mechanism. It is confirmed that the Cyc2 protein is capable of EET without the need for soluble iron or other accessory proteins. These results confirm the critical role of bound metal ions in the A. ferrooxidans EET mechanism, and it is expected that homologous monoheme OMPs will have similar conduction pathways.

A novel in-silico model explores LanM homologs among Hyphomicrobium spp

Investigating microorganisms in metal-enriched environments holds the potential to revolutionize the sustainable recovery of critical metals such as lanthanides (Ln3+). We observe Hyphomicrobium spp. as part of a Fe2+/Mn2+-oxidizing consortia native to the ferruginous bottom waters of a Ln3+-enriched lake in Czechia. Notably, one species shows similarities to recently discovered bacteria expressing proteins with picomolar Ln3+ affinity. This finding was substantiated by developing an in-silico ionic competition model and recombinant expression of a homolog protein (Hm-LanM) from Hyphomicrobium methylovorum. Biochemical assays validate Hm-LanM preference for lighter Ln3+ ions (from lanthanum to gadolinium). This is comparable to established prototypes. Bioinformatics analyses further uncover additional H. methylovorum metabolic biomolecules in genomic proximity to Hm-LanM analogously dependent on Ln3+, including an outer membrane receptor that binds Ln3+-chelating siderophores. These combined observations underscore the remarkable strategy of Hyphomicrobium spp. for thriving in relatively Ln3+ enriched zones of metal-polluted environments.

Various microbes used for the recovery of rare earth elements from mine wastewater

Microbes used for the recovery of rare earth elements (REEs) from mining wastewater indicated traces of Escherichia coli (E. coli, 2149.6 μg/g), Bacillus sphaericus (1636.6 μg/g), Bacillus mycoides (1469.3 μg/g), and Bacillus cereus (1083.9 μg/g). Of these, E. coli showed an affinity for REEs than non-REEs (Mn and Zn). The amount of heavy REEs adsorbed (1511.1 μg/g) on E. coli was higher than light REEs (638.0 μg/g) due to the process of increasing adsorption with decreasing ionic radius. Additionally, E. coli demonstrated stability in the recovery of REEs from mining wastewater, as evidenced by 4 cycles. SEM-EDS, XPS and FTIR showed that REEs had a disruptive effect on cells, REEs absorbed and desorbed on the cell surface including ion exchange with ions such as Na+, ligand binding with functional groups like -NH2. Finally, the cost assessment confirmed the economically feasible of E. coli in recovery of REEs from mining wastewater.

Overexpression of a Designed Mutant Oxyanion Binding Protein ModA/WtpA in Acidithiobacillus ferrooxidans for the Low pH Recovery of Molybdenum and Rhenium

Molybdenum and rhenium are critically important metals for a number of emerging technologies. We identified and characterized a molybdenum/tungsten transport protein (ModA/WtpA) of Acidithiobacillus ferrooxidans and demonstrated the binding of tungstate, molybdate, and chromate. We used computational design to expand the binding capabilities of the protein to include perrhenate. A disulfide bond was engineered into the binding pocket of ModA/WtpA to introduce a more favorable geometric coordination and surface charge distribution for oxyanion binding. The mutant protein experimentally demonstrated a 2-fold higher binding affinity for molybdate and 6-fold higher affinity for perrhenate. The overexpression of the wild-type and mutant ModA/WtpA proteins in A. ferrooxidans cells enhanced the innate tungstate, molybdate, and chromate binding capacities of the cells to up to 2-fold higher. In addition, the engineered cells expressing the mutant protein exhibited enhanced perrhenate binding, showing 5-fold and 2-fold higher binding capacities compared to the wild-type and ModA/WtpA-overexpressing cells, respectively. Furthermore, the engineered cell lines enhanced biocorrosion of stainless steel as well as the recovered valuable metals from an acidic wastewater generated from molybdenite processing. The improved binding efficiency for the oxyanion metals, along with the high selectivity over nontargeted metals under mixed metal environments, highlights the potential value of the engineered strains for practical microbial metal reclamation under low pH conditions.

Constructing Two-Dimensional (2D) Heterostructure Channels with Engineered Biomembrane and Graphene for Precise Scandium Sieving

The special properties of rare earth elements (REE) have effectively broadened their application fields. How to accurately recognize and efficiently separate target rare earth ions with similar radii and chemical properties remains a formidable challenge. Here, precise two-dimensional (2D) heterogeneous channels are constructed using engineered E. coli membranes between graphene oxide (GO) layers. The difference in binding ability and corresponding conformational change between Lanmodulin (LanM) and rare earth ions in the heterogeneous channel allows for precisely recognizing and sieving of scandium ions (Sc3+). The engineered E. coli membranes not only can protect the integrity of structure and functionality of LanM, the rich lipids and sugars, but also help the Escherichia coli (E. coli) membranes closely tile on the GO nanosheets through interaction, preventing swelling and controlling interlayer spacing accurately down to the sub-nanometer. Apparently, the 2D heterogeneous channels showcase excellent selectivity for trivalent ions (SFFe /Sc≈3), especially for Sc3+ ions in REE with high selectivity (SFCe/Sc≈167, SFLa/Sc≈103). The long-term stability and tensile strain tests verify the membrane's outstanding stability. Thus, this simple, efficient, and cost-effective work provides a suggestion for constructing 2D interlayer heterogeneous channels for precise sieving, and this valuable strategy is proposed for the efficient extraction of Sc.

Biochemical and biophysical interaction of rare earth elements with biomacromolecules: A comprehensive review

The growing utilization of rare earth elements (REEs) in industrial and technological applications has captured global interest, leading to the development of high-performance technologies in medical diagnosis, agriculture, and other electronic industries. This accelerated utilization has also raised human exposure levels, resulting in both favourable and unfavourable impacts. However, the effects of REEs are dependent on their concentration and molecular species. Therefore, scientific interest has increased in investigating the molecular interactions of REEs with biomolecules. In this current review, particular attention was paid to the molecular mechanism of interactions of Lanthanum (La), Cerium (Ce), and Gadolinium (Gd) with biomolecules, and the biological consequences were broadly interpreted. The review involved gathering and evaluating a vast scientific collection which primarily focused on the impact associated with REEs, ranging from earlier reports to recent discoveries, including studies in human and animal models. Thus, understanding the molecular interactions of each element with biomolecules will be highly beneficial in elucidating the consequences of REEs accumulation in the living organisms.

Rare earth elements in biology: From biochemical curiosity to solutions for extractive industries

Rare earth elements (REEs) are critical for our modern lifestyles and the transition to a low-carbon economy. Recent advances in our understanding of the role of REEs in biology, particularly methylotrophy, have provided opportunities to explore biotechnological innovations to improve REE mining and recycling. In addition to bacterial accumulation and concentration of REEs, biological REE binders, including proteins (lanmodulin, lanpepsy) and small molecules (metallophores and cofactors) have been identified that enable REE concentration and separation. REE-binding proteins have also been used in several mechanistically distinct REE biosensors, which have potential application in mining and medicine. Notably, the role of REEs in biology has only been known for a decade, suggesting their considerable scope for developing new understanding and novel applications.

Microbial recovery of rare earth elements from various waste sources: a mini review with emphasis on microalgae

Rare Earth Elements (REEs) are indispensable in contemporary technologies, influencing various aspects of our daily lives and environmental solutions. The escalating demand for REEs has led to increased exploitation, resulting in the generation of diverse REE-bearing solid and liquid wastes. Recognizing the potential of these wastes as secondary sources of REEs, researchers are exploring microbial solutions for their recovery. This mini review provides insights into the utilization of microorganisms, with a particular focus on microalgae, for recovering REEs from sources such as ores, electronic waste, and industrial effluents. The review outlines the principles and distinctions of bioleaching, biosorption, and bioaccumulation, offering a comparative analysis of their potential and limitations. Specific examples of microorganisms demonstrating efficacy in REE recovery are highlighted, accompanied by successful methods, including advanced techniques for enhancing microbial strains to achieve higher REE recovery. Moreover, the review explores the environmental implications of bio-recovery, discussing the potential of these methods to mitigate REE pollution. By emphasizing microalgae as promising biotechnological candidates for REE recovery, this mini review not only presents current advances but also illuminates prospects in sustainable REE resource management and environmental remediation.