Table of Contents
Rare earth elements (REEs), often referred to as “industrial vitamins”, possess unique physicochemical properties that make them vital components in high-tech industries including optics, electronics, and magnetism. However, conventional extraction and separation techniques—such as sulfate roasting, alkali leaching, and ammonium salt leaching—are associated with severe environmental pollution and ecological damage. In response, microbial bioleaching and biosorption using bacteria represent a promising green alternative, offering an eco-friendly, cost-effective, and sustainable strategy for REE recovery. In this project, we first validated the REEs adsorption capacity of Pseudomonas kunmingensis (P. kunmingensis) HL22-2T, and subsequently engineered the strain with key genes involved in polyphosphate metabolism. By reprogramming its phosphate regulatory network, we aim to enhance the strain’s ability to adsorb REEs, aligning with the principles of sustainable biotechnology.
Currently, the extraction and purification of REEs predominantly rely on chemical methods including sulfuric acid roasting, alkaline leaching, and ammonium salt extraction. These processes contribute significantly to environmental contamination through pollution emissions and ecosystem disruption. In contrast, bacterial-based recovery systems leverage inherent advantages such as wide availability, low cost, ease of cultivation, ecological compatibility, and biodegradability. Such bio-based approaches have been successfully applied to recover REE ions from electronic waste, acid mine drainage, and industrial effluents.
In REE tailings and mining regions, bacteria belonging to the genus Pseudomonas are frequently among the dominant microbiota. Known for their environmental resilience and metabolic versatility, Pseudomonas species represent ideal candidates for biosorption and resource recovery of REEs. In 2014, Fuhong Xie et al. isolated a Gram-negative, rod-shaped, exopolysaccharide-producing, strictly aerobic bacterium with a single polar flagellum from a phosphate mining area in Kunming, Yunnan, China. This strain was taxonomically classified as P. kunmingensis, with the type strain designated HL22-2T.
On the molecular level, functional groups including carboxyl, phosphate, and hydroxyl moieties present on microbial cell surfaces play critical roles in the complexation and stabilization of REEs. These groups form coordination bonds with REE ions, facilitating adsorption, enrichment, and recovery. Phosphate-containing groups, in particular, are among the most effective ligands for REE binding. Bacterial cell wall constituents—such as teichoic acids, lipopolysaccharides, and phosphorylated proteins—expose phosphate groups that electrostatically attract and strongly chelate REE³⁺ ions. Upon saturation of surface binding sites and under conditions of localized high phosphate and REE concentration, this process can further promote the precipitation of rare earth phosphate nanoparticles (e.g., REEPO₄) on the cell surface or within the periplasm.
Intracellular polyphosphate (polyP) acts as a dynamic reservoir and supplier of soluble phosphate, directly influencing the concentration and flux of free intracellular phosphate. This regulation, in turn, affects the amount of phosphate available for release into the extracellular environment. Therefore, this project first seeks to confirm the phosphate-solubilizing capacity and REE adsorption performance of P. kunmingensis HL22-2T. We will then use synthetic biology approaches to genetically modify key enzymes in the polyP metabolism pathway of this strain. The objective is to enhance its REE adsorption efficiency through targeted manipulation of phosphate metabolism.
Phosphate-solubilizing microorganisms (PSMs) secrete organic acids that liberate inorganic phosphate from insoluble mineral compounds. Their cell surfaces are rich in functional groups—including phosphate and carboxyl groups—that effectively adsorb REE ions. Strains with moderate phosphate-solubilizing activity avoid extreme acidification or abrupt phosphate release, thereby maintaining conditions favorable for sustained REE complexation with surface functional groups. This enables efficient biosorption and surface enrichment of REEs.
To evaluate these traits, P. kunmingensis HL22-2T will be cultured in PVK medium with tricalcium phosphate as the sole phosphorus source, along with reference PSMs including Shewanella oneidensis MR-1, Escherichia coli DH5α, and Corynebacterium glutamicum ATCC 14067. Phosphate dissolution will be quantified using the molybdenum-antimony spectrophotometric method. Following confirmation of phosphate solubilization, the capacity of P. kunmingensis HL22-2T to adsorb REEs will be assessed via inductively coupled plasma-mass spectrometry (ICP-MS).
PolyP metabolism is governed by enzymes such as polyphosphate kinase 1/2 (PPK1/2), exopolyphosphatase (PPX), and endopolyphosphatase (PPN). Among these, PPK1, PPK2, and PPX are central regulators of intracellular inorganic phosphate (Pi) homeostasis. Their expression and activity are finely tuned by cellular Pi levels, enabling polyP to serve as a dynamic buffer under varying environmental conditions.
We plan to heterologously express ppk1, ppk2, and ppx in P. kunmingensis HL22-2T to modulate its polyP metabolism. Following genetic modification, the engineered strain will be evaluated for enhanced adsorption efficiency toward REEs compared to the wild-type strain. This comparison will serve to validate the feasibility of our proposed strategy.
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