Rare Earth Element (REE) Extraction Pathways

Why this is interesting

• REEs (La, Ce, Pr, Nd … Lu, Y) and co-occurring strategic metals (Co, Ni, Mn, Cu, Zn, Fe) are critical for magnets, batteries, and electronics
• Conventional extraction is chemically harsh and produces difficult-to-separate mixtures of chemically similar lanthanides
• Biomining offers a milder, tunable alternative driven by living chassis
• Biology already does lanthanide chemistry: lanthanide-dependent enzymes are widespread in methylotrophs
• Goal: program microbial or plant chassis to do selective metal recovery

Target scope

REEs: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y

Strategic co-occurring metals: Co, Ni, Mn, Cu, Zn, Fe

System goals

1. Detect metal ions in complex matrices
2. Mobilize metals via leaching (acidolysis, redoxolysis)
3. Bind and sequester selectively
4. Concentrate intracellularly
5. Trigger controlled export into recovery solution

Key biology: lanthanides are biological

• Methylotrophs use lanthanide-dependent methanol dehydrogenases (XoxF-type MDHs) — Ln³⁺ sits in the active site
• Lanmodulin (lanM) is a compact, highly REE-selective EF-hand binding protein [PMID:35814700]
• Lanthanophores (e.g. methylolanthanin) are secreted small molecules that mobilize REEs, analogous to siderophores for iron
• Dedicated lut uptake clusters import lanthanides into the cell [PMID:35875576]
• These natural parts are the building blocks for engineered recovery pathways

Approach: AI-assisted gene review → design

• Use the AI gene review framework to map candidate genes to chassis and annotate GO terms
• Decompose recovery into four modules: detect → mobilize/leach → capture/uptake → sequester/concentrate → export/release
• For each module: pick candidate gene systems, candidate chassis, and selectivity strategies
• Conceptual blueprint (placeholder loci flagged TBD where biology is not yet mapped)

Sensing → [TF/Riboswitch] → Leaching agents → Solubilized ions
       → Selective binding → Uptake → Sequestration → Export → Recovery

Module 1: Biosensors + leaching control

Sense target metals, induce leaching agents (acidolysis + redoxolysis)

• Metal-responsive regulators: NikR (Ni), MntR (Mn), CnrX/CnrY/CnrH (Co/Ni)
• Acidolysis via PQQ-dependent gluconate: gcd + pqqABCDE/pqqFG [PMID:7665488; PMID:9043136]
• Redox shuttles: phenazine operon phzA–G [PMID:8586283; PMID:28871340]
• Fe(II) oxidation bioleaching: rus operon (cyc2, cyc1, coxBACD, rus)

METEA lanthanophore loci: mll cluster MexAM1_META1p4132–4138, regulators mluA/R/I META1p4129–4131, PQQ META1p1748/1751 → methylolanthanin secretion

Module 2: Selective binding + uptake

Capture solubilized metals with high selectivity, transfer into the cell

• Lanthanide-binding protein: lanM (lanmodulin)
• Lanthanide uptake cluster: lutH (TonB receptor) + lut genes (META1_1778–1787, includes lanM and lutD)
• Ln-dependent MDH module: xoxF + xoxG + xoxJ [PMID:31017712]
• Ni uptake: nikABCDE + NikR [PMID:9882686]; Mn uptake: mntH [PMID:21908668]

METEA loci: lanM META1p1786; xoxF1 META1p1740; xoxG META1p1741; xoxJ META1p1742

Selectivity: geometry matched to REE ionic radius; competitive exclusion of Fe/Cu/Zn; pH-gated binding

Module 3: Sequestration + concentration

Store metals safely and concentrate to useful levels

• Metallothionein: smtA (cyanobacterial; transferable module) [PMID:1607014]
• Polyphosphate granules: ppk1/ppk2 polyphosphate kinases [PMID:12486232; PMID:17360677]
• Engineered protein nanocages / bacterial microcompartments

Readouts: fluorescent REE reporters; growth-linked selection for high accumulators

METEA status: lanthasome-like polyphosphate storage phenotype reported, but specific storage loci not yet mapped — a discovery priority

Module 4: Inducible export + recovery

Release concentrated metals into recovery solution after harvest

• Co/Zn/Cd efflux: czcCBA RND pump [PMID:12867443]
• Co/Ni efflux: cnrCBA + cnrYXH regulation [PMID:10671463]
• Ni/Co efflux: rcnA + rcnR [PMID:15805538]; Mn efflux: mntP

Timing for fractionation:

T0 Harvest → T1 Induce REE export → T2 Collect REE fraction
           → T3 Induce Co/Ni/Mn export (secondary fraction)

METEA status: REE/Co/Ni/Mn efflux loci remain TBD — to map once transporters are identified

Chassis comparison

Chassis scoring (heuristic)

Weights: REE selectivity 0.45, leaching strength 0.35, metal tolerance 0.20

Heuristic placeholders — tune after pilot assays (leaching rates, uptake specificity, survival).

Challenges

• Selectivity among chemically similar lanthanides, and REE vs Fe/Cu/Zn competition in real matrices
• Chassis engineering: acid tolerance, genetic stability under metal stress, growth vs accumulation tradeoff
• Sparse annotation: METEA storage and export loci still TBD; biology not fully mapped
• Avoiding non-specific uptake of toxic metals
• Biosecurity and containment (kill-switches)

Conclusions & future directions

Status: IN_PROGRESS — modular conceptual blueprint with chassis-mapped candidate genes

Next steps

• Map candidate genes to chassis species and annotate GO terms
• Identify literature for REE-binding proteins and lanthanide transport systems
• Prototype biosensor circuits with tunable thresholds and logic gates
• Pilot small-scale bioleaching with complex ore simulants

Milestones: reporter strains → leaching induction → selective uptake → intracellular concentration → triggered export with measured purity