Rubidium element Critical Material
Rubidium (Rb), represented as an element distinct from its ions, compounds, isotopes, host minerals, alloys and commodity streams. The US 2025 designation names the rubidium commodity and does not designate every compound or host material. A laboratory experiment tracked light-dependent uptake of rubidium-86 ions by intact phototrophic bacteria as a probe of potassium transport. This does not establish rubidium recovery from ore, brine or waste.
Identifiers & Classification
| System | Identifier | Label |
|---|---|---|
| CHEBI |
CHEBI:33322
|
rubidium atom |
Criticality Designations
| Jurisdiction | List Name | Edition / Document | Classification | Source |
|---|---|---|---|---|
| United States of America | U.S. Department of the Interior Final List of Critical Minerals | 2025 (published November 7, 2025; 90 FR 50494-50497) | critical |
usgs_2025
|
Microbial & Biochemical Mechanisms (1)
Light-dependent rubidium ion uptake by intact strain 2.4.1 cells
other
Microbial
Associated Organisms:
Cereibacter sphaeroides (NCBITaxon:1063)
Intact cells reported as Rhodopseudomonas sphaeroides strain 2.4.1 accumulated rubidium-86 ions under illumination in defined laboratory buffers. Light/dark transitions, competing cations, ionophores and uncouplers supported an electrogenic transport interpretation linked to the membrane potential. The transported species was an ion supplied as rubidium chloride, not neutral elemental rubidium. No particular transporter gene or purified protein was identified, and rubidium was a potassium-transport tracer, not an isolated recovered product.
Experimental Context & Analytical Limits:
Hellingwerf et al., pp. 1183-1184, 1187-1189: strain 2.4.1 was grown anaerobically in light in Sistrom medium at 30 degrees Celsius; 50 micrograms/mL chloramphenicol was added before harvest. Rubidium-86 chloride was measured by automated flow dialysis. Figure 4 used cells extensively washed five to eight times over two days at room temperature to deplete endogenous energy reserves, with paired absence/presence of 20 micromolar valinomycin (preincubated for at least 1 h at 25 degrees Celsius). Figure 4A used 67 mM Na2SO4, 20 mM MES, 20 mM HEPES, 5 mM MgSO4, 2 mM sodium EDTA and 15 micrograms/mL chloramphenicol, comparing pH 5.5 and 7.5. Figure 4B instead used 100 mM choline chloride, 20 mM PIPES, 5 mM MgSO4 and 10 micrograms/mL chloramphenicol, pH 7.5; 35 mM sodium formate was added during the trace. Figure 5 used cells washed seven times over two days, with 67 mM Na2SO4, 40 mM sodium phosphate, 1 mM MgSO4 and 15 micrograms/mL chloramphenicol at pH 8, varying actinic light intensity. These buffers and washed intact cells are separate from the article's sodium-transport chromatophore and phosphate-uptake assays. The exact rubidium loading, assay temperature and replicate count for Figures 4-5 are not specified in their captions; growth and valinomycin-preincubation temperatures are not substituted for assay temperature.
Hellingwerf et al., pp. 1183-1184, 1187-1189: strain 2.4.1 was grown anaerobically in light in Sistrom medium at 30 degrees Celsius; 50 micrograms/mL chloramphenicol was added before harvest. Rubidium-86 chloride was measured by automated flow dialysis. Figure 4 used cells extensively washed five to eight times over two days at room temperature to deplete endogenous energy reserves, with paired absence/presence of 20 micromolar valinomycin (preincubated for at least 1 h at 25 degrees Celsius). Figure 4A used 67 mM Na2SO4, 20 mM MES, 20 mM HEPES, 5 mM MgSO4, 2 mM sodium EDTA and 15 micrograms/mL chloramphenicol, comparing pH 5.5 and 7.5. Figure 4B instead used 100 mM choline chloride, 20 mM PIPES, 5 mM MgSO4 and 10 micrograms/mL chloramphenicol, pH 7.5; 35 mM sodium formate was added during the trace. Figure 5 used cells washed seven times over two days, with 67 mM Na2SO4, 40 mM sodium phosphate, 1 mM MgSO4 and 15 micrograms/mL chloramphenicol at pH 8, varying actinic light intensity. These buffers and washed intact cells are separate from the article's sodium-transport chromatophore and phosphate-uptake assays. The exact rubidium loading, assay temperature and replicate count for Figures 4-5 are not specified in their captions; growth and valinomycin-preincubation temperatures are not substituted for assay temperature.
Curated Evidence Points:
Support
Source:
primary
Results, potassium (rubidium) ion transport, pp. 1187-1188 and Figures 4-5, directly show condition-dependent tracer uptake and light-dependent changes. Competing unlabeled potassium or rubidium inhibited labeled uptake, while protonophorous uncouplers inhibited uptake. This supports ion transport by prepared intact bacterial cells under the tested conditions, not universal selectivity or recovery performance.
Partial
Source:
primary
Figure 4B and p. 1188 retain an explicit null: washed cells in the choline-chloride buffer at pH 7.5 did not accumulate rubidium without valinomycin until sodium formate was added. Valinomycin itself facilitated rubidium transmembrane transfer and increased efflux; ionophore-containing traces cannot be attributed solely to the native bacterial transport system.
Partial
Source:
primary
The Discussion, p. 1190, states that an absolute membrane-potential threshold cannot be assigned because calculating it assumes accumulated rubidium is unbound. The apparent threshold varied with pH, and a molecular explanation was unresolved. The absence of effects from arsenate and dicyclohexylcarbodiimide is reported as evidence against direct phosphate-bond-energy coupling in this system; the ATP-synthesis inhibition result is described as data not shown. These pharmacological observations do not identify a transporter or exclude every alternative coupling mechanism.
Partial
Source:
primary
Figures 4-5 provide illustrative traces without replicate counts or error bars, and the full flow-dialysis implementation refers to earlier methodological work. The experiment neither tests a real mineral/brine feed nor measures selective rubidium product isolation, desorption recovery, industrial deployment or a rubidium redox reaction. The authors also acknowledge previously reported differences between potassium and rubidium transport; using rubidium as a potassium analogue does not make the two ions interchangeable in every organism.
Sources & References
-
chebi_rubidium: ChEBI:33322 - rubidium atom; elemental symbol Rb, net charge zero.
https://www.ebi.ac.uk/chebi/CHEBI:33322 (accessed 2026-10-11) -
usgs_2025: U.S. Geological Survey (2025). Final 2025 List of Critical Minerals. Federal Register 90 FR 50494-50497, November 7, 2025; Rubidium row in the final commodity table. The adjacent Yes/No column describes byproduct/co-product occurrence, not criticality.
https://www.govinfo.gov/content/pkg/FR-2025-11-07/html/2025-19813.htm (accessed 2026-10-11) -
primary: Hellingwerf, Friedberg, Lolkema, Michels and Konings (1982). Energy Coupling of Facilitated Transport of Inorganic Ions in Rhodopseudomonas sphaeroides. Journal of Bacteriology 150(3):1183-1191. DOI 10.1128/jb.150.3.1183-1191.1982; PMID 6281239; PMC216339. Full nine-page article scans, especially pp. 1183-1184 and 1187-1190, Figures 4-5.
https://doi.org/10.1128/jb.150.3.1183-1191.1982 (accessed 2026-10-11) -
ncbi_sphaeroides: NCBI Taxonomy 1063 - Cereibacter sphaeroides, species; Rhodopseudomonas sphaeroides is the listed basionym. Species identity does not independently establish the experimental strain 2.4.1.
https://api.ncbi.nlm.nih.gov/datasets/v2/taxonomy/taxon/1063/dataset_report (accessed 2026-10-11)
Curation History
| Timestamp | Curator | Action | Summary |
|---|---|---|---|
2026-10-11T07:53:00Z |
codex (GPT-6) | CREATE | Curated light-dependent rubidium tracer uptake by intact strain 2.4.1 with ionophore and null-arm limitations, dated designation and verified identities; attached the actual canonical CREATE sidecar. |
Record source: data/records/rubidium.yaml