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Chromium element Critical Material

Chromium (Cr), represented as an element. Its identity differs from chromate oxyanions, potassium chromate reagent, chromium-bearing precipitates and chromite mineral species. The dated US designation names the chromium mineral commodity and is mapped to its elemental identity here; it is not inherited by every chromium compound or waste.

Identifiers & Classification

System Identifier Label
CHEBI CHEBI:28073 chromium 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)

MtrC/OmcA-associated reduction of Cr(VI) by resting MR-1 cells

redox transformation Microbial

Associated Organisms: Shewanella oneidensis MR-1 (NCBITaxon:211586)

Resting Shewanella oneidensis MR-1 cells removed aqueous Cr(VI) supplied as potassium chromate. Deletion comparisons and purified protein electrocatalysis support contributions from the outer-membrane cytochromes MtrC and OmcA to reduction. Residual activity in the double mutant excludes an exclusive dependence on these two proteins. Chromium-bearing precipitates were observed, but their assignment to Cr(III) was inferred from earlier studies, not independently established by oxidation-state spectroscopy in this experiment.

Experimental Context & Analytical Limits:
Laboratory resting-cell assays used wild-type MR-1 and its delta-mtrC, delta-omcA and double-deletion derivatives. Cells were grown aerobically for 16 h in dextrose-free tryptic soy broth at 30 degrees C, harvested and washed once in 30 mM sodium bicarbonate buffer reported at pH 8. Sealed, gas-purged assays contained 0.2 mM K2CrO4, 10 mM sodium lactate and 2 x 10^8 cells/mL at 30 degrees C and 25 rpm; the paper reports CO2:N2 of 80:20 without a post-purge pH measurement. Soluble Cr(VI) was measured by diphenylcarbazide, with equal-density heat-killed wild-type controls and triplicate measurements. Kinetic Figure 1 covers 24 h; selected microscopy observations extend to 120 h. Separate purified-protein films were tested electrochemically in pH 7.4 phosphate-buffered saline at the reported 273 K, with a 30 mV/s scan, 3000 rpm electrode rotation and analysis at -0.33 V versus the standard hydrogen electrode. Those electrode conditions are not whole-cell cultivation conditions.
Curated Evidence Points:
Methods (Cr(VI) reduction), Results (In vivo Cr(VI) reduction) and Figure 1 show lower Cr(VI)-loss rates in the deletion strains than in wild type and no observed reduction in the heat-killed control. Results (In vitro Cr(VI) reduction), Figures 4-5 and Table 1 report chromate-dependent catalytic currents from purified MtrC and OmcA films, distinguishable from bare-electrode responses. Together these support a cytochrome contribution, beyond a gene-expression correlation.
All deletion strains retained activity; the double mutant's reported 4 h rate was 31.1% of wild type. Reduction remained incomplete, with little further change reported from 24 to 120 h (later kinetics not shown). Figures 2-3 locate electron-dense material and detect elemental Cr in selected particles; EDX does not establish valence or identify every dark inclusion. Figure 2C is labeled delta-mtrC in the image but delta-omcA in its caption, preventing an unambiguous assignment of that panel to one single mutant. The study establishes neither a recovered chromium-product mass balance nor industrial deployment.

Sources & References

  1. chebi_chromium: ChEBI:28073 - chromium atom (Cr; atomic number 24; net charge zero)
    https://www.ebi.ac.uk/chebi/CHEBI:28073 (accessed 2026-10-11)
  2. usgs_2025: U.S. Geological Survey (2025). Final 2025 List of Critical Minerals. Federal Register 90 FR 50494-50497, November 7, 2025; chromium commodity entry on page 50496.
    https://www.govinfo.gov/content/pkg/FR-2025-11-07/html/2025-19813.htm (accessed 2026-10-11)
  3. belchik_2011: Belchik et al. (2011). Extracellular Reduction of Hexavalent Chromium by Cytochromes MtrC and OmcA of Shewanella oneidensis MR-1. Applied and Environmental Microbiology 77(12), 4035-4041. DOI 10.1128/AEM.02463-10; PMID 21498755; PMCID PMC3131624.
    https://journals.asm.org/doi/full/10.1128/aem.02463-10 (accessed 2026-10-11)
  4. ncbi_mr1: NCBI Taxonomy: Shewanella oneidensis MR-1 (Taxonomy ID 211586; no rank)
    https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=211586&mode=Info (accessed 2026-10-11)

Curation History

Timestamp Curator Action Summary
2026-10-11T04:36:03Z Codex (GPT-6; Codex) create Created chromium element identity, dated US2025 criticality and a primary-supported MR-1 chromate reduction mechanism. Retained residual mutant activity, incomplete transformation, inferred precipitate valence and the Figure 2C label conflict; attached the separate canonical CREATE session before independent review.

Record source: data/records/chromium.yaml