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Boron element Critical Material

The element B, distinct from boric acid, borate ions, borate mineral species and commercial borates. The unqualified boron entry in Annex II of the original EU 2024 critical-raw-materials regulation is mapped to this element; the separately metallurgy-qualified strategic entry is not broadened here.

Identifiers & Classification

System Identifier Label
CHEBI CHEBI:27560 boron atom

Criticality Designations

Jurisdiction List Name Edition / Document Classification Source
European Union Regulation (EU) 2024/1252, Annex II, critical raw materials Original OJ text of 3 May 2024; Annex II section 1(g) critical eu_2024_original

Microbial & Biochemical Mechanisms (1)

Culture-associated boron-bearing accretions in Streptomyces cultures

other Microbial

Associated Organisms: Streptomyces (NCBITaxon:1883)

Cultures reported as Streptomyces sp. 053 and Streptomyces sp. 128 contained extracellular accretions with boron and oxygen signals after growth with boric acid. This supports an association of boron-bearing solids with the prepared culture material. Biologically induced precipitation is a proposed interpretation; a specific mineral phase, metabolic necessity and recovery of elemental boron are not established.

Experimental Context & Analytical Limits:
Laboratory isolates from boron-containing soils in Salta, Argentina; strain labels are retained without assigning a species or strain-level taxonomy ID. For microscopy, living cultures were grown for 72 h at 30 degrees C and 250 rpm in minimal medium with nominal 20 or 40 mM boric acid. Medium contained glucose 10 g/L, L-asparagine 0.5 g/L, K2HPO4 0.5 g/L, MgSO4.7H2O 0.20 g/L and FeSO4.7H2O 0.01 g/L. Boric acid was sterilized separately; the reported post-addition pH values were 5.58 and 5.44. Culture methods used 650 microliters of pregrown inoculum added to 20 mL medium in 100 mL flasks. Pellets were fixed with glutaraldehyde, dehydrated through ethanol, critical-point dried and gold/palladium coated before SEM-EDS. Controls contained growing cells without added boric acid. A matched uninoculated boric-acid medium taken through the same preparation was not described. This is a liquid-culture observation, not a demonstrated soil treatment or industrial deployment.
Curated Evidence Points:
Section 3.2 and Figure 3 show accretions in boric-acid cultures of both strains compared with no-boric-acid culture controls. Figure 4 and Table 2 report B and O in selected accretions: strain 053 at nominal 20 mM and strain 128 at nominal 40 mM. These observations support the narrow culture-associated solid-material claim; they do not localize intracellular uptake or measure an isolated yield.
Table 2 calls its composition atomic percentage while its column is labeled Weight (%), and expresses a calculated composition as oxides including B2O3. No diffraction-based phase determination is reported. The oxide calculation is not evidence of pure boron oxide, and morphology plus EDS cannot distinguish all possible borate phases or preparation-associated deposits. Raw solid boric acid compared in Figures 6-7 is not a processed abiotic-medium control. The protein bands in Figures 8-9 were not identified and do not demonstrate a transporter or causal precipitation pathway.
Solution depletion in Section 3.1 and Table 1 was estimated using azomethine-H colorimetry of 0.45-micrometer filtrates. The Figure 1 caption labels nominal 20 and 40 mM boric acid as 1.22 and 2.78 mg/L, inconsistent with those molar concentrations; the plotted axis instead carries a factor of 10^3. The specified stock additions and final volume also do not reproduce the nominal doses exactly; the 0.25-2 mg/L calibration description does not document the sample dilution needed to reconcile the scales. Accordingly, no removal percentage or capacity is adopted here. Duplicate culture sampling and duplicate analyses are described, but no quantitative solid-phase mass balance connects solution loss to the selected accretions. Nominal doses and source discrepancies remain explicit rather than being silently corrected.

Sources & References

  1. chebi_boron: ChEBI:27560 - boron atom; formula B, charge 0
    https://www.ebi.ac.uk/chebi/CHEBI:27560 (accessed 2026-10-11)
  2. eu_2024_original: European Parliament and Council, Regulation (EU) 2024/1252 of 11 April 2024, original Official Journal text published 3 May 2024; Annex II section 1(g), boron; contrasted with Annex I section 1(c), boron - metallurgy grade.
    https://eur-lex.europa.eu/eli/reg/2024/1252/oj (accessed 2026-10-11)
  3. moraga_2017: Moraga, Irazusta, Amoroso and Rajal (2017). Bio-precipitates produced by two autochthonous boron tolerant Streptomyces strains. Journal of Environmental Chemical Engineering 5:3373-3383. DOI 10.1016/j.jece.2017.06.044; PMC5828252 author manuscript. Sections 2.1-2.5 and 3.1-3.3, Figures 1-9 and Tables 1-2.
    https://doi.org/10.1016/j.jece.2017.06.044 (accessed 2026-10-11)
  4. ncbi_streptomyces: NCBI Taxonomy 1883 - Streptomyces; genus, not a strain identifier
    https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=1883&mode=Info (accessed 2026-10-11)

Curation History

Timestamp Curator Action Summary
2026-10-11T07:25:31Z Codex (GPT-6; Codex) CREATE Created elemental boron identity, original EU2024 criticality and qualitative culture-associated B-bearing solids evidence; preserved dosing, calibration, phase and abiotic-control limitations.

Record source: data/records/boron.yaml