Zhao 2025 — Bioaccumulation, chemical forms, and gene expression of As/Cd/Pb in Sargassum fusiforme (hijiki) under controlled exposure

Zhao and colleagues exposed cultured Sargassum fusiforme (hijiki) shoots to controlled concentrations of As(III), As(V), cadmium, and lead in enriched seawater for 6 days, then measured tissue accumulation, chemical-form distribution (arsenic speciation and sequential extraction of Cd/Pb), and the transcriptomic (RNA-Seq) response, to elucidate uptake and detoxification mechanisms. This is a laboratory exposure and mechanism study, not a food-occurrence survey: the exposed-tissue concentrations reflect experimental dosing, not natural or market contamination, and are therefore not usable as occurrence inputs for the Category 6 seaweed standard (consistent with the standard practice — e.g., Lynch 2014 — of excluding intentional-exposure/uptake experiments from food-occurrence pooling). Its directly food-relevant data point is the unexposed control baseline: natural S. fusiforme total arsenic 77.4 mg/kg, cadmium 1.84 mg/kg, and lead 0.16 mg/kg dry weight. Mechanistically it reports that absorbed As(III) is oxidised to As(V) in the tissue (no As(III) detected in As(III)-exposed tissue), Cd is detoxified largely by pectate/protein binding, and Pb is immobilised mainly as insoluble (oxide/phosphate) compounds.

Key numbers

All dry weight, mean ± SD, n=3.

Control (unexposed) baseline — the only natural-composition values (Table 1):

• Total arsenic: 77.36 ± 2.40 mg/kg
• Cadmium: 1.84 ± 0.04 mg/kg
• Lead: 0.16 ± 0.02 mg/kg
• Control seawater (for context): total As 1.4 µg/L, Cd 0.2 µg/L, Pb 0.1 µg/L.

Control arsenic speciation (water-extractable; Table 2): As(V) 23.38 mg/kg (30.22% of total As); DMA 4.35 (5.62%); an unidentified species labelled “AsS” (3.2-min retention; likely an arsenosugar) 16.89 (21.84%); MMA not detected in control. As(III) was not detected in any As(III)-exposed tissue, indicating essentially complete oxidation of absorbed As(III) to As(V); a trace of As(III) (0.31%, 1.99 mg/kg) appeared only in the highest (10 µM) As(V) group — so the tissue inorganic arsenic is essentially all As(V).

Experimental exposure values — NOT food occurrence (controlled 6-day dosing at 0.1–10.0 µM; Table 1), recorded only to document the dose-response/mechanism:

• Total As rose to 334.88 (As(III) treatment) and 336.46 mg/kg (As(V) treatment) at 10 µM (~4× control).
• Cd rose to 114.27 mg/kg at 10 µM; Pb to an extreme 420.62 mg/kg at 10 µM (Pb bioaccumulation capacity > Cd).
• Intracellular water-extractable As(V) rose dose-dependently from 30.93% to 68.63% of total As; the source’s §3.2 text states the 10 µM As(V) group reached As(V) 207.25 mg/kg (61.90% of total). (Source-internal inconsistency: the paper’s Table 2 places 207.25 mg/kg / 61.90% in the 10 µM As(III) row and shows 230.93 mg/kg / 68.63% for the 10 µM As(V) row — the table and text disagree on which 10 µM group the 207.25 value belongs to.)

Cited literature context (not measured here): inorganic arsenic occupies ~80% of total arsenic in hijiki (Sanders 1979); inorganic arsenic in S. fusiforme has ranged 0.32–117 mg/kg (Almela 2006).

Mechanism/chemical-form findings:

• Cd: the 1 M NaCl-extractable fraction (pectate/protein-bound) was 59.7% in controls, rising to 70.9% at 10 µM — Cd detoxified mainly by organic-ligand/pectate binding.
• Pb: the 0.6 M HCl-extractable (oxide-bound) fraction rose from 14.1% (control) to 86.8% (10 µM); water-soluble Pb fell from 41.1% to 1.6% — Pb immobilised as insoluble compounds.
• Transcriptomics: differentially expressed genes (DEGs) were metal-specific — As(III) 210 DEGs (free-radical-scavenging/phenylpropanoid/flavonoid pathways), As(V) 566 DEGs (oxidative phosphorylation), Cd 10,286 DEGs (largest response), Pb 4,564 DEGs (ribosomal biogenesis/translation).

Methods (brief)

Mature S. fusiforme from Wenzhou (Zhejiang, China) were acclimatised and cultured; short shoots were exposed to As(III), As(V), Cd, or Pb at 0.10/1.0/5.0/10.0 µM in 1% PES-enriched seawater for 6 days (controls without added metals), in triplicate. Total As, Cd, and Pb were determined by ICP-MS (PerkinElmer ELAN DRC II) after digestion, validated against CRM laver GBW 10023 (recoveries 94.3–98.7%). Water-extractable arsenic species were extracted per the National Standard Method of China (2009) (deionised water + acetic acid, sonication) and separated by HPLC-ICP-MS (IonPac AS19 anion-exchange column); species [As(III), As(V), MMA, DMA] mapped to certified standards, with unidentified species quantified as As(V) equivalents (conservative). Cd and Pb chemical forms were determined by stepwise sequential extraction (80% ethanol, deionised water, 1 M NaCl, 2% acetic acid, 0.6 M HCl, residue). RNA was extracted from controls and the 10 µM groups and sequenced (Illumina HiSeq, Trinity de novo assembly, edgeR DEGs at FDR<0.05 and |log2 ratio|>1), validated by qRT-PCR. All values dry weight.

Implications

• Certification (HMTc): a peer-reviewed primary study, but a controlled-exposure mechanism/transcriptomics experiment rather than a food-occurrence survey — so it is NOT a pooling input for the Category 6 seaweed-kelp-foods P97 (its exposed-tissue concentrations are experimental dosing, not market/natural food levels; the occurrence-extraction pass should exclude it, consistent with excluding intentional-uptake experiments). Its food-relevant contributions are narrow but real: (1) a natural S. fusiforme/hijiki baseline composition (total As 77.4, Cd 1.84, Pb 0.16 mg/kg dw) for a single cultured Chinese batch, reinforcing hijiki as a high-total-As, moderate-Cd seaweed; and (2) mechanistic support that inorganic arsenic in hijiki tissue is present as As(V) (absorbed As(III) is oxidised), which is relevant to how the row treats inorganic-arsenic speciation. It also documents the high Pb/Cd bioaccumulation capacity of S. fusiforme, relevant to the Cd platform of the row.
• Microbiome/mechanism and metal pages: contributes detoxification-mechanism content (Cd pectate/protein binding; Pb insoluble immobilisation; As(III)→As(V) oxidation; metal-specific transcriptomic pathways) to the seaweed ingredient and the As/Cd/Pb metal pages.

Verification notes

• raw_handle MFK_1-s2.0-S0147651325007936-main from the PDF filename; raw_path under “raw/Manual Fetch Kimi /June 8 Inorganic Arsenic Seaweed/“. DOI 10.1016/j.ecoenv.2025.118453 confirmed on the article header. Open-access CC-BY-NC-ND-4.0 (stated), recorded in license.
• Evidence tier A reflects peer-reviewed primary study quality; but POOLABILITY is the key flag: this is a controlled-exposure/mechanism study, NOT food occurrence. Only the unexposed control baseline reflects natural composition; the exposed values are experimental dosing and are recorded only to document dose-response/mechanism, explicitly NOT for occurrence pooling. Flagged for the extraction pass to exclude (or use only the single control baseline as EF-4 context).
• Speciation: total As and inorganic As reported; the inorganic arsenic measured/reported is As(V) (As(III) not detected in As(III)-exposed tissue, indicating complete oxidation; only a 0.31% / 1.99 mg/kg trace of As(III) in the 10 µM As(V) group) — lifted as iAs with the As(V)-specific basis stated. Cd and Pb are total-metal measurements (chemical-form fractions are sequential-extraction operational fractions, not separate analytes). Methylated As (MMA, DMA) and arsenosugars measured but not lifted to frontmatter. metals frontmatter [tAs, iAs, Cd, Pb] covers the four studied analytes (all in the HMTc panel and the row’s iAs/tAs/Cd platform plus Pb).
• Units/basis preserved exactly (mg/kg dry weight; seawater in µg/L). Control vs exposed values clearly separated.
• sample_n=1 (one cultured S. fusiforme batch; control + four exposure levels × four metals, n=3 replicates each), described in sample_population.
• Read coverage: introduction, methods, and results (Tables 1–2, Figs 1–4) read in full from the article body; the later discussion/transcriptomics-detail pages and references continue the mechanism narrative (summarised here). No additional food-occurrence concentration tables beyond Tables 1–2.
• Matrix fit: S. fusiforme = hijiki (Sargassum/Hizikia fusiforme), a brown alga, not a kelp (Laminaria) — kelp omitted from matrices. It is a seaweed-kelp-foods member, hence the product link, but routed with the not-poolable mechanism caveat above.
• Brand firewall: not engaged (cultured research material; no products or brands).
• Jurisdiction CN (Wenzhou, Zhejiang; Chinese institutions). sampling_locations records the origin.
• Instrument/CRM names (PerkinElmer ELAN DRC II, IonPac AS19, CRM laver GBW 10023, Illumina HiSeq) retained in Methods as permitted scientific reporting.
• Audit subagent (2026-06-08, fresh-context) returned REVISE; it confirmed the critical points correct — the NOT-poolable / controlled-exposure flagging (5 places), control-baseline-only natural composition, iAs=As(V) speciation discipline (no DMA/MMA/AsS mislabeled), kelp omission, metals/jurisdiction/license, and all Table 1/2 values, Cd/Pb extraction percentages, and DEG counts (210/566/10,286/4,564, each verified as up+down vs Fig 3). Two wording ⚠️ applied: (1) narrowed the over-broad “As(III) not detected in any tissue” to “not detected in any As(III)-exposed tissue; trace 0.31% in the 10 µM As(V) group”; (2) softened the “AsS” arsenosugar label to “unidentified species, likely an arsenosugar” per the source’s “unknown arsenic species.” Also footnoted the source-internal Table-2-vs-text inconsistency on the 207.25 mg/kg value (page reproduces the source prose faithfully).

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