Integration of bioaccumulations, chemical forms and gene expression responses to understand the transformation and detoxification of inorganic arsenic, cadmium and lead in the brown seaweed Sargassum fusiforme

Overview

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 cadmium (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 lead (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 arsenic (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.

Update history

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