Distribution of Water-Soluble Arsenic Species in the Sub-thallus of Australian Brown Macroalgae

Overview

Siregar and colleagues measured total arsenic and water-soluble arsenic speciation (As(V), methylated arsenicals, and four arsenosugars) by HPLC-ICP-MS in seven wild brown macroalgae taxa from Mosquito Bay (NSW, Australia), sectioned into holdfast, old stipe, young stipe, and blade. Total arsenic ranged 32.3–70.2 µg/g dry weight (whole thallus), arsenosugars dominated (66–97% of arsenic, predominantly the phosphate arsenosugar), and inorganic arsenic (measured as As(V)) ranged 0.04–14.0 µg/g dw, highest in Fucus vesiculosus (blade) and generally higher in upper/younger sections (young stipe, blade) than in holdfast/old stipe. Inorganic arsenic in almost all taxa and sections exceeded the Australia–New Zealand 1 mg/kg seaweed limit, although these are wild seaweeds not harvested for consumption; estimated dietary-intake risk indices (THQ, CR) were below thresholds given low Australian seaweed consumption.

Key numbers

Concentrations in µg/g dry weight (= mg/kg dw), mean ± SD, n=3. Inorganic arsenic is reported by the source as As(V).

Total arsenic, whole thallus (Table 1):

Inorganic arsenic (As(V)) by taxon, range across thallus sections (Table 3; µg/g dw):

Additional source-reported facts:

• Arsenosugars were the dominant species in all samples (66–97% of arsenic); phosphate (PO4) arsenosugar was the most abundant (14–45 µg/g), followed by glycerol (0.1–25 µg/g), sulfate (0.1–22 µg/g), and sulfonate (2–18 µg/g). Methylated species: DMA > MA (MA <1 µg/g, sometimes not detected). As(V) was the second-most-abundant species class after arsenosugars.
• Arsenic was unevenly distributed within the thallus: upper sections (young stipe, blade) tended to have higher arsenic than lower parts (holdfast, old stipe), with a significant taxon × section interaction.
• Risk assessment (Table 4): estimated daily intake (EDI) of inorganic arsenic 0.001–0.29 µg/kg bw/day; target hazard quotient (THQ) 0.004–0.97 (all <1); carcinogenic risk (CR) 1.6×10⁻⁶–4.4×10⁻⁴ (within the 10⁻⁶–10⁻⁴ acceptable range). Reference dose 0.3 µg/kg bw/day; cancer slope factor 1.5×10⁻³ (µg/kg bw/day)⁻¹.
• Regulatory context: the Australia–New Zealand maximum level for inorganic arsenic in seaweed is 1 mg/kg; inorganic arsenic in almost all taxa and sections in this study exceeded 1 mg/kg, but the samples are wild seaweeds not harvested for consumption. Inorganic arsenic in imported seaweed on the Australian market is reported below 0.5 mg/kg except imported dried hijiki at 7.8 mg/kg (ANZFS 2021). The source benchmarks the imported-Australian-market-seaweed iAs EDI (0.01–0.16 µg/kg bw/day) as below the JECFA BMDL₀.₅ (2 µg/kg bw/day) and the EFSA BMDL₀.₁ (0.3 µg/kg bw/day); the study’s own wild-seaweed EDIs (max 0.29 µg/kg bw/day) are also below both.

Methods (brief)

Seven dominant brown macroalgae taxa were collected within a <100 m² area at Mosquito Bay (NSW; −35.7857, 150.2332) on 8 April 2021, transported in seawater, rinsed, dissected into holdfast/old-stipe/young-stipe/blade sections (triplicate), lyophilised, and ball-milled (<20 µm); whole-thallus composites were also prepared. Total arsenic was determined by ICP-MS (PerkinElmer NexIon 300D) after microwave digestion (US EPA 3051, HNO3 + H2O2). Water-soluble arsenic speciation used a methanol:water (1:1) extraction (Taylor & Jackson 2016 method) and HPLC-ICP-MS with a Phenomenex PRP-X100 anion-exchange column (malonic-acid gradient, pH 5.6) for As(V)/DMA/MA/arsenosugars and a Supelcosil LC-SCX cation-exchange column (pyridine buffer pH 2.6) for cationic species/OH-arsenosugar confirmation; ¹³⁰Te internal standard. Quality control used CRM NMIJ 7405-b hijiki (total-As recovery ~90% of the reference value; measured As(V), SO4- and OH-arsenosugar within 89–113% of certified). All values dry weight.

Implications

• Certification (HMTc): a direct-evidence occurrence-and-speciation source for the Category 6 seaweed-kelp-foods row (iAs/tAs platform), adding Australian brown-macroalgae data on the dry, as-sold basis. It shows that beyond hijiki and Laminaria digitata, the fucoid brown alga Fucus vesiculosus can also carry elevated inorganic arsenic (As(V) up to 14.0 µg/g dw, the second-most-abundant species class after arsenosugars), and that several other brown taxa (Cystoseira, Lobospira) reach a few µg/g — broadening the high-iAs concern within brown algae and reinforcing a species- and section-aware treatment for the row. Caveat for weighting: these are wild-harvested field specimens (intra-thallus distribution study), not retail products, so the occurrence-extraction pass should weight them as field/wild data (similar to Ronan 2017), distinct from retail-market surveys.
• App: contributes Australian brown-macroalgae total and inorganic arsenic to the seaweed ingredient contamination_profile.

Verification notes

• raw_handle MFK_s12403-025-00708-1 from the PDF filename; raw_path under “raw/Manual Fetch Kimi /June 8 Inorganic Arsenic Seaweed/“. DOI 10.1007/s12403-025-00708-1 confirmed on the article header. Open-access CC-BY-4.0 (stated in the article), recorded in license.
• Evidence tier A: peer-reviewed primary speciation study, HPLC-ICP-MS, CRM (NMIJ 7405-b) validated, triplicate.
• Speciation: total As and inorganic As (As(V)) reported and kept distinct; lifted to frontmatter as tAs and iAs. The inorganic measure is specifically As(V) (As(III) was a calibration standard; in these aerobic dried samples As(V) is the inorganic species reported) — the As(V)-specific basis is stated in Key numbers. Methylated species (MA, DMA) and the four arsenosugars (PO4/OH/SO3/SO4) were measured and summarised but NOT lifted to frontmatter (not HMI analyte-vocabulary tokens); recording any of them as iAs would be a speciation error.
• UNIT NOTE: Table 3 reports concentrations in µg/g (= mg/kg dry weight); the As(V) range is 0.04–14.0 µg/g dw. The Discussion text states the inorganic-arsenic range as “0.04–13.97 µg/kg,” which is an apparent typo for µg/g (it matches the Table 3 As(V) min/max of 0.04 and 14.0±4 µg/g). Values here follow the Table 3 unit (µg/g dw); no conversion applied.
• sample_n=7 brown-macroalgae taxa; each sectioned into four thallus parts (≈26 section-samples) plus whole-thallus composites, described in sample_population. iAs values are tabulated per taxon as the across-section range; per-section means are in the source’s Table 3.
• Matrix fit: all seven are brown macroalgae (Phaeophyta; fucoids/Dictyotales), wild-harvested. kelp omitted from matrices — none of the taxa are Laminariales (kelp); they are fucoid/other brown algae.
• Brand firewall: not engaged (wild specimens; no products or brands).
• Jurisdiction AU (Mosquito Bay, NSW); sampling_locations records the site; sampling_year_range 2021.
• Instrument/CRM/vendor names (PerkinElmer NexIon/NexSAR, Phenomenex PRP-X100, Supelcosil, NMIJ 7405-b) retained in Methods as permitted scientific reporting.
• Audit subagent (2026-06-08, fresh-context) returned REVISE; all Table 1/3/4 values, the As(V)/iAs speciation discipline (no arsenosugar/methylated value mislabeled), the µg/g unit handling, slugs (kelp correctly omitted), license, and both firewalls verified clean — and it confirmed the page correctly did NOT propagate two source-internal typos (the Discussion’s “all below 0.2 µg/kg/day” contradicting its own 0.29 max, and Table 4’s stray “C. fragile” 8th-taxon row). One ❌ applied: the PRP-X100 anion-exchange column vendor was given as “Hamilton” but the source (p.1026) attributes it to Phenomenex — corrected (Hamilton is a common real-world PRP-X100 maker, the likely source of the slip). One ⚠️ applied: the BMDL comparison was reattributed to the imported-seaweed EDI (the range the source actually benchmarks), noting the study’s own EDIs are also below both BMDLs.
• Cross-source note: comparisons to Ronan 2017, Taylor & Jackson 2016, Sim 2023, Huang 2022, Wolle 2021 etc. (the source’s Table 1/2 cross-study comparisons) belong to the Part 9 synthesis pass; this page reports only this study’s own measurements.

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