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 inorganic arsenic (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/total arsenic (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.

Update history

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