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High proportions of inorganic arsenic in Laminaria digitata but not in Ascophyllum nodosum samples from Ireland

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Ronan and colleagues measured total arsenic and inorganic arsenic (as As(V)) by ICP-MS and HPLC-ICP-MS across the different thallus parts of two wild Irish brown seaweeds: the kelp Laminaria digitata and the intertidal wrack Ascophyllum nodosum.

Author Karen Pendergrass Cite this pageVersion history
Page snapshot
Cited by5 pages
Metals measured2
Evidence tierA
Year2017

Overview

Ronan and colleagues measured total arsenic and inorganic arsenic (as As(V)) by ICP-MS and HPLC-ICP-MS across the different thallus parts of two wild Irish brown seaweeds: the kelp Laminaria digitata and the intertidal wrack Ascophyllum nodosum. Total arsenic was similar between species (36–131 mg/kg dry weight in L. digitata, 38–111 in A. nodosum), but the inorganic fraction diverged sharply: inorganic arsenic was always below 1% of total in A. nodosum (≤0.703 mg/kg DW) yet reached 2.2–87 mg/kg DW in L. digitata, rising along the thallus from the stipe to the decaying distal blades and exceeding 50% of total arsenic in the middle, distal, and decaying-distal blades. This establishes that high inorganic arsenic in edible seaweed is not confined to hijiki: kelp harvested from a clean site carried inorganic arsenic an order of magnitude above the French (3 mg/kg) and Australia–New Zealand (1 mg/kg) seaweed limits.

Key numbers

Concentrations in mg/kg dry weight (DW). Method LOD/LOQ: inorganic As 0.001/0.01; total As 0.002/0.02.

Laminaria digitata (kelp), n=5 thalli, 6 parts each:

  • Total arsenic: 36–131 mg/kg DW across all parts (highest in distal blade 131 and decaying distal blade 129); mean of thalli 78 ± 24.
  • Inorganic arsenic: 2.2–87 mg/kg DW, increasing from the stipe toward the distal parts; highest 87 mg/kg DW in decaying distal tips.
  • Inorganic arsenic comprised >50% of total arsenic in most middle, distal, and decaying-distal blades, but <50% in stipe, meristem, and young blade.
  • iAs in stipe/meristem/young blade was significantly lower than in middle/distal/decaying-distal blades (one-way ANOVA p=0.0009).
  • Blade-only range (Table 1, stipe and meristem excluded, across five thalli): total As 59–114, inorganic As 30–62 mg/kg DW.

Ascophyllum nodosum (knotted wrack), n=25 thalli, annual segments:

  • Total arsenic: 38–111 mg/kg DW, increasing with thallus-part age, highest (111) in 5-year-old sections; mean of thalli 64 ± 20.
  • Inorganic arsenic: <1% of total throughout; 0.007–0.070 mg/kg DW across yearly sections, with two older (4- and 5-year) sections higher at 0.178 and 0.703 mg/kg DW.
  • Total arsenic was significantly higher in L. digitata than A. nodosum (p=0.026); inorganic arsenic significantly higher in L. digitata (p<0.01).

Additional source-reported facts:

  • The site (Galway Bay) is a routinely-monitored, relatively uncontaminated bay; seawater arsenic is typical of unpolluted Irish west-coast waters.
  • Regulatory comparison: mean L. digitata inorganic arsenic in the young/middle/distal/decaying blades exceeded the French food limit (3 mg/kg DW) and the Australia–New Zealand limit (1 mg/kg) typically by an order of magnitude; A. nodosum inorganic arsenic always satisfied both. Applying the EU feed Directive 2003/100/EC basis (40 mg/kg total As at 12% moisture), mean total As in both species exceeded the limit (69 and 57 mg/kg respectively).
  • The authors’ Table 1 (comparison with other studies) places this study’s L. digitata alongside Taylor & Jackson 2016 (New England: total As 50–106, iAs 2.8–20 mg/kg DW), García-Salgado 2012 (France: iAs 77 ± 3 mg/kg as As(V)), and others, confirming wide between-study variability in kelp inorganic arsenic.
  • Context cited: EFSA (2014) estimated hijiki at 11 mg/kg iAs (lower than the 96 mg/kg of Rose 2007 and 117 mg/kg of Almela 2006); UK FSA (2010) and the Belgian Superior Health Council (2015) advise against hijiki consumption.

Methods (brief)

Whole thalli were collected at low tide (Spiddal, Galway Bay; 53.2409 N, −9.3095 W) in October 2015, rinsed, scraped of epiphytes, freeze-dried, and homogenised. Inorganic arsenic was determined as As(V): 0.100 g freeze-dried algae microwave-extracted (Mars 5) with 1% HNO3 + 2% H2O2 to oxidise As(III) to As(V), then HPLC-ICP-MS (Agilent 1260 HPLC + 8800 ICP-MS, Hamilton PRP-X100 anion-exchange column, 30 mM ammonium carbonate pH 8.5, germanium internal standard, O2 reaction gas, As measured as AsO+ at m/z 91). Total arsenic was determined after overnight concentrated-HNO3 pre-digestion and Ethos Up microwave digestion to 200 °C, by ICP-MS in MS/MS mode (m/z 91). Quality control used CRMs BCR-211 (rice; certified inorganic As) and ERM-CD200 (Fucus vesiculosus; certified total As): total-As recovery 90.4 ± 5.7% (BCR-211) and 96.4% (ERM-CD200); inorganic-As recovery 97.6% (BCR-211). The unequivocal determination of inorganic As as As(V) was confirmed by parallel HPLC-ICP-MS/ESI-MS showing no arsenosugar co-elution interference (acid extraction collapses arsenosugars to the aglycone-free sugar without generating inorganic As). All values are dry weight.

Implications

  • Certification (HMTc): direct-evidence speciation data for the Category 6 seaweed-kelp-foods row (iAs/tAs platform), and the most consequential finding so far for that row’s structure — kelp (Laminaria digitata), a primary component of “kombu”/sea-vegetable products, can itself carry inorganic arsenic up to 87 mg/kg DW (>50% of total in the blades), comparable to hijiki and an order of magnitude above the French and ANZ seaweed limits, even from a clean site. This argues that the within-row high-iAs concern is not hijiki-exclusive and that kelp/Laminaria warrants explicit treatment in the row’s clean/dirty structuring. The strong intra-thallus gradient (stipe low → decaying distal high) and species contrast (A. nodosum negligible) are processing/sourcing-relevant signals. All on the dry-weight, as-harvested basis.
  • App: contributes to the seaweed ingredient contamination_profile for total and inorganic arsenic, with the species- and thallus-part-specific caveats (kelp blades high; knotted wrack low).

Verification notes

  • raw_handle MFK_ronan2017 from the PDF filename; raw_path under “raw/Manual Fetch Kimi /June 8 Inorganic Arsenic Seaweed/“. DOI 10.1016/j.chemosphere.2017.07.076 confirmed on the accepted-manuscript cover. This is the accepted-manuscript version (Chemosphere, CHEM 19609).
  • Evidence tier A: peer-reviewed, HPLC-ICP-MS speciation with CRM-based QC and ESI-MS confirmation of inorganic As as As(V).
  • Speciation: total As and inorganic As (measured as As(V) after As(III)→As(V) oxidation) both reported and kept distinct; lifted to frontmatter as tAs and iAs. No mercury/cadmium/lead/chromium measured.
  • Units/basis preserved exactly (mg/kg dry weight). Inorganic-As ranges (2.2–87 for L. digitata; 0.007–0.703 for A. nodosum) and total-As ranges (36–131; 38–111) taken from the Results text; per-thallus-part mean values shown only graphically (Figure 2) and in supplementary Table S1 (not reproduced numerically here beyond the text-stated ranges and gradient, to avoid reading approximate values off a bar chart). The blade-only ranges (total As 59–114, iAs 30–62) are the source’s Table 1 “this study” row, footnoted as blades of five thalli with stipe and meristem excluded.
  • sample_n=30 reflects the L. digitata thallus-part samples (5 thalli × 6 parts); the 25 A. nodosum thalli (annual-segment pooled samples) are described in sample_population — recorded this way because L. digitata is the food-relevant (kelp/kombu) subject for the seaweed-kelp-foods row.
  • Matrix fit: L. digitata is harvested as a sea vegetable/kombu and for feed; A. nodosum is mainly feed/fertiliser/alginate but is edible macroalgae. Both are in-scope for the seaweed/macroalgae ingredient and the seaweed-kelp-foods product row as wild-harvested raw material; this is a wild intra-thallus distribution study, not a retail-product survey (noted for the occurrence-extraction pass, which may weight retail surveys differently).
  • Brand firewall: not engaged (wild-harvested specimens; no commercial products or brands).
  • Jurisdiction IE; sampling_locations records the Galway Bay (Spiddal) site; sampling_year_range 2015.
  • matrices [edible-seaweed, macroalgae, kelp, dry-weight] per corpus convention; kelp is directly warranted (L. digitata).
  • Instrument/CRM/vendor names (Agilent, Hamilton, Mars 5/Ethos Up, BCR-211, ERM-CD200) retained in Methods as permitted scientific reporting.
  • Audit subagent (2026-06-08, fresh-context) returned PROMOTE on all five checks; speciation fidelity (tAs vs iAs-as-As(V) by species and thallus part) verified exact with no transposition. One ⚠️ applied: intro prose understated the A. nodosum inorganic-As maximum as “≤0.70”; verified against source (0.703 mg/kg DW, Results) and corrected to “≤0.703”. Matrices-vocabulary ⚠️ left as-noted (corpus convention, documented above).

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