Nour et al. 2025 — Heavy metals in seaweeds from Djibouti
Six seaweed species from the Djiboutian coastline were analyzed for nutritional composition and heavy metal content (Cr, Ni, As, Cd, Pb) by ICP-MS, providing one of the first published occurrence datasets for edible marine algae from the Horn of Africa region. Results showed wide inter-species variation, with arsenic concentrations notably elevated in the brown alga Turbinaria decurrens (70.2 µg/g dry weight) and chromium highest in the green alga Ulva clathrata (29.61 µg/g DW). The paper documents both nutritional value and contamination risk for seaweeds increasingly harvested and consumed in East African coastal communities and entering global health-food markets.
Key numbers
All values in µg/g dry weight (= mg/kg DW = ppm DW). Method: ICP-MS.
Species results from Table 2:
| Species | Cr | Ni | As | Cd | Pb |
|---|---|---|---|---|---|
| Padina pavonica | 9.91 | 8.68 | 12.5 | 1.37 | 1.25 |
| Sargassum ilicifolium | 2.45 | 1.25 | 37.4 | 1.25 | 0.32 |
| S. latifolium | 2.84 | 2.71 | 37.3 | 1.77 | 0.54 |
| Turbinaria decurrens | 2.21 | 0.65 | 70.2 | 1.00 | 0.11 |
| Ulva clathrata | 29.61 | 3.38 | 7.31 | 0.22 | 1.90 |
| Hypnea musciformis | 8.44 | 5.24 | 6.72 | 0.65 | 2.26 |
Key observations: Arsenic is dramatically elevated in the brown algae Turbinaria decurrens (70.2 µg/g DW) — substantially above EU limits for seaweed products. Chromium is highest in Ulva clathrata at 29.61 µg/g DW, an order of magnitude above other species. Cadmium ranges 0.22–1.77 µg/g DW across species. Lead ranges 0.11–2.26 µg/g DW. All values are dry weight; the iAs/tAs split was not performed — arsenic reported as total arsenic only.
Note on units: 1 µg/g DW = 1 mg/kg DW = 1000 µg/kg DW = 1000 ppb DW. At a typical seaweed dry-to-wet conversion factor of approximately 8:1 (seaweeds are ~87% water), wet-weight concentrations are roughly 8× lower than the DW values tabulated above.
Methods (brief)
Seaweed samples were collected from two sites along the Djiboutian coast (Red Sea coast near Djibouti City and Gulf of Aden). Species were identified taxonomically. Dried and powdered samples were acid-digested and analyzed by ICP-MS for simultaneous multi-element detection. Nutrient composition (proximate analysis, minerals) was also reported. Speciation of arsenic into iAs and organic forms was not performed. Chromium reported as total Cr only.
Implications
Certification: Seaweeds and sea vegetables are increasingly used as food ingredients in health products, functional foods, and dietary supplements. This dataset establishes baseline heavy metal levels for East African seaweeds. As concentrations in T. decurrens (70.2 µg/g DW) are well above EU limits for food supplements from algae (typically 3 mg/kg DW for iAs), though this is tAs and speciation is unknown.
Courses: Illustrates species-level variation in heavy metal uptake in marine algae; demonstrates need for species-specific risk assessment rather than treating “seaweed” as a uniform matrix.
App: Seaweed ingredient category — confirm tAs as primary concern at high end; Cr and Cd also relevant.
Microbiome: Not applicable.