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

Abstract

Seaweed can accumulate inorganic arsenic (iAs) from seawater as hydrogen arsenate (HAsO₄^2-) in place of the phosphate anion (HPO₄^2-). While it is rapidly metabolised to organoarsenic species, predominantly arsenosugars and arsenolipids, iAs may be present in seaweed biomass and this poses a potential health concern for consumers of seaweed products. Here, the distribution of total (As_TOT) and iAs was determined in thallus parts of the kelp Laminaria digitata and the intertidal fucoid Ascophyllum nodosum (both Phaeophyceae) using inductively-coupled plasma mass spectrometry (ICP-MS) and high performance liquid chromatography - ICP-MS (HPLC-ICP-MS). As_TOT ranged from 36 to 131 mg kg^-1 dry weight (DW) in L. digitata, and from 38 to 111 mg kg^-1 DW in A. nodosum, with no statistically significant differences between different thallus parts. iAs was detected in all A. nodosum samples, comprising less than 1% of the As_TOT content. Concentrations of iAs in L. digitata were significantly higher, ranging from 2.2 to 87 mg kg^-1, increasing through the thallus from the stipe to the decaying distal blades. iAs comprised more than 50% of As_TOT in the middle to decaying distal blades. This finding has potential implications for harvesting, processing and use of Laminaria digitata in agri-, food and health applications.

Keywords: Arsenic; Brown seaweed; HPLC-ICPMS; Macroalgae; Speciation.