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

Seaweed can accumulate inorganic arsenic (iAs) from seawater as hydrogen arsenate (HAsO42-) in place of the phosphate anion (HPO42-). 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 (AsTOT) 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). AsTOT 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 AsTOT 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 AsTOT 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.