Why is NanoSIMS elemental imaging of arsenic in seaweed (Laminaria digitata) important for understanding of arsenic biochemistry in addition of speciation information

Brown seaweed such as Laminaria digitata is known to accumulate arsenic to more than 100 mg/kg. How the algae can tolerate such high level of arsenic has traditionally been studied by arsenic speciation analysis using HPLC-ICPMS, but the knowledge of its molecular forms has not yet given any answers. Here we use HPLC-ICPMS/MS coupled simultaneously to ESI-MS to determine the molecular forms of all arsenic and additionally use the complimentary method of NanoSIMS to identify how arsenic is distributed in the cells and cell substructures. The majority of 117 mg/kg arsenic in L. digitata fronds was in the form of inorganic arsenic (53%) and arsenosugars (32%) and only 1.5% of total arsenic as arsenolipids (mainly as AsHC and AsPL). The majority of arsenic was found in the cell walls and cell membrane, the inside of the cell was almost arsenic free, which is not expected if the majority of arsenic species are hydrophilic. Whether inorganic arsenic and the arsenosugars are bound directly to the polymeric carbohydrates alginates or fucoidans is unclear and needs further investigations.