Arsenic is not stored as arsenite - phytochelatin complexes in the seaweeds Fucus spiralis and Hizikia fusiforme

Abstract

Environmental context Seaweeds hyperaccumulate the toxic metalloid arsenic, but seemingly achieve detoxification by transformation to arsenosugars. The edible seaweed hijiki is a notable exception because it contains high levels of toxic arsenate and arsenite. Terrestrial plants detoxify arsenic by forming arsenite–phytochelatin complexes. The hypothesis that seaweeds also synthesise phytochelatins to bind arsenite as a means of detoxification before arsenosugar synthesis is tested in this investigation. Abstract Phytochelatins (PCs), generic structure [γ-Glu-Cys]n-Gly, are peptides synthesised by terrestrial plants to bind toxic metal(loid)s such as cadmium and arsenic. Seaweeds are arsenic hyperaccumulators, seemingly achieving detoxification via arsenosugar biosynthesis. Whether seaweeds synthesise PCs to aid detoxification during arsenic exposure is unknown. Hizikia fusiforme (hijiki) and Fucus spiralis were used as model seaweeds: the former is known for its large inorganic arsenic concentration, whereas the latter contains mainly arsenosugars. F. spiralis was exposed to 0, 1 and 10 mg L–1 arsenate solutions for 24 h, whereas hijiki was analysed fresh. All samples contained AsIII, glutathione and reduced PC2, identified using HPLC-ICP-MS/ES-MS. Although hijiki contained no AsIII–PC complexes, arsenate exposed F. spiralis generated traces of numerous arsenic compounds that might be AsIII–GS or AsIII–PC2 complexes. AsIII–PC complexes seem not to be a principal storage form for long-term arsenic storage within seaweeds. However, 40 times higher glutathione concentrations were found in hijiki than F. spiralis, which may explain how hijiki deals with its high inorganic arsenic burden.