The closed anaerobic decomposition extracts of Fucus distichus incubated with seawater and sediment, and without sediment as control, were subjected to extractions and isolation on Sephadex LH 20 and Cellulose Thin Layer Chromatography. The decomposition extracts and isolates were analyzed by using both the Hydride Generation Gas Chromatography Atomic Absorption Spectrometry (HG-GC-AAS) and High Performance Liquid Chromatography Inductively Coupled Plasma Mass Spectrometry (HPLC-ICPMS) to identify the arsenic species in the equilibrium mixtures of the seaweed and filtrates separately. In the methanol seaweed extract, equilibrium mixture of arsenosugars (AS) AS1 and AS2 and their biotransformation products of dimethylarsinoylethanol (DMAE) and dimethylarsinic acid (DMAA) were identified. In the methanol filtrate extract of the mixture, only DMAE and DMAA were identified. However, in the control methanol filtrate extract five organoarsenic species, AS1 and AS2, one unidentified hidden organoarsenic species, DMAE and DMAA were identified in the equilibrium mixture. This result confirmed that the hidden organoarsenic species in Fucus distichus, AS1 and AS2, and an unidentified organoarsenic compounds are biotransformed to only DMAE and DMAA under an anaerobic condition. This also suggests that DMAE and DMAA are strong intermediate candidates for the generation of arsenobetaine, from arsenoribosides in the marine food webs. 1. Introduction Arsenic is a very ubiquitous element in the environment [1]. Inorganic and organic arsenic compounds have extremely different toxicological effects [1, 2]. Concerns of possible human health effects of the toxic arsenic from dietary intake have continued to stimulate research on arsenic biogeochemistry including its bioaccumulation and biotransformation in both the freshwater and marine food chains [2]. Arsenic enters the marine environment from both the natural and anthropogenic point and diffuse sources [3, 4]. Some of the highest concentrations of arsenic in biota are encountered in marine organisms because they have the ability to concentrate arsenic from different food sources [5]. Marine plants, particularly algae, are known to accumulate high arsenic contents [6, 7]. As primary producers, phytoplankton and marine macroalgae accumulate inorganic arsenic and biotransform them into complex organic molecules that are either water- or lipid-soluble arsenic compounds. When fed upon by the higher trophic level, the water- and lipid-soluble organoarsenicals are further metabolized into other distinct arsenicals and/or they
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