Environmental context. Despite the widespread occurrence of arsenobetaine in marine animals the origin of this arsenic compound remains unknown. A current hypothesis is that arsenobetaine is formed from more complex arsenic compounds found in marine algae. To test this hypothesis, we examined the arsenic compounds in a mangrove ecosystem where algae play a limited role in primary productivity.

Abstract. Marine algae are known to bioaccumulate arsenic and transform it into arsenosugars, which are thought to be precursors of the major arsenic compound, arsenobetaine, found in marine animals. Marine ecosystems based on mangrove forests have high nutrient input from mangrove leaves, and thus provide a unique opportunity to study the cycling of arsenic in a marine system where algae are not the dominant food source. Two mangrove forests in Phuket, Thailand were selected as sampling sites for this study. For comparison, samples were also collected from two coral reef sites at and near Phuket. The samples collected included mangrove leaves, corals, algae, molluscs, fish and crustaceans. Arsenic contents in the samples and in aqueous extracts of the samples were determined by hydride generation atomic absorption spectrometry following a dry-ashing mineralisation procedure, and arsenic species were determined in the aqueous extracts by HPLC-MS (mainly ICPMS). Mangrove leaves contained only low concentrations of total arsenic (0.10–0.73 mg kg^–1 dry mass) and the aqueous extracts thereof contained inorganic arsenic species, methylarsonate and dimethylarsinate, but arsenosugars were not detected. The total mean arsenic contents (3.2–86 mg kg^–1 dry mass) of the animals from the mangrove ecosystem, however, were typical of those found in animal samples from other marine ecosystems. Similarly the arsenic compounds present were typical of those in animals from other marine ecosystems comprising mainly arsenobetaine with smaller quantities of other common arsenicals including arsenosugars, arsenocholine, tetramethylarsonium ion, trimethylarsine oxide and dimethylarsinate. A trimethylated arsenosugar, which is not commonly reported in marine organisms, was a significant arsenical (6–8% of total As) in some gastropod species from the mangrove ecosystem. The coral samples contained mainly arsenosugars and arsenobetaine, and the other animals collected from the coral ecosystem contained essentially the same pattern of arsenicals found for the mangrove animals. The data suggest that food chains based on algae are not necessary for animals to accumulate large concentrations of arsenobetaine.