Biotransformation of Arsenate to the Tetramethylarsonium Ion in the Marine Polychaetes Nereis diversicolor and Nereis virens

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Abstract
Arsenic compounds in aqueous extracts of the marine polychaetes Nereis diversicolor and Nereis virens were determined by HPLC−ICP-MS. Both polychaetes contained most of their water-soluble arsenic as arsenobetaine (∼60%), and the tetramethylarsonium ion was also a significant arsenic constituent (∼20% in N. diversicolor and ∼30% in N. virens). Trimethylarsoniopropionate and arsenosugars were present in the Nereis species as minor constituents, and traces of arsenocholine, trimethylarsine oxide, and arsenate were also detected. When N. diversicolor and N. virens were exposed in laboratory experiments to different concentrations of arsenate in seawater (10, 50, 100, 500, and 1000 μg of As L-1), both species accumulated arsenic in a dose-dependent manner. At the highest exposure level, they achieved mean arsenic concentrations (dry mass) of ∼70 mg of As kg-1 (N. virens) and 90 mg of As kg-1 (N. diversicolor), which represented about a 5-fold increase when compared with the control animals. The arsenic taken up by the polychaetes was readily methylated, and the major metabolite was the tetramethylarsonium ion (up to ∼85% of the accumulated arsenic). Two other products of arsenic methylation, methylarsonate and trimethylarsine oxide, were also produced to a small extent; however, dimethylarsinate, another likely intermediate in the formation of the tetramethylarsonium ion, could not be detected in these experiments. The remaining accumulated arsenic was present as unchanged arsenate. This is the first report of significant biomethylation of arsenic to the tetramethyl stage and provides a ready explanation for the widespread occurrence of tetramethylarsonium ion in marine animals.