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The metabolism of arsenate by the unicellular marine phytoplankton Dunaliella tertiolecta was studied using the isotope 74As. During a 45-min-exposure period the algal cells synthesized three polar arsenic metabolites which were identified as arsenite, monomethylarsonic acid, and dimethylarsinic acid. A large fraction of assimilated arsenic appeared in the lipid fraction and three arsenolipids could be separated by thin-layer chromatography. The turnover of dimethylarsinic acid and arsenolipids was rapid since all four metabolites reached equilibrium levels within the exposure period. Two of the arsenolipids were found to be labile in acid solution, and the primary acid hydrolysis product in both cases was identified as arsenite. One of these acid-labile arsenolipids behaved as an anionic compound on diethylaminoethyl cellulose ion-exchange columns and was also susceptible to hydrolysis by phospholipase D. A chromatographically identical compound could be synthesized from phosphadityl inositol and arsenite. The acid-labile anionic phosphatide produced by algae is therefore proposed to be a complex between arsenite and phosphadityl inositol.Key words: arsenic, arsenite, dimethylarsinic acid, phytoplankton, arsenolipids, phosphadityl inositol

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