Uptake and metabolism of arsenate, methylarsonate and arsenobetaine by axenic cultures of the phytoplankton Dunaliella tertiolecta

Axenic cultures of the phytoplankton Dunaliella tertiolecta were dosed with either arsenate [As(V)], methylarsonate (MA) or arsenobetaine (AB) at environmentally realistic concentrations (2 mu g l(-1)) to investigate incorporation and transformation of arsenic species. Total arsenic concentrations in cultures dosed with As(V) were significantly higher than those dosed with MA and AB (6-10 mu g g(-1) compared to 1-3 mu g g(-1)). Arsenic concentrations in As(V)-dosed cultures increased over time, whereas arsenic concentrations in MA- and AB-dosed cultures remained constant, indicating that As(V) is rapidly and continuously incorporated into the cell in comparison to MA and AB. Arsenic was found predominantly in the residue fractions (29-55%) of all cultures, irrespective of culture age. This was attributed to the ability of inorganic arsenic as arsenite [As(III)] to bind to proteins and/or phytochelatins. 2-Dimethylarsinoylethanol (DMAE) was the main arsenic species found in lipid cell fractions. Within the water-soluble and residue cell fractions, arsenic was isolated primarily as As(V), although it is likely the extraction procedure may have converted some As(III) to As(V). Small amounts of As(III), MA, dimethylarsinate (DMA) and arsenoribosides were also found in the lipid and water soluble As fractions. All cultures showed an increase in DMA and arsenoriboside species over time. The significance of these results is that while AB, the major arsenical in marine animals, is not produced or accumulated by this phytoplankton species, the possible precursors to AB formation are produced.