Toxicity of arsenic species to Lemna gibba L. and the influence of phosphate on arsenic Bioavailability

The toxicity of arsenic (As) species to Lemna gibba L. and the influence of PO43- on As bioavailability and uptake were tested in batch culture. L. gibba were exposed to six test concentrations of NaHAsO4 . 7H(2)O and NaAsO3 with 0, 0.0136,13.6, and 40 mg L-1 KH2PO4. In batch culture As toxicity to L. gibba did not relate linearly to As concentration. The growth rate, related to frond number as recommended by OECD and ISO/DIN, was significantly inhibited in fronds exposed to 20-50 mug L-1 As(III) compared with fronds exposed to AsM. The growth rate was stimulated when plants were exposed to 50-250 mug L-1 of both As(III) and As(V). After exposure to 300-800 mug L-1 growth inhibition was significantly higher for As(III) than for As(V), whereas above 800 mug L-1 As(V) was inhibited the most. The bioaccumulation of As(III) and As(V) was significantly higher for P-deficient cultures (0.98 +/- 0.08 and 1.02 +/- 0.19 g kg(-1) respectively for 0.0136 mg L-1 PO43-) than for P-sufficient cultures (243 and 343 mg kg(-1) for 40 mg L-1, respectively). Plants exposed to As(V) had uptake and accumulation values slightly higher than did plants exposed to As(III). No significant differences in bioaccumulation were found between plants exposed to a concentration of As(l 11) > 1 mg L-1 and those exposed to AsM at the same concentration. This indicates a direct relationship to P content in the culture. Toxicity may result from the uptake of As(V) instead of PO43- as a result of ion competition during uptake because of close thermodynamic properties, which may change the interaction among components in the media. The toxicity pattern is interpreted as a manifestation of changing speciation in the batch culture and of the oxidation of As(III) to As(V) in an oxygen-rich environment. (C) 2004 Wiley Periodicals, Inc.