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 PO(4) (3-) on As bioavailability and uptake were tested in batch culture. L. gibba were exposed to six test concentrations of NaHAsO(4). 7H(2)O and NaAsO(3), with 0, 0.0136, 13.6, and 40 mg L(-1) KH(2)PO(4). 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 microg L(-1) As(III) compared with fronds exposed to As(V). The growth rate was stimulated when plants were exposed to 50-250 microg L(-1) of both As(III) and As(V). After exposure to 300-800 microg L(-1) growth inhibition was significantly higher for As(III) than for As(V), whereas above 800 microg 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) PO(4) (3-)) 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(III) >1 mg L(-1) and those exposed to As(V) 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 PO(4) (3-) 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.