RESUMO
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.
