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

Abstract

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.

Potential of macrophyte for removing arsenic from aqueous solution

“…At this concentration, L. gibba accumulated about 10-fold less As than when exposed to 0.008 μmol L -1 phosphate, corroborating the data of Mkandawire et al (2004). In the present study, L. gibba removed about 4% As from the solution containing 0.25 μmol L -1 phosphate, whereas in solution without phosphate it removed about 40% As from the solution (Mkandawire et al, 2004), showing its potential for extracting the metalloid from the environment.…”

“…In this study, three species of aquatic macrophytes were used (Azolla caroliniana, Salvinia minima and Lemna gibba) which, according to the literature, have shown potential for remediation of impacted aquatic environments (Jain et al, 1990;Zayed et al, 1998;Axtell et al, 2003;Mkandawire et al, 2004;Mkandawire & Dudel, 2005).…”

Phosphate regulates uranium(VI) toxicity to Lemna gibba L. G3

“…The synthetic mine water was approximately a 5Â dilution of standard Hutner medium (Table I), adjusted to physicochemical properties and values within the range observed in surface waters of abandoned uranium mines at Lengenfeld and Mechelgrün in eastern Germany (Mkandawire et al, 2004b(Mkandawire et al, , 2005a. Selected physicochemical and water quality parameters of interest from the field are presented in Table II.…”

“…semicontinuous mode maintained steady-state conditions in the culture by discharging and recharging 20% of the media, while harvesting the excess L. gibba biomass every 2 days (Mkandawire et al, 2004b(Mkandawire et al, , 2005a. The culture vessels were shaken continuously to maintain homogeneity and to allow exchange of gases that accumulated during L. gibba growth (Mkandawire et al, 2004b(Mkandawire et al, , 2005a.…”

“…The culture vessels were shaken continuously to maintain homogeneity and to allow exchange of gases that accumulated during L. gibba growth (Mkandawire et al, 2004b(Mkandawire et al, , 2005a. The ecotron was programmed to a 16 and 8 h day-night cycle attaining a maximum temperature of 248C (for an 8 h period) during the day and minimum temperature of 168C (for an 8 h period) during the night.…”