Organic acids in two arsenic hyperaccumulators and a non-hyperaccumulator of Pteris exposed to elevated arsenic concentrations

Abstract

Six organic acids (oxalic, malonic, malic, citric, palmitic and linolenic acid) in two arsenic (As) hyperaccumulators (Pteris multifida and Pteris vittata) and a non-hyperaccumulator fern (Pteris semipinnata) exposed to different As concentrations (0, 5, 20 and 40 mg As L−1) under hydroponic conditions were determined using gas chromatography-mass spectrometry (GC-MS). Three fern species were collected from an uncontaminated site, but two As-hyperaccumulators were also collected from an As-contaminated site. Furthermore, the time-course effect of organic acid production in P. multifida and P. semipinnata collected from the uncontaminated site were also studied under 0 and 20 mg As L–1 treatments with a sampling interval at 0, 6, 12, 24 and 36 h. After esterifying (H2SO4–CH3OH), extracting (CH2Cl2), washing (saturated NaCl) and drying (anhydrous Na2SO4), these organic acids were isolated (Column: HP Ultra 2, 50 m × 0.2 mM × 0.33 µM; Carrier gas: He). There was no significant increase in the concentrations of the six organic acids in the fronds and roots of the three fern species sampled from an uncontaminated site under 5 or 40 mg As L−1 treatments. In addition, there were also no significant differences in the concentrations of organic acids among the three fern species under the same As concentration treatments. Concentrations of malic, oxalic and linolenic acids in the fronds and roots of two As hyperaccumulators collected from an As-contaminated or uncontaminated site and the non-hyperaccumulator collected from an uncontaminated site followed the sequence of malic acid > oxalic acid > linolenic acid. As for time-course effect, compared to P. semipinnata, a significant increase was observed in concentrations of oxalic and linolenic acid in P. multifida over time under 20 mg As L−1 treatment. In general, the changes of organic acid concentrations in As hyperaccumulators did not directly contribute to As accumulation.