Speciation and distribution of arsenic in the non-hyperaccumulator macrophyte Ceratophyllum demersum L.

Abstract

Although arsenic is a common pollutant worldwide, many questions about As metabolism in non-hyperaccumulator plants remain. Concentration and tissue dependent speciation and distribution of arsenic was analysed in the aquatic plant Ceratophyllum demersum to understand arsenic metabolism in non-hyperaccumulator plants. Speciation was analysed chromatographically (HPLC-(ICP-MS)-(ESI-MS)) in whole-plant extracts and by tissue-resolution confocal X-ray absorption spectroscopy (µ XANES) in intact shock-frozen hydrated leaves, which were also used for analysing cellular element distribution through X-ray fluorescence (µ XRF). Chromatography revealed up to 20 As-containing species binding >60% of accumulated As. Of these, eight were identified as thiol-bound (phytochelatins; PCs, glutathione; GSH and cystein) species including three newly identified complexes: Cys-As(III)-PC2, Cys-As-(GS)2 and GS-As(III)-desgly-PC2 complex. Confocal µ XANES showed As(V), As(III), As-(GS)3 and As-PCs with varying ratios in various tissues. The epidermis of mature leaves contained the highest proportion of thiol- (mostly PC-) bound As, while in younger leaves a lower proportion of As was thiol-bound. At higher As concentrations, the percentage of unbound As(III) increased in the vein and mesophyll of young mature leaves. At the same time, µ XRF showed an increase of total As in the vein and mesophyll but not in the epidermis of young mature leaves while it was reverse for Zn distribution. Thus, As toxicity was correlated with a change in As distribution pattern and As species, rather than general increase in many tissues.