Arsenic biotransformations in terrestrial organisms: A study of the transport and transformation of arsenic in plants, fungi, fur and feathers, using conventional speciation analysis and X-ray absorption spectroscopy
Smith, Paula Graham
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Arsenic taken up by plants and fungi from contaminated soils can subsequently be introduced into food chains. Given the toxic properties of some arsenic compounds, this may be a cause for concern. Much remains to be learned about how these compounds are transformed and distributed in terrestrial organisms. Radishes, white button mushrooms, fur, and feather samples were thus investigated to gain a better understanding of arsenic biotransformations in terrestrial organisms. In this study, we utilized two analytical techniques for the detection and identification of arsenic compounds (“arsenic speciation analysis”). High performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) provided a highly sensitive method for detecting low levels (ng•g-1) of methanol:water extractable arsenic compounds. X-ray absorption spectroscopy (XAS) techniques provided direct arsenic speciation analysis of tissues, resulting in a more representative arsenic profile of the original organisms, without the need to extract arsenic first. Overall, the results for speciation analysis underline the complementary nature of the HPLC-ICP-MS and XAS techniques. Mushrooms contained organic arsenic compounds which were not identified in the radish. In particular arsenobetaine (AB), which is usually found as a minor constituent of terrestrial organisms, was a predominant arsenic compound found in mushroom extracts. How AB is synthesized in the environment remains unclear; however, results presented here suggest it was a product of fungal biotransformation and we speculate it may play a role in osmoregulation. In radish, fur and feather samples, direct analysis identified arsenic(III)-sulphur compounds not observed using HPLC-ICP-MS. In plants, these compounds are likely to be metal/metalloid binding phytochelatin proteins the formation of which has yet to be confirmed in planta. In radish plant vasculature, XAS imaging revealed segregation of pentavalent and trivalent arsenic compounds, suggesting differences in arsenic transport. In hair and feathers the formation of arsenic(III)-sulphur compounds may be evidence of arsenic binding to keratin proteins which has been hypothesized to occur, and may contribute to the observed reduction of exogenous arsenic contamination.