Arbuscular mycorrhizal fungi alter Hg root uptake and ligand environment as studied by X-ray absorption fine structure

- In non-AM plants Hg is bound in di-thiolate and di-amino complexes.
- In AM plants Hg-tetra-thiolate complexes are also present.
- In AM plants the amount of tetra-thiolate complexes increases with arbuscular density.
- Hg-tetra-thiolate complexes are attributed to the fungal partner.

Mercury (Hg) - plant - fungal interactions are only poorly studied. Hg speciation and ligand environment in maize roots inoculated with arbuscular mycorrhizal (AM) fungi were investigated in order to better understand the role of AM in Hg soil to root transfer.The maize plants were grown in Hg polluted substrate (50 μg g−1 as dissolved HgCl2) and inoculated with AM fungi originating from: a) highly Hg polluted environment of a former Hg smelting site in Idrija, Slovenia, (Glomus sp. - sample AmI), and b) non-polluted environment (commercial AM inoculum Symbivit® - sample AmC). Hg speciation and ligand environment in maize roots was studied by Hg-L3 XANES and EXAFS with emphasis on XAS methodology - modelling and fitting the XAFS spectra to extract in a reliable way as much information on Hg coordination as possible.The AmI plants developed more arbuscules and less vesicles than the AmC plants, and also accumulated more Hg in the roots. A clear difference in Hg coordination between the AM (AmC & AmI) and the control (ConC & ConI) plants is recognized in Hg L3-edge EXAFS analysis: in the ConC & ConI maize roots 73-80% of Hg is attached between two sulphur atoms at the distance of 2.34 Å. The remaining ligand is nitrogen at 2.04 Å. In AmI & AmC roots another Hg-S attachment encompassing four thiol groups at the S-distance of ∼2.50 Å are identified, accounting for 21-26%. AM fungi can modify Hg ligand environment in plant roots, thus playing an important role in biogeochemical cycling of Hg in terrestrial ecosystems.

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