Mercury Localization and Speciation in Plants Grown Hydroponically or in a Natural Environment

Abstract

Better understanding of mercury (Hg) accumulation, distribution, and speciation in plants is required to evaluate

potential risks for the environment and to optimize phytostabilization strategies for Hg-contaminated soils. The behavior of Hg

in alfalfa (Medicago sativa) plants grown under controlled conditions in a hydroponic system (30 μM HgCl2) was compared with

that of naturally occurring Horehound (Marrubium vulgare) plants collected from a mining soil polluted with Hg (Almadenejos,

Spain) to characterize common mechanisms of tolerance. Synchrotron X-ray Fluorescence microprobe (μ-SXRF) showed that

Hg accumulated at the root apex of alfalfa and was distributed through the vascular system to the leaves. Transmission electron

microscopy (TEM) implied association of Hg with cell walls, accompanied by their structural changes, in alfalfa roots. Extended

X-ray absorption fine structure (EXAFS) determined that Hg was principally bound to biothiols and/or proteins in M. sativa

roots, stems, and leaves. However, the major fraction of Hg detected in M. vulgare plants consisted of mineral species, possibly

associated with soil components. Interestingly, the fraction of Hg bound to biothiols/proteins (i.e., metabolically processed Hg)

in leaves of both plants (alfalfa and M. vulgare) was similar, in spite of the big difference in Hg accumulation in roots, suggesting

that some tolerance mechanisms might be shared.