Mercury Localization and Speciation in Plants Grown Hydroponically or in a Natural Environment Sandra Carrasco-Gil,* ,‡,§,† Hagar Siebner, ∥,†,¶ Danika L. LeDuc, ⊥ Samuel M. Webb, # Rocío Milla ́ n, § Joy C. Andrews, ⊥,# and Luis E. Herna ́ ndez ‡ ‡ Laboratory of Plant Physiology, Department of Biology, Universidad Autó noma de Madrid, 28049 Madrid, Spain § Centro de Investigaciones Energe ́ ticas, Medioambientales y Tecnoló gicas, Avd. Complutense, 22, 28040 Madrid, Spain ∥ Department of Geological and Environmental Sciences, Stanford University, 367 Panama St., Stanford, California 94305-2115, United States ⊥ Department of Chemistry and Biochemistry, California State University, East Bay 25800 Carlos Bee Boulevard, Hayward, CA # Stanford Synchrotron Radiation Lightsource, 2575 Sand Hill Road, SLAC MS 69 Menlo Park, CA * S Supporting Information 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 HgCl 2 ) 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. ■ INTRODUCTION Mercury is highly toxic to humans and ecosystems and is considered a global pollutant because it is highly mobile and extremely persistent in the environment. 1 The occurrence of heavily Hg-polluted soils is frequently associated with natural deposition, which is derived from the weathering of Hg- containing bedrock minerals, and anthropogenic dispersion from mining and smelting activities as is the case for Hg in the mining Almade ́ n district, Spain. 2 In such areas, the phytostabi- lization of pollutants using the innate properties of plants to accumulate them in below ground organs might be the most feasible phytoremediation strategy to use, creating a vegetation cover to limit Hg dispersion in the environment. 3 In addition, Hg uptake by aboveground organs increases the risk of its entry to the food chain; a problem aggravated by agricultural practices performed in the surroundings of abandoned mines and smelting facilities. 4 Mercury can be found in the environment in several forms including the monatomic metallic form (Hg 0 ), ionic Hg (Hg + , Hg 2+ ) as a component of metal ores (e.g., in cinnabar; HgS), or in methylated forms (CH 3 Hg + , (CH 3 ) 2 Hg), which cause the highest toxic effect in humans. 5 Depending on environmental conditions, Hg might also undergo transformations, generally through reduction or methylation/demethylation, 6,7 affecting its toxicity and distribution in the environment. Exposure to Received: August 15, 2012 Revised: December 22, 2012 Accepted: February 13, 2013 Published: February 13, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 3082 dx.doi.org/10.1021/es303310t | Environ. Sci. Technol. 2013, 47, 3082−3090