Using X-ray Microscopy and Hg L₃ XANES To Study Hg Binding in the Rhizosphere of Spartina Cordgrass

Abstract

San Francisco Bay has been contaminated historically by mercury from mine tailings as well as contemporary industrial sources. Native Spartina foliosa and non-native S. alterniflora-hybrid cordgrasses are dominant florae within the SF Bay estuary environment. Understanding mercury uptake and transformations in these plants will help to characterize the significance of their roles in mercury biogeochemical cycling in the estuarine environment. Methylated mercury can be biomagnified up the food web, resulting in levels in sport fish up to one million times greater than in surrounding waters and resulting in advisories to limit fish intake. Understanding the uptake and methylation of mercury in the plant rhizosphere can yield insight into ways to manage mercury contamination. The transmission x-ray microscope on beamline 6-2 at the Stanford Synchrotron Radiation Lightsource (SSRL) was used to obtain absorption contrast images and 3D tomography of Spartina foliosa roots that were exposed to 1 ppm Hg (as HgCl2) hydroponically for one week. Absorption contrast images of micron-sized roots from S. foliosa revealed dark particles, and dark channels within the root, due to Hg absorption. 3D tomography showed that the particles are on the root surface, and slices from the tomographic reconstruction revealed that the particles are hollow, consistent with microorganisms with a thin layer of Hg on the surface. Hg L3 XANES of ground-up plant roots and Hg L3 micro-XANES from microprobe analysis of micron-sized roots (60–120 microns in size) revealed three main types of speciation in both Spartina species: Hg-S ligation in a form similar to Hg(II) cysteine, Hg-S bonding as in cinnabar and metacinnabar, and methylmercury-carboxyl bonding in a form similar to methylmercury acetate. These results are interpreted within the context of obtaining a “snapshot” of mercury methylation in progress.

Phase retrieval using polychromatic illumination for transmission X-ray microscopy

“…Different research areas ranging from material science [10,13] to biomedical studies [11,14] have benefitted from this technique. In cases that investigate weakly absorbing samples (mainly with low Z elements), phase contrast imaging (PCI) and, subsequently, quantitative phase retrieval are especially important.…”

TXM-Wizard: a program for advanced data collection and evaluation in full-field transmission X-ray microscopy

“…The unique capability of imaging three-dimensional structure with spatial resolution down to a few tens of nanometers (Chao et al, 2005;Schneider et al, 2010; over a field of view of several tens of micrometers has made TXM a well recognized tool for research ranging from materials science (Chen et al, 2008;Li et al, 2009;Nelson et al, 2011) to biological studies (Larabell & Le Gros, 2004;Andrews et al, 2010;Patty et al, 2009;Schneider et al, 2010).…”

Applications of Hard X‐ray Full‐Field Transmission X‐ray Microscopy at SSRL

“…Different research areas, including materials investigation [1][2], environmental science [3], and biological studies [4][5] have benefitted from this technique. Illuminated by x-rays from a 56-pole, 0.9-Tesla wiggler, the TXM at BL6-2C at SSRL [4] has multiple sets of x-ray optics (capillary condensers and Fresnel zone plates) to cover the working energy range from 5.0 keV to 15.0 keV.…”