DATE: Friday, February 01, 2019 TIME: 2:30 pm (refreshments at 2:15 pm) PLACE: ENR building, room 223 14 College Farm Road, New Brunswick, NJ
Sara Thomas
Molecular Environmental Geochemistry Group, Princeton University
MERCURY (Hg) COORDINATION IN BACTERIA BY HIGH ENERGY RESOLUTION (HR)-XANES SPECTROSCOPY
Anaerobic bacteria and archaea possessing the hgcAB gene cluster can transform inorganic Hg(II) into methylmercury (MeHg), a potent neurotoxin that biomagnifies in aquatic food webs. The microbially-mediated conversion of Hg(II) to MeHg is the primary source of MeHg in the environment. For Hg methylation to occur, Hg(II) must be internalized by the cell and hence interact with ligands in the cell membrane and cytoplasm. Hg(II) has a high affinity for reduced sulfur which exists both as thiols and sulfides in bacteria, and thus, Hg(II) is likely transported into the cell while bound to reduced sulfur. However, the pathway(s) for Hg uptake and the internalized Hg species have yet to be identified.
Understanding the cell-associated Hg(II) coordination environments under conditions that lead to high as well as low MeHg production could provide insight into the Hg(II) uptake mechanisms. This presentation provides an overview of the recently developed technique high energy resolution (HR)-XANES and shows how HR-XANES spectroscopy can be used to directly determine the Hg coordination environment in bacteria. HR-XANES spectra have more distinguishing features compared to conventional XANES spectra, which offer (1) the ability to distinguish closely related Hg species (i.e., Hg(SR)4 vs. β-HgS) and (2) lower Hg detection limits. As a result, bacterial suspensions exposed to Hg concentrations as low as 50 nM (~0.5 - 1 µg Hg per g bacteria) can be assessed, approaching environmentally-relevant conditions. From HR-XANES, the average Hg coordination number to sulfur is greatest under exposure conditions that are correlated with high Hg(II) uptake and MeHg production, suggesting that these Hg species may be involved in Hg(II) uptake.