B24C-05
CORRELATIVE CRYO-TEM, CRYO-STXM AND CRYO-SHXM INVESTIGATION OF SELENIUM BIOREDUCTION IN A CONTAMINATED AQUIFER
Tuesday, 15 December 2015: 17:00
2008 (Moscone West)
Sirine Fakra1,2, Birgit Luef3, Tolek Tyliszczak1, Cindy J Castelle4, Sean William Mullin5, Laura A Hug4, Kenneth Hurst Williams6, Matthew Marcus6 and Jillian F Banfield2, (1)Lawrence Berkeley National Laboratory, Advanced Light Source, Berkeley, CA, United States, (2)University of California Berkeley, Earth and Planetary Science, Berkeley, CA, United States, (3)Norwegian University of Science and Technology, Trondheim, Norway, (4)University of California Berkeley, Berkeley, CA, United States, (5)California Institute of Technology, Pasadena, CA, United States, (6)Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Abstract:
Accurate mapping of the composition and ultrastructure of minerals and cells is key to understanding biogeochemical process in contaminated environments. Here we developed two apparatus that allow correlation of cryogenic transmission electron microscopy (TEM), synchrotron hard X-ray microprobe (SHXM) and scanning transmission X-ray microscopy (STXM) datasets. These cryogenic methods enabled precise determination of the distribution, valence state and structure of selenium in intact biofilms sampled during a biostimulation experiment in a contaminated aquifer near Rifle, CO, USA. Results were replicated in the laboratory via anaerobic selenate-reducing enrichment cultures. 16S rRNA analyses of field-derived biofilm indicated the dominance of Betaproteobacteria from the Comamonadaceae family, and uncultivated members of the Simplicispira genus. The major product in field and culture-derived biofilms consists of ~25-300 nm red amorphous Se0 aggregates of colloidal nanoparticles. Correlative analyses of the cultures provided direct evidence for microbial dissimilatory reduction of Se(VI) to Se(IV) to Se0. X-ray diffraction and Se K-edge extended X-ray absorption fine structure spectroscopy revealed red amorphous Se0 with a first shell Se-Se interatomic distance of 2.339 ± 0.003 Å. STXM showed that these aggregates are strongly associated with a protein-rich biofilm matrix containing acidic polysaccharides. From Rifle groundwater, we isolated a strain that shares 98.9% 16S rRNA gene sequence identity with Dechloromonas aromatica RCB and grows anaerobically by oxidizing acetate and reducing selenate. We refer to this isolate as Dechloromonas selenatis strain RGW99. 3D cryo-electron tomography showed that Se0 particles do not form inside the cytoplasm but rather originate in the cell membrane. The end product of selenate reduction by D. selenatis is 240 ± 66 nm diameter red amorphous Se0 colloidal aggregates. This product was found to be stable for months. Overall, these results established a role for D. selenatis RGW99 in selenate reduction in the Rifle aquifer and provided new insights into the nature and stability of selenium bioreduction products in the subsurface.
