title: Structural and spectral features of selenium nanospheres produced by Se-respiring bacteria.
abstract: Certain anaerobic bacteria respire toxic selenium oxyanions and in doing so produce extracellular accumulations of elemental selenium [Se(0)]. We examined three physiologically and phylogenetically diverse species of selenate- and selenite-respiring bacteria, Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter shriftii, for the occurrence of this phenomenon. When grown with selenium oxyanions as the electron acceptor, all of these organisms formed extracellular granules consisting of stable, uniform nanospheres (diameter, approximately 300 nm) of Se(0) having monoclinic crystalline structures. Intracellular packets of Se(0) were also noted. The number of intracellular Se(0) packets could be reduced by first growing cells with nitrate as the electron acceptor and then adding selenite ions to washed suspensions of the nitrate-grown cells. This resulted in the formation of primarily extracellular Se nanospheres. After harvesting and cleansing of cellular debris, we observed large differences in the optical properties (UV-visible absorption and Raman spectra) of purified extracellular nanospheres produced in this manner by the three different bacterial species. The spectral properties in turn differed substantially from those of amorphous Se(0) formed by chemical oxidation of H(2)Se and of black, vitreous Se(0) formed chemically by reduction of selenite with ascorbate. The microbial synthesis of Se(0) nanospheres results in unique, complex, compacted nanostructural arrangements of Se atoms. These arrangements probably reflect a diversity of enzymes involved in the dissimilatory reduction that are subtly different in different microbes. Remarkably, these conditions cannot be achieved by current methods of chemical synthesis.
authors.affiliation: Water Resources Division, U.S. Geological Survey, Menlo Park, California 94025, USA. roremlan@usgs.gov
authors.forename: Ronald S
authors.initials: RS
authors.lastname: Oremland
authors.forename: Mitchell J
authors.initials: MJ
authors.lastname: Herbel
authors.forename: Jodi Switzer
authors.initials: JS
authors.lastname: Blum
authors.forename: Sean
authors.initials: S
authors.lastname: Langley
authors.forename: Terry J
authors.initials: TJ
authors.lastname: Beveridge
authors.forename: Pulickel M
authors.initials: PM
authors.lastname: Ajayan
authors.forename: Thomas
authors.initials: T
authors.lastname: Sutto
authors.forename: Amanda V
authors.initials: AV
authors.lastname: Ellis
authors.forename: Seamus
authors.initials: S
authors.lastname: Curran
mesh_terms: D000693:Anaerobiosis; D001407:Bacillus / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure; D001419:Bacteria / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D003470:Culture Media; D003594:Cytoplasmic Granules / Q000737:chemistry / Q000378:metabolism; D020565:Epsilonproteobacteria / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D012643:Selenium / Q000737:chemistry* / Q000378:metabolism*; D013059:Spectrum Analysis, Raman
chemical_list: D003470:Culture Media; D012643:Selenium