HR: 0830h
AN: B41D-0914    [PDF]
TI: Rare Arsenic-Antimony-Sulphide Bio-immobilization and Bacterial S-layer Preservation in Siliceous Sediments from Champagne Pool Hot-Spring, Waiotapu, New Zealand
AU: * Phoenix, V R
EM: vernon@geology.utoronto.ca
AF: Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Renaut, R W
EM: robin.renaut@usask.ca
AF: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2 Canada
AU: Jones, B
EM: brian.jones@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3 Canada
AU: Ferris, F G
EM: ferris@geology.utoronto.ca
AF: Dept. of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: Champagne Pool is a large (65 m diameter, 150 m deep) hot spring in the Waiotapu geothermal area of North Island, New Zealand. The spring discharges water of a mildly acid chloride type, with a pH of 5.2, a constant temperature of 75 $^{o}$C and a silica concentration of 460 ppm. Siliceous sinter, loose sediments, and flocs suspended in the spring water are composed of opaline silica and metal-rich sulfides that contain many well-preserved, mineralized microbes. Detailed analysis by transmission electron microscopy and energy dispersive spectrometry has shown that bacterial cell wall and capsular material is preserved by the immobilization of high levels of As (up to 33 wt%), Sb (up to 60 wt%), and S (up to 20 wt%) in the organic matrix. Significant precipitate formation is absent (and when present only small microcrysts form), suggesting much of the As-Sb-S has accumulated through adsorption processes. When extensive biomineral precipitates are present upon the cell wall, they are composed of Al rich amorphous silicates. This suggests a 2-step biomineralization process whereby As and Sb sulfur complexes are adsorbed onto the cell surface polymers first, followed by inorganically driven precipitation of the supersaturated amorphous silica phase. Despite the lack of detailed preservation, biomineralization commonly preserves S-layers, an ordered mosaic of proteins on the outer surface of the cell wall. These are the finest ultrastructural details thus far found preserved by hot-spring biomineralization. Preserved S-layers exhibited either a hexagonal (p6) or square (p4) lattice structure with unit-unit spacing of 9.7 $\pm$ 1.6 nm. Because S-layer morphology varies considerably between species it can be used as a fingerprint to aid identification of microfossils. By considering both S-layer morphology and the hot-spring habitat (pH, Eh, temp etc) it is suggested the S-layers preserved here belong to {\it Clostridium thermohydrosulfuricum} or {\it Desulfotomaculum nigrifacans}. To determine possible mechanisms behind S-layer preservation, geochemical speciation calculations were undertaken to determine the probable inorganic species present in the hot spring water, using the REACT component of the Geochemists Workbench software package (GWB version 3.2.1). Mineral saturation indices were also calculated using this code. Results suggest that arsenic and antimony are present in the spring water as negative and neutrally charged thio or hydroxide complexes such as HAs$_{2}$S$_{4}$$^{-}$, H$_{3}$AsO$_{3}$ and HSb$_{2}$S$_{4}$$^{-}$. Furthermore, all As, Sb and S mineral phases were undersaturated. This explains the lack of notable As-Sb-S precipitate upon the cells and adds credence to the idea that the As-Sb-S phases may have been adsorbed onto the organic polymers. The observation that S-layers are preserved whilst other ultrastuctures are not may indicate a novel mechanism is involved in S-layer preservation at this hot-spring site. S-layers are dominated by amine groups which would be positively charged at the pH of the spring water. Although tentative, it is speculated the early adsorption of negative As-S and Sb-S complexes onto reactive amine groups on S-layers is paramount in the excellent preservation of these delicate ultrastructures.
DE: 0400 Biogeosciences
DE: 1045 Low-temperature geochemistry
DE: 8424 Hydrothermal systems (8135)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting