HR: 08:45h
AN: B21A-04    [PDF]
TI: Determining the Feasibility of Organosilicate Utilization by Diatoms for Silica Biomineralization
AU: * Sahai, N
EM: sahai@geology.wisc.edu
AF: University of Wisconsin, 1215 West Dayton Street, Madison, WI 53711
AB: The utilization of silicon by microscopic, unicellular, silica-precipitating organisms such as diatoms, sponges and radiolaria controls the cycling of silicon in oceans. A long-standing question has been the chemical form (aqueous speciation) in which dissolved silicon is utilized by the organisms. Silicic acid and organosilicates are the most commonly proposed forms in the literature. In this study, ab initio calculations at the 6-311+G(2d,p)/HF//6-31G* level were used to calculate optimized structures, formation energies, and Si-29 NMR isotropic chemical shifts of organosilicates putatively involved in diatom metabolism. Quadra-coordinated, penta-coordinated and hexa-coordinated complexes of silicates with polyalcohols, polysaccharides, polycarboxylic acids, and amino-acids were investigated. The Si-29 peak at -101 ppm experimentally observed by Kinrade et al. (1999, 2001), is matched by spirocyclic, penta-coordinated organosilicates in 5-membered rings, but not by the 7-membered rings previously suggested in the literature. A heptet observed at -141 ppm in the H-1-Si-29 coupled spectrum (Kinrade et al., 2001) is best explained by hexa-coordinated 5-membred spirocyclic rings where Si bonds to oxygens atoms such that six symmetrically equivalent H atoms in H-C-O-Si bonds are formed, consistent with recent experimts (Benner et al., 2003). But even for this structure, the difference between calculated shift (-152 ppm) and observed shift (-141 ppm) is slightly beyond computational error. The hyper-coordinated organosilicates were observed in the NMR experiments at extreme pHs, and high starting concentrations of the organic and of silicic acid. Energy calculations, however, suggest that penta- and hexa-coordinated organosilicates are unlikely to be stable at the pHs and total dissolved silicon concentrations of ambient ocean water, and at the ambient pH ~ 5 found within the silica deposition vesicle of the diatom. Further, results show that it is silicic acid is the most likely form taken up from ocean water, and that it is energetically favorable to form quadra-coordinated organosilicates for storing silicon intracellularly, thus increasing silicon solubility until required for silica precipitation.
UR: http://www.geology.wisc.edu/people/display.html?id=36
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL
SC: Biogeosciences [B]
MN: 2003 Fall Meeting