Silicon and germanium uptake and cell growth by the marine diatom Nitzschia frustulum Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2v23vx54m

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  • Diatoms are single celled algae that form cell walls made primarily of silicon dioxide (SO₂). The metabolic machinery that gives diatoms the ability to biogenically form highly ordered solid silica from soluble silicon extracted from their external environment provides a unique platform to create novel nanostructured materials inexpensively and with little environmental impact. Of particular interest are nanocomposite materials made from silicon doped with germanium, which can display unique characteristics, e.g. photoluminescence. Therefore, this investigation focused on a method to introduce germanium to diatom systems in such a way that diatoms are still able to grow, which implies the ability to continue making nanostructured materials. Diatoms were cultivated in bubble column photobioreactors, with a doubling time of approximately 36 hours, to a cell density of approximately 0.3 g L⁻¹, at which point their external medium was depleted of silicon. Upon reaching a state of silicon starvation Phase Two of diatom cultivation began with the addition of varying amounts of Ge andlor Si to the cell culture. Phase Two concentrations as high as 11.52 mg L⁻¹ elemental Ge and molar Ge/Si ratios as high as 0.83 mol Ge mol Si⁻¹ were measured. It was found that cultures which received germanium only during Phase Two initially consumed the soluble germanium only to efflux most of the Ge back to the bulk medium within four hours and never experienced an increase in cell mass density. All diatom cultures that were given silicon or a combination of Si/Ge at the onset of Phase Two experienced an increase in cell density regardless of germanium concentration, with doubling times of approximately 100 hours. Germanium was not effluxed by cultures receiving both Si and Ge as it was by the Ge only cultures. These results show that when diatom cultures are grown to silicon starvation and then fed a Ge/Si combination the diatom cell is able to process the germanium with silicon in such a way as to permanently incorporate the germanium into the cell mass. When silicon starved diatom cultures in stined tank photobioreactors were given one time additions of germanium the rate of uptake was found to follow Michaelis-Menten kinetics with a maximum uptake rate of 90.5 ± 18.9 mg Ge g dry cell mass⁻¹ hr⁻¹ with a half saturation constant of 5.02 ± 3.17 mg Ge L⁻¹.
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