Williams, Paul; (1995) The production of cadmium sulphide crystallites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this thesis was to determine factors affecting the large-scale production and recovery of cadmium sulphide crystallites from the fission yeast Schizosaccharomyces pombe. Most microbial work to date on crystallite formation has concentrated on the small chelating peptides induced on contact with cadmium ions. However, the complete mechanism of cadmium sulphide crystallite production needs to be understood before this novel bio-mineralisation can be optimised and exploited. Uptake of cadmium ions by S.pombe during different growth phases was studied by fully instrumented batch fermentation and the effect of the fermentation conditions on crystallite production was determined. It was concluded that under the conditions studied, the addition of cadmium ions to a batch culture of S.pombe during the mid-exponential growth phase best facilitated production of cadmium sulphide crystallites. Production of crystallites within S.pombe cells was found to be at a limited level, irrespective of culture conditions. This means that an increased yield per fermentation must be achieved by a greater S.pombe biomass per fermentation. Chemostat fermentations were therefore set-up and indicated an optimum specific growth rate of 0.1h-1 for S.pombe which was then used in fed-batch fermentations to maximise biomass. The release of intracellular cadmium sulphide crystallites from S.pombe by high pressure homogenisation has been characterised and indicated complete release after two passes at 1300 bar. Intracellular crystallites were also released from S.pombe cells by the effects of freeze-thaw, thus avoiding the need for cell breakage. Because only 8% of the total intracellular protein content was also released as a result of freeze-thaw, this has major implications for the ease of large-scale production and purification of cadmium sulphide crystallites from the microbial source. Intracellular cadmium sulphide crystallites (a minimum of 100μg/g.DCW) have been purified from S.pombe cells by chromatographic methods.

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