AbstractThe growth response of batch culture of Saccharomyces cerevisiae to magnesium-limitation, under conditions of micro-aerophillic glucose-repression, was shown to be hyperbolic, thus indicating a Monod relationship. The maximum growth rate was found to be 0.20hrs-1 whilst Ks was equal to 36μM and the yield co-efficient at 1/2μmax equal to 1.9 grams of cells formed per milligram of magnesium removed from the medium. The suggested downward revision of the yeast's exogenous magnesium requirements from 2-4mM (Jones and Greenfield, 1984) to 0 5mM (Jones, 1986) is confirmed, with a specific level of 0.65mM for a minimal medium at 2.5% w/v glucose being recommended. A molasses complex medium was found to contain levels in excess of 40mM although the possibility of magnesium-limitation with in an industrial context still exists.
Magnesium uptake patterns were established over a range of limiting (11-650μM) and non-limiting (650-4000μM) concentrations of exogenous magnesium. The uptake patterns obtained corresponded to the changes in the metabolic activities of the population further indicating the highly regulated nature of the ion within the yeast cell (Jones and Greenfield, 1984). The patterns of uptake and release suggests a means of 'priming' the cells to establish control over the fermentative capacity of the organism through the timing of the inoculum transfer.
The investigation of the physiological states of S. cerevisiae a range of growth rates using a magnesium-limited chemostat was carried out with the results agreeing in the main with the trends established in the batch cultures. At growth rates lower than μmax, the cells reveal a physiological state characteristic of non-carbon-limited growth. This involves an increase in the biomass yield from magnesium and alterations in the relative contributions of fermentation and respiration to the overall-fermentative activity as the growth rate increases towards μmax. At growth rates greater than μmax, the cells exhibit pseudo-hyphal growth consistent with the involvement of magnesium in cell morphology.
Studies on the metabolic behaviour of S. cerevisiae when released from magnesium-limitation within a chemostat have been carried out and indicate that magnesium-limitation is primarily exerting its effect within the cell division cycle.
|Date of Award||Mar 1993|
|Supervisor||Graeme Walker (Supervisor)|