Fermentation and gas bubble formation in psychrophilic yeasts
Saaiman, Susanna Elizabeth
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In recent research on the production of intracellular gas bubbles in yeast during fermentation, bubble-like structures were observed in 16 organisms during fermentation. These structures were absent or present in small numbers during respiration. Different parameters have been explored in the search for gas bubbles, but bubble-like structures have only been observed during 30°C fermentations. In this study, the production of gas bubbles during low temperature fermentations in psychrophilic yeasts was explored. The yeasts were cultivated in fermentable and non-fermentable media. The fermentable media was used at two different concentrations. Bubble-like structures were observed in the yeasts cultivated in fermentable media using Light Microscopy, Transmission Electron Microscopy (TEM) and Nano Scanning Auger Microscopy. Additionally, finger-like structures were observed on the cell surface of some of the psychrophilic yeast cells with TEM and Scanning Electron Microscopy. When cultivating the yeasts in non-fermentable media, the gas bubbles were absent or present in low numbers. Mrakia curviuscula was used as a control since this yeast does not have the ability to ferment. An increase in bubble-like structures were observed when the glucose concentration of the fermentable media was increased, therefore to confirm that the bubble-like structures are gas bubbles rather than lipid droplets, the total lipid content of the yeast cells was determined and Confocal Laser Scanning Microscopy (CLSM) using BODIPY stain was done. No significant increase in lipid content was observed, suggesting that the structures are gas bubbles and a clear difference between lipid droplets and gas bubbles were observed with CLSM. The difference between the weight of a fermenting and non-fermenting yeast cell was determined to determine fermenting cells containing gas weigh less than a respiring yeast cell, however the results were not significant. The bubble-like structures observed in this study were similar to the structures observed by Swart and Co-workers in Saccharomyces. The presence of gas bubbles in psychrophilic yeasts confirms that these structures are conserved in yeast. Future research might include the biotechnological effect of gas in yeast cells and investigating the mechanism of release of the gas bubbles to the cell environment.