Pyrimidine and hopanoid synthesis in anaerobic yeasts

J. Bouwknegt

Research output: ThesisDissertation (TU Delft)

329 Downloads (Pure)

Abstract

Products resulting from alcoholic fermentation by microorganisms have been used by mankind for over 11,000 years. Alcoholic fermentation is involved in production of bread, alcoholic beverages such as wine and beer, distilled spirits such as vodka and rum, and bio-ethanol. The popularity of the most intensively used microorganism for these alcoholic-fermentation-related processes, baker’s yeast (Saccharomyces cerevisiae), is related to its ability to grow anaerobically. In large-scale industrial bioreactors, aeration is expensive and homogenous oxygen concentrations are extremely difficult to obtain, especially during production of ethanol, which, during fermentation of hexose sugars, coincides with the production of equimolar amounts of CO2. Moreover, to maximize the yield of dissimilation products such as ethanol on the carbohydrate feedstock, aerobic respiration should be prevented.
A fermentative pathway is a prerequisite for anaerobic growth in yeasts, but although many yeasts can ferment sugars to ethanol, only few are able to grow in the complete absence of oxygen. Yeasts that are able to grow anaerobically have specific nutritional requirements, which are needed to bypass biosynthetic reactions that either directly require molecular oxygen or are dependent on respiration. For example, synthesis of sterols and unsaturated fatty acids in S. cerevisiae requires oxygen. Sources of these compounds are therefore routinely included in defined media for anaerobic cultivation. Other yeast species may additionally require oxygen for other processes, including synthesis of pyrimidines, vitamins and/or redox-cofactor balancing. However, the exact oxygen requirements for industrially relevant non-Saccharomyces yeasts remain to be elucidated.
Oxygen requirements of yeasts are mostly investigated in laboratory-scale anaerobic cultivation systems. In contrast to the situation in industrial scale bioreactors, it is extremely difficult to achieve fully anaerobic conditions in these small set-ups. Even entry of minute amounts of oxygen may already be enough to meet minimum requirements for biosynthetic processes and thereby obscure oxygen requirements that are highly relevant at industrial scale. Moreover, this challenge in experimental design is likely to have contributed to contradicting literature regarding the oxygen requirements of yeast strains and species. The research described in this PhD thesis was aimed to obtain a deeper insight in adaptations of non-Saccharomyces yeasts that enable anaerobic pyrimidine synthesis and in adaptation mechanisms of such yeasts to sterol depletion under anaerobic conditions.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Pronk, J.T., Supervisor
  • Daran, J.G., Supervisor
Award date10 Mar 2022
Print ISBNs978-94-6384-288-4
DOIs
Publication statusPublished - 2022

Funding

ERC advanced grant 694633

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