In industrial biotechnology, micro-organisms are used for making a wide range of products, including neutraceuticals, transport fuels, pharmaceuticals and plastics. The raw materials for these microbial processes are often sugars derived from natural resources. Industrial biotechnology offers an advantage with respect to the petrochemical synthesis of these products with respect to its sustainability in CO2 emissions. Yeasts are important industrial workhorses in industrial biotechnology, as they are used for making many of the abovementioned products. Baker’s yeast – Saccharomyces cerevisiae – is used for the large-scale production of, for example, bioethanol, farnesene, human insulin and succinic acid. To achieve cost-efficient production processes, the high-yield conversion of substrate (e.g. glucose) to the product of interest is essential. A possible approach to achieve this goal is to uncouple microbial product formation from growth. This approach is challenging since, during evolution, microbial metabolism has been optimized for the use of substrate for growth and maintenance of cellular integrity and viability. For dissimilatory products, whose synthesis by micro-organisms results in the net production of ATP, such as ethanol for S. cerevisiae, uncoupling of growth and product formation has previously been investigated under anaerobic conditions. For non-dissimilatory products such as succinic acid, farnesene and proteins, a net input of ATP is required and their production is in direct competition with the use of ATP and energy substrate for growth and maintenance. High-yield production of non-dissimilatory products therefore requires a high energy efficiency in dissimilation. During fully respiratory growth of S. cerevisiae, the ATP yield from glucose dissimilation is eight-fold higher than during fermentative growth. Aerobic, respiratory dissimilation of glucose is therefore highly favorable for non-dissimilatory product formation. Systematic characterization of yeasts can be performed under strictly controlled conditions in bioreactors. When bioreactors are operated as steady-state chemostat cultures, the specific growth rate is determined by the dilution rate, which is set by the experimenter. Chemostat cultivation therefore permits comparisons between strains and/or conditions independent of the specific growth rate. Due to technical limitations in the rate of medium supply, chemostat cultivation in bench-top laboratory bioreactors is practically not feasible at extremely low dilution rates (< 0.015 h-1). The retentostat, a modification of the chemostat in which 100 % biomass retention in the reactor is achieved by placing a filter in the outflow line, offers a suitable alternative for the investigation of such extremely low growth rates.
|Qualification||Doctor of Philosophy|
|Award date||31 Oct 2019|
|Publication status||Published - 25 Sep 2019|