ALBERT

Description: Physiologic responses of yeast to oscillatory environments as they appear in the liquid phase in large-scale bioreactors were studied in the past. So far, however, the impact on the sterol household and intracellular regulation remains to be investigated. Since oxygen is a co-factor in several reaction steps within the sterol metabolism, changes in oxygen availability, as it occurs in aerated bioreactors of production scales, might have an influence on the regulation and incorporation of free sterols into the cells’ lipid layer. Therefore, sterol and fatty acid synthesis in a two- and three-compartment scale-down Saccharomyces cerevisiae cultivation were both studied and compared with typical values obtained in homogeneous lab-scale cultivations. While cells were exposed to oscillating substrate and oxygen availability in the scale-down cultivations, growth was reduced and accumulation of carboxylic acids was increased. Sterol synthesis was elevated up to ergosterol at the same time. The higher fluxes led to increased concentrations of esterified sterols. The cells, thus, seem to utilize the increased availability of pre-cursors to fill their sterol reservoirs, however, this seems to be limited in the three-compartment reactor cultivation due to a prolonged exposure to oxygen limitation. Besides, a larger heterogeneity within the single-cell size distribution was observed under oscillatory growth conditions with 3-dimensional holographic microscopy. Hence, the impact of gradients is also observable at the morphologic level. The consideration of such a single-cell based analysis provides useful information about the homogeneity of responses among the population.