Lipid production from lignocellulosic material by oleaginous yeasts

Oleaginous yeasts are a potential lipid source for production of fuels, chemicals and food or feed and use of lignocellulosic biomass as feedstock is considered a sustainable approach. Pre-treatment of lignocellulose is required to release the major carbon sources, glucose, xylose and other sugars for lipid production by oleaginous yeasts, but also releases inhibitory compounds. Aim of this thesis was to examine the potential for producing lipids from different lignocellulosic substrates using oleaginous yeasts and to develop analytical methods for monitoring the kinetics of lipid accumulation, as a basis for further investigations of physiological differences in oleaginous yeasts on different substrates. Investigations of 29 different oleaginous yeast strains revealed considerable differences in xylose utilisation capacity, even among very closely related strains. Some strains were very efficient in accumulating lipids from all carbon sources in lignocellulose hydrolysate, others showed no or only weak growth on xylose and in one case intracellular lipid degradation during consumption of xylose was observed. Further investigation demonstrated that it is possible to combine furfural production from hemicellulose and microbial lipid or ethanol production from the cellulose fraction of wheat straw material. An investigation of lipid production from birch wood hemicellulose containing high amounts of xylose and acetic acid revealed that by establishing a pH-regulated feeding strategy, acetic acid could be utilised as an additional carbon source and no growth inhibition was observed. Target parameters when studying lipid-accumulating yeasts are intracellular lipid content and lipid profile. However, classical extraction-based analytical methods are time- and workintensive. Therefore, a non-invasive method based on high-throughput Fourier transform infrared (FTIR) spectroscopy was established. Overall, large diversity among oleaginous yeasts was revealed, especially when converting xylose. Promising strains for lipid production from different substrates were identified, providing a baseline for further studies on the physiology of oleaginous yeasts and on biotechnological production of microbial lipids.