Growth of oleaginous yeasts on mixed C5 and C6 sugar streams to generate lipid feedstocks for renewable hydrocarbon production
Creators: Okafor, Damaris Chinwendu
Publisher: name: University of Alberta Library
Year: 2022

<p>Microbial lipids are a promising feedstock to produce renewable fuels. Lignocellulosic feedstocks can provide multiple carbon sources for microbial growth, such as hexoses (C6) and pentoses (C5). Oleaginous yeasts can convert C5 and C6 sugars to microbial lipids that can be further processed into renewable fuels and value-added chemicals. The major challenge to the production of microbial lipids for biofuel production using mixed sugar streams is the inability of oleaginous yeasts and many other microbes to utilize the C5 and C6 sugars simultaneously. The challenge is caused by carbon catabolite repression (glucose repression), which is characterized by the global regulatory mechanism that prevents the transcription of genes responsible for the expression/synthesis of enzymes responsible for secondary carbon (C5) utilization in the presence of preferred primary carbon (C6).<br>
Many preliminary screening research reports have identified some oleaginous microbes that can simultaneously utilize xylose and glucose for microbial lipid production. However, further studies to explore comparatively these non-conventional oleaginous yeasts endowed with the ability to simultaneously use C5 and C6 sugars for lipid production will add more knowledge to our understanding of the behavior of these oleaginous yeasts, and improve their industrial application in the renewable energy sector. The present study investigates the mixed sugar (i.e. C5 and C6) utilization abilities of Pseudozyma tsukubaensis, Pseudozyma hubeiensis, and Cystobasidium iriomotense in both shake flask and 5 L scale. The lipids extracted from biomass grown using nitrogen-limiting medium and fully automated 5 L bioreactors were subjected to lipid pyrolysis to generate renewable hydrocarbons. The products of lipid pyrolysis were then characterized. <br>
Comparative studies of P. tsukubaensis, P. hubeiensis, and C. iriomotense grown in shake flasks and bioreactors using the fermentation strategy deployed in this study confirmed the gradual simultaneous assimilation of the xylose and glucose sugars, rather than the preferential utilization of glucose that is typically observed. This research revealed that P. tsukubaensis and P. hubeiensis efficiently utilized the mixed C5 and C6 carbon sources within 240 h. Conversely, C. iriomotense could not completely utilize both sugars within the same cultivation period. P. hubeiensis and P. tsukubaensis produced higher lipid levels, 59.0 ± 2.3% and 58.1 ± 0.1%, respectively, than C. iriomotense, which displayed lipid yields of 27.0 ± 07%. The n-hexane soluble extracts (recovered fatty acids) when derivatized were abundant in unsaturated fatty acid methyl esters (FAMEs): 86.8% for P. tsukubaensis, 64.8% for P. hubeiensis, and 56.4% for C. iriomotense. <br>
The composition of the renewable hydrocarbon products generated from lipid pyrolysis was identified. n-alkanes, 1-alkenes, internal alkenes, branched hydrocarbons, aromatics, cyclics, and some fatty acids were present. These classes of hydrocarbons had a high C to H ratio indicating that some of the lipid feedstocks were subjected to saturation, decarbonylation, and/or decarboxylation reactions. The most abundant hydrocarbon was the n-alkanes series in both the gaseous and liquid fractions. The observed composition of renewable hydrocarbons in this research is consistent with fossil fuels and thus can be used in existing infrastructure where fossil fuels are already in operation. Thus, the hydrocarbon products generated in these studies are promising alternatives to fossil fuel towards reduced/zero carbon emissions, sustainability, and cost-effectiveness.</p>

<p>Taken together, this study provides a comparative study of three oleaginous yeasts and demonstrated that they could simultaneously use xylose and glucose in nitrogen-limiting (C: N = 100:1) fermentation conditions to generate large amounts of lipids. Pyrolysis experiments yielded liquid renewable hydrocarbons that could potentially be used as drop-in fuels. Finally, this research demonstrated that the microbes that are best candidates for simultaneously using C5 and C6 sugars from lignocellulosic feedstocks to generate lipids for biofuel applications were P. tsukubaensis and P. hubeiensis.</p>