LIPID PRODUCTION FROM LIGNOCELLULOSE BY OLEAGINOUS YEAST CRYPTOCOCCUS CURVATUS

Xiaochen Yu

Microbial oils produced from lignocellulose by oleaginous yeasts are promising feedstocks for renewable biofuel. In the dissertation research, the key issues on microbial oil production utilizing lignocellulose were investigated, including tolerance to the inhibitors, utilization of mixed sugars and feasibility and improvement of a simultaneous saccharification and fermentation (SSF) process. It was demonstrated that Cryptococcus curvatus could grow at furfural up to 3 g/L, p-hydroxybenzaldehyde (PHB) up to 1.0 g/L, vanillin up to 1.5 g/L, and syringaldehyde up to 2.0 g/L, respectively. Generally, little significant differences in cell biomass and lipid contents were observed between glucose and xylose being used as a sole carbon source in the presence of the inhibitory compounds. Compared to other reported strains, C. curvatus showed its excellent tolerance to the inhibitors derived from lignocellulose. Simultaneous co-utilization of cellobiose and xylose at similar rates can be achieved by C. curvatus. At a total sugar of 70 g/L, the consumption rates kept constant at about 0.6 g/L/h regardless of the sugar compositions. Compared to those on glucose as a sole carbon source, the cell biomass and lipid production were comparable in the presence of cellobiose and xylose. Further, the underlying mechanism of simultaneous co-fermentation of cellobiose and xylose by C. curvatus was studied. The work on location of β-glucosidase (BGL) activity revealed that cellobiose were firstly transported into the intracellular environment and then hydrolyzed by BGL into glucose for further metabolism. It was also found that xylose had little inhibition on BGL activities in the presence of cellobiose. D-xylose uptake experiments proved that C. curvatus assimilated xylose with little inhibition of cellobiose in the presence of both sugars. Finally, the role of BGL in the SSF of ammonia fiber explosion (AFEX) treated corn stover was evaluated. Without extra BGL addition, C. curvatus reached the comparable highest lipid concentration in a shorter time than those in the presence of BGL at 60 CBU/g glucan. In the absence of extra BGL supplementation, a smaller amount of hemicellulases was required. This study elucidated that C. curvatus was an excellent candidate for the economical microbial oil production from lignocellulose.

LIPID PRODUCTION FROM LIGNOCELLULOSE BY OLEAGINOUS YEAST CRYPTOCOCCUS CURVATUS

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