15-19: Development of a novel strategy for co-utilization of mixed sugars by oleaginous yeast Cryptococcus curvatus for lipid production

Producing microbial lipid as an alternative feedstock for producing hydrocarbon as “drop-in” fuel by utilizing lignocellulosic biomass has attracted increasing attention. However, one of the major challenges is simultaneous fermentation of mixed sugars in lignocellulosic hydrolysates. Our previous work indicated that oleaginous yeast Cryptococcus curvatus could efficiently produce lipids from lignocellulosic hydrolysates; however, it still lacked the capability of simultaneously utilizing C6 and C5 sugars. To overcome this limitation, we investigated the feasibility of co-utilization of xylose and cellobiose to bypass glucose repression in C. curvatus. Interestingly, it was found that the cells could co-ferment xylose and cellobiose at essentially similar rates.  Generally a cellobiose utilization pathway requires cellodextrin transport system and the enzyme β-glucosidase (BGL) for uptake and hydrolysis of cellobiose into glucose for further fermentation. To study the mechanism behind this scenario, we explored the performances of the sugar transport and BGL activity in C. curvatus. C14-labeled xylose kinetic studies elucidated the presence of at least two transporters for xylose in the cell membrane and that glucose exerted an inhibitory effect on xylose transport while cellobiose did not. On the other hand, the results showed that BGL only existed intracellularly and was induced by cellobiose but not glucose and xylose. Moreover, addition of glucose to cultures containing cellobiose severely depressed BGL activity. Therefore, this intensive research on sugar utilization of C. curvatus demonstrated an effective approach circumventing glucose repression for microbial lipid production through xylose/cellobiose co-fermentation.