Lipid accumulation by oleaginous yeasts in synthetic and authentic AFEXTM corn stover hydrolysate

Microbial oil is a potential alternative to food/plant-derived oil for use in production of biodiesel and other oleochemicals. Our screening studies identified 17 new oleaginous (lipid accumulating) yeast species, accumulating intracellular lipids that represented 20-60% of their cellular dry weight when they were grown in a defined medium known to stimulate lipid accumulation, containing glucose as the carbon source. Conversion of lignocellulosic hydrolysates presents challenges that are not observed in laboratory medium including presence of pentoses, growth inhibitors, and unidentified compounds.  In this study, 36 yeast strains were selected based on their growth, lipid contents and/or buoyancy in 20% glycerol. The ability of these yeasts to grow and accumulate lipids was further investigated in synthetic (SynHA) and authentic AFEX™-pretreated corn stover hydrolysate (ACSH).  SynHA is laboratory medium that mimics the known compound composition of the ACSH.  Pre-culturing yeasts in SynHA media with xylose as the sole carbon source enabled the strains to assimilate both glucose and xylose in the subsequent hydrolysate medium, thereby reducing culturing time and accelerating lipid accumulation.  Cryptococcus humicola UCDFST 10-1004 isolated from a fungal fruiting body in the rain forest, Sulawesi, Indonesia produced 40 g/L dry cellular mass and accumulated over 40% lipids in ACSH in shake flask studies. This strain was scaled up in bioreactor studies to evaluate biodiesel production. This study demonstrates that ACSH is a suitable medium for these oleaginous yeasts to convert lignocellullosic sugars to triacylglycerols for production of biodiesel and other oleochemicals.

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