Conversion of lignocellulose and crude glycerol to lipids by oleaginous yeasts - physiology and diversity

Oleaginous yeasts are organisms capable of accumulating lipids. Some of them can grow on various substrates considered to be too toxic for many other microorganisms. Crude glycerol (CG) and hemicellulose hydrolysate (HH) are two examples of such substrates. Lipid quantification by extraction is a time-consuming process which requires usage of organic solvents and strong acids when applied on yeasts. To address this, we developed a new method using Fourier-transform near infra-red (FT-NIR) spectroscopy to quantify lipids within yeast cells. The resulting model for Rhodotorula toruloides had a R2 of 98% and a 5% error in prediction when compared with the traditional lipid extraction method. The method was used to follow lipid formation kinetics in subsequent experiments.Out of 27 tested oleaginous yeast strains, less than half could grow on crude glycerol. Two strains, R. toruloides CBS14 and Rhodotorula glutinis CBS3044 grew well on this substrate, and were even activated when it was supplemented with hemicellulose hydrolysate (producing up to 12.5 g/L lipids). RNA sequencing in R. toruloides CBS14 revealed increased transcription of genes related to energy metabolism, mitochondrial enzymes and genes involved in protein synthesis. There were only little differences in genes related to glycerol metabolic pathways. Probably, cells grown in CGHH have a more efficient energy metabolism and thus, more ATP to build up biomass and lipids, and to take up substrate from the medium.The new method for lipid quantification and the identified regulated genes can be the basis for further manipulations of yeast metabolism to reach sustainable microbial oil production from residual substrates.