Digital holographic microscopy is suitable for lipid accumulation analysis in single cells of Yarrowia lipolytica

Digital holographic microscopy (DHM) is a label-free analytical technique for the determination of the cells' volume and their cytosolic refractive index. Here, we demonstrate the suitability of DHM for the quantification of total lipid accumulation in the oleaginous yeast Yarrowia lipolytica. Presently, microbial lipids are gaining increasing attention due to their nutritional value in feed and food applications. Their microbiological synthesis in algae and yeast is subject to optimization studies, which necessitates rapid quantification of total lipids for faster progress and the possibility of process control. So far, quantification of the total intracellular long-chain fatty acid concentration in yeast cells is time-consuming though when common chromatography for a volumetric analysis or staining and flow cytometry for a single-cell based analysis are used. This study, however, demonstrates that 3D-DHM facilitates a quasi-real-time measurement that allows for a rapid quantification of total intracellular lipid accumulation on a single-cell level without cell staining. Data from wild-type and lipid overproducing Y. lipolytica strains with specific yields of long-chain fatty acids in a range between 70 and 360 mg/gCDW show a good correlation with the optical volume determined by DHM, as the total lipid accumulation in the cell is typically well correlated with the long-chain fatty acid concentration. The results further correlate with data obtained from gas chromatography and flow cytometry of Nile Red-stained cells, which proves the reliability of DHM for lipid quantification in Y. lipolytica.