Synthese und Optimierung von Cellobioselipiden und Mannosylerythritollipiden

Techno-Economic Analysis as a Driver for Optimisation of Cellobiose Lipid Fermentation and Purification

Cellobiose lipids (CL) are glycolipids synthesized by Ustilaginaceae species with potential application as detergents or in cosmetics. This study identified process optimisation potential for CL fermentation based on process modelling and techno-economic analysis. Using a stoichiometric equation based on laboratory data, we calculated the maximum possible CL yield YP/S of 0.45 gCL·gglucose−1 at the biomass yield of 0.10 gBiomass·gglucose−1 with an Ustilago maydis strain. Due to substrate inhibition that may occur at high glucose concentrations, a fed-batch process to increase biomass and CL concentrations was considered in our model. Simulation of different process scenarios showed that the choice of aeration units with high oxygen transfer rates and adaptation of power input to oxygen uptake can significantly decrease electricity consumption. We further assessed scenarios with different fermentation media and CL purification methods, suggesting additional process optimisation potential. Here the omission of vitamins from the fermentation medium proved to be a possible mean to enhance process economy, without compromising CL productivity.

Factors Affecting the Synthesis of Cellobiose Lipids by Sporisorium scitamineum

Cellobiose lipids (CL) are extracellular glycolipids that are produced by many microorganisms from the family Ustilaginaceae. The sugarcane smut fungus Sporisorium scitamineum has been long known as a producer of the glycolipids mannosylerythritol lipids (MEL) and was recently described to additionally secrete CL as a byproduct. In fact, we identified 11 homologous genes in S. scitamineum by in silico analysis sharing a high similarity to the CL biosynthesis gene cluster of Ustilago maydis. We here report the first systematic cultivation of S. scitamineum targeting the synthesis of CL with high product titers and its transfer to the bioreactor. In an initial screening we examined different fermentation media compositions, consisting of a mineral salts solution with vitamins and/or trace elements, three carbon sources (glucose, fructose, sucrose), three pH values (2.5, 4.0, 6.7) and three levels of C/N values (42.2, 83.8, 167.2 mol_C⋅mol_N ^-1) with urea as nitrogen source. A pH of 2.5 proved to result in the highest product titers. An increase of urea concentration from 0.6 to 1.2 g⋅L^-1 had a positive effect on biomass formation, however the glycolipid formation was favored at a C/N ratio of 83.8 mol_C⋅mol_N ^-1, using 0.6 g⋅L^-1 urea. Amongst the examined carbon sources, sucrose resulted in an increase in the secretion of cellobiose lipids, compared to glucose. Comparing different media compositions, vitamins were identified as not necessary for CL synthesis. We obtained a concentration of cellobiose lipids of 8.3 ± 1.0 g⋅L^-1 in shaking flasks. This increased to 17.6 g⋅L^-1 in the 1 L bioreactor with additional feeding of carbon source, with a final purity of 85-93%. As a side product, erythritol and mannosylerythritol lipids (MEL) were also synthesized. Via HPTLC coupled MALDI-TOF MS we were able to analyze the secreted CL structures. S. scitamineum produces a mixture of acylated low molecular weight D-glucolipids, linked to a 2,15,16-trihydroxy-hexadecanoic acid via their ω-hydroxyl group (CL-B). The produced cellobiose lipids precipitate as needle like crystals at an acidic pH value of 2.5.

pH-Driven Self-Assembly of Acidic Microbial Glycolipids

Microbial glycolipids are a class of well-known compounds, but their self-assembly behavior is still not well understood. While the free carboxylic acid end group makes some of them interesting stimuli-responsive compounds, the sugar hydrophilic group and the nature of the fatty acid chain make the understanding of their self-assembly behavior in water not easy and highly unpredictable. Using cryo-transmission electron microscopy (cryo-TEM) and both pH-dependent in situ and ex situ small angle X-ray scattering (SAXS), we demonstrate that the aqueous self-assembly at room temperature (RT) of a family of β-d-glucose microbial glycolipids bearing a saturated and monounsaturated C18 fatty acid chain cannot be explained on the simple basis of the well-known packing parameter. Using the "pH-jump" process, we find that the molecules bearing a monosaturated fatty acid forms vesicles below pH 6.2, as expected, but the derivative with a saturated fatty acid forms infinite bilayer sheets below pH 7.8, instead of vesicles. We show that this behavior can be explained on the different bilayer membrane elasticity as a function of temperature. Membranes are either flexible or stiff for experiments performed at a temperature respectively above or below the typical melting point, TM, of the lipidic part of each compound. Finally, we also show that the disaccharide-containing acidic cellobioselipid forms a majority of chiral fibers, instead of the expected micelles.