Volatile fatty acids as sustainable feedstocks for tailored microbial lipid production.

BACKGROUND: Interest in single-cell oils from oleaginous yeasts is growing with respect to their potential as sustainable alternatives to plant- and fossil-based oils. The yeast Cutaneotrichosporon oleaginosus is a promising candidate for microbial lipid production due to its broad substrate range and high lipid accumulation capacity. Among potential feedstocks, volatile fatty acids (VFAs) derived from organic waste streams have attracted increasing attention as low-cost, sustainable carbon sources. These VFA mixtures typically contain C2-C6 volatile fatty acids, including acetic, propionic, butyric, valeric, isovaleric, and caproic acids. However, a systematic understanding of how individual VFAs affect growth, lipid accumulation, and fatty acid composition in C. oleaginosus is still lacking. RESULTS: This study investigates the effects of individual VFAs in fed-batch fermentations with C. oleaginosus ATCC 20509 under controlled, elemental carbon-equivalent conditions, using acetic, propionic, butyric, valeric, and caproic acid as well as the branched-chain acids 2-methylpropionic, 2-methylbutyric, and 3-methylbutyric acid. It was demonstrated that carbon chain length strongly influences these parameters, with growth and lipid production generally decreasing as chain length increases. The highest lipid titers were observed in acetic acid- and butyric acid-based fermentations. Even-chain VFAs primarily resulted in the synthesis of even-chain fatty acids. In contrast, odd-chain VFAs promoted the formation of odd-chain fatty acids, with propionic acid emerging as a particularly promising substrate for the targeted production of odd-chain fatty acids. Fermentations with branched VFAs revealed that growth and lipid accumulation are strongly reduced, potentially due to steric effects associated with methyl substituents. However, the presence of the methyl group did not significantly alter the overall fatty acid profile compared to unbranched carbon chains, as confirmed by NMR analysis. CONCLUSION: These findings demonstrate that the choice of substrate has a major impact on targeted lipid production. By enabling the modulation of lipid composition and supporting efficient biomass and lipid accumulation, volatile fatty acids offer a high-value alternative to conventional carbon sources. Overall, these results highlight the potential of VFAs to contribute to a circular bioeconomy by converting low-value waste streams into valuable microbial oils.