High-yield microbial lipid synthesis through organic waste co-fermentation guided by reverse carbon and nitrogen...

Microbial lipids derived from organic waste fermentation broths offer a promising route for sustainable biodiesel production. However, yields are often limited by broth-derived stressors and the need for costly pre-treatment. In this study, we rebalance carbon and nitrogen fluxes to improve lipid enrichment by Yarrowia lipolytica cultivated on volatile fatty acid (VFA)-rich food waste (FW) broths. We first delineated lipid-accumulation performance across 20-75 g/L VFAs concentrations and a range of carbon-to-nitrogen (C/N) ratios in a defined medium. We then produced VFAs from organic waste and implemented the co-fermentation of FW with rice straw (RS) to refine the C/N balance. Compared with FW mono-fermentation, FW/RS co-fermentation at a 2:1 ratio increased the lipid concentration from 1.94 to 2.31 g/L and lipid content from 47 wt% to 59 wt%. Optical photothermal infrared (O-PTIR) spectroscopy indicated a shift in cellular metabolism toward lipid storage under an optimized flux regime. Consistently, transcriptomics revealed strengthened detoxification and antioxidative programs, ion homeostasis, and nitrogen limitation-associated regulation, alongside the repression of lipid-degradation pathways, supporting lipid accumulation by alleviating inhibitor stress and reinforcing carbon flux redistribution. These results demonstrate that organic waste co-fermentation guided by reverse carbon and nitrogen regulation is an effective strategy for upgrading waste-derived fermentates into lipid-producing substrates, and provide mechanistic insight into how the selected hydrolysate promotes lipid accumulation in Y. lipolytica.