Ferric oxide enhances mixed fungal-bacterial microbiome for medium-chain fatty acid production via endogenous electron donors

The establishment of a bacterial-fungal system provides a promising route for medium-chain fatty acid production from food waste using endogenous electron donors (ED). However, its broader application has been limited by restricted electron transfer between bacteria and fungi. In this study, ferric oxide was incorporated to enhance caproate production from food wastes (FW). The findings reveal that 5 g/L ferric oxide addition resulted in 1.3 times increase in the caproate production than the control. The endogenous ethanol production reached 10.6 ± 1.2 g COD/L, which was further indicated as the dominant ED by linear analysis. Mechanistic analysis revealed that ferric oxide improved the fermentation environment by lowering pH, stimulating dissimilatory iron reduction, enhancing extracellular polymeric substance secretion, and increasing electron transport system activity. Furthermore, ferric oxide supplementation promoted the enrichment of the caproate-producing bacterium Caproiciproducens , with its relative abundance increasing by 5.28-fold compared to the control. Candida was identified as the dominant ethanol-producing yeast. Metagenomic analysis indicated that ferric oxide primarily increased the abundances of genes associated with the reverse β -oxidation pathway and extracellular electron transfer. This study elucidates the mechanism by which ferric oxide enhances bacterial-fungal synergy and electron transfer to drive endogenous ethanol-driven chain elongation, providing a feasible strategy for the valorization of organic waste.