Performance of a mixed inoculum of sludge and pit mud for short and medium-chain fatty acids production: Insight into key microbiome and functional potential in anaerobic fermentation inoculum

In this study, medium-chain fatty acids (MCFA) production from food waste with ethanol as electron donor using three different inoculants (active sludge, pit mud and mixture of sludge and pit mud). The results showed that the R3 (mixed inoculum of sludge and pit mud) produced 8539.1 mg COD/L of MCFA, which was significantly higher than that produced by R1 (active sludge, 2712.6 mg COD/L) and R2 (pit mud, 3585.9 mg COD/L). The major MCFA was caproate and heptanoate, and the short-chain fatty acids (SCFA) and MCFA selectivity reaches 69.2%. Microbial community analysis revealed that the relative abundance levels of Clostridium_sensu_stricto_1, Clostridium_sensu_stricto_12 and Oscibacter were positively correlated with MCFA production. Further analysis of the enzymes related genes involved in the MCFA biosynthesis indicates that they were mainly related to enzymes of Clostridium, Oscibacter and Ruminiclostridium. These were the key genera involved in the chain elongation process. Metagenomic analysis revealed that fatty acid biosynthesis (FAB) pathway was the main pathway of MCFA synthesis in all three fermentation reactors. The abundance levels of all FAB pathway related-genes and MCFA functional microorganisms were higher in R3. Additionally, the enrichment of the main gene abundance of metabolic pathway revealing that the mixed inoculum of sludge and pit mud combined the ability of fatty acid biosynthesis and carbohydrate metabolism, with higher fermentation potential. Overall, this study provides new strategies and insights for MCFA production.