Nano zero-valent iron enhances medium-chain fatty acids production from wine cellar sludge: Insights into metabolic pathways and interspecies cooperation

This study utilized anaerobic digestion sludge and wine cellar sludge as inocula, to which nano zero-valent iron (NZVI) was added at varying concentration gradients. It was found that the addition of the same concentration of NZVI to the wine cellar sludge was more beneficial for the formation of caproate than the anaerobic digestion sludge. Among them, the caproate concentration of 20 g/L NZVI nearly reached the highest of 22 g/L, the selectivity reached 96 %, which was a 1.57-fold increase compared to the control. Concentrations of odd carbon MCFAs, butanol and hexanol in the cellar sludge group were significantly lower than those in the anaerobic sludge group, which reduced the diversion of carbon sources and was conducive to the formation of caproate, indicating that the cellar sludge was an ideal chain elongation inoculum. Through microbial analysis, the dominant genera of different inocula were significantly different, and metagenomic analysis showed that the methanogen Methanothrix may be involved in the chain elongation process through the Wood-Ljungdahl pathway, and the abundance of functional enzymes in the FAB pathway in wine cellar sludge was higher. Metatranscriptomic analysis showed that there was a cooperative relationship between species in the process of chain elongation to promote the chain elongation reaction.