Metabolic interactions in chain elongation system with granular activated carbon for medium-chain carboxylates production

Abstract

The chain elongation bioprocess for the production of medium-chain carboxylates (MCCs) is promising for resource recovery from organic wastes. In this study, the microbiome development and metabolic interactions in chain elongation system with 10% granular activated carbon (GAC, size between 1.5–2 mm) were investigated. The bioreactor achieved an electron recovery rate over 70%, and the carbon distribution towards caproate was over 64%. Additionally, the microbial community structure shifted and chain elongation microbiome rapidly acclimated from traditional anaerobic fermentation consortia. The taxonomic composition shifted in favor of MCCs production by enriching the key microbes associated with caproate production. Caproate-producing Clostridium (75% abundance) and proteolytic Proteiniphilum remarkably predominated in the microbial community. Further metagenomics analysis recovered genome bins of Clostridium kluyveri, Clostrium indolis, Clostridium aminovalericum and Proteiniclasticum ruminis, which together represented the majority of the microbial community. Among them, the complete ethanol-acetate fermentation for caproate production via the reversed β-oxidation pathway (RBO) and fatty acid biosynthesis (FAB) pathway were fully recovered, suggesting that both the pathways contributed to MCCs formation. Further metabolic pathways analysis suggested that microbial metabolic exchanges between chain elongator and symbiotic bacteria, such as cross-feeding of acetate and acetyl-CoA, is vital to shape a robust microbial ecosystem.