Light-driven N-doped carbon quantum dots facilitate microbial chain elongation: Bridging process enhancement to...

Microbial chain elongation (CE) converts low-value substrates into medium-chain fatty acids (MCFAs), but its efficiency is often constrained by limited electron availability and incomplete elongation from C₄ to C₆ products. This study demonstrates that nitrogen-doped carbon quantum dots (NCQDs), under visible light irradiation, significantly improved CE performance and product selectivity. At 1.5 g/L NCQDs, caproate concentration increased to 3.76 g/L, representing a 276% improvement over the control, while butyrate accumulation decreased, indicating enhanced elongation toward longer-chain products. Electrochemical characterization showed that NCQDs exhibited visible light absorption, a 2.31 eV bandgap, measurable photocurrent responses, and reduced charge-transfer resistance, reflecting enhanced redox activity at the system level. Metagenomic analysis revealed increased relative abundance of Bacillus and enrichment of functional genes associated with reverse β-oxidation and fatty acid biosynthesis pathways. In addition, quorum sensing-related genes (LuxI, LuxR, RpfF) and Hnd hydrogenase-associated gene clusters were enriched, indicating enhanced functional potential for microbial coordination and redox-related metabolism. These coordinated shifts in electrochemical behavior, microbial community composition, and functional gene abundance were consistent with improved caproate production and metabolic selectivity. This work provides an effective hybrid photochemical-microbial strategy associated with enhanced MCFAs production and offers a promising approach for waste valorization into value-added biochemicals.