Low-dose nitrogen-doped carbon quantum dots boost medium-chain fatty acids production via enhanced proton-electron transfer.

Carbon-based materials are effective enhancers for medium-chain fatty acids (MCFAs) production, but their high dosage requirements (typically >1500 mg·(g VSS)) elevate operational costs. This study shows that nitrogen-doped carbon quantum dots (NCQDs) can serve as a highly efficient conductive material, achieving a maximal MCFAs yield of 13.34 g COD L (3.9-fold higher than the Control) at a low dosage of 150 mg·(g VSS), less than 1/10 of conventional carbon material requirements. Mechanistic investigations indicated that NCQDs polarized cellular amide groups to sustain active proton-coupled electron transfer and significantly upregulated key proton-pump genes (Complex I, III, and V). Gene expression such as CYC encoding cytochrome c in the electron transfer system was enhanced by 191.2% in NCQDs group. Besides, NCQDs upregulated chemotaxis/flagellar genes, aiding microbe movement, nutrient search, and electron transfer. Chain elongation pathway analysis further supported that these proton-electron transfer enhancements promoted carbon assimilation into MCFAs, thereby resulting in the high MCFAs yield. Collectively, these interconnected mechanisms underpinned a stable and highly productive chain elongation process throughout 100 days continuous operation. This study proposes NCQDs as dual mediators of extracellular proton and electron transfer system, providing a new strategy for optimizing MCFAs biosynthesis.