Microalgae-based bioremediation is a sustainable strategy for nitrate (NO3−-N) removal and resource recovery, yet its efficiency is hindered by suboptimal light utilization and high demand of energy and reductant supply of NO3−-N assimilation. Here, we developed tannic-acid-derived carbon dots (TA-CDs) as intracellular photoelectronic mediators to enhance photosynthetic electron availability and its functional coupling with NO3−-N assimilation in Chlorella vulgaris. At an optimal dosage of 0.5 mg/L, TA-CDs accelerated NO3−-N depletion by 9.2-fold relative to the control, and increased biomass-N incorporation by 3.4-fold. This improvement translated into substantial biomass valorization, with biomass yield and protein accumulation rising by 66.7% and 71.6%, respectively. Distinct from carbon supplementation or spectral conversion alone, TA-CDs penetrated algal cells and improved Photosystem II operating efficiency and electron transport output without elevating non-photochemical energy dissipation. This amplified photochemical output was preferentially coupled to NO3−-N reduction, as evidenced by a ~ 14-fold induction of nitrate reductase activity. Integrated transcriptomics and metabolomics further demonstrated that TA-CDs triggered coordinated metabolic responses: reinforced photosynthetic electron transfer components, upgraded central carbon metabolism and transport capacity, and enhanced amino‑nitrogen sink formation, particularly via glutamine/glutamate and the ornithine-arginine axis. These results support a mechanistic framework in which carbon dots enhance photosynthetic electron transport and carbon‑nitrogen coupling to overcome intrinsic kinetic bottlenecks in microalgal NO3−-N assimilation, enabling efficient NO3−-N removal and biomass valorization.