Biogenic Carbon Nanodots Enable a Closed‐Loop Microalgal Nano‐Biohybrid for Simultaneous Wastewater Remediation and Bioproduct Enhancement

The present study investigates the role of carbon nanodots (CNDs) in enhancing microalgal growth, biochemical productivity, and wastewater remediation within a circular bioeconomy framework. Carbon nanodots synthesized from biomass‐derived precursors were characterized and applied at different concentrations during the cultivation of microalgae in sewage wastewater. The effects of CND supplementation on growth kinetics, pigment accumulation, biomass productivity, lipid synthesis, lutein production, and nutrient removal efficiency were systematically evaluated. Among the tested treatments, the optimally supplemented CND1 condition exhibited superior performance, achieving maximum chlorophyll content (24.44 μg·mL ⁻¹ ), carotenoid accumulation (2.25 μg·mL ⁻¹ ), biomass concentration (0.62 g·L ⁻¹ ), lipid productivity (0.45 g·L ⁻¹ ), and lutein accumulation (2.21 mg·g ⁻¹ ). Significant enhancement in pollutant remediation was also observed, with removal efficiencies of 85.41% ammoniacal nitrogen, 82.22% nitrate nitrogen, 88.03% phosphorus, 88.76% biochemical oxygen demand (BOD), and 87.08% chemical oxygen demand (COD). The improved performance was attributed to enhanced photosynthetic efficiency, electron transfer, nutrient assimilation, and mixotrophic metabolism induced by the physicochemical properties of CNDs. Correlation analysis further demonstrated that CND supplementation altered metabolic interactions among biomass, pigments, and lipids, promoting distinct biochemical partitioning compared with conventional cultivation conditions. However, higher CND concentrations exerted inhibitory effects due to possible oxidative stress and light attenuation. Overall, the findings demonstrate that optimally dosed carbon nanodots act as effective photosynthetic biostimulants, significantly improving phycoremediation efficiency and value‐added metabolite production, thereby offering a sustainable strategy for advanced wastewater treatment and resource recovery.