Synergistic Effect of Indole-3-Acetic Acid and Homogeneously Dispersed Ulva prolifera-Derived Carbon Quantum Dots in Enhancing CO2 Fixation, Biomass Production, and Macromolecules in Chlorella vulgaris.

To address light attenuation caused by self-shading in dense microalgal cultures within photobioreactors, this study proposes a synergistic strategy employing Ulva prolifera-derived carbon quantum dots (U-CQDs) combined with indole-3-acetic acid (IAA). Hydrothermally synthesized U‑CQDs exhibiting fluorescence emission at 580 nm were homogeneously suspended in the culture, converting UV‑blue light into orange‑red wavelengths readily absorbed by microalgae, thereby improving light distribution uniformity. The combination of 1 mg·L-1 U-CQDs and 5 mg·L-1 IAA increased biomass production to 2.17 g·L-1 (30.01% higher than IAA alone), elevated the CO2 fixation rate to 313.19 mg·L-1·d-1, and enhanced light‑use efficiency to 16.69% (a 27.51% improvement). This synergistic biomass yield is comparable to results achieved with carbon dots under optimized photoperiods, yet our strategy additionally integrates IAA to simultaneously enhance CO2 fixation and light‑use efficiency. Unlike mixotrophic approaches relying on organic carbon supplementation, our method remains autotrophic, avoiding additional costs and carbon footprint. Low U‑CQDs concentrations promoted protein and carbohydrate accumulation, whereas higher concentrations (10 mg·L-1) induced lipid synthesis via oxidative stress. IAA further optimized carbon flux, increased the lipid accumulation rate by 33.02%, effectively scavenged reactive oxygen species, and alleviated oxidative damage. This study elucidates the synergistic mechanism between U‑CQDs and IAA, providing a novel approach for developing efficient microalgal carbon fixation systems.