Fe₃O₄-Doped Blue-Fluorescent Carbon Dots Improved Photoconversion with Photosynthetic Phosphorylation to Enhance CO₂ Fixation in <i>Nannochloropsis oceanica</i>

To address the issues of high toxicity to microalgal organisms and low electron transfer efficiency in conventional carbon dots for converting solar ultraviolet light into visible light, blue-fluorescent carbon dots doped with Fe3O4 (CDs@Fe3O4) with low toxicity and high electron transfer efficiency were used to improve photosynthetic carbon fixation with microalgae. Transcriptomic analysis clarified that upregulated photosynthesis-antenna protein with CDs@Fe3O4-excited blue light enhanced the light-trapping capacity of microalgae. Nitrogen-doped carbon dots and strongly conductive Fe3O4 induced charge delocalization to provide more photosynthetic electrons, which accelerated photosynthetic phosphorylation in ATP synthesis for the energy supply. CDs@Fe3O4 enhanced the porphyrin metabolic pathway to synthesize more chlorophyll and light-sensing biliverdin, while cytochrome C associated with respiration decreased. CDs@Fe3O4 enhanced carbon concentration mechanisms and two auxiliary pathways in the TCA cycle and glycine-serine conversion during photorespiration of microalgae to promote efficient CO2 recycling. Under full-spectrum irradiation, microalgae incubated with 8 mg/L CDs@Fe3O4 exhibited the highest photosynthetic carbon sequestration performance. This caused a 38.5% increase in maximum photosynthetic electron transfer rate, 40.8% increase in chlorophyll a concentration, 39.4% increase in carotenoids concentration, and 30.7% increase in average biomass growth rate. These findings open new avenues for the development of advanced biocompatible photosensitizers for microalgae carbon sequestration technology. This technology provides a highly efficient biological solution for capturing and using CO2 from industrial flue gas.