Carbon dots alleviate photoinhibition and enhance photosynthesis in Chlorella pyrenoidosa

• High-intensity light causes photoinhibition in C. pyrenoidos a, reducing its biomass. • CDs with wide absorption (300–700 nm) and low fluorescence quantum efficiency (2.11 %). • CDs act as sunlight absorbers and can absorb excess sunlight around C. pyrenoidosa. • CDs alleviate photoinhibition caused by high-intensity light in C. pyrenoidosa. • CDs enhance C. pyrenoidosa biomass in both normal and high-intensity light conditions. High-intensity light induces oxidative stress in plants, disrupting the photosynthetic system and causing photoinhibition . Herein, carbon dots (CDs) with wide absorption (300–700 nm) and low fluorescence quantum efficiency (2.11 %) effectively absorb excess sunlight around Chlorella pyrenoidosa . Additionally, the surface functional groups of CDs bind to free radicals, scavenging reactive oxygen species (ROS). Under high-intensity light stress, CDs activate signal transduction (hormones and MAPK), initiating diterpenoid pathways and antioxidant processes to scavenge ROS and enhance photosynthetic capacity through light absorption and heat dissipation mechanisms. Specifically, CDs up-regulate CpGA20ox expression, enhance the AsA-GSH cycle, and increase the SOD and CAT activities by 24.15 % and 136.0 %, respectively, reducing ROS levels by 52.0 %. Furthermore, CDs down-regulate the expression of light-harvesting proteins ( CpLHCb2 and CpLcyB ) while increasing the serine/threonine kinase-encoded genes to promote state transition from PSII to PSI, reducing exciton accumulation at Q A . Moreover, CDs increase pigment content (chlorophyll a , b , total chlorophyll, and carotenoids by 2.24, 2.00, 1.42, and 2.17 times, respectively) and elevate the expression of carbon fixation genes ( CpALDO and CpGAPDH ). Ultimately, CDs enhance the antioxidant and photosynthetic capacities of Chlorella pyrenoidosa , increasing its biomass by 5.85 % (compared to that under normal light) and 17.1 % (compared to that under high-intensity light).