Influence of light irradiance on the toxicity of graphitic carbon nitride to algae: Intensified nano-bio interactions driven by particle reassembly.

Graphitic carbon nitride (g-C3N4) is increasingly applied in various fields involving water treatment, surely resulting in environmental release and ecological exposure. Nonetheless, the effect of light intensity on the toxicity of g-C3N4 and related mechanisms remain unexplored. Our study demonstrated that exposure to g-C3N4 sheets (1-3 µm, 0.5-10 µg/mL) at 10,500 lx induced significantly greater ultrastructure damage and growth inhibition (maximally 17.9 %) in freshwater algae (Chlorella vulgaris) in comparison to those at 1980 (6.45 %) and 5000 lx (11.9 %). Oxidative damage resulted from extracellular reactive oxygen species (ROS) was a predominant contributor to the toxicity. The increase of light irradiance down-regulated citrate cycle, amino acid, and fatty acid metabolisms, closely linked to the adverse effects triggered by g-C3N4. The distinct reassembly of g-C3N4 under varying light irradiances lead to differential nano-bio interactions with algae. At 10,500 lx, the introduction of negatively charged groups increased the hydrophilicity, strengthening the envelopment effect of g-C3N4 toward algae. The photoreactivity of g-C3N4 sheets was augmented due to bandgap reconfiguration, exacerbating the abiotic ROS generation. Our findings provide insights into the significant role and affecting mechanisms of light irradiance on the ecotoxicity of g-C3N4 sheets, which are critical for their ecological risk assessment in aquatic environments.