Taming the algal toxicity of black phosphorus nanosheets: Fulvic acid as both accomplice and antidote in aquatic environments

Abstract

While black phosphorus nanosheets (BPNS) demonstrate considerable phytotoxicity toward microalgae, current risk assessments often overlook crucial environmental mediators such as natural organic matter. This study systematically examines the dynamic interactions between BPNS and fulvic acid (FA) - a widespread aquatic organic constituent - in modulating the physiological responses of Chlamydomonas reinhardtii. Our results indicate that BPNS induces dose-dependent phytotoxicity, manifesting as ultrastructural deformation, ROS overproduction (2.35-fold increase), and PSII photoinhibition (16.9 % reduction in Fv/Fm) at 100 mg/L. Intriguingly, FA exhibits concentration-dependent biphasic effects: low-to-medium concentrations (1–10 mgC/L) intensify oxidative damage (over 50 % ROS elevation) and PSII photoinhibition, whereas high FA levels (20 mgC/L) substantially reduce toxicity through radical scavenging, surface complexation, and antioxidant activation. Multi-omics analysis reveals BPNS-induced disruptions in porphyrin and chlorophyll metabolism, particularly upregulation of pheophorbide a (19 % expression increase). High-concentration FA restores metabolic homeostasis by boosting amino acid metabolism (1.35-fold amino acid metabolites increase) and lipid turnover (54 % fatty acid upregulation), while also enhancing photosynthetic electron transport and antioxidant defenses, thereby stabilizing photosynthetic activity. This work elucidates the molecular interactions between engineered nanomaterials and natural organic ligands, offering valuable insights for ecological risk assessment and sustainable nanomaterial design in complex aquatic systems.