Nanobiochar Mitigates Photosynthetic Impairment in Rice Caused by Antibiotic Cocontamination: A Mechanistic Elucidation of Dual-Site Photoinhibition and Metabolic Reprogramming

Antibiotic cocontamination poses a severe but poorly understood threat to crop photosynthesis. This study employed an integrated multiscale framework to uncover the phytotoxicity of tetracycline (TC) and ciprofloxacin (CIP) on rice seedlings and evaluate the mitigation potential of nanobiochar (nBC). Coexposure to TC and CIP amplified antibiotic residues in both root and shoot, exacerbating growth inhibition and photosynthetic pigment depletion, with the net photosynthetic rate plummeting to 1.44 μmol CO2 m–2 s–1. Mechanically, chlorophyll fluorescence and transmission electron microscopy revealed complementary photoinhibition pathways: TC impaired the donor side of photosystem II, whereas CIP blocked the acceptor side, jointly disrupting the electron flow and causing extensive thylakoid disintegration. Critically, nBC amendment effectively reduced tissue antibiotic burden, restored photosynthetic function (light-saturated rate recovered to 3.94 μmol CO2 m–2 s–1), and attenuated metabolomic disturbance, decreasing differential metabolites from 981 to 858. These coordinated recoveries demonstrate that nBC mitigates toxicity primarily by reducing antibiotic bioavailability. This work establishes a novel diagnostic framework linking physiological impairment to subcellular damage and metabolic reprogramming, providing a mechanistic basis for using nBC to safeguard crop productivity against antibiotic mixtures in agroecosystems.