Differentiated molecular defense strategies and survival trade-offs in the marine ciliate Euplotes vannus exposed to structurally homologous tetracyclines.

Antibiotic contamination, particularly by tetracycline hydrochlorides, poses a severe threat to marine benthic ecosystems. This study investigates the differentiated toxicological effects and molecular defense mechanisms of the marine ciliate Euplotes vannus when exposed to three structurally similar tetracyclines: chlortetracycline hydrochloride (CTC), doxycycline hydrochloride (DOX), and tetracycline hydrochloride (TCH). Through acute (6 h) and chronic (144 h) exposure assessments, we integrated population dynamics, biochemical enzyme assays, and comparative transcriptomics. The 24-h EC₅₀ values established a toxicity gradient of CTC (4.858 mg/L) > TCH (8.336 mg/L) > DOX (9.023 mg/L). Biochemical assays revealed severe lipid peroxidation across all treatments alongside distinct antioxidant enzyme activation patterns: CTC significantly induced catalase and peroxidase, whereas DOX primarily elevated superoxide dismutase activity. Transcriptomic analysis elucidated compound-specific molecular responses and survival strategies for each antibiotic. CTC exposure triggered global metabolic reprogramming, accelerating lipid and branched-chain amino acid degradation to fuel cytochrome P450-mediated detoxification and transmembrane efflux. Conversely, DOX induced profound endoplasmic reticulum stress and disrupted protein folding, necessitating the upregulation of molecular chaperones and energy systems. TCH exposure resulted in translational arrest and the global downregulation of energy metabolism, leaving E. vannus with a diminished compensatory antioxidant response despite sustaining the most severe lipid damage. These findings demonstrate that E. vannus does not deploy a uniform defense against tetracycline hydrochlorides; instead, it executes complex, compound-specific adaptive strategies involving energy reallocation, macromolecular quality control, and targeted detoxification. This study provides critical insights into the comparative ecotoxicology of marine protozoa.