Cadmium stress drives remodeling of earthworm mucus metabolism: Hormetic antioxidant response and synergistic amino acid defence mechanisms.

Sensitive biomarkers are urgently required for the ecological risk assessment of heavy metal-contaminated soils. However, the synergistic defense mechanism involving oxidative stress and mucus metabolism in earthworms under cadmium (Cd) stress remains poorly understood, limiting insights into soil animal adaptation strategies. This study employed the earthworm 'Eisenia fetida' as a model organism. Key findings include: (1) Antioxidant enzymes (SOD, POD, CAT) exhibited a characteristic biphasic effect of "low-concentration induction followed by high-concentration inhibition". Metallothionein (MT) levels initially increased then decreased, while malondialdehyde (MDA) and glutathione S-transferase (GST) activities showed a sustained upward trend, collectively revealing the dynamic equilibrium of oxidative stress. (2) Mucus composition underwent dose-dependent restructuring: primarily consisting of proteins, amino acids, and carbohydrates, mucus pH decreased significantly, whereas electrical conductivity (EC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), and metal element content increased dose-dependently. (3) Metabolomic analysis identified the top 10 core differential metabolites, including glutamine (Gln), arginine (Arg), and alanine (Ala). Notably, amino acid dynamics displayed a significant positive correlation with key oxidative stress indicators (SOD, POD, MT), indicating an "enzyme-mediated antioxidant - amino acid chelation" synergistic defense mechanism. This study establishes, for the first time, a three-tiered defense mechanism in earthworms under Cd stress: "antioxidant biphasic response - mucus component regulation - enzymatic-metabolic coupling". It provides a novel perspective for understanding heavy metal adaptation in soil fauna and lays the foundation for screening pollution biomarkers and developing ecological restoration strategies in contaminated soils.