Combined toxicity of decabromodiphenyl ethane and Pb on earthworms (Eisenia fetida) based on multiple biomarker responses

Purpose Decabromodiphenyl ethane (DBDPE) and lead (Pb), two prevalent contaminants in e-waste dismantling areas, pose significant threats to soil ecosystem health. However, the ecotoxicological effects of their co-exposure on soil invertebrates remain poorly characterized. Methods Avoidance behavior, survival rate, and growth-inhibition assays were performed to assess the combined toxic effects of DBDPE and Pb on earthworm ( Eisenia fetida ) in an artificial soil system over exposure periods of 7–21 d. Responses from multiple biochemical biomarkers, including superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA), glutathione S-transferase (GST), and acid phosphatase (ACP) were also measured, and the data were integrated by the biomarker response index (BRI). Interaction between Pb and DBDPE on BRI was further analyzed by calculation of effect addition index (EAI). Results Avoidance behavior emerged as the most sensitive endpoint and exhibited concentration-dependent escalation under co-exposure conditions. While individual exposure to DBDPE (0–0.75 mg/kg) induced negligible impacts on growth and mortality, its combination with Pb (1000 mg/kg) resulted in significant synergistic effects. Similarly, biochemical biomarkers—including SOD, CAT, MDA, GST, and ACP—showed markedly greater alterations in co-exposure groups compared to single DBDPE treatments. The BRI revealed only slight alterations in biological health across most treatments. The EAI values indicated that antagonistic effects dominated the interaction between DBDPE and Pb. Conclusion Our findings demonstrated significant variation in biomarker sensitivity to pollutant exposure. Notably, short-term sub-lethal co-exposures may elicit synergistic effects on avoidance, growth, and mortality, revealing a time-concentration toxicity dependency. These results provide novel insights into the interaction between heavy metals and organo-pollutants and highlight the ecological hazards posed by emerging brominated flame retardants (e.g., DBDPE) in heavy metal-contaminated soil systems.