A multi-biomarker approach to study the toxic effects of thallium on earthworms (Eisenia fetida) fed with different food sources

Thallium (Tl) is a highly toxic heavy metal. It is widely spread in soil. However, the effects of Tl on soil invertebrates have received limited attention. Eisenia fetida, a sensitive and widely used bioindicator, is important in assessing ecological risks in soil ecosystems. It is conceivable that the stress resistance of E. fetida may vary depending on its diet, potentially influencing the assessment of ecological risks associated with contaminants. This study aims to assess the toxicological effects of Tl in soil on E. fetida, focusing on mechanisms involving Tl-induced oxidative stress, disruption of antioxidant defenses, and diet-mediated differences in physiological tolerance. E. fetida was nourished with yogurt waste or cow dung as their primary food source before exposure. The research showed a significant correlation between the increase in soil Tl levels and its bioaccumulation in E. fetida. The highest Tl accumulation was observed in E. fetida fed with yogurt waste (5.55 μg g⁻¹), exceeding those fed with cow dung (4.77 μg g⁻¹). Tl inhibited the growth of E. fetida and induced oxidative stress responses. The activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) initially increased at lower concentrations and earlier time points but were suppressed at higher Tl concentrations and longer exposures. In contrast, glutathione S-transferase (GST) and glutathione peroxidase (GPx) activities were generally elevated, especially in yogurt waste-fed worms. Additionally, reduced glutathione (GSH) levels declined over time, while malondialdehyde (MDA) levels increased significantly, indicating lipid peroxidation and oxidative damage. Furthermore, the Integrated Biomarker Response index indicated that cow dung-fed E. fetida exhibited a higher level of toxic stress when compared to those fed with yogurt waste. In a comparative analysis, despite accumulating more Tl, yogurt waste-fed E. fetida exhibited a lower overall toxic response than their cow dung-fed counterparts. Our results suggest that the diet, specifically yogurt waste, can enhance Tl tolerance in E. fetida. Hence, when assessing the ecological risk of Tl concerning earthworms, it is imperative to consider their dietary sources to increase the scientific validity of evaluation results.