Evaluation of combined noxious effects of siduron and cadmium on the earthworm Eisenia fetida

Environmental contaminants do not often occur as individual chemicals but rather in complex mixtures whose joint effects can create a strong toxicity to surrounding organisms. To determine the combined harmful effects of siduron (herbicide) and cadmium (heavy metal) toward Eisenia fetida earthworms, samples of worm's coelomocytes were subjected to siduron and cadmium (Cd) at sublethal concentrations (lower than LC₅₀)-siduron 0, 0.8, 2.4, and 7.2 μg cm^-2 Cd 0, 0.4, 0.8, and 1.6 μg cm^-2 in filter paper contact assay, both as individual compounds and combinations. The CI-isobologram model was utilized to reveal the types of toxicological interactions between siduron and cadmium in inducing DNA damage toward earthworms. The results indicated that tail DNA percentage (TDNA %) at individual siduron and cadmium concentrations (with the exception of the lowest concentration of Cd 0.04 μgcm ^-2) were highly significant compared to those of the control (p < 0.01). Tail moments (TM) at individual Cd concentrations (0.8 and 1.6 μg cm ^-2) were highly significant compared to those of the control (p < 0.05), while the increase of TM for individual siduron was only significant (p < 0.05) at 7.2 μg cm ^-2 which is the highest dose/concentration of siduron used in this study. The combinations of siduron and Cd indicated a significant synergism (CI < 1) at the lower effect levels and a significant antagonism (CI > 1) at the higher effect levels. The synergistic effect for a particular combination of chemicals suggests that there might be a possible risk connected to the coincidence of these chemicals.