Divergent molecular mechanisms underlie differential toxicity of trivalent lanthanum and divalent cadmium in Enchytraeus crypticus

Co-contamination of soil by trivalent rare earth elements (REEs) and divalent non-essential metals has raised increasing ecological concerns. However, the differential toxic mechanisms of these two metal categories in soil invertebrates, particularly under combined exposure, remain poorly understood. Here, we investigated the single and equi-effective mixture toxicities of lanthanum (La3 +) and cadmium (Cd2+) in the model soil oligochaete Enchytraeus crypticus across individual, physiological, and molecular levels. Single exposure toxicity tests revealed that Cd2+ was approximately tenfold more toxic than La3+. Metabolomic profiling showed that both metals significantly perturbed metabolic profiles at environmentally relevant concentrations, with Cd2+ inducing more pronounced disruptions. Notably, La-Cd mixtures at equivalent toxic units exhibited additive effects without inducing additional disturbed pathways, suggesting no synergistic molecular interaction. Integrated physiological and metabolomic analyses further revealed that both metals impaired energy and carbohydrate metabolism and induced DNA damage, whereas Cd2+ specifically triggered oxidative stress and membrane disruption. Collectively, our findings provide comprehensive insights into the differential toxicological profiles of La3+ and Cd2+ at sublethal and field realistic exposure scenarios, thereby advancing the mechanistic understanding necessary for more accurate ecological risk assessments of metal-contaminated soils, particularly under low-level and co-exposure conditions.