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 (La 3 + ) and cadmium (Cd 2+ ) in the model soil oligochaete Enchytraeus crypticus across individual, physiological, and molecular levels. Single exposure toxicity tests revealed that Cd 2+ was approximately tenfold more toxic than La 3+ . Metabolomic profiling showed that both metals significantly perturbed metabolic profiles at environmentally relevant concentrations, with Cd 2+ 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 Cd 2+ specifically triggered oxidative stress and membrane disruption. Collectively, our findings provide comprehensive insights into the differential toxicological profiles of La 3+ and Cd 2+ 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. • Cd induced an almost 10-folds higher toxicity compared to La at organismal level. • Cd induced more intensive alteration on the metabolomic profiles than that of La. • An additive effect of La and Cd at molecular level was found for their mixtures. • La and Cd affected the synthesis of energy and carbohydrate and caused DNA damage. • Cd specifically caused oxidative stress with membrane disturbance.