Rethinking ecotoxicological paradigms: microbial-mediated tolerance and life-history trade-offs of Folsomia candida in hydrocarbon-contaminated groundwater

The ecological relevance of soil invertebrates in contaminated groundwater systems remains largely unexplored, particularly under chronic exposure to petroleum-derived compounds. Here, we provide the first evidence of stable colonization and functional persistence of the model collembolan Folsomia candida within a hydrocarbon-polluted aquifer in Gaeta (central Italy). Long-term monitoring (2019-2023) revealed persistent groundwater contamination, with ethyl tert-butyl ether (ETBE) > 10,000 μg/L and total petroleum hydrocarbons (TPH) > 1000 μg/L. Despite these extreme conditions, F. candida populations were consistently detected on the water surface of quiescent piezometers and taxonomically confirmed via COI DNA barcoding. To disentangle tolerance and potential adaptive responses, we performed ecotoxicological bioassays using three populations (laboratory control, native contaminated population, and a depurated lineage) across aqueous and soil exposure scenarios with site-specific groundwater. Survival remained high (>70%) across all treatments which indicates a limited acute toxicity of ETBE-dominated mixtures. In contrast, reproduction was significantly stimulated under contaminated conditions, suggesting a shift from toxic to trophic effects mediated by hydrocarbon-associated microbial communities, including Aeromonas and Sulfuricurvum, likely providing both nutritional subsidy and oviposition substrates. However, under soil conditions, this enhanced reproductive output was coupled with reduced body length, revealing a contamination-driven life-history trade-off between reproduction and somatic growth. These results demonstrate that organism-microbe-contaminant interactions can modulate ecotoxicological outcomes beyond direct toxicity paradigms, enabling persistence and even functional exploitation of polluted environments. Our findings redefine the ecological boundaries of Collembola and support their use as bioindicators of hydrocarbon contamination pathways across terrestrial and groundwater interfaces, with implications for risk assessment frameworks in subsurface ecosystems.