Sedimentary co-exposure to bis(2-ethylhexyl) phthalate and titanium dioxide nanoparticles aggravate ecotoxicity and ecological risks through disrupted bioenergetics in Caenorhabditis elegans

Emerging contaminants in estuarine sediments, such as bis(2-ethylhexyl) phthalate (DEHP) and titanium dioxide nanoparticles (nTiO₂), pose ecotoxicological risks that may be exacerbated by co-contamination. This study investigated the impacts of DEHP, nTiO₂, and their combinations at environmentally relevant concentrations (1, 10, and 100 μg/g) on the soil nematode Caenorhabditis elegans in estuarine-like sediment (14.25‰ salinity). Life history traits and bioenergetics endpoints were examined, with a sample size of ≥ 45 worms or 9 technical repeats per treatment. While individual exposures did not affect growth, the combination of DEHP (1 μg/g) and nTiO₂ (100 μg/g) significantly reduced body length by 19%. Single exposure reduced total offspring by 18-41%, whereas the combination of DEHP and nTiO₂ synergistically worsened reproductive toxicity (52-74% inhibition), as revealed by Loewe's additivity model and Bliss's independence. DEBtox modeling revealed a shift in physiological mode of action from "increased reproductive costs" in singular exposures to "increased growth and reproductive cost" in co-exposure. Moreover, co-exposure significantly intensified the impacts on bioenergetics-related endpoints, including ATP level (single exposure: 33-34%; co-exposure: 56%), mitochondrial damage (single exposure: 15-17%; co-exposure: 40%), and oxidative stress (single exposure: 5-7%; co-exposure: 13%). Risk quotients based on reproductive toxicity EC₁₀ and DEBtox-derived zb suggested that environmental concentrations of DEHP and nTiO₂ pose high risks in global estuarine sediments, with a 2-fold increase during co-exposure. This study demonstrates that co-contamination of DEHP and nTiO₂ synergistically aggravates ecotoxicities through disrupted energy allocation, highlighting the importance of assessing mixture toxicity in environmental risk assessment of estuarine sediments.