Increased phenanthrene toxicity to <i>Eisenia fetida</i> upon co-exposure to <i>o</i>-xylene

Polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylene (BTEX) compounds are often found in contaminated soil and have been shown to be toxic to the ecosystem, but their combined environmental risks and ecological effects remain poorly understood. Herein, <i>Eisenia fetida</i> was exposed to phenanthrene (PHE) and <i>o</i>-xylene (OX) in artificial soil to assess their toxic effects (including mortality, reproduction, antioxidant enzyme activities, and malondialdehyde levels) and bioaccumulation. The 48 h LC₅₀ values for single contaminated PHE and OX were 71.6 and 121 mg kg^-1, respectively, while 28 day EC₅₀ values for reproduction were 4.73 and 5.20 mg kg^-1, respectively. Co-exposure to PHE and OX intensified toxicity, reducing fecundity by over 15% compared to exposure to PHE alone. The synergistic effect was confirmed <i>via</i> a mathematical model based on probabilities. Furthermore, distinct biomarker responses were found between earthworms exposed to PHE and OX, implying different toxic mechanisms. However, similar biomarker responses were detected in earthworms exposed to PHE and combined treatments, suggesting the predominant role of PHE in the combined toxicity. In the combined treatments, OX exhibited a catalyst-like effect, enhancing the accumulation and toxicity of PHE in earthworms. These findings highlight the importance of accounting for the combined effects of pollutants when assessing the ecological risks of co-contaminated soils.