Antagonism or synergism? Contrasting toxicity mechanisms of combined antibiotic and metal pollution on Eisenia Fetida

Abstract

The pervasive occurrence of combined metal and antibiotic pollution (CMAP) in agricultural soils is increasingly being recognized as a novel threat to ecosystems. However, the toxicity variations of CMAP compared to single pollution and the mechanisms underlying these changes remain poorly understood. Herein in this study, the toxicities of copper (Cu)/erythromycin (ERY) and lead (Pb)/norfloxacin (NOR) to earthworms (Eisenia Fetida) were investigated. These results indicated that a single exposure to ERY and NOR at environmental concentrations had negligible effects on physiological processes. Combined Cu/ERY exposure induced more significant oxidative stress, disrupted energy metabolism, and caused cellular damage than Cu alone, as indicated by altered antioxidant enzyme activities, malondialdehyde and adenosine triphosphate content, elevated reactive oxygen species levels, and apoptosis rates in coelomocytes. Conversely, these adverse effects were mitigated by Pb/NOR exposure compared to Pb treatment alone. Further analysis of the gut microbiota revealed that Cu/Pb-tolerant Bacillus spp. play a critical mediating role in the contrasting toxicity profiles. ERY reduced the abundance of Bacillus spp., diminishing their ability to secrete soluble phosphate to immobilize Cu in the gut and leading to increased Cu absorption and toxicity. NOR enriches Bacillus spp. in the gut, facilitating Pb immobilization and reducing Pb bioavailability and toxicity. The contrast toxicity profile revealed the response of the gut microbiota taxa is the primary determinant of the variation in CMAP toxicity. These findings advance our understanding of the impact of CMAP on soil organisms and highlight the need for comprehensive ecological risk assessments to inform regulatory strategies.