Molecular response of Daphnia magna exposed to disinfected effluents

Water scarcity is a growing global challenge, necessitating innovative strategies such as the reuse of treated wastewater. Disinfection is a critical step in wastewater reuse, effectively inactivating pathogens but potentially generating harmful disinfection by-products. This study evaluated the molecular responses of Daphnia magna to municipal treated wastewater both undisinfected and disinfected with ozone (O₃) and peracetic acid (PAA) via acute (48 h) and chronic (21 day) exposures. Stress-related gene expression (catalase, dehydrogenase, and glutathione S-transferase) was quantified using qPCR, with 18S rRNA as a reference. Acute exposure to undisinfected and PAA-disinfected treated wastewater considerably upregulated catalase (4- to 5-fold) and glutathione S-transferase (3- to 5-fold), while ozonated effluents induced milder increases (1.9- to 2.7-fold). Chronic exposure to treated wastewater before and after ozonation downregulated all tested genes (up to 3-fold lower), suggesting antioxidant depletion under prolonged oxidative stress. In contrast, PAA-disinfected effluents sustained upregulation of glutathione S-transferase (1.1-fold) and dehydrogenase (2.0-fold), with no notable change in catalase, indicating ongoing stress mitigation. HPLC-ESI MS/MS analysis identified a variety of disinfection by-products, predominantly carboxylic acids, such as tricarballylic acid and adipic acid, with ozonation producing more complex compounds than peracetic acid. Notably, our study identified novel compounds formed during disinfection with PAA and O₃, while also confirmed the presence of others previously documented in the literature. These findings emphasize the necessity of optimizing disinfection protocols to balance microbial inactivation with ecological safety, offering insights into molecular toxicity mechanisms and the environmental impact of disinfection by-products.