https://hdl.handle.net/10012/21785

Aquatic organisms, such as fishes, are in constant contact with various anthropogenic stressors present within their environments, including contaminants from wastewater treatment plant effluents (WWTPE). WWTPE represents a pervasive source of pharmaceutically active compounds (PhACs) in surface waters, with the serotonin and norepinephrine reuptake inhibitor venlafaxine (VEN), frequently detected at elevated concentrations in receiving waters worldwide. VEN is a chiral pharmaceutical administered as a racemic (50:50) mixture of R- and S-enantiomers, which exhibit distinct pharmacological activities and can be selectively degraded or enriched in effluent depending on treatment processes. However, little is known about the stereoselective toxicological effects of VEN in fish, particularly in the context of multi-contaminant WWTPE exposure.

This thesis investigates the systemic effects of WWTPE and enantiomer-specific VEN exposure on Etheostoma spp. (darters) using transcriptomic profiling, oxidative stress assays, and biochemical markers of cellular damage. Wild darters (rainbow, greenside, fantail, and johnny darter) were collected upstream and downstream of the Waterloo municipal WWTP outfall (Grand River, ON). Additionally, a laboratory-controlled VEN exposure was conducted using rainbow darters, where liver tissue was analysed for transcriptomic responses and brain tissue was assessed for oxidative stress markers. Findings reveal that WWTPE alters hepatic function and stress responses, with species-specific transcriptional responses. However, key biological pathways related to lipid metabolism, immune regulation, and mitochondrial function were consistently disrupted across species. Laboratory exposure to VEN demonstrated stereoselective effects, where R-VEN primarily influenced amino acid metabolism, vesicle trafficking, and immune pathways, while S-VEN affected chromatin remodelling, epigenetic regulation, and extracellular matrix organization. Despite some overlap in lipid metabolism and inflammatory pathways, few transcripts were shared between VEN-exposed fish and WWTPE-exposed wild populations, highlighting the complex interactions of multi-contaminant mixtures in effluent-exposed fish. In the brain, VEN exposure induced oxidative stress and cellular damage, as evidenced by increased superoxide dismutase (SOD) and glutathione peroxidase (GPX) activity, as well as elevated lipid peroxidation byproducts. S-VEN had a more rapid oxidative impact, suggesting a higher potential for neurotoxicity, which may contribute to behavioural alterations in exposed fish.

Overall, this research contributes to the development of biomonitoring practices by identifying molecular markers and key biological pathways disrupted by WWTPE and VEN exposure. Findings support the refinement of Adverse Outcome Pathway (AOP) frameworks for pharmaceutical contaminants and provide high-quality transcriptomic reference data for use in high-throughput ecotoxicological monitoring. These results raise considerations for improving monitoring, regulation, and treatment strategies to better address the potential ecological impacts of effluent-derived pollutants in freshwater ecosystems.

https://hdl.handle.net/10012/21785