Transcriptomics and Toxcast Data Identify Bioeffects in Zebrafish Embryos Exposed to Chemical Mixtures in an Effluent-dominated Stream

Abstract

Wastewater treatment plant (WWTP) effluent-dominated streams provide critical habitat for aquatic organisms but also continually expose them to complex mixtures of pharmaceuticals and other contaminants of emerging concern (CECs) that can potentially impair growth, behavior, and reproduction. Limited toxicity data on the adverse biological impacts of in vivo exposure to these mixtures make it difficult to assess risk for aquatic organisms, particularly with respect to pharmaceuticals whose designed bioactivity often extends beyond conventionally monitored biological pathways. To address this knowledge gap, the goal of this thesis was to identify biomarkers of exposure to complex CEC mixtures relevant to specific chemicals and mixtures in an effluent-dominated stream. RNA sequencing and targeted gene expression were used to identify novel gene and pathway-based impacts of exposure to complex mixtures of pharmaceuticals and other CECs as they related to developmental stage in zebrafish (Danio rerio) larvae and seasonal changes in the chemical mixture composition of the stream. High throughput screening data from the ToxCast database were used to predict biological impacts based on measured chemical concentrations. Together, these data suggest several associations between chemicals (diphenhydramine and thiabendazole) and impacts observed in zebrafish exposures (histaminergic and cardiovascular). While some pathway-based impacts observed in zebrafish exposures were also predicted by ToxCast, many were not (e.g., visual system, musculoskeletal, and metabolic). The results of this work demonstrate a need to expand the ToxCast assay library to encompass bioeffects specific to fish and relevant to pharmaceutical mixtures in WWTP effluent.