Differential Gene Expression in Zebrafish (Danio rerio) Following Exposure to Gaseous Diffusion Plant Effluent and Effluent Receiving Stream Water

Problem statement: The expression of six genes known to serve as bioindicators of environmental stress were examined using real-time quantitative PCR in liver tissue extracted from zebrafish (Danio rerio, Hamilton) exposed to effluent and effluent containing stream water associated with the Paducah Gaseous Diffusion Plant (PGDP). Approach: The PGDP, the only active uranium enrichment facility in the US, is located in western Kentucky and discharges treated effluents into several surrounding streams. Environmentally relevant concentrations of several heavy metals and polychlorinated biphenyls (PCBs) can be found in effluents emerging from the plant as well as in receiving streams. Fish were exposed in the laboratory to water from both effluents and downstream areas as well as to water from an upstream reference site. Expression of six genes known to be altered by metal and/or PCB exposure was quantified at both 7 and 14 day time points. Results: Transcription of the biomarker enzyme cytochrome P4501A1 (CYP1A1) was significantly elevated in fish exposed to one plant effluent at both the 7 (16 fold) and 14 (10 fold) day time points. Sediment PCB levels from this site were the highest observed in the study, indicating PCBs may be contributing to the elevated CYP1A1 mRNA. Additionally, catalase, an enzyme responsible for hydrogen peroxide detoxification and known to be impacted by metal contamination, demonstrated significant alterations in expression in the effluent containing the highest concentrations of most metals observed in this study. Interestingly, despite the presence of metal levels consistent with the induction of metallothionein in other studies, no metallothionein induction was observed. All other stress biomarker encoding genes were likewise unaffected by effluent water exposure. Conclusion/Recommendations: These results indicate that contaminant levels observed in this system altered transcription of catalase and CYP1A1 but failed to significantly alter metallothionein, superoxide dismutase, or glutathione peroxidase, providing important data linking pollutant levels and physiological effects.