Exposure to municipal wastewater effluent impacts stress performance in rainbow trout

Abstract

The objective of the study was to examine the impact of municipal wastewater effluents on the functioning of the cortisol stress axis in rainbow trout ( Oncorhynchus mykiss). Juvenile rainbow trout were caged upstream (reference) and downstream (100% and 10% effluent) of a tertiary-treated municipal wastewater treatment plant outfall and sampled at 14 d later (0 time samples). A second set of fish were then subjected to a 5 min handling disturbance and sampled at 1 and 24 h post-stressor exposure. Plasma cortisol, glucose and lactate concentrations, liver and brain glucocorticoid receptor (GR) protein levels, head kidney mRNA abundances of corticosteroidogenesis genes, including steroidogenic acute regulatory protein (StAR), cytochrome P450 side chain cleavage (P450scc), 11β-hydroxylase and melanocortin 2 receptor (MC2R), and key liver metabolic enzyme activities, were measured. Exposure to effluent for 14 d significantly elevated plasma cortisol and lactate levels in 100% effluent group compared to the reference and 10% effluent sites. There was a significantly higher StAR mRNA abundance in the effluent groups compared to the upstream control. GR protein levels in the liver, but not the brain, were significantly higher in the 100% effluent group compared to the upstream control group. Chronic exposure to 100% effluent for 14 d significantly lowered liver hexokinase and glucokinase activities, but did not affect glycogen content or the activities of phosphoenolpyruvate carboxykinase, pyruvate kinase, lactate dehydrogenase, aspartate aminotransferase and alanine aminotransferase compared to the other two groups. Subjecting these fish to a secondary acute stressor elicited a physiological stress response, including significant transient elevation in plasma cortisol, glucose and lactate levels at 1 h which dropped to pre-stress levels at 24 h after stressor exposure, in the control and 10% effluent groups, but this conserved stress response was impaired in the 100% effluent group. The 100% effluent group fish also had significantly higher StAR and P450scc mRNA abundances at 1 h post-stress, while transcript abundances of all the major corticosteroidogenesis genes were suppressed at 24 h post-stressor compared to the control and 10% effluent groups. Considered together, exposure to full-strength MWWE for 14 d elicits a chronic stress response in rainbow trout, and perturbs the conserved adaptive response to an acute stressor. Our results reveal that the impact of tertiary-treated MWWE on stress performance in rainbow trout is abolished by 90% effluent dilution.