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Title: Molecular and physiological responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar)

item TAKLE, HARALD - Nofima
item FELIP, OLGA - University Of Barcelona
item SELSET, ROGER - Nofima
item GOOD, CHRISTOPHER - Freshwater Institute
item ASGARD, TORBJORN - Nofima

Submitted to: Aquatic Toxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2012
Publication Date: 11/15/2012
Citation: Kolarevic, J., Takle, H., Felip, O., Ytteborg, E., Selset, R., Good, C.M., Baeverfjord, G., Asgard, T., Terjesen, B. 2012. Molecular and physiological responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar). Aquatic Toxicology. 124-125:48-57.

Interpretive Summary: As a final product of metabolic processes that break down proteins in most fishes, ammonia is produced and excreted into the surrounding environment across the gills. The toxicity of ammonia at elevated concentrations is well documented for a number of fish species. Elevated ammonia in aquaculture systems can hinder ammonia excretion or cause net uptake of ammonia from the environment with adverse effects on fish performance (growth, feed utilization) and welfare. Fish use a number of different strategies to address ammonia toxicity such as decreased ammonia production, conversion of ammonia to less toxic compounds, or increased ammonia excretion through the gills. To gain further insight into ammonia detoxification physiology in fish, we exposed Atlantic salmon parr to four ammonia concentrations and two feeding regimes for 15 weeks, assessing several key biomarkers, and identifying genes important to ammonia excretion during long term exposure. These findings can guide further research into ammonia toxicity studies in salmon and other aquatic species.

Technical Abstract: The objective of this study was to determine the underlying physiological and molecular responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar) parr. Previous studies have pre- dominately focused on mechanisms during acute, short-term exposure. For that purpose Atlantic salmon parr were exposed to four ammonia concentrations between 4 and 1800 fl,mol l-1 total ammonia nitro- gen (TAN), and subjected to two feeding regimes for 15 weeks. Elevated environmental ammonia and full feeding strength caused an initial increase in plasma ammonia levels ([Tamm ]) after 22 days of exposure, which thereafter declined and remained similar to the control animals towards the end of the study. On the other hand, a progressive decrease in plasma urea levels was evident throughout the entire exposure period and depended on the concentration of environmental ammonia, with the largest decrease in urea levels observed at the highest ammonia concentrations (1700 and 1800 fl,mol l-1 TAN). We hypothesized that the successful adaptation to long-term elevated ammonia levels would involve an increased capac- ity for carrier-facilitated branchial excretion. This hypothesis was strengthened by the first evidence of an up-regulation of branchial transcription of the genes encoding the Rhesus (Rh) glycoproteins, Rhcg1 and Rhcg2, urea transporter (UT) and aquaporin 3a (Aqp3a), during long-term exposure. Of the Rhesus glycoprotein (Rh) mRNAs, Rhcg1 was up-regulated at all tested ammonia levels, while Rhcg2 showed a concentration-sensitive increase. Increased transcription levels of V-type H+ -ATPase (H+ -ATPase) were observed at the highest ammonia concentrations (1700 and 1800 fl,mol l-1 TAN) and coincided with an up-regulation of Rhcg2 at these concentrations. Transcription of UT and Aqp3a was increased after 15 weeks of exposure to low ammonia levels (470 and 480 fl,mol l-1 TAN). A significant increase in brain glutamine (Gln) concentration was observed for full fed Atlantic salmon after 22 days and in fish with restricted feeding after 105 days of exposure to 1800 and 1700 fl,mol l-1 TAN, respectively, without any concomitant decrease in brain glutamate (Glu) concentrations. These results suggest that Gln synthesis is an ammonia detoxifying strategy employed in the brain of Atlantic salmon parr during long-term sub- lethal ammonia exposure. Full feed strength had an additive effect on plasma [Tamm ], while the restricted feeding regime postponed the majority of the observed physiological and molecular responses. In con- clusion, Atlantic salmon parr adapts to the long-term sublethal ammonia concentrations with increased branchial transcription levels of ammonia and urea transporting proteins and ammonia detoxification in the brain.