Effects of photoperiod regimes and salinity on Na+, K+-ATPase α-isoform expression in gills and kidney during smoltification in Atlantic salmon (Salmo salar) in recirculating aquaculture systems

Understanding the molecular mechanisms of smoltification and seawater adaptation in Atlantic salmon (Salmo salar) is crucial for optimizing smolt production in aquaculture. This study investigated the expression patterns of Na+/K+-ATPase (NKA) subunits during smoltification in domesticated salmon under different light regimes and salinities in gills and kidney. The experiment was carried out in four photoperiodic treatments: 1) No Winter (NW): continuous light (24L:0D), 2) Early Winter (EW): Standard production light regime (6 weeks winter signal (12L:12D) followed by summer signal (24L:0D)), 3) Late Winter (LW): Delayed winter signal using standard production light regime (6 weeks winter signal (12L:12D) followed by summer signal (24L:0D)) and 4) Late Long Winter (LLW): Delayed and prolonged winter signal (20 weeks winter signal (12L:12D) followed by summer signal (24L:0D)). There were initially 8 tanks with freshwater (FW) (0-1 ppt) which split into brackish water (BW) (12 ppt) for all photoperiodic light regimes after summer signal was initiated. NKAa1a mRNA abundance in gills decreased during smoltification in all groups. NKAa1b mRNA abundance in gills did not increase as expected in gills, but NKAa1b protein levels increased. NKAa1c and NKAa3 isoforms had low and stable expression in the gills in FW. NKAa1c may have a compensatory function under specific conditions. In the kidney, NKAa1c was the most abundant isoform. NKAa1b mRNA abundance in the gills increased with higher salinity, supporting its role in salinity adaptation. NKAa1a mRNA expression increased in response to brackish water in the kidney, while NKAa1b showed no significant changes. NKAa1c and NKAa3 had minimal responses to salinity in the kidney. Monitoring the decrease in NKAa1a mRNA abundance may indicate smolt readiness and seawater tolerance. The increase in NKAa1b mRNA and protein abundance is associated with salinity tolerance and can therefore be a indicator for successful smoltification.Masteroppgave i biologiBIO399MAMN-HAVSJMAMN-BI