Interactive effect of high environmental ammonia and nutritional status on ecophysiological performance of European sea bass (Dicentrarchus labrax) acclimated to reduced seawater salinities

We investigated the interactive effect of ammonia toxicity, salinity challenge and nutritional status on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to normal seawater (32 ppt), to brackish water (20 ppt and 10 ppt) and to hyposaline water (2.5 ppt). Following acclimation to different salinities for two weeks, fish were exposed to high environmental ammonia (HEA, 20 mg/L ∼1.18 mM representing 50% of 96 h LC50 value for ammonia) for 12 h, 48 h, 84 h and 180 h, and were either fed (2% body weight) or fasted (unfed for 7 days prior to HEA exposure). Biochemical responses such as ammonia (Jamm) and urea excretion rate, plasma ammonia, urea and lactate, plasma ions (Na+, Cl− and K+) and osmolality, muscle water content (MWC) and liver and muscle energy budget (glycogen, lipid and protein), as well as branchial Na+/K+-ATPase (NKA) and H+-ATPase activity, and branchial mRNA expression of NKA and Na+/K+/2Cl− co-transporter (NKCC1) were investigated in order to understand metabolic and ion- osmoregulatory consequences of the experimental conditions. During HEA, Jamm was inhibited in fasted fish at 10 ppt, while fed fish were still able to excrete efficiently. At 2.5 ppt, both feeding groups subjected to HEA experienced severe reductions and eventually a reversion in Jamm. Overall, the build-up of plasma ammonia in HEA exposed fed fish was much lower than fasted ones. Unlike fasted fish, fed fish acclimated to lower salinities (10 ppt-2.5 ppt) could maintain plasma osmolality, [Na+], [Cl−] and MWC during HEA exposure. Thus fed fish were able to sustain ion-osmotic homeostasis which was associated with a more pronounced up-regulation in NKA expression and activity. At 2.5 ppt both feeding groups activated H+-ATPase. The expression of NKCC1 was down-regulated at lower salinities in both fed and fasted fish, but was upregulated within each salinity after a few days of HEA exposure. Though an increment in plasma lactate content and a decline in energy stores were noted for both feeding regimes, the effect was more severe in feed deprived fish. Overall, several different physiological processes were disturbed in fasted sea bass during HEA exposure while feeding alleviated adverse effects of high ammonia and salinity challenge. This suggests that low food availability can render fish more vulnerable to external ammonia, especially at reduced seawater salinities.