| In recent years, the available fresh water resources become increasingly scared, while there are a lot of saline water unutilized. Only a few organisms can live in these areas, due to its high salinity, alkalinity and complex ion component. Oreochromis niloticus can survive and grow at high concentrations of salinity-alkalinity water after acclimation, it is an important candidate species for salinity-alkalinity breeding. In order to assess the tolerance and growth performance of the second selected generation in salinity-alkalinity waters and the ammonia metabolism pathway of Oreochromis niloticus under alkalinity stress, the second selected generation of Nile tilapiawere used in experiments, change of serum ammonia concentration, carbonic anhydrase(CA) and glutamine synthetase(GS) activity, GS, CA-5A, GLS2, CPS1 and Rhcgl2 gene expression under alkalinity at different times were compared, the major results were as follows:(1) In order to assess the salinity-alkalinity tolerance of the second selected generation of Oreochromis niloticus, was compared under different salinity(18 g/L, 20 g/L, 22 g/L, 24 g/L, 26 g/L), NaHCO3 alkalinity(2 g/L, 4 g/L, 6 g/L, 8 g/L, 10 g/L, 12 g/L) and salinity-alkalinity(Salinity 10 g/L, 15 g/L, 20 g/L; Alkalinity 2 g/L, 4 g/L, 6 g/L. their semi-lethal salinity, NaHCO3 alkalinity and concentration of NaHCO3 under different salinities at 24 h, 48 h, 72 h and 96 h were calculated. The results showed that the 96 h semi-lethal salinity, NaHCO3 alkalinity was 21.809 g/L, 8.514 g/L for the second selected generation, concentration of NaHCO3 under 10 g/L, 15 g/L, 20 g/L salinities was 5.925 g/L, 4.583 g/L, 1.897 g/L, respectively, there was no significantly difference between the second generation and the first selected generation(P>0.05), but significantly higher than the base generation(P<0.05).(2)In order to assess the salinity-alkalinity tolerance and growth performance of the second selected generation of Nile tilapia, chronic lethal experiment was carried out for young fish and the average growth rate in different salinity-alkalinity waters(S0A0, S10A0, S10A2, S10A4, S10A6)was compared during 60 days culture experiment. The results showed that the median lethal salinity of the second selected generation was 57.37±1.56 g/L, and the control group was 48.71±1.58 g/L, the median lethal alkalinity was 19.09±0.76 g/L, 16.32±0.45 g/L for the second selected generation and the control group, respectively. The growth comparison showed that the average growth rate decreased with the elevation of salinity-alkalinity. There was no significant difference between the two groups in S0A0 treatment(P>0.05), but in S10A0, S10A2, S10A4 and S10A6 treatments, the average growth rate of the second selected generation was bigger than that of the control group(P<0.05). Salinity-alkalinity tolerance and growth performance was greatly improved for the selected generation.(3)In order to understand the ammonia metabolism pathway of Oreochromis niloticus under alkalinity stress. We selected several ammonia metabolism related genes: GS(Glutamine synthetase), CA-5A(Carbonic anhydrase 5A), GLS2(Glutaminase 2), CPS1(Carbamyl phosphate synthetase 1), Rhcgl2(Ammonium transporter Rh type C-2 like), the serum ammonia concentration, gene expression and enzyme activity were investigated after acute carbonate alkalinity gradient stresses. The serum ammonia concentration increased at a raising carbonate alkalinity gradient, and showed a increasing and then decreasing change trend with the highest at 12 h. Ammonia metabolism related genes in different tissues were upregulated at various degrees after different alkalinity stresses, the expression change trends showed increasing and then decreasing over time, their expression increased significantly at 12-24 h, then recovered gradually to stable levels. Rhcgl2 was mainly expressed in gill, CA-5A, GS, CPS1 were mainly expressed in liver, GLS2 was mainly expressed in kidney and gill. Enzyme activity results showed that CA and GS activities increased along the increased alkalinity, CA acted most significantly in gill while GS in liver. Over all, the results showed that carbonate alkalinity stress could result in the raising of serum ammonia concentration, and ammonia metabolism related genes in gill, liver and kidney played different roles in adjusting to ammonia metabolism, excreting the ammonia directly in gill, producing glutamine and urea in liver, and decreased the serum ammonia concentration. |
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