| Nile tilapia(Oreochromis niloticus)is widely cultivated all over the world because of its fast growth and strong stress resistance.Nile tilapia belongs to wide salt fish and has good adaptability to saline and alkaline water bodies.Carbonate alkalinity is an important ionic component in saline and alkaline waters,and it is also a common abiotic factor in the process of aquaculture.Exploring the relationship between environmental factors and tilapia survival and immune system characteristics is helpful to develop a healthy and efficient tilapia culture model and implement the important deployment of green and healthy aquaculture.Therefore,in this study,Nile tilapia with different body mass was used as experimental materials to explore the tolerance of Nile tilapia to carbonate alkalinity through experiments such as acute alkali stress and chronic alkali acclimation;Then,Nile tilapia infected with Aeromonas hydrophila after long-term alkalinity stress was used as experimental materials to explore the effect of chronic alkali stress on the immune response of Nile tilapia by counting the cumulative mortality,measuring the activity of peripheral blood leukocytes,and the expression of immune related enzymes and genes in different tissues(liver,spleen and gill).The results are as follows:(1)In this study,we examined two sizes of Nile tilapia [large:(21.56±0.27)g,small:(5.92±0.03)g] to explore their tolerance to carbonate alkalinity.The results show that the half lethal alkalinities of Nile tilapia upon acute stress for 96 h range from 6.25 g/L to9.01 g/L.The alkaline tolerance of tilapia is weaker than that of fishes which adapted to extreme alkaline environment(e.g.Gymnocyprinus przewalskii),but it is still stronger than that of most freshwater cultured fishes.At the juvenile stage,three times or more weight gain increases the alkaline tolerance of tilapia significantly.Tilapia can be acclimated by daily alkaline increment of 2 g/L in production.And the relationship of“time-alkalinity-mortality” upon acute alkaline stress can be well fitted by the complementary log-log model.(2)We examined Nile tilapia(291.1±47.2 g)which was exposed to 1 g/L and 3 g/L of sodium bicarbonate alkaline water(AW)to explore the effects of chronic alkalinity stress on immune response in Nile tilapia.At 31 d post alkalinity stress,the fish were vaccinated by Aeromonas hydrophila,and the cumulative mortality within 7 days was calculated;The immune related indexes of peripheral blood cells,enzyme activities(LYS,ALP,C3,Ig M)and gene expression(TLR5,IL-1β,γ-IFN)in liver,spleen and gill of Nile tilapia were assessed at 1d / 31 d post alkalinity stress and 1d / 7 d post vaccination.The results showed that the cumulative mortality of 1 g/L AW was 26.7%,which was significantly lower than 66.7% of 3 g/L AW and 50% of freshwater(FW).At 1 d post vaccination,the respiratory burst,lymphocyte proliferation rates of the 1 g/L AW were higher than those of 3 g/L AW.ALP activity of 1 g/L AW was higher than that of 3 g/L AW in most tissues after vaccination.At 31 d post alkalinity stress and 7 d post vaccination,the TLR5 m RNA expression level in liver and spleen of 1 g/L AW was higher than that of 3 g/L AW and initial group.At 7 d post vaccination,the γ-IFN m RNA expression level in gill and spleen of 1 g/L AW was higher than that of 3 g/L AW.The IL-1β m RNA expression level in spleen of 3 g/L alkalinity group was significantly higher than those of 1 g/L alkalinity group almost in all sampling tissues and time points.At 7 d post vaccination,the activities or expression level of LYS,ALP,C3,Ig M,TLR5 and γ-IFN in spleen of 3 g/L AW were lower than those of 1 g/L AW.The above results showed that long-term feeding of Nile tilapia in 1 g/L carbonate alkaline water can promote immune function and reduce the mortality of Aeromonas hydrophila infection. |
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