研究了不同作用时间(0、24、48、96和120 h)下不同质量浓度氨氮(对照组、1、3、5和10 mg·L-1)对罗非鱼(GIFT Oreochromis niloticus)血清SOD、AKP、溶菌酶和补体C3活性的影响,以及不同质量浓度氨氮(对照组、1、3、5和10 mg·L-1)胁迫下罗非鱼对海豚链球菌的易感性.结果显示:在1.0 mg·L-1氨氮质量浓度下,罗非鱼血清SOD活性在24 h受到诱导;在3.0、5.0和10.0 mg·L-1氨氮质量浓度下,罗非鱼血清SOD活性均受到抑制,最大抑制率分别达18.85%、23.47%和38.46%.在1.0 mg·L-1氨氮质量浓度下,罗非鱼血清AKP活性在0～96 h呈先降低后恢复的趋势;5.0和10.0 mg·L-1实验组的血清AKP活性在24～120 h则一直受到抑制.1.0 mg·L-1实验组的罗非鱼血清溶菌酶活性在24～120 h一直无显著变化,3.0、5.0和10.0mg·L-1实验组的血清溶菌酶活性随着氨氮质量浓度的升高而降低.1.0 mg·L-1实验组的罗非鱼血清补体C3活性在各个采样时间下均大于对照组,而5.0和10.0 mg·L-1实验组补体C3活性始终受到抑制.罗非鱼对海豚链球菌的易感性实验研究显示,置于3.0、5.0和10.0 mg-L-1的罗非鱼的累积死亡率分别为43.33%、46.67%和63.33%,显著高于对照组和1.0 mg·L-1实验组,表明氨氮质量浓度超过3.0 mg·L-1能够增加罗非鱼对海豚链球菌的易感性.本实验认为,超过3 mg·L-1的氨氮质量浓度胁迫能够显著降低罗非鱼的免疫力,增加对海豚链球菌的易感性.因此在罗非鱼健康养殖中,调控并降低氨氮质量浓度非常重要.%The experiment, including the susceptible test of tilapia exposed to Streptococcus iniae and the test on effect of immune-related enzymes (superoxide dismutaso, SOD; lysozyme; alkaline phosphates, AKP and supplement C3) to tilapia under different ammonia, was carried out to reveal the relationship between ammonia and its stress effect on the susceptible to tilapia and the immunity to fish exposed to Streptococcus iniae. The results showed that, There was an induction trend of SOD activity when fish were exposed to 1.0 mg·L-1 ammonia, but instead of suppression when the tilapia were exposed to 3, 5.0 and 10.0 mg·L-1 group,maximum suppression ratio achieved to 18.85％, 23.47％ and 38.46％ respectively. In 1.0 mg·L-1 group, AKP activity decreased, and then recovered to normal level. AKP activity decreased when fish were exposed to 5.0 and 10.0 mg·L-1 ammonia. There was no significant difference in LSZ when tilapia fish were exposed to 1.0 mg·L-1, and with the elevation of ammonia, the LSZ activity decreased in some high concentration groups. In 1 mg·L-1 group, complement C3 was always higher than control in every sampling time, but decreased when tilapia fish were transferred to 5.0 and 10.0 mg·L-1 ammonia. The cumulative mortality of S. iniae injected-fish held in 3.0, 5.0 and 10.0 mg·L-1 achieved to 43.33％, 46.67％ and 63.33％ respectively, the cumulative mortality was significantly higher than that of injected-fish held in 0.1 mg·L-1 (control) and 1.0 mg·L-1. It was concluded that more than 3.0 mg·L-1 ammonia in water caused suppression of the immune response and mortality incidence of tilapia by the S. iniae infection. The results indicated that high ammonia could cause the inhibition of immunity of tilapia, and then enhanced the susceptibility of pathogenic bacteria, and make the mortality increase. Thus, it is so important to regulate the concentration of ammonia in aquatic environment as to prevent the break out of Streptococcus iniae.
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