The Effect Of NH₃-N_t On Energy Budget And Toxicological Responses Of Meretrix Meretrix

Meretrix Meretrix is an important shellfish of aquaculture. In recent years, the phenomenon of large numbers of deaths has occurred. One of the important factors is the accumulation of ammonia in the environment. In this paper, experimental ecological method was applied to study the effects of total ammonia-N (NH3-Nt) on the survival, growth, energy allocation, oxidative stress, cell damage of M. Meretrix. And investigate the effects of NH3-Nt on the bio-energetics of M. Meretrix and the probable toxicity and mechanism. The main results are as follows:There was a significant effect of NH3-Nt on the survival of M. Meretrix. In the conditions, water temperature (24.5±0.5)℃, salinity 21.5, pH 8.0, the 96hLC50 and SC of NH3-Nt to M. Meretrix with shell length of 3.85~4.35cm were 92.37mg.L-1 (4.18mg.L-1 for NH3-Nm), 9.237mg.L-1 (0.418mg.L-1 for NH3-Nm) respectively.Changes of feeding, breathing, excretion and energy budget of M. Meretrix with shell length of 3.82~4.50cm at different level of NH3-Nt, from 0 to 36.0mg.L-1, were studied. The test results display: the ingestion rate, assimilation efficiency decreased significantly along with the increasing of NH3-Nt, as NH3-Nt≧4.0mg.L-1, the ingestion rate and assimilation efficiency has decreased significantly. The effect of NH3-Nt on the oxygen consumption rate and ammonia excretion rate of M. Meretrix showed a gradual increase in low concentration section and the gradual suppression in high concentration segment. The oxygen consumption rate and ammonia excretion rate acquired maximum value at NH3-Nt 9.0mg.L-1, 4.0mg.L-1 respectively. There was a phenomenon of ammonia penetrated the body at NH3-Nt 18.0 mg.L-1. Instantaneous growth rate and scope for growth in the energy allocation model of M. Meretrix all shows that there was no significant different in growth of M. Meretrix when NH3-Nt≦2.0 mg.L-1(NH3-Nm≦ 0.090mg.L-1) and the growth of M. Meretrix was inhibited when NH3-Nt≧ 4.0mg.L-1(NH3-Nm≧ 0 .180mg.L-1). An exposure experiment was conducted to study the toxicological effects of ammonia (NH3-Nt) at different concentrations (0, 0.5, 1.0, 2.0, 4.0, 9.0, 18.0 mg.L-1) and different times (3, 7, 14, 21d) to M. Meretrix. The results show that: the T-AOC, T-SOD, CAT in gill could be induced by different concentrations NH3-Nt, from 0.5 to18.0 mg.L-1 at 3th day. The T-AOC, T-SOD, CAT in gill of M. Meretrix were first increased and then decreased along with the increasing of NH3-Nt, and acquired maximum value at NH3-Nt 4.0 mg.L-1. In the late stage of stress (21th day), the T-AOC, T-SOD, CAT, GSH-PX in gill and the T-AOC, GSH-PX in liver decreased significantly when NH3-Nt≧9 .0 mg.L-1. This indicates that high concentration of ammonia stimulation for a long period could injury antioxidant defense system of M. Meretrix. The change of CAT activity in gill and liver of M. Meretrix is a sensitive indicator of ammonia stress. The change of antioxidant enzyme activity and the result of cell damage observed by TEM all show gill is more sensitive than liver under ammonia stress. The GSH content in gill reduced significantly under 9.0~18.0 mg.L-1 ammonia stimulation and the GSSG content in liver increased significantly under 0.5~18.0mg.L-1 ammonia stimulation since 7th day, show that the high concentration (9.0~18.0mg.L-1) ammonia stress of M. Meretrix can damage glutathione system in tissue. The LSZ, ACP in gill increased significantly and the AKP in liver decreased significantly from 7th to 21th day when NH3-Nt≧ 2.0mg.L-1. It indicates that the ammonia stress at high concentration (9.0~18.0 mg.L-1) for a long time could affect the activity of lysosomal enzymes in M. Meretrix vivo significantly, and enhance the ability of lysozyme but decrease its cell metabolism.When NH3-Nt≧ 4.0 mg.L-1, the AST activity in gill and liver increased significantly from 7th to 21th day. This means there are inflammatory and can not eliminate, the damage may already exist. The result of cell damage in gill and liver observed by TEM as follows: when NH3-Nt≦ 1.0mg.L-1, there was no significant different in cells of gill and liver. When NH3-Nt≧2 .0mg.L-1, cells of gill damage occurs, mainly as mitochondrial swelling, hollow, ridge reduced; endoplasmic reticulum; nuclear condensation; chromatin margination; membrane rupture; apoptotic cells increased and so on, and with the increase of NH3-Nt damage increased. When NH3-Nt≧ 4.0mg.L-1, cells of liver damage occurs and the performance and organelles in damage similar to gill, and also with the increase of NH3-Nt damage increased.