Evaluation of Ammonia Nitrogen Exposure in Immune Defenses Present on Spleen and Head-Kidney of Wuchang Bream (Megalobrama amblycephala)

Dietary arginine regulates the growth performance, antioxidant capacity, and immune response in Culter alburnus

Culter alburnus is sensitive to stressors. Arginine is a precursor of nitric oxide, which can effectively relieve the level of oxidative stress and improve the antioxidant and immune capacity of fish. The effect of different arginine levels on topmouth culter (Culter alburnus) fry development performance, liver antioxidant capacity, and immune parameters were investigated in this study. Five diets (1.96%, ARG1, control group; 2.28%, ARG2; 2.52%, ARG3; 2.81%, ARG4; 3.09%, ARG5) were used to feed fry (initial weight 0.31 ± 0.01 g) for 8 weeks. The data showed that the final weight (FW), weight gain rate (WGR), and specific growth rate (SGR) of the ARG3 and ARG4 groups were significantly improved, while the feed conversion ratio (FCR) reduced significantly. Compared with the ARG1 group, all groups remarkably reduced the crude ash content of the whole body. The activity of hepatic superoxide dismutase (SOD) and the content of hepatic glutathione (GSH) were significantly increased in the ARG3 and ARG4 groups, while the malondialdehyde (MDA) content was significantly decreased. Compared with the ARG1 group, arginine levels in ARG2, ARG3, and ARG4 groups up-regulated the expression levels of Nrf2, down-regulated the gene expression level of Keap1 in the liver. And the expression of Nrf2/Keap1 pathway downstream genes Mn-SOD and CAT was up-regulated in ARG2 and ARG3 groups. Furthermore, the expression levels of MyD88 and IL-1β were down-regulated, and the anti-inflammatory gene TGF-β expression levels were up-regulated in the ARG2, ARG3, and ARG4 groups. Additionally, compared to the ARG1 group, there was a significant increase in the relative expression levels of the C3 and C4 genes in the ARG4 group. In conclusion, 2.28-2.81% dietary arginine levels improved the growth performance, promoted antioxidant capacity, and enhance immune response. The optimal level of arginine was determined by the quadratic regression analysis of SGR and FCR to be 2.55% of diet (5.43% of dietary protein) and 2.53% of diet (5.38% of dietary protein), accordingly.

New insights into endocrine reproductive toxicity of Microcystis aeruginosa combined with ammonia exposure in zebrafish

The ecological risk posed by MCs-producing M. aeruginosa and elevated ammonia to fish in actual aquatic environments remains uncertain. To address this knowledge gap, we conducted simulations to investigate the endocrine-reproductive toxicity of prolonged exposure (45 d) to Microcystis aeruginosa (2 × 10^6 cells/mL) and 30 mg/L total ammonia nitrogen (TAN) in zebrafish under environmentally relevant conditions. Our results showed that exposure to M. aeruginosa significantly inhibited the body weight, increased gonadosomatic index (GSI), delayed oocyte development, and disrupted endocrine hormonal balance (reduced gonadotropin-releasing hormone (GnRH), and increased estradiol (E2) and testosterone (T)). Mechanistically, it should be attributed to the over-expression of hypothalamic-pituitary-gonadal-liver (HPGL) axis-related genes (cyp11a and cyp17) induced by M. aeruginosa. On the other hand, TAN exposure caused mild damage to zebrafish ovarian tissue and promoted an increase of T levels by inducing the upregulation of steroid hormone synthesis gene (3βhsd) expression in the ovary. It is worth noting that the dysregulation of E2/T ratio in zebrafish ovaries may be attributed to the inhibition of cyp19a1a by both M. aeruginosa and TAN. These results were further confirmed by changes in steroidogenic enzymes activities in the M. aeruginosa or TAN treated groups. Our findings indicated that exposure to M. aeruginosa and TAN had adverse impacts on the reproductive system of zebrafish. And the combined exposure of M. aeruginosa and TAN had more severe effects on the body weight, GSI, pathological changes, hormone levels and HPGL-axis related gene expression in female zebrafish. These results provide compelling evidence regarding the potential risks for reproductive health associated with M. aeruginosa and TAN in eutrophic water bodies experiencing M. aeruginosa blooms, and contribute to the development of effective strategies for monitoring and managing these toxins in aquatic ecosystems.

Effect of polypropylene microplastics on virus resistance in spotted sea bass (Lateolabrax maculatus)

Microplastics (MPs) pollution is a hot issue of global concern. Polypropylene microplastics (PP-MPs) age quickly in the marine environment and break down into smaller particles because of their relatively low temperature resistance, poor ultraviolet resistance, and poor antioxidant capacity, making them one of the major pollutants in the ocean. We assessed whether long-term exposure to micron-sized PP-MPs influences fish susceptibility to viral diseases. We found that exposure to PP-MPs (1-6 μm and 10-30 μm) at concentrations of 500 and 5000 μg/L resulted in uptake into spleen and kidney tissues of Lateolabrax maculatus. Increased activation of melanomacrophage centers was visible in histopathological sections of spleen from fish exposed to PP-MPs, and greater deterioration was observed in the spleen of fish infected by largemouth bass ulcerative syndrome virus after PP-MPs exposure. Additionally, exposure to PP-MPs led to significant cytotoxicity and a negative impact on the antiviral ability of cells. PP-MPs exposure had inhibitory or toxic effects on the immune system in spotted sea bass, which accelerated virus replication in vivo and decreased the expression of the innate immune- and acquired immune related genes in spleen and kidney tissues, thus increasing fish susceptibility to viral diseases. These results indicate that the long-term presence of micron-sized PP-MPs might impact fish resistance to disease, thereby posing a far-reaching problem for marine organisms.

The IL17 signaling pathway: A potential signaling pathway mediating gill hyperplasia and inflammation under ammonia nitrogen stress was identified by multi-omics analysis

Ammonia nitrogen is extremely toxic to aquatic animals, and is also the most common pollutant in the aquatic environment. In order to investigate the effect of high concentration of ambient ammonia nitrogen on fish gills, two groups, including a high ammonia group (T group: TAN = 2.5 mg/L, 10 % 96 h LC50) and a control group (Z group: total ammonia nitrogen (TAN) = 0 mg/L) were set up in this study. The effects of chronic ammonia stress on the gills of Pelteobagrus fulvidraco were investigated by histopathological, enzymatic, transcriptomic and proteomic analyses after 28 d of stress at different ammonia nitrogen concentrations. Histopathological observations revealed significant inflammatory cell infiltration, necrotic and abscission at the base of the gill filaments, and massive proliferation of cells at the base of the gill lamellae. Ammonia nitrogen stress led to increased reactive oxygen species (ROS) content and decreased catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GSH-PX) activities in gills, indicating significant oxidative stress in gills. And further transcriptomic analysis revealed that 807 differential expression genes (DEGs) were generated in the gills, of which 587 DEGs were up-regulated and 220 DEGs were down-regulated. In addition, proteomics analysis identified 1073 differential expression proteins (DEPs) in gills, including 983 up- and 90 down-regulated DEPs. Pathway enrichment analysis of the DEGs and DEPs revealed that multiple inflammation-related signaling pathways were activated in the gill, including the significantly enriched IL17 signaling pathway. This suggests that IL17 signaling pathway might have a significant impact during signaling transduction. Further analysis of network regulation by mapping DEGs and DEPs to KEGG pathway revealed that IL17 signaling pathway mediated inflammation and cell proliferation in gills under ammonia stress. The results of this study provided new insights into the response of fish gills to ammonia nitrogen stress, and the IL17 signaling pathway may be a potential therapeutic target for reducing ammonia nitrogen gill toxicity.

Responses of Micropterus salmoides under Ammonia Stress and the Effects of a Potential Ammonia Antidote

Ammonia is a common environmental limiting factor in aquaculture. To investigate the effects of ammonia stress and explore the protective effect of N-carbamylglutamate (NCG) on Micropterus salmoides (M. salmoides), tissue sections and parameters related to oxidative stress and the inflammatory response in M. salmoides were carried out during the ammonia stress test and feeding test. The results demonstrated that the LC50 for 24 h, 48 h, 72 h, and 96 h under ammonia stress in M. salmoides were 25.78 mg/L, 24.40 mg/L, 21.90 mg/L, and 19.61 mg/L, respectively. Under ammonia stress, the structures of the tissues were damaged, and the GSH content decreased, while the MDA content increased with the increase in stress time and ammonia concentration. The NO content fluctuated significantly after the ammonia nitrogen stress. In the 15-day feeding test, with the increased NCG addition amount and feeding time, the GSH content increased while the MDA and NO contents decreased gradually in the NCG addition groups (NL group: 150 mg/kg; NM group: 450 mg/kg; NH group: 750 mg/kg) when compared with their control group (CK group: 0 mg/kg). In the ammonia toxicology test after feeding, the damage to each tissue was alleviated in the NL, NM, and NH groups, and the contents of GSH, MDA, and NO in most tissues of the NH group were significantly different from those in the CK group. The results suggested that ammonia stress caused tissue damage in M. salmoides, provoking oxidative stress and inflammatory response. The addition of NCG to the feed enhances the anti-ammonia ability of M. salmoides. Moreover, the gill and liver might be the target organs of ammonia toxicity, and the brain and kidney might be the primary sites where NCG exerts its effects. Our findings could help us to find feasible ways to solve the existing problem of environmental stress in M. salmoides culture.