Nitrite Stress Induces Oxidative Stress and Leads to Muscle Quality Decreased in Wuchang Bream (Megalobrama amblycephala Yih) Juveniles

To determine the effects of nitrite exposure on muscle quality and physiological functions in Wuchang bream (Megalobrama amblycephala), we exposed M. amblycephala juveniles to acute nitrite (0, 1, 5, 10, 20 mg/L), and the muscle and blood samples were measured at 12, 24, 48, and 96 h. The results showed that when exposed to nitrite for 12 h, the concentrations of blood glucose, cortisol, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) in the 20 mg/L experimental group had the maximum value. The activity of lactate dehydrogenase (LDH) increased significantly in a dose-dependently manner and peaked at 96 h in the 20 mg/L group. During 96 h of exposure to nitrite, the total antioxidant capacity (T-AOC) and catalase (CAT) activity in the liver of the 20 mg/L experimental group were significantly higher than those of the control group, while the concentration of muscle glycogen showed a downtrend. At 12 h and 96 h, the hardness of the four experimental groups were significantly higher than that of the control group. Our research shows that acute sodium nitrite exposure will not only cause oxidative stress and decreased muscle quality in M. amblycephala juveniles but also will be accompanied by changes in serum biochemical index, liver antioxidant capacity, muscle physiological characteristics, and muscle physical characteristics. Preliminary speculation may be that acute nitrite exposure may cause M. amblycephala juveniles to choose to reduce muscle quality and activate antioxidant systems.

Comparison of Myosepta Development and Transcriptome Profiling between Blunt Snout Bream with and Tilapia without Intermuscular Bones

Intermuscular bones (IBs) are small spicule-like bones located in the myosepta of basal teleosts, which negatively affect the edibleness and economic value of fish. Blunt snout bream (Megalobrama amblycephala, with epineural and epipleural IBs) and tilapia (Oreochromis niloticus, without epineural and epipleural IBs) are two major aquaculture species and ideal models for studying the formation mechanisms of fish IBs. Here, we compared myosepta development between M. amblycephala and O. niloticus, based on histological analysis, transcriptome profiling, and expression analysis of bone-related genes. The histological results showed that dye condensation began to appear in the myosepta 20 days post hatching (dph) in M. amblycephala, and IBs could be clearly observed 50 dph in the myosepta, based on different staining methods. However, in O. niloticus, dye condensation was not observed in the myosepta from 10 to 60 dph. Differentially expressed genes (DEGs) at different developmental stages were screened by comparing the transcriptomes of M. amblycephala and O. niloticus, and KEGG analysis demonstrated that these DEGs were enriched in many bone-related pathways, such as focal adhesion, calcium, and Wnt signaling pathways. Quantitative PCR was performed to further compare the expression levels of some bone-related genes (scxa, scxb, runx2a, runx2b, bgp, sp7, col1a2, entpd5a, entpd5b, phex, alpl, and fgf23). All the tested genes (except for alpl) exhibited higher expression levels in M. amblycephala than in O. niloticus. A comparison of the dorsal and abdominal muscle tissues between the two species also revealed significant expression differences for most of the tested genes. The results suggest that scxa, scxb, runx2a, runx2b, entpd5a, col1a2, and bgp may play important roles in IB development. Our findings provide some insights into the molecular mechanisms of IB formation, as well as clues for further functional analysis of the identified genes to better understand the development of IBs.

The Sustainable Treatment Effect of Constructed Wetland for the Aquaculture Effluents from Blunt Snout Bream (Megalobrama amblycephala) Farm

In aquaculture, constructed wetland (CW) has recently attracted attention for use in effluent purification due to its low running costs, high efficiency and convenient operation,. However, less data are available regarding the long-term efficiency of farm-scale CW for cleaning effluents from inland freshwater fish farms. This study investigated the effectiveness of CW for the removal of nutrients, organic matter, phytoplankton, heavy metals and microbial contaminants in effluents from a blunt snout bream (Megalobrama amblycephala) farm during 2013–2018. In the study, we built a farm-scale vertical subsurface flow CW which connected with a fish pond, and its performance was evaluated during the later stage of fish farming. The results show that CW improved the water quality of the fish culture substantially. This system was effective in the removal of nutrients, with a removal rate of 21.43–47.19% for total phosphorus (TP), 17.66–53.54% for total nitrogen (TN), 32.85–53.36% for NH4+-N, 33.01–53.28% NH3-N, 30.32–56.01% for NO3−-N and 42.75–63.85% for NO2−-N. Meanwhile, the chlorophyll a (Chla) concentration was significantly reduced when the farming water flowed through the CW, with a 49.69–62.01% reduction during 2013–2018. However, the CW system only had a modest effect on the chemical oxygen demand (COD) in the aquaculture effluents. Furthermore, concentrations of copper (Cu) and lead (Pb) were reduced by 39.85% and 55.91%, respectively. A microbial contaminants test showed that the counts of total coliform (TC) and fecal coliform (FC) were reduced by 55.93% and 48.35%, respectively. In addition, the fish in the CW-connected pond showed better growth performance than those in the control pond. These results indicate that CW can effectively reduce the loads of nutrients, phytoplankton, metals, and microbial contaminants in effluents, and improve the water quality of fish ponds. Therefore, the application of CW in intensive fish culture systems may provide an advantageous alternative for achieving environmental sustainability.

Nutrient metabolism in the liver and muscle of juvenile blunt snout bream (Megalobrama amblycephala) in response to dietary methionine levels

A 75-day rearing trial was designed to study the response of juvenile Megalobrama amblycephala to dietary methionine (Met) levels. Three practical diets with graded Met levels (0.40%, 0.84% and 1.28% dry matter) were prepared to feed the juvenile fish. The results showed that the 0.84% Met diet significantly improved the growth compared with 0.40% diets. Compared with 0.84% and 1.28% Met, 0.40% Met significantly increased the hepatic lipid content, while decreasing the muscular lipid and glycogen contents. 0.40% Met decreased the protein levels of phospho-Eukaryotic initiation factor 4E binding protein-1 (p-4e-bp1), 4e-bp1 and Ribosomal protein S6 kinase 1 in the liver, compared with 0.84% diet, while an increasing trend was observed in the muscle. Met supplementation tended to decrease and increase lipid synthesis in the liver and muscle, respectively, via changing mRNA levels of sterol regulatory element-binding protein 1, fatty acid synthetase and acetyl-CoA carboxylase. 1.28% dietary Met promoted fatty acid β-oxidation and lipolysis in both the liver and muscle by increasing carnitine palmitoyl transferase 1, peroxisome proliferator activated receptor alpha, lipoprotein lipase and lipase mRNA levels. Compared with 0.40% and 0.84% dietary Met, 1.28% Met enhanced the mRNA levels of hepatic gluconeogenesis related genes phosphoenolpyruvate carboxykinase (pepck), and glucose-6-phosphatase, and muscular glycolysis related genes phosphofructokinase (pfk), and pyruvate kinase (pk). The mRNA levels of hepatic pfk, pk and glucokinase were markedly downregulated by 1.28% Met compared with 0.84% level. Muscular pepck, glycogen synthase, and hepatic glucose transporters 2 mRNA levels were induced by 1.28% Met. Generally, deficient Met level decreased the growth of juvenile Megalobrama amblycephala, and the different nutrient metabolism responses to dietary Met were revealed in the liver and muscle.

Transcriptome Analysis Reveals Sexual Disparities between Olfactory and Immune Gene Expression in the Olfactory Epithelium of Megalobrama amblycephala

The olfactory organ is an important chemoreceptor in vertebrates. However, the sexual disparities in gene expression patterns in the olfactory organ in fish remain unstudied. Here, we conducted a transcriptome analysis of the olfactory epithelium (OE) of male and female blunt snout bream (Megalobrama amblycephala) to identify the differences. The histological analysis showed that there were 22 leaf-like olfactory lamellaes on one side of the OE of the adult blunt snout bream. The sensory area of OE is enriched with ciliated receptor cells and microvilli receptor cells. The transcriptome analysis showed that only 10 out of 336 olfactory receptor genes (224 ORs, 5 V1Rs, 55 V2Rs, and 52 TAARs) exhibited significant expression differences between males and females, and most of the differentially expressed genes were related to the immune system. We also validated these results using qPCR: 10 OR genes and 6 immunity-related genes significantly differed between males and females. The FISH analysis results indicated that the ORs were mainly expressed at the edge of the olfactory lamellae. Collectively, our study reveals that gender is not an important factor influencing the expression of olfactory receptors, but the expression of immune genes varies greatly between the genders in blunt snout bream.