The application of fermented soybean meal (FSBM) is an effective strategy to alleviate the shortage of fish meal (FM) in aquaculture. However, an excessive substitution ratio often reduces fish growth and induces liver oxidative stress, while the mechanism remains poorly understood. Here, an 8-week feeding trial was conducted in largemouth bass (initial weight: 6.82 ± 0.09 g) to establish an oxidative stress model by replacing 50% of FM with FSBM (fermented by Bacillus subtilis). The results showed that FSBM substitution significantly reduced the growth performance of largemouth bass, including the weight gain rate and specific growth rate. Moreover, FSBM significantly reduced the contents of essential amino acids and total free amino acids in muscle, along with the mRNA expression of amino acids and small peptide transporters. Enzyme activity detection and liver sections showed that FSBM substitution caused liver oxidative stress, indicating the successful construction of an oxidative stress model. An integrated analysis of transcriptomic and metabolomic data revealed that FSBM substitution impaired glycine, serine and threonine metabolism, as well as glutathione metabolism. In addition, the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) was decreased in the FSBM group, which may explain the mechanism of oxidative stress caused by FSBM substitution. Considering that glycine is an important component of glutathione synthesis, key genes involved in glycine metabolism (glya, gnmt and agxt) and dietary glycine supplementation should be valued to improve the availability of FSBM. This study reveals for the first time the importance of non-essential amino acids in improving the utilization of plant-based protein sources and provides original insight for the optimization of aquatic feeds.

The liver of fish is an essential metabolic organ that also serves an immune regulatory role. In this study, we constructed a model of largemouth bass (Micropterus salmoides) infected with Nocardia seriolae by injection to explore the immune and antioxidant functions of the liver. The results showed that N. seriolae infection caused severe pathological changes in the liver, including cell necrosis, granuloma formation, and leukocyte infiltration. The level of mRNA expression of immune-related genes in the liver was significantly increased 2 days post-infection. Moreover, the combined analysis of transcriptome and metabolome showed that N. seriolae infection markedly affected liver metabolism, including glutathione metabolism, arginine and proline metabolism, arachidonic acid metabolism, as well as starch and sucrose metabolism. Additionally, multiple key biomarkers were identified as involved in regulating responses to N. seriolae infection, including arginine, glutathione, gpx, GST, PLA2G, GAA, and PYG. To further elucidate the regulatory effects of arginine on the immune and antioxidant processes in the liver, primary hepatocytes were isolated and cultured. The results demonstrated that arginine supplementation significantly reduced the expression of LPS-induced apoptosis-related genes (bax, cas3, cas8, and cas9) by up to 50% while increasing the expression of antioxidant genes (gpx, GST) by up to 700% at 24 h. Through the analysis of metabolic changes and immune responses in the liver following N. seriolae infection, combined with in-vitro experiments, this study elucidated the anti-apoptotic and antioxidant effects of arginine, revealing the immune response mechanisms in fish liver and laying the groundwork for using nutritional strategies to improve fish health.

An 8-week feeding trial was conducted to assess the effects of different dietary lipid levels on the growth performance, feed utilization, and body composition of juvenile kelp grouper Epinephelus moara. Six diets with varying lipid levels of 2.82%, 5.30%, 7.83%, 11.76%, 14.19%, and 16.32% (designated as CL1 to CL6) were carefully formulated. A cohort of 324 juvenile fish (initial body weight of 5.87 ± 0.09 g fish⁻¹) were randomly divided into six groups with three replicates in each group. The results showed that weight gain (WG) significantly improved as the dietary lipid level increased to 7.83%, followed by a decline with further increases. Fish fed the diet with 7.83% lipids also exhibited the highest feed efficiency (FE) and protein efficiency ratio. Although daily nitrogen intake and daily energy intake varied, no significant differences in nitrogen retention and energy retention were detected among the groups. With the increase in dietary lipid levels, daily lipid intake and daily lipid gain significantly increased, but lipid retention showed a consistent decline. Additionally, the viscerosomatic index and intraperitoneal fat ratio and the lipid content of the whole body, dorsal muscle, and liver increased significantly with increasing dietary lipid levels. The hepatic glycogen content inversely decreased with the elevation in dietary lipid levels. The lowest total cholesterol content in the serum was detected in the 16.32% dietary lipid treatment. According to the second-order polynomial regression analysis of WG and FE, a dietary lipid level of 6.56–9.31% was optimal for the growth performance and feed efficiency of juvenile kelp grouper.

Water temperature significantly affects the physiological balance of aquatic organisms like crustaceans, and heat shock proteins (HSPs) are crucial for stress resistance and pathogen defense. This study conducted a genome-wide analysis to explore the functional characteristics of the Hsp70 gene family in Procambarus clarkii. Fifteen Hsp70 family members were identified, with several genes showing upregulation under non-lethal heat shock (NLHS) and pathogen challenges. RNA-Seq and qPCR analyses confirmed increased expression of certain PcHsp70s during NLHS, indicating NLHS activation of the Hsp70 family to enhance immune regulation. dsRNA-mediated silencing of Hsp70 led to downregulation of TLR pathway genes (e.g., TLR1, TLR6), suggesting Hsp70 regulates the TLR signaling pathway for immune responses. These findings reveal that NLHS-induced Hsp70 upregulation improves pathogen resistance, offering insights for addressing temperature fluctuations and disease outbreaks in aquaculture to optimize management practices.

Background/Objective: Over an eight-week period, this study assessed the influence of dietary carbohydrate levels on growth, metabolism, and immunity in Pacific white shrimp (Litopenaeus vannamei) raised within a biofloc technology (BFT) system. Methods: Five isonitrogenous and isolipidic diets, spanning carbohydrate levels from 11% to 47%, were evaluated. Results: The results showed that dietary carbohydrate significantly impacted both growth performance and feed utilization. The diet containing 38% carbohydrate yielded the best outcomes, resulting in the highest weight gain, specific growth rate, and an optimal feed conversion ratio in the shrimp. Hepatopancreatic metabolic analysis revealed that the shrimp adapted to diets high in carbohydrates through the upregulation of glycolytic enzymes (PK, PFK) and downregulation of gluconeogenic enzymes (PEPCK, G6Pase). By optimizing the water quality and supplementing microbial nutrition, L. vannamei in the BFT system exhibited enhanced dietary carbohydrate utilization and strengthened innate immunity. Specifically, SOD and CAT activities remained largely unaffected by varying carbohydrate levels. However, excessive carbohydrate intake still induced oxidative stress. The high-sugar group (47%) exhibited a significant increase in hemolymph MDA content (p < 0.05), with corresponding metabolic alterations observed in glucose, triglyceride, and total protein levels. On the basis of the results of this study, the BFT system may mitigate the adverse effects of a high-carbohydrate diet by enhancing lysosomal enzyme activity (e.g., ACP) and increasing total protein levels. Conclusions: These findings suggest that the BFT system enhances shrimp immunity and mitigates the potential adverse effects of imbalanced dietary components. Piecewise regression analysis determined the optimal dietary carbohydrate level for shrimp within the BFT system to be 31.44–31.77%.

This study examines the impact of the arginine/lysine ratio in feed on the growth, serum amino acids, arginine metabolism, and antioxidant capacity of juvenile largemouth bass (5.95 ± 0.02 g). Five isonitrogenous and isolipidic diets with varying arginine/lysine ratios were formulated and administered over an eight-week period. The results indicated that the treatments had no significant effect on protein efficiency ratio (PER), daily feed intake (DFI), or morphological indices of juvenile largemouth bass (p > 0.05). When the arginine/lysine ratio was 0.85 (2.25/2.65; 2.54/3.00), liver antioxidant capacity was maximized, and inflammatory factors were suppressed. Conversely, a ratio of 2.25/2.99 significantly reduced weight gain (WG) and specific growth rate (SGR) in juvenile largemouth bass, inhibited arginase activity, and increased serum total nitric oxide synthase (T-NOS) activity. When lysine was in excess (2.25/2.99 group), elevating arginine content (2.54/3.00 group) enhanced growth, antioxidant, and immune performance. Analysis of glutathione metabolism and innate immune-related pathway revealed that an optimal arginine/lysine ratio mitigates inflammatory damage induced by oxidative stress. An arginine/lysine imbalance significantly elevated liver malondialdehyde (MDA) content while reducing total antioxidant capacity (T-AOC), superoxide dismutase (SOD), catalase (CAT) activities, and glutathione (GSH) content, thereby increasing the expression levels of inflammatory factors (IL1B, IL8, TGFB1, BAX). These findings demonstrate that an imbalance in arginine/lysine adversely affects the growth, metabolism, and antioxidant capacity of largemouth bass. When lysine is in excess, increasing the arginine content to achieve an arginine/lysine ratio of 0.85 alleviates the negative effects of antagonism, suggesting arginine supplementation may regulate oxidative damage caused by lysine excess.