Yu Huang

Trachinotus ovatus (golden pompano) represents one of the dominant high-value mariculture fish species farmed across coastal regions of China. In intensive farming modes, intestinal damage and immune suppression are frequent issues in golden pompano; the widespread application of antibiotics in disease control has also triggered increasing worries about ecological risks and aquatic product safety. Host-origin probiotics have been widely recognized as a green and safe strategy to mitigate health risks and promote growth in farmed fish. Nevertheless, the regulatory roles and mechanisms of autochthonous E. faecalis strains on the intestinal health of golden pompano are still poorly understood. In this study, a 42-day feeding trial was conducted using golden pompano reared in a large-capacity floating net-cage system. The experimental fish were fed diets supplemented with different concentrations of E. faecalis SZ07 (1 × 10⁵ to 1 × 10⁹ CFU/g feed). Dietary supplementation with SZ07 at suitable doses (1 × 10⁷ to 1 × 10⁸ CFU/g feed) notably improved intestinal tissue structure, elevated the activities of digestive, antioxidant, and immune enzymes, and modulated the mRNA levels of key immune genes in the spleen. Analysis of the gut microbiota revealed that dietary E. faecalis SZ07 reshaped the intestinal microbial community, with an increased abundance of beneficial taxa and a decreased proportion of opportunistic pathogens. Taken together, our results indicate that autochthonous E. faecalis SZ07 acts as an effective probiotic to enhance intestinal health and immune function of golden pompano and offers technical support for the healthy development of its aquaculture industry.

Non-specific cytotoxic cells (NCCs) are critical first-line defenses against bacterial pathogens in teleosts, yet the molecular mechanisms governing their activity remain poorly understood. This study characterized LOC109197777, a novel long non-coding RNA (lncRNA) from Nile tilapia (Oreochromis niloticus), as a positive regulator of NCC-mediated immunity against Streptococcus agalactiae. Bioinformatic and experimental analyses confirmed its non-coding potential and predominant cytoplasmic localization. In vitro, overexpression of LOC109197777 in tilapia head kidney cells upregulated NCC activation receptors while downregulating effector molecules and pro-inflammatory cytokines, indicating a "primed but restrained" activation state; conversely, siRNA-mediated knockdown reduced NCC cytotoxicity. In vivo, intraperitoneal delivery of LOC109197777 significantly enhanced survival following S. agalactiae challenge and reduced bacterial burdens in the head kidney and spleen. Furthermore, tissue-specific expression profiling revealed that LOC109197777 orchestrates spatially distinct regulatory programs: rapid activation in the head kidney, sustained responses in the spleen, early responses in the liver, and prolonged mucosal immunity in the intestine. Notably, the lncRNA differentially regulated perforin-1 and granzyme B in a tissue and time-dependent manner, balancing effective pathogen clearance with immune homeostasis. These findings establish LOC109197777 as the first characterized lncRNA regulator of NCC activity in teleosts, operating through a tissue-specific, temporally coordinated mechanism. This study provides a novel molecular target for genetic enhancement of disease resistance in tilapia aquaculture and offers a theoretical foundation for developing lncRNA-based immunomodulatory strategies.

Sialic acid-binding immunoglobulin-type lectin 2 (Siglec2, also known as CD22) is a key immunoregulatory molecule in mammals; however, its functional role in antibacterial immunity remains poorly understood in teleost fish. In this study, we identified and characterized a Siglec2 homolog from Nile tilapia (Oreochromis niloticus), designated On-Siglec2. The open reading frame of On-Siglec2 is 1767 bp and encodes a type I transmembrane protein of 588 amino acids. On-Siglec2 was broadly expressed in all examined tissues, with highest transcript levels detected in the head kidney. Its expression was significantly upregulated following Streptococcus agalactiae (S. agalactiae) infection. Recombinant On-Siglec2 (rOn-Siglec2) exhibited binding affinity to both Gram-positive and Gram-negative bacteria and suppressed the growth of S. agalactiae and Aeromonas hydrophila (A. hydrophila), but did not significantly alter macrophage phagocytosis activity. In vivo administration of rOn-Siglec2 enhanced survival rates and reduced bacterial burdens in the liver and spleen of infected fish. These protective effects were accompanied by the coordinated regulation of pro- and anti-inflammatory cytokines, immune signaling molecules, and apoptosis-related genes. Collectively, these results demonstrate that On-Siglec2 functions as an immunomodulatory factor in antibacterial defense, likely through pathogen recognition and fine-tuning of host immune responses.

Thioredoxin (TRX) is an important redox-related protein, which plays a key role in maintaining redox balance in cells. However, the role of TRX in ammonia exposure of Oreochromis niloticus is still unknown. In this study, we successfully cloned the TRX gene (OnTRX) of O. niloticus and performed systematic bioinformatics analysis. The results of multi-sequence alignment and phylogenetic tree analysis showed that OnTRX is highly conserved in vertebrates, but exhibits low sequence similarity with TRX homologs from arthropods (crustaceans). The tissue distribution of OnTRX and its transcriptional response to ammonia challenge were determined by qRT-PCR, and we further investigated the relationship between OnTRX and the response of the Nile tilapia epithelial cell line (TSE-04) to NH₄Cl treatment. The results showed that OnTRX exhibited tissue-specific expression, with a relatively high expression level in gill tissue. Ammonia exposure could significantly induce the expression of OnTRX in the gill, head kidney, intestine, skin, liver, and spleen of tilapia. In TSE-04 cells, OnTRX overexpression was associated with reduced NH₄Cl-induced morphological damage, a lower proportion of apoptotic cells, and altered transcript levels of several stress-related genes. Collectively, these findings indicate that OnTRX is likely involved in the regulatory response of Nile tilapia to ammonia-induced stress, while the underlying molecular mechanisms thereof remain to be further elucidated.

Although numerous studies have focused on the potential application of heterotrophic nitrification–aerobic denitrification (HNAD) bacteria in wastewater treatment, research exploring their potential in aquaculture biofloc systems remains limited. In this study, a promising HNAD strain, identified as Stutzerimonas stutzeri MJ20, was isolated from mature biofloc. This strain efficiently utilized low-cost carbon sources (e.g., glucose) and small-molecule carbon sources (e.g., sodium acetate and sodium succinate). Under conditions with glucose as the carbon source, a carbon-to-nitrogen (C/N) ratio of 15, pH 6–9, temperature 25–35 °C, salinity 0–35‰, and shaker speed of 0–150 rpm, it achieved removal rates of 95–100% for NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N at initial concentrations of 100 mg/L each. Even at higher concentrations (up to 200 mg/L NH₄⁺-N and 500 mg/L for both NO₂⁻-N and NO₃⁻-N), removal rates exceeded 99%. Under mixed nitrogen sources, strain MJ20 demonstrated efficient nitrogen removal, preferentially utilizing NH₄⁺-N, with only minimal and transient accumulation of nitrite and nitrate. Genomic analysis revealed that MJ20 carries key denitrification genes, including napA, nirS, norB and nosZ, and possesses complete pathways for nitrate reduction to nitrogen gas and ammonia assimilation, although typical autotrophic nitrification genes were not detected. Combined genomic data and autotrophic culture experiments indicated that, in addition to utilizing various organic carbon sources, the strain also exhibited certain autotrophic growth capabilities. Furthermore, MJ20 showed strong flocculation ability (flocculation rate > 96% within 16 h), sensitivity to multiple common antibiotics, and no toxicity to zebrafish, demonstrating favorable biosafety. In simulated seawater aquaculture wastewater with a C/N ratio of 5, it achieved a total nitrogen removal rate exceeding 94% within 72 h. These results indicate that strain MJ20 possesses comprehensive advantages, including efficient nitrogen removal, broad carbon source adaptability, strong environmental resilience, minimal accumulation of intermediate nitrogen products, excellent flocculation ability, and high biosafety. These traits highlight its potential for application in biofloc systems and in treating aquaculture tail water with a low C/N ratio. This study provides theoretical insights and practical guidance for screening HNAD bacteria suitable for biofloc systems.

This study aimed to elucidate the key characteristics of the humoral immune response and the tissue distribution of specific antibody-secreting cells (ASCs) in Nile tilapia (Oreochromis niloticus). A specific immune model was established by immunizing fish with human IgG. Lymphocytes were isolated from the head kidney, spleen, and peripheral blood and subjected to antigen stimulation in vitro. The MTT assay, reflecting cell metabolic activity and viability, identified the optimal culture conditions as a cell concentration of 2.5 × 10⁶ cells/mL, an antigen concentration of 2 μg/mL, and a culture duration of 72 h. Under these conditions, peripheral blood lymphocytes exhibited the most increase in metabolic activity, followed by head kidney lymphocytes, while splenic lymphocytes showed no significant response. Subsequent dynamic monitoring of antibody-secreting cells (ASCs) using ELISPOT revealed that, in the absence of antigen stimulation, ASC numbers from all three tissues declined over time. Notably, head kidney ASCs retained approximately 50% of their initial number by day 5, whereas ASCs in peripheral blood and spleen decayed to barely detectable or completely undetectable levels, respectively. These findings suggest that the head kidney may serve as a primary site for ASC persistence during the effector phase, potentially contributing to sustained humoral immunity. Although antigen stimulation did not induce significant ASC expansion, it significantly slowed their decay rate (p < 0.05), indicating an antigen-dependent maintenance role. ELISA detection of antibody levels in the culture supernatants showed a consistent trend with the ELISPOT results, further confirming the sustained functional support of antigen for ASCs. Additionally, LPS stimulation experiments demonstrated that all three tissues contained plasmablasts activatable by non-specific stimuli, with peripheral blood showing the highest proliferation fold (4–6 times). In conclusion, this study provides insights into the tissue-specific distribution, in vitro persistence, and antigen-dependent maintenance of ASCs in Nile tilapia, providing insights into the cellular basis that may contribute to humoral immune memory and laying a theoretical foundation for the rational design and application of tilapia vaccines.

Research on heterotrophic bioflocs is extensive, whereas investigations into autotrophic bioflocs remain limited. This study established an in situ autotrophic biofloc (ABF) system for intensive Pacific white shrimp (Penaeus vannamei) farming, aiming for zero water exchange and optimized water quality. A 120-day indoor experiment tested three stocking densities (300 (T1), 250 (T2), and 200 shrimp per m³ (T3)) with no water exchange. Water quality was monitored every two days, and bacterial communities were analyzed on days 10 and 70. The results indicated that ABF maturation was achieved by day 70 across all treatments, marked by three key indicators: (1) synchronous declines in nitrite and nitrate concentrations; (2) concurrent decreases in pH and total alkalinity approaching maturation; and (3) sustained high nitrogen removal efficiency (nitrite < 0.7 mg/L, ammonia < 0.6 mg/L). All density groups displayed similar patterns in both water quality dynamics and microbial community evolution. Bacterial analysis revealed that dominant genera such as Ruegeria, Bacillus, Muricauda, SM1A02, and Nitrospira played critical roles in toxic nitrogen removal, while pathogenic Klebsiella and Vibrio significantly decreased post-maturation. Heterotrophic nitrification and aerobic denitrification microorganisms (HNADMs) were identified as potentially responsible for nitrite accumulation. Nitrite accumulation was found in all groups. T2 and T3 achieved satisfactory breeding performance despite pre-maturation nitrate peaks exceeding 40 mg/L, whereas T1 suffered a low survival rate (27.47%) due to severe nitrite accumulation (>50 mg/L). A biofloc volume (BFV) of 4–8 mL/L effectively managed daily feed inputs of 75–110 g/m³. These findings lay a theoretical and technical foundation for the application of in situ ABF cultivation in intensive farming and enhance the sustainability of aquaculture.

The Echinorhynchidae has a long research history, but its mitochondrial genome evolution remains poorly understood, hindering phylogenetic resolution. In this study, we report the first complete mitochondrial genome of the genus Echinorhynchus , obtained from its type species, Echinorhynchus gadi . The circular mitogenome was 17,696 bp in length and contained 39 genes: 12 protein-coding genes (lacking atp8 ), two ribosomal RNA genes, and 25 transfer RNA genes, including two extra copies of trnW and one extra copy of trnV . Five non-coding regions were identified; the major non-coding region contained tandem repeats and pseudogene fragments, consistent with a tandem duplication and random loss mechanism. Phylogenetic analysis based on the concatenated amino acid sequences of the 12 protein-coding genes placed E. gadi and E. truttae in a well-supported monophyletic clade representing the genus Echinorhynchus . This clade was sister to Aspersentis megarhynchus , supporting a close relationship between Echinorhynchidae and Heteracanthocephalidae. Because the published E. truttae mitogenome is incomplete, this study fills a critical genomic gap and provides a valuable molecular resource for future taxonomic, systematic, and evolutionary studies of Acanthocephala.