探索糖海带岩藻聚糖（Fucoidan）调节大西洋鲑免疫反应与肠道菌群的生物活性特性及其在功能性饲料中的应用前景

本研究针对大西洋鲑养殖中因应激和疾病导致的经济损失问题，通过体外和体内实验探究了从挪威糖海带（Saccharina latissima）中提取的岩藻聚糖（Fucoidan）作为功能性饲料添加剂的潜力。研究发现，岩藻聚糖能够增强鲑鱼头肾细胞的活性和抗氧化基因表达，调节头肾白细胞中TNF-α、HSP70和IL-10等免疫相关蛋白的产生，并显著改变肠道菌群结构（如增加Bacillus相对丰度）。此外，多组学分析表明，岩藻聚糖能上调肠道中与淋巴管发育、趋化性和伤口愈合相关的通路，并增强抗原呈递标志物（MHC II和CD3）的表达。该研究为开发基于海藻的健康促进型功能性饲料提供了重要依据，对提升鲑鱼养殖业的可持续性和经济效益具有重要意义。

　　

aquaculture industry faces significant challenges due to stress-induced mortality in Atlantic salmon, driven by factors such as handling, water quality issues, and pathogen exposure. Concurrently, the expansion of seaweed cultivation in regions like Norway is limited by economic and legislative constraints. To address these issues, researchers are exploring the potential of bioactive compounds from seaweed, such as fucoidan—a sulfated polysaccharide known for its immunomodulatory and antioxidant properties—as functional feed additives. This study investigates the effects of fucoidan extracted from sugar kelp (Saccharina latissima) on the immune response and gut microbiota of Atlantic salmon, aiming to enhance fish health and resilience while promoting the valorization of seaweed products.

The research employed a combination of in vitro and in vivo approaches. In vitro assays used the salmon head kidney cell line (SHK-1) to assess cell viability and immune-related gene expression after incubation with fucoidan and Microbe-Associated Molecular Patterns (MAMPs) from Aeromonas salmonicida. In vivo, a five-week feeding trial was conducted with pre-smolt Atlantic salmon fed diets containing varying levels of fucoidan (0.1%, 0.2%, and 0.4%) or a commercial β-glucan (MacroGard?) as a positive control. Key techniques included cell viability assays (alamarBlue), gene expression analysis (RT-qPCR), protein biomarker detection (indirect ELISA), 16S rRNA sequencing for microbiota characterization, RNA sequencing (RNA-seq) for transcriptomics, immunohistochemistry (IHC) for antigen presentation markers, and proteomics (iTRAQ labeling coupled with 2D LC-MS/MS). Samples were collected from head kidney leukocytes (HKLs) and distal intestine (DI), with analyses focusing on immune modulation, microbial composition, and molecular pathways.

3.1. Cell viability and expression of immune-related biomarkers in SHK-1 cells

Exposure to fucoidan at 500 μg mL^-1 for 72 hours significantly increased SHK-1 cell viability. At 100 μg mL^-1, fucoidan upregulated antioxidant genes (sod at 6 hpi and prx2 at 72 hpi), while co-incubation with A. salmonicida MAMPs modulated inflammatory responses, suggesting a protective role against bacterial challenges.

3.2. Overall health status and fish performance at in vivo level

No mortalities or adverse health effects were observed during the trial. Fucoidan supplementation did not significantly affect feed intake, growth rate, or feed conversion ratio, indicating its safety as a feed additive.

3.3. HKLs from fish fed fucoidan exposed to MAMPs from A. salmonicida

HKLs from fish fed 0.2% fucoidan (F2 group) showed increased production of TNF-α and HSP70 at 6 hpi, similar to the MacroGard? group. In contrast, the 0.4% fucoidan group (F4) exhibited elevated IL-10 levels at 24 hpi, indicating dose-dependent immunomodulatory effects, with higher doses promoting anti-inflammatory responses.

3.4. DI morphometry and microbiota

No morphological changes were detected in the distal intestine. Microbiota analysis revealed Firmicutes as the dominant phylum, with a dose-dependent increase in Bacillus abundance in fucoidan-fed groups. Alpha-diversity decreased at higher fucoidan levels, driven by shifts in lactic acid bacteria (e.g., Limosilactobacillus and Lactobacillus), while beta-diversity remained unchanged. Feed was identified as the primary source of gut microbiota, with minimal influence from water.

3.5. DI multi-omic data and specific immune-related biomarkers

Transcriptomics of the F2 group showed 1,863 differentially expressed genes (DEGs), with upregulation of pathways related to lymph vessel development, chemotaxis, wound healing, and oxidoreductase activity. IHC confirmed increased levels of MHC II and CD3 proteins, indicating enhanced antigen presentation. Gene expression analysis revealed upregulation of gata3, a key regulator of Th2 immune responses. Proteomics identified higher abundance of proteins involved in chaperones, DNA repair, and the ubiquitin system, alongside reduced levels of transporters and glycosaminoglycan-binding proteins, suggesting improved cellular stress management and immune coordination.

The study demonstrates that dietary fucoidan from sugar kelp can modulate both innate and adaptive immune responses in Atlantic salmon. At 0.2% inclusion, fucoidan enhances antioxidant capacity, promotes beneficial shifts in gut microbiota (e.g., increased Bacillus), and upregulates intestinal pathways involved in immune surveillance and tissue repair. Higher doses (0.4%) induce anti-inflammatory effects, highlighting the importance of dosage in immunomodulation. The findings underscore the potential of fucoidan as a functional feed additive to improve salmon health and resilience, thereby reducing economic losses in aquaculture. Furthermore, this research supports the development of value-added seaweed products, contributing to the sustainability and economic viability of both the aquaculture and seaweed cultivation industries. Future studies should explore long-term effects and specific mechanisms underlying fucoidan's impact on T-cell polarization and antibody production in fish.