Key Laboratory of Aquatic Animal Diseases and Immune Technology of Heilongjiang Province, Heilongjiang River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Harbin, China.
Front Immunol. 2022 Jun 29;13:900793. doi: 10.3389/fimmu.2022.900793. eCollection 2022.
Carbonate alkalinity is a major environmental stress factor affecting aquatic feed configuration, which easily causes oxidative stress and hypoimmunity for fish. Hence, the purpose of the study is to assess the potential effect of phosphorus on growth, intestinal oxidation resistance, physical barrier function, and microflora for Songpu mirror carp ( Songpu) (initial average weight of 2.95 ± 0.21 g) reared at the high-concentration carbonate alkalinity environment. A two-factor, three-level (2 × 3) design was applied, in which diets with three different phosphorus levels (3.6, 7.0, and 10.5 g/kg dry matter) were randomly assigned to 0 and 15 mmol/L carbonate alkalinity groups with three replicate aquariums. After the 8-week trial, we found that weight gain rate (WGR), specific growth rate (SGR), protein efficiency ratio (PER), and lipase and amylase activities in the intestine significantly ( < 0.05) declined with increasing carbonate alkalinity. Carbonate alkalinity of 15 mmol/L significantly reduced glutathione peroxidase (GSHPx) activities in the intestine ( < 0.05). The relative expressions of nuclear factor (erythroid-derived 2)-like 2 (Nrf2), glutathione peroxidase 1a (GPX1a), Clautin3, Clautin11, and tumor necrosis factor β (TNF-β) in the intestine were markedly downregulated by increasing carbonate alkalinity levels ( < 0.05), whilst the relative expressions of interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) in the intestine were markedly upregulated ( < 0.05). At the 15 mmol/L carbonate alkalinity treatment, Songpu mirror carp suffer from hypoimmunity status with failed digestion, antioxidant, inflammation, and immune response, thereby inducing impaired growth. Additionally, significant increments in the abundance of Proteobacteria and a significant decrease in the abundance of Fusobacteria and the Firmicutes/Bacteroidetes ratio were caused due to excessively high carbonate alkalinity (15 mmol/L) and excessively low dietary phosphorus supply (3.6 g/kg). Collectively, 7.0 g/kg dietary phosphorus supplementation was effective in promoting intestinal antioxidant enzyme activities and the corresponding gene expression the Keap1-Nrf2 signaling pathway and in enhancing intestinal immunity by upregulating anti-inflammatory and downregulating pro-inflammatory genes. Appropriate dietary phosphorus supply could promote the formation of beneficial microflora in freshwater, and it has the potential ability to transfer the adverse effect of carbonate alkalinity stress to the structural composition of intestinal microflora. Hence, consideration should be given to suitable phosphorus supply for fish under the chronic carbonate alkalinity stress.
碳酸盐碱度是影响水产饲料配制的主要环境胁迫因子，容易导致鱼类氧化应激和低免疫力。因此，本研究旨在评估磷对生长、肠道抗氧化、物理屏障功能和微生物区系的潜在影响。在高浓度碳酸盐碱度环境中养殖的松江鲈鱼（Songpu）（初始平均体重为 2.95±0.21g），采用两因素三水平（2×3）设计，三种不同磷水平（3.6、7.0 和 10.5g/kg 干物质）的饲料随机分配到 0 和 15mmol/L 碳酸盐碱度组，每组三个重复水族箱。8 周试验后，我们发现随着碳酸盐碱度的增加，增重率（WGR）、特定生长率（SGR）、蛋白质效率比（PER）以及肠内脂肪酶和淀粉酶活性显著（<0.05）下降。15mmol/L 的碳酸盐碱度显著降低了肠内谷胱甘肽过氧化物酶（GSHPx）活性（<0.05）。肠内核因子（红细胞衍生 2）样 2（Nrf2）、谷胱甘肽过氧化物酶 1a（GPX1a）、Clautin3、Clautin11 和肿瘤坏死因子 β（TNF-β）的相对表达水平随碳酸盐碱度水平的升高而明显下调（<0.05），而肠内白细胞介素 1β（IL-1β）和肿瘤坏死因子 α（TNF-α）的相对表达水平明显上调（<0.05）。在 15mmol/L 的碳酸盐碱度处理下，松江鲈鱼表现出免疫功能低下，消化、抗氧化、炎症和免疫反应失败，从而导致生长受损。此外，由于过高的碳酸盐碱度（15mmol/L）和过低的膳食磷供应（3.6g/kg），导致变形菌门的丰度显著增加，而 Fusobacteria 和 Firmicutes/Bacteroidetes 比值的丰度显著降低。总之，7.0g/kg 膳食磷补充有效地促进了肠道抗氧化酶活性和相应基因的表达，激活了 Keap1-Nrf2 信号通路，通过上调抗炎基因和下调促炎基因增强了肠道免疫功能。适当的膳食磷供应可以促进淡水有益微生物的形成，并有可能将碳酸盐碱度胁迫的不利影响转移到肠道微生物区系的结构组成上。因此，在慢性碳酸盐碱度胁迫下，应考虑鱼类的适宜磷供应。
