Coming along with high water reuse in sustainable and intensive recirculating aquaculture systems (RASs), the waste products of fish in rearing water is continuously accumulated. Nitrate, the final product of biological nitrification processes, which may cause aquatic toxicity to fish in different degrees when exposed for a long time. Therefore, the present study was conducted to evaluate the impact of chronic nitrate exposure on intestinal morphology, immune status, barrier function, and microbiota of juvenile turbot. For that, groups of juvenile turbot were exposed to 0 (control check, CK), 50 (low nitrate, L), 200 (medium nitrate, M), and 400 (high nitrate, H) mg L⁻¹ nitrate-N in small-sized recirculating aquaculture systems. After the 60-day experiment period, we found that exposure to a high concentration of nitrate-N caused obvious pathological damages to the intestine; for instance, atrophy of intestinal microvilli and necrosis in the lamina propria. Quantitative real-time PCR analysis revealed a significant downregulation of the barrier forming tight junction genes like occludin, claudin-like etc. under H treatment (P < 0.05). Intestinal MUC-2 expression also decreased significantly in the nitrate treatment groups compared to that in the control (P < 0.05). Additionally, the expression of HSP70 and HSP90 heat-shock proteins, toll-like receptor-3 (TLR-3), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) significantly increased (P < 0.05), whereas that of transforming growth factor-β (TGF-β), lysozyme (LYS), and insulin-like growth factor-I (IGF-I) significantly decreased with H treatment (P < 0.05). The results also revealed that intestinal microbial community was changed following nitrate exposure and could alter the α-diversity and β-diversity. Specifically, the proportion of intrinsic flora decreased, whereas that of the potential pathogens significantly increased with M and H treatments (P < 0.05). In conclusion, chronic nitrate exposure could weaken the barrier function and disturb the composition of intestinal microbiota in marine teleosts, thereby harming their health condition.

伴随着可持续和集约化循环水产养殖系统（RASs）中的高水回用，饲养水中鱼的废物不断积累。硝酸盐，生物硝化过程的最终产物，长时间暴露可能对鱼类产生不同程度的水生毒性。因此，本研究旨在评估长期硝酸盐暴露对幼年turbo的肠道形态，免疫状态，屏障功能和微生物群的影响。为此，将青少年菱t的组暴露于0（对照检查，CK），50（硝酸盐，L），200（硝酸盐，M）和400（硝酸盐，H）mg L ^-1小型循环水产养殖系统中的硝酸盐氮。经过60天的实验期后，我们发现暴露于高浓度的硝酸盐-N会对肠道造成明显的病理损害。例如，肠道微绒毛萎缩和固有层坏死。实时定量PCR分析显示，在H处理下，形成紧密连接基因的屏障（如闭合蛋白，claudin样等）显着下调（P  <0.05）。与对照组相比，硝酸盐处理组的肠道MUC-2表达也显着降低（P  <0.05）。此外，HSP70和HSP90热休克蛋白，toll​​样受体3（TLR-3），白介素1β（IL-1β）和肿瘤坏死因子-α（TNF-α）的表达显着增加（P  <0.05），而H处理显着降低了转化生长因子-β（TGF-β），溶菌酶（LYS）和胰岛素样生长因子-I（IGF-1）的表达（P  <0.05）。结果还表明，硝酸盐暴露后肠道微生物群落发生变化，并可能改变α-多样性和β-多样性。具体来说，内源菌群的比例下降，而潜在病原体的比例随M和H处理而显着增加（P  <0.05）。总之，长期暴露于硝酸盐会削弱海洋硬骨鱼的屏障功能并扰乱肠道微生物群的组成，从而损害其健康状况。