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Heavy metal lead (Pb) is a primary pollutant that pollutes aquatic environments and can be enriched in aquatic animals. Here, the effects of waterborne Pb on grass carp (Ctenopharyngodon idella) within 96 h of exposure were detected, and the association between microbiota alterations in the intestines and water was analysed. Results revealed that the intestinal barrier was damaged, including the intestinal morphology change, the structure‐related genes (villin‐1 and Claudin‐7) expression down‐regulation, the antioxidant (GST and GSH‐Px) and immune‐related genes (TNF‐α and IL‐8) expression up‐regulation. The MiSeq sequencing data revealed a decrease in diversity and an increase in microbiota richness in the intestines and water after Pb exposure. Linear discriminant analysis (LDA) results revealed an increase in the dominant bacteria (P_proteobacteria and C_Gammaproteobacteria) in the water, which enriched bacteria in the intestines. Canonical correlation analysis (CCA) found that there were correlations between intestinal microbiota and water microbiota, intestinal structure morphology and immune factors. Based on these results, we speculated that the effects of Pb on the grass carp intestinal barrier may be related to changes in the intestinal structure of grass carp and the water microbial community. These findings improve our understanding of the effects of heavy metals on the fish–microbe relationships after polluting aquatic habitats. [ABSTRACT FROM AUTHOR]

The effects of replacing fish meal (FM) with low‐gossypol cottonseed meal (LCSM) on the intestinal barrier in different segments (PI, proximal intestine; MI, middle intestine; DI, distal intestine) were studied in juvenile golden pompano (Trachinotus ovatus) using 6‐week feeding trials. FM of 0% (diet_1), 20% (diet_2), 40% (diet_3) and 60% (diet_4) was replaced by LCSM. The FM‐based diet (diet_1) containing 25% FM was used as control group. The intestinal morphology, mRNA expression of tight junction‐related genes (ZO‐1, Occludin, Claudin‐12), antioxidant enzyme‐related genes (CAT, GPx, GR, CuZnSOD) and non‐specific immune response‐related genes (IL‐8, TNF‐α, C4) and microbiota of different intestine segments were detected. The results showed that the number and length of microvilli and intestinal wall thickness decreased significantly in PI of diet_3 and diet_4 groups. Meanwhile, mRNA expression of Occludin, Clauding‐12 and C4 was up‐regulated in PI and DI of diet_3 group; the expression of GR and CuZnSOD was up‐regulated in PI and MI with increasing LCSM replacement; the expression of TNF‐α was up‐regulated in the three segments of diet_3 group. Further, 16S rRNA amplicon sequencing revealed a significant reduction in the richness and diversity of intestinal microbial in diet_3 and diet_4 groups. At the family level and genera level, the abundance of Nocardiaceae and Rhodococcus was up‐regulated in the three segments of diet_3 and diet_4 groups respectively. These results suggested that the replacement of LCSM by more than 20% causes the intestinal barrier function of juvenile golden pompano to be affected. These provide a reference for sustainable aquaculture. [ABSTRACT FROM AUTHOR]