Intestinal Inflammation, Dysfunction of Intestinal Digestion, and Disorder in the Intestinal Microbiota and Their Metabolites Caused by Oral Microcystis Exposure in Common Carp (Cyprinus carpio)

Cyanobacterial blooms are a common water pollution phenomenon worldwide now. Among them, <i>Microcystis</i> blooms occur most frequently internationally, and most of them can produce hepatotoxins-microcystins (MCs). When the bloom occurs in water bodies, common carp, as the top consumers in the aquatic food chain, inevitably ingest a large amount of cyanobacteria. The effects on the intestinal digestion, immunity, and microbiota of the carp remain to be revealed. In the present study, 21-day gavage exposure to <i>Microcystis</i> paste (equivalent to 378.25 μg/kg of MC-LR) was conducted on juvenile carp mainly to investigate the adverse effects of MCs on the fish intestinal digestion, immunity, and microbiota. The results showed that <i>Microcystis</i> exposure caused intestinal barrier damage, intestinal structure destruction, and increased permeability. Analysis of the activities of three main intestinal digestive enzymes showed that <i>Microcystis</i> exposure affected the intestinal digestion of fish. Targeted metabolome analysis and 16S rRNA sequencing showed that the abundance of opportunistic pathogens such as Proteobacteria and Aeromonas increased in the exposed fish, and the contents of intestinal metabolites lipopolysaccharide (LPS) and trimethylamine oxide (TMAO) increased. However, the abundance of Firmicutes and Bacillus that produce short-chain fatty acids (SCFAs) was significantly reduced. These results indicated that <i>Microcystis</i> exposure could imbalance the intestinal flora and increase the abundance of Gram-negative bacteria, thereby activating the inflammatory response in the common carp. In summary, oral <i>Microcystis</i> exposure by the common carp can negatively affect fish digestion, immunity, and flora homeostasis of the fish intestinal tract. This study also provides a theoretical basis for a comprehensive evaluation of the beneficial effects of SCFAs on regulating metabolic disorders, activating lipid catabolism, and nutrient utilization of fish in aquaculture.