# 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)

## TL;DR

Exposure to Microcystis in common carp causes intestinal damage, reduced digestion, and harmful changes in gut bacteria, affecting fish health.

## Contribution

This study reveals the negative impact of oral Microcystis exposure on intestinal digestion, immunity, and microbiota in common carp.

## Key findings

- Microcystis exposure caused intestinal barrier damage and increased permeability in common carp.
- Exposure increased harmful bacteria like Proteobacteria and reduced beneficial bacteria producing short-chain fatty acids.
- Intestinal inflammation and reduced digestive enzyme activity were observed in exposed fish.

## Abstract

Cyanobacterial blooms in aquatic environment are often accompanied by the release of microcystins (MCs), which can threaten the health of aquatic organisms. In the present study, intestinal barrier damage, oxidative stress, and inflammation were found to occur in the common carp after the ingestion of Microcystis aeruginosa. At the same time, the digestive function of the carp decreased, and the homeostasis of intestinal flora was disordered, accompanied by content reduction in the beneficial metabolites from fish intestinal flora. These findings provide an important theoretical basis for a comprehensive evaluation of the intestinal toxicity of MCs on fish.

Cyanobacterial blooms are a common water pollution phenomenon worldwide now. Among them, Microcystis 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 Microcystis 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 Microcystis exposure caused intestinal barrier damage, intestinal structure destruction, and increased permeability. Analysis of the activities of three main intestinal digestive enzymes showed that Microcystis 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 Microcystis 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 Microcystis 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.