# Microcystin-LR-Induced Oxidative Stress, Transcriptome Changes, Intestinal Microbiota, and Histopathology in Rana chensinensis Tadpoles

## TL;DR

This study explores how microcystin-LR affects Chinese brown frog tadpoles by causing oxidative stress, changing gene activity, altering gut bacteria, and damaging organs.

## Contribution

The study provides a comprehensive analysis of MC-LR toxicity in anuran tadpoles using multiple biological endpoints.

## Key findings

- MC-LR exposure caused oxidative stress and downregulated digestion-related genes in tadpole livers.
- Intestinal microbiota richness and dominant taxa abundances were altered by MC-LR stress.
- MC-LR exposure led to structural damage in the liver and intestine of tadpoles.

## Abstract

Microcystin-LR (MC-LR), a toxic byproduct of harmful algal blooms, has emerged as a global health threat to aquatic organisms. However, its toxic effects on amphibian tadpoles remain incompletely understood. This study presents a comprehensive analysis of MC-LR-induced toxicity in the liver and intestine of Chinese brown frog (Rana chensinensis David, 1875) tadpoles by integrating assessments of oxidative stress, liver transcriptome, gut microbiota, and histopathology. Our results demonstrated that exposure to MC-LR induced oxidative stress and downregulated genes involved in digestion in tadpole livers. The richness of the intestinal microbiota and the abundances of the dominant taxa were altered under MC-LR stress. Further, MC-LR caused structural damage in both the liver and intestine. These findings will expand our understanding of the toxic mechanisms induced by MC-LR in anuran tadpoles.

Microcystin-LR (MC-LR), produced by Cyanobacteria, is being detected in many types of waters, posing a universal threat to aquatic animals. However, there have been few comprehensive endpoints assessed, including oxidative stress, transcriptome changes, intestinal microbiota, and histopathology, in anurans exposed to MC-LR. In this study, all these effects of MC-LR on Chinese brown frog (Rana chensinensis David, 1875) tadpoles were investigated by exposing the tadpoles to MC-LR at different concentrations (0, 0.1, 1.0, 5.0, and 10.0 μg/L) for 7 days. Our results revealed that treatment of tadpoles with the high MC-LR dosage (10.0 μg/L) induced a significant increase in malondialdehyde (MDA) content and decreases in superoxide dismutase (SOD) activity, glutathione peroxidase (GPx) activity, and total antioxidant capacity (TAC). RNA-seq analysis of the liver showed that the number of differentially expressed genes (DEGs) was 2361 under lower MC-LR stress (1.0 μg/L), while the number of DEGs increased to 3185 under higher MC-LR stress (10.0 μg/L). Gene Ontology analysis showed that several biological processes and molecular functions related to digestion were enriched in both MC-LR treated groups, such as digestion, serine-type endopeptidase activity, and serine-type peptidase activity. KEGG enrichment analysis also indicated that the digestion for pancreatic secretion, protein digestion and absorption, and fat digestion and absorption pathways were significantly enriched in the treatment groups. Additionally, the bacterial richness was elevated by MC-LR exposure. At the phylum level, treatment with MC-LR changed the relative abundances of Desulfobacterota, Fusobacteriota, and Actinobacteriota. At the genus level, MC-LR caused significant alterations in the abundances of 23 genera. Furthermore, examination of sections obtained from the livers and intestines of tadpoles in the treatment groups showed damaged histological structure. The knowledge from this study will have potential value for understanding the mechanisms related to MC-LR toxicity in anurans.