Microcystin exposure alters gut microbiota composition in fish: An in-Situ analysis of post-bloom effects in Hulun Lake, China

Cyanobacterial blooms are one of the most common stressors aquatic plants and animals encounter in freshwater ecosystems such as rivers and lakes. Following such outbreaks, some cyanobacteria release toxins, notably microcystins, which are highly toxic. Although numerous studies have explored the effects of microcystins on fish, their in-situ effects on the fish gut microbiome remain unexamined. Our objectives were to examine the fish gut microbiome before (Ju) and after (Au) cyanobacterial blooms and to monitor water quality. We collected Ju and Au fish gut and water samples from Hulun Lake. Using 16S rRNA full-length sequencing, we analyzed the composition, structure, and function of the fish gut microbiome. Results revealed significant disparities in microcystin concentrations between the Ju and Au water samples. In addition, the microcystin concentration in the carp gut was significantly higher than that in its muscle after the cyanobacterial bloom outbreak. Notably, the Au group demonstrated an increase in the microcystin level in water and a marked reduction in fish gut microbiota diversity compared with the Ju group. The cyanobacterial bloom decreased gut microbiome diversity in fish, with the Au group exhibiting a significantly reduced abundance of bacteria related to gut stability and microcystin degradation compared to the Ju group. Furthermore, we observed an upregulation of disease- associated bacterial metabolic functions in the Au group. In conclusion, these findings suggest that microcystins influence the composition and function of fish gut microbiota, improving our understanding of the interaction between fish gut microbiome and their environment. This study offers new perspectives on the adaptive mechanisms of aquatic organisms to cyanobacterial blooms.