Environmental deterioration and toxic cyanobacterial blooms (Microcystis and Anabaena) induce severe pathophysiology in Labeo rohita.

Cyanobacteria are widely distributed in water worldwide, and their harmful effects on water quality have become a global issue. It has long been recognized that cyanobacterial blooms in aquaculture ponds can cause sudden, large-scale fish mortalities. However, their impact on cellular and molecular responses in fish remains poorly understood. In this study, the impact of cyanobacterial blooms, which have been associated with mass mortality events in the freshwater fish Labeo rohita, was investigated by examining their detrimental effects on the host's cellular and molecular responses. The fish had a swollen abdomen and a red, enlarged anus. When gentle pressure was applied, fluid dripped from the anus, possibly due to an algal bloom in the culture system. Microscopic and molecular tests confirmed a cyanobacterial bloom caused by Microcystis sp. and Anabaena sp., likely responsible for the high fish mortality. During the infection, the fish exhibited severe histopathological alterations in the liver, kidney, and gills. The fish's immune system responded strongly through differentially expressed nonspecific and specific immune responses, involving significant effect on the inflammation (TNF-α, IL-1β, iNOS), immune activation (TLR 4, C3, MYD88, NOD 1), and innate and adaptive immune response (IFN-γ, Hsp70, Mx, IgM) genes in the L. rohita liver, kidney, and gill tissue samples. This suggests that cyanobacterial blooms influence the host immune system, creating conditions that promote immunotoxicity in fish, which can lead to infection and mortality. Here, we evaluated the effects of cyanobacterial blooms (Microcystis sp. and Anabaena sp.) on the cellular and molecular responses of fish during a severe mass mortality event. Hence, the present study may contribute to a deeper understanding of the interactions between the aquaculture water environment and the fish immune system.