E. Hamedi - Faculty of Specialized Veterinary Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran
A.E. Khajeh-Rahimi - Department of Marine Science and Technology, Islamic Azad University, North Tehran Branch, North Tehran, Iran.
R. Kazempoor - Department of Biology, Roudehen Branch, Islamic Azad University, Roudehen, Iran.
B. Nowruzi - Department of Biotechnology, Faculty of Converging Sciences and Technologies, Islamic Azad University, Science and Research Branch, Tehran, Iran.

Cyanobacteria, ancient photosynthetic prokaryotes, form harmful blooms that degrade water quality and pose many risks to human health. The main objective of this study is to identify toxic cyanobacteria present in fish ponds, evaluate their effects on Zebrafish (Danio rerio) tissue, and investigate their characteristics. Samples were collected from the walls and floors of rainbow trout raceway ponds at Alborz Caspian Company, situated in Alborz province, Iran. The purpose of this collection was to isolate and examine cyanobacteria colonies. As a result of meticulous microscopic and macroscopic observations, three pure samples of Calothrix sp., Nostoc sp., and Microcystis sp. species were successfully identified. These samples were then transported to the laboratory and processed using the Z۸ solid culture medium within a growth chamber. Cyanobacteria were identified based on their morphology using a light microscope and validated identification keys. The DNA extraction was performed using the cetyl-trimethyl-ammonium bromide method. Zebrafish were acclimated and fed with fish food containing lyophilized cyanobacteria for a period of ۳۰ days. At specific intervals, fish were collected for histopathological analysis and measurement of antioxidant enzyme activity. The histopathological examination of intestinal tissue in the treatment groups exposed to lyophilized cyanobacteria revealed lesions including hyperplasia of enterocytes, reduction in their length, vacuolation of enterocytes, hyperplasia of goblet cells, and infiltration of lymphocytes. The gill samples from the treatment group exhibited severe histopathological abnormalities such as displacement of epithelial cells, fusion of lamellae, epithelial necrosis, and lymphocyte infiltration. These symptoms diminished over time. Hepatocellular lipid changes and vacuolation were observed in the treatment group's liver samples, peaking on the ۳۰th day. The activity of superoxide dismutase (SOD) enzyme significantly increased in the exposed Zebrafish on the ۳۰th day compared to the control group, and similar significant increases were observed on the ۲۰th and ۳۰th days (p<۰.۰۵). There were no significant differences in Catalase (CAT) activity between the control and treatment groups (p>۰.۰۵). Generally, the study identified specific histopathological abnormalities in the fish exposed to lyophilized cyanobacteria and observed changes in antioxidant enzymes activity. These findings contribute to understanding the impact of cyanobacteria on fish health status.

Cyanobacteria, Cyanotoxin, Zebrafish, Oxidative stress, Histopathology, Toxicology