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• Nostoc muscorum can bio-accumulate large amount of pyridaphenthion (PY). • N. muscorum induced PY detoxification & degradation to less toxic compound. • PY induced photosynthesis inhibition & oxidative stress, particularly in Anabaena laxa. • Improved antioxidative capacity of N. muscorum explains its high stress tolerance. • N. muscorum is a promising candidate for PY remediation. Excessive use of organophosphorus pesticides such as pyridaphenthion (PY) to constrain insects induced crop loss, results in soil and water sources contamination. Cyanobacteria are sensitive biological indicators and promising tools for bioremediation of soil and water pollutants. To understand PY toxicity, detoxification and degradation in cyanobacteria, we performed a comparative study in the two diazotrophic cyanobacteria; Anabaena laxa and Nostoc muscorum. They were exposed to mild (5 mg/L) and high (10 mg/L) concentrations of PY for 7 days. Compared to A. laxa, N. muscorum efficiently showed high PY accumulation and degradation to a safe environmentally product; 6-hydroxy-2-phenylpyridazin-3(2 H)-one. PY inhibited cell growth and reduced Chl a content and photosynthesis related enzymes (PEPC and RuBisCo) activities in both species, but to less extend in N. muscorum. It also induced oxidative damage, particularly in A. laxa , as indicated by high H 2 O 2 , lipid peroxidation and protein oxidation levels and increased NADPH oxidase enzyme activity. N. muscorum invested more in antioxidants induction, i.e., induced ascorbate and glutathione cycle, however, these antioxidants increments in A. laxa were less pronounced. Overall, this study provides more in-deep insights into the PY toxicity and the role of N. muscorum as a promising PY remediator. [ABSTRACT FROM AUTHOR]