A UV resistant mutant of Calothrix braunii has been isolated after repeated exposure to UV-C (254 μM) radiation. LD50 for wild type against UV-B was 1.74 h and 100% lethality was achieved after 3.5 h. Whereas, UV resistant mutant showed LD50 at 3.33 h and loss of complete survival after 5 h exposure. The growth rate of mutant was about 29 per cent greater than that of the wild type. The photosynthetic pigments, chlorophyll a and phycoerythrin were stimulated by 36.2 and 41.2 per cent, respectively over wild type. There were no differences in nitrogenase activity nitrate reductase activity, extracellular ammonia production and of plasmid DNA.
UV radiation, mutation, cyanobacteria
