Ionizing-radiation resistance in the desiccation-tolerant cyanobacterium Chroococcidiopsis

The effect of X-ray irradiation on cell survival, induction, and repair of DNA damage was studied by using 10 Chroococcidiopsis strains isolated from desert and hypersaline environments. After exposure to 2.5 kGy, the percent ages of survival for the strains ranged from 80 to 35%. In the four most re sistant strains, the levels of survival were reduced by 1 or 2 orders of ma gnitude after irradiation with 5 kGy; viable cells were recovered after exp osure to 15 kGy but not after exposure to 20 kGy. The severe DNA damage evi dent after exposure to 2.5 kGy was repaired within 3 h, and the severe DNA damage evident after exposure to 5 kGy was repaired within 24 h. The increa se in trichloroacetic acid-precipitable radioactivity in the culture supern atant after irradiation with 2.5 kGy might have been due to cell lysis and/ or an excision process involved in DNA repair. The radiation resistance of Chroococcidiopsis strains may reflect the ability of these cyanobacteria to survive prolonged desiccation through efficient repair of the DNA damage t hat accumulates during dehydration.