Ionizing-radiation resistance in the desiccation-tolerant cyanobacterium Chroococcidiopsis

Abstract

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 percentages of survival for the strains ranged from 80 to 35%. In the four most resistant strains, the levels of survival were reduced by 1 or 2 orders of magnitude after irradiation with 5 kGy; viable cells were recovered after exposure to 15 kGy but not after exposure to 20 kGy. The severe DNA damage evident 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 increase in trichloroacetic acid-precipitable radioactivity in the culture supernatant 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 that accumulates during dehydration.

FIG. 1

Survival of 10 Chroococcidiopsis strains after irradiation with 2.5 kGy. The values are mean survival rates based on one trial with four replicates.

FIG. 2

Representative survival curves for four Chroococcidiopsis strains and controls. The values are means based on two independent trials with three replicates per trial.

FIG. 3

Electron micrograph of ultrathin section of Chroococcidiopsis sp. strain 029, showing single cells in different developmental and physiological states within the same aggregate. The cells differ in size, in the thickness of the cell envelope, in the presence of a division septum (arrow), and in the abundance of thylakoid membranes in the cytoplasm. Bar = 0.5 μm.

FIG. 4

Ability of Chroococcidiopsis sp. strain 171 to recover from DNA damage following exposure to 5 kGy of irradiation. (A) Lane 1, 1-kb DNA ladder; lane 2, genomic DNA extracted from unirradiated cells; lane 3, genomic DNA extracted immediately after irradiation; lanes 4 and 5, DNA extracted 12 h (lane 4) and 24 h (lane 5) after exposure. (B) Lane 1 contained a 1-kb DNA ladder. When lanes were loaded immediately after extraction, nucleic acids from Chroococcidiopsis sp. strain 015 (lane 2) and strain 171 (lane 3) lacked the shearing in the rRNA and in the upper band (asterisk) of the total DNA. (C) Lane 1, 1-kb DNA ladder; lanes 2 and 3, genomic DNA from irradiated strain 171 loaded immediately after extraction (lane 3) and after a cycle of freezing and thawing (lane 2).

FIG. 5

Release of [³H]thymidine-labeled DNA from three Chroococcidiopsis strains after 2.5 kGy of irradiation.