Temporal gene expression of the cyanobacterium Arthrospira in response to gamma rays

The edible cyanobacterium Arthrospira is highly resistant to ionising radiation. How theses cells can escape, protect or repair the radiation damage is not known, and requires additional molecular investigation. Therefore, in this study, Arthrospira cells were shortly exposed to different doses of 60Co gamma rays and the dynamic response of Arthrospira cells to irradiation was investigated by monitoring its gene expression and cell physiology, after irradiation. The dynamic transcriptome and associated physiological traits showed clearly two events: (1) an intense global emergency response with general reprogramming of carbon and energy metabolism and (2) a recovery period. Genes expressed during early response indicated a reduction of photosynthesis and reduced tricarboxilic acid (TCA) and Calvin Benson Cycles, while a likely activation of pentose phosphate pathway. Activation of biosynthesis of additional carbon storage molecules, compatible solutes, vitamins and sugars transport occurred after irradiation. In addition the cells enhance the restoration of the redox homeostasis. The repair mechanism of Arthrospira cells involve mainly proteases for cellular cleaning and removal of damaged proteins, single strand DNA repair and restriction modification system. During recovery, Arthrospira relies on the powerful antioxidant Glutathione molecule, for ROS detoxification. The exposed cells expressed during recovery period also again highly the arh genes, coding for a group of novel proteins which were detected in our previous irradiation studies, which confirms our hypothesis that they are a key element in the radiation resistance and merit further detailed investigation. This study provides new insights into phasic response and the cellular pathways involved the radiation resistance of microbial cells, in particularly for photosynthetic organisms as the cyanobacterium Arthrospira. The irradiation was performed using BRIGITTE facility at the Belgian Reactor N°2 (BR2). The irradiation was done inside a closed canister under water, surrounded by 60Co gamma rays sources (Energy of 1.33 Mev and 1.17 Mev). Different doses of gamma rays were given, i.e. 800 Gy, 1600 Gy and 3200 Gy, at a constant dose rate of 20000 Gy h-1. The time required for irradiation was dose-dependent, i.e. 2.4 minutes for 800 Gy, 4.8 minutes for 1600 Gy and 9.6 minutes for 3200 Gy. During irradiation, the cultures were in the dark, and the temperature inside the irradiation canister was automatically monitored and ranged between 26-27°C. In parallel, three other cultures were maintained at the same conditions but outside the irradiation facility, as non-irradiated controls.