Abstract
The edible cyanobacterium Arthrospira is resistant to ionising radiation. The cellular mechanisms underlying this radiation resistance are, however, still largely unknown. Therefore, additional molecular analysis was performed to investigate how these cells can escape from, protect against, or repair the radiation damage. Arthrospira cells were shortly exposed to different doses of 60Co gamma rays and the dynamic response was investigated by monitoring its gene expression and cell physiology at different time points after irradiation. The results revealed a fast switch from an active growth state to a kind of 'survival modus' during which the cells put photosynthesis, carbon and nitrogen assimilation on hold and activate pathways for cellular protection, detoxification, and repair. The higher the radiation dose, the more pronounced this global emergency response is expressed. Genes repressed during early response, suggested a reduction of photosystem II and I activity and reduced tricarboxylic acid (TCA) and Calvin-Benson-Bassham (CBB) cycles, combined with an activation of the pentose phosphate pathway (PPP). For reactive oxygen species detoxification and restoration of the redox balance in Arthrospira cells, the results suggested a powerful contribution of the antioxidant molecule glutathione. The repair mechanisms of Arthrospira cells that were immediately switched on, involve mainly proteases for damaged protein removal, single strand DNA repair and restriction modification systems, while recA was not induced. Additionally, the exposed cells showed significant increased expression of arh genes, coding for a novel group of protein of unknown function, also seen in our previous irradiation studies. This observation confirms our hypothesis that arh genes are key elements in radiation resistance of Arthrospira, requiring further investigation. This study provides new insights into phasic response and the cellular pathways involved in the radiation resistance of microbial cells, in particularly for photosynthetic organisms as the cyanobacterium Arthrospira.
Highlights
Arthrospira is a motile filamentous planktonic non-N2-fixing cyanobacterium
Three independent lab cultures were propagated and used separately for irradiation. They were grown aerobically in 1000 ml Zarrouk-UBP medium in 2000 ml Erlenmeyers [14] on a rotatory shaker 120 rpm in an incubator at 30°C illuminated with a photon flux density of ~ 42 μE m-2 s-1 provided by three Osram daylight tubes (Binder KBW 400), to mid-exponential growth phase corresponding to an optical density of OD750nm ~1 and a Zarrouk-UBP medium pH of ca. 10.5
There was one specific set of genes, the arh genes, which remained highly expressed throughout the full recovery period
Summary
Arthrospira is a motile filamentous planktonic non-N2-fixing cyanobacterium. It grows naturally in alkaline lake water environments in regions with strong sunlight and high temperature [1]. The response of Arthrospira to such environmental stresses has mainly been investigated based on morphological and physiological traits [13]. We were the first to study the damage of radiation stress (i.e. high doses of gamma rays) at molecular level, using a combination of genomic, transcriptomic and proteomic data [12]. Cells exposed to high doses of 3200 or 5000 Gy of gamma rays, causing significant cell damage, showed organised shutdown of photosynthesis and carbon fixation, decreased pigment, lipid and secondary metabolite synthesis; and induced thiol-based antioxidant systems, photo-sensing and signalling pathways
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