Arthrospira are cyanobacteria with a high protein content and rich in carbohydrates, essential fatty acids, and a variety of minerals, vitamins, and nutritional pigments such as beta-carotene. Hence, Arthrospira species have a long history of human consumption (now being commercialised under the name ‘Spirulina’) and are also used worldwide as feed for fish, poultry, and farm animals. Besides their nutritional value Arthrospira species are known for their clinical relevance (e.g. as antioxidants and immunity modulators), their potential in bioremediation and biosequestration, and their applied use in CO2 mitigation and biofuel technology. The Arthrospira sp. PCC 8005 strain was selected by the European Space Agency as an oxygen producer as well as a nutritional end-product for the life support system MELiSSA.
When we investigated the genomic stability of irradiated PCC 8005 cells (since this strain would be used in space missions hence subjected to cosmic radiation) we were surprised to find that this strain could withstand up to 5,000 Gray of gamma radiation. To put this in perspective, the average dose for 50% lethality (LD50) in mammals is about a thousand times less, or 5 Gy. The classical dogma of radiobiology is that cellular damage by ionising radiation (IR) lays primarily at the genetic level i.e. either during or immediately after the transfer of radiation energy to genomic DNA. Indeed, IR causes DNA double stranded breaks in the narrow range of ~0.002-0.008 DSB/Gy/Mb. These DNA lesions are highly cytotoxic because replication cannot proceed. Hence, it was first thought that IR-resistant organisms would possess very efficient DNA repair mechanisms. Although this generally holds true, IR-resistant and IR-sensitive organisms suffer the same number of DNA DSBs for an equivalent dose of IR and often possess similar DNA repair pathways. It now has been recognized that proteins are major targets of radiation damage through the action of reactive oxygen species (ROS). These ROS, which also cause oxidative damage to DNA, lipids, and other biomolecules, mainly result from the radiolysis of water since water is the most abundant chemical in living cells. Oxidative damage of a protein may render it dysfunctional with a potential detrimental effect on the cell depending on the its function and role. Particularly, an efficient protection of DNA repair enzymes against ROS attacks seems paramount to an enhanced radioresistance. This protection involves antioxidant or ROS-scavenging enzymes, pigments, metal ions (e.g. Mn[II]-complexes) or other ions, compatible solutes, as well as free aminoacids or small peptides.
In addition, IR-resistant bacteria seem to have acquired molecular adaptations to their DNA repair proteins, giving raise to novel enzymatic functions as well as to new substrate specificities. In fact, an IR-sensitive bacterium like E. coli can acquire a 10,000 fold increase of IR-resistance by undergoing iterative cycles of irradiation and outgrowth. Full-genome sequencing of isolates that evolved such extreme IR-resistance showed that mutations within these isolates were mainly situated within ten loci whose genes could be generally grouped in four functional catagories: (i) suppression of oxidative damage (rsxBD, gsiB, fnr), (ii) DNA repair and replication (recA, dnaBT/priAC, yfjK), (iii) basic respiration (glpD), and (iv) cell wall biogenesis (clpXP, wcaMKC, nanET).
The Arthrospira sp. PCC8005 genome was sequenced by us (Janssen et al., JBact 192(9): 2465–2466) (v1) and a Nimblegen tiling array ('Arthrospira HX12') holding >135,000 probes was designed. These probes were mapped to the improved sequence data (v5 - six ordered contigs; EMBL database acc. nr. GCA_000176895) of the genome, covering a total of 5,865 coding regions (CDS) and 3,141 intergenic regions. Detailed genome data were made available by us via the genome annotation platform MaGe (Genoscope, Evry, France – subject to account registration). These data indicate that Arthrospira sp. PCC 8005 makes use of the RecFOR pathway for ds-DNA repair and has normal sets of ss-DNA repair proteins (e.g., UvrABCD, MutST) while it lacks other proteins known for their involvement in DNA repair (e.g., the regulatory protein LexA, and RecBCD). On the level of protection against protein oxidation, a number of ROS-scavenging proteins and enzymes were identified, e.g. superoxide dismutase and at least two peroxiredoxins, but no catalase.
Using the above tiled array of probes we performed gene expression profiling of Arthrospira sp. PCC 8005 via the extraction of total RNA from cells exposed to 3,200 and 5,000 Gy of gamma radiation. The association analysis between the induction or repression of genes following irradiation and the underlying genetic mechanisms and biochemical pathways however proved to be difficult because most IR-affected genes were 'conserved hypothetical' (i.e. genes whose geneproduct show only a sequence match to database proteins of unkown function) or 'hypothetical' (i.e. genes that code for proteins with no match to the public protein databases). Nonetheless, a number of radiation-affected genes were assigned to photosynthesis, pigment biosynthesis, energy production, carbon fixation, oxidative stress response, heat-shock proteins, and DNA repair.