Assessment of intrageneric environmental limits of the extremophilic cyanobacterium Chroococcidiopsis (Pleurocapsales) and its implications for space exploration





The cyanobacterium Chroococcidiopsis contains species found in extreme environments, providing the opportunity to study adaptation to a range of environments within the confines of a single genus. Due to its extremophile nature, it has attracted attention for space colonization as well as a model of life elsewhere.

In this study, eight unialgale strains from diverse habitats, isolated in unialgale culture and cultured under laboratory conditions, were characterized for their ability to survive a range of extreme environments, including UVC radiation (254 nm) , oxidative damage, desiccation and repeated freezing/freezing. thawing. The study revealed two previously uncharacterized saltwater isolates of Chroococcidiopsis that were more resistant to radiation than most other isolates.

Isolate CCMP 1991 from Hawaii survived up to 1750 J m-2, and isolate CCMP 3184 from Samoa survived up to 1000 J m-2 (254 nm UVR) versus 250 J m- 2 for most other isolates tested. These two UV-resistant isolates are closely related phylogenetically, but inhabit different environments. Each was further characterized for its ability to repair DNA damage, assessed by UV-induced thymine dimer repair and for tolerance to oxidative damage via resistance to H2O2-induced (oxidative) damage.

Both isolates repaired thymine dimers faster in light than in dark, with the Hawaiian isolate repairing faster than the Samoan isolate in light, suggesting photoreactivation repair. The Hawaiian isolate was more tolerant to H2O2 exposure than the Samoan isolate, indicating a possible role of antioxidants in cell protection. Both isolates were more tolerant than the other isolates tested to freezing/thawing in liquid nitrogen, which is also known to cause DNA damage.

Spectral absorbance analyzes were performed to detect pigments in each isolate. While all exhibited peaks likely to be chlorophyll a, carotenoids, phycocyanin, scytonemin, and MAAs, the Hawaiian isolate contained a pigment absorbed at approximately 325 nm that none of the other isolates contained. Although this pigment is outside the UVC absorbance range, it is speculated that it may play a role in DNA protection as a UV sunscreen or as an antioxidant. The phenotypic similarities in radiation resistance and freeze/thaw resistance among Hawaiian, Somoan, and Negev isolates appear to be the result of environmental adaptation rather than phylogenetic markers, as the first of between them was part of a saltwater clade, while the Negev strain belongs to a freshwater clade.

As pigmentation and probable resistance to other environmental conditions can be induced, these data provide a baseline study of strains in an uninduced state. Thus, the true environmental limits of Chroococcidiopsis probably go beyond our current knowledge. Implications of this are discussed in relation to space exploration

John R. Cumbers, View ORCID ProfileLynn J. Rothschild
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Declaration of Competing Interests
The authors have declared no competing interests.

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