Response of endolithic Chroococcidiopsis strains from the polyextreme Atacama Desert to light radiation

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Abstract
The Atacama Desert is known to be the place on Earth with one of the highest solar radiation limiting the presence of life to endolithic microhabitats and soil microbial ecosystems. Endolithic microbial communities are supported by photosynthetic primary producers, mainly cyanobacteria, which can be injured by UVR. Nevertheless, cyanobacteria exposed to high solar radiation and its harmful effects have developed a series of defense mechanisms: avoidance, antioxidant systems or production of photoprotective compounds such as scytonemin among others. Scytonemin is a liposoluble pigment whose absorption maxima are located in UVA and UVC range and highly absorbing in the UVB range. In order to elucidate the protection capacity of endolithic cyanobacteria against harmful radiation, two cyanobacterial strains from Chroococcidiopsis genus were isolated from different endolithic microhabitats in the Atacama Desert: UAM813 strain, originally from the cryptoendolithic microhabitat of halite (NaCl), and UAM816 strain from chasmoendolithic microhabitat of calcite (CaCO 3 ). Both were exposed to PAR and UVR+PAR conditions studying their short-term response, as oxidative stress and long-term response, as scytonemin production, metabolic activity and ultrastructural damage. The observed response of both strains reveals a high sensitivity to direct light exposure, even to PAR. The differences in their acclimation suggest specific adaptation strategies related to their original microhabitat, revealing their protective potential and the strain specific environmental pressure selection to inhabit different microhabitats and exposed to different light conditions. Importance: Cyanobacteria are photosynthetic prokaryotes that inhabit most types of illuminated environments, even the endolithic microhabitats in cold and hot deserts. The environmental pressure caused by the extreme solar irradiation in the Atacama Desert involve that only those cyanobacterial strains able to cope with it can be found in these endolithic communities, usually dominated by members belonging to the extremotolerant Chroococcidiopsis genus. Here, a comprehensive analysis of multiple lines of defense against harmful sun radiation was conducted to diagnose the response of two Chroococcidiopsis strains isolated from different endolithic microhabitats and lithic substrates, and identify its relation with the original microenvironmental conditions of each strain. Our results contribute to a better understanding of the acclimation strategies developed by these cyanobacterial strains and its potential protective role for the whole endolithic microbial community.
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