The green alga Klebsormidium is found worldwide in terrestrial and freshwater habitats. A novel strain was isolated from biological soil crusts on Breinosa Mountain in Svalbard (High Arctic, 78°N) to investigate its adaptation to extreme Arctic conditions, characterized by low temperatures and short periods of snow-free ground. Genetic analyses confirmed the strain's identity as Klebsormidium flaccidum, showing high genetic similarity to strains from polar and temperate habitats. This study explored the effects of nitrogen starvation, cold stress, desiccation, and freezing, focusing on molecular responses and stress resilience. Nitrogen starvation strongly impacted metabolic activity, while cold and desiccation stress had more subtle effects, with K. flaccidum showing permanent stress-related gene expression instead of acute responses. Cold acclimation was found to enhance desiccation and freezing tolerance, while nitrogen starvation worsened the effects of these stresses. These results suggest that K. flaccidum CCALA 1182 is inherently capable of withstanding fluctuating environmental stresses, highlighting its potential for survival in diverse and harsh ecosystems.

Centre for Biology Geosciences and the Environment Faculty of Education University of West Bohemia Chodské náměstí 1 Plzeň 306 14 Czech Republic

Centre for Polar Ecology Faculty of Science University of South Bohemia Na Zlaté stoce 3 České Budějovice 370 05 Czech Republic

Department of Ecology Faculty of Science Charles University Albertov 6 Prague 128 00 Czech Republic

Department of Experimental Plant Biology Faculty of Science University of South Bohemia Branišovská 1645 31a České Budějovice 370 05 Czech Republic

Department of Phycology Institute of Botany CAS Dukelská 135 Třeboň 379 01 Czech Republic

Institute for Plant Sciences University of Cologne Zülpicher Str 47b Cologne 50674 Germany

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