| Summary: | Melting polar and alpine ice surfaces frequently exhibit blooms of dark pigmented algae. These microbial extremophiles significantly reduce the surface albedo of glaciers, thus accelerating melt rates. However, the ecology, physiology and taxonomy of cryoflora are not yet fully understood. Here, a Swiss and an Austrian glacier dominated either by filamentous <i>Ancylonema nordenskioeldii</i> or unicellular <i>Mesotaenium berggrenii</i> var. <i>alaskanum</i>, were sampled. Molecular analysis showed that both species are closely related, sharing identical chloroplast morphologies (parietal-lobed for <i>Ancylonema</i> vs. axial plate-like for <i>Mesotaenium sensu stricto</i>), thus the unicellular species was renamed <i>Ancylonema alaskana</i>. Moreover, an ecophysiological comparison of the two species was performed: pulse–amplitude modulated (PAM) fluorometry confirmed that they have a high tolerance to elevated solar irradiation, the physiological light preferences reflected the conditions in the original habitat; nonetheless, <i>A. nordenskioeldii</i> was adapted to higher irradiances while the photosystems of <i>A. alaskana</i> were able to use efficiently low irradiances. Additionally, the main vacuolar polyphenol, which effectively shields the photosystems, was identical in both species. Also, about half of the cellular fatty acids were polyunsaturated, and the lipidome profiles dominated by triacylglycerols were very similar. The results indicate that <i>A. alaskana</i> is physiologically very similar and closely related but genetically distinct to <i>A. nordenskioeldii</i>.
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