Cyanobacteria form extensive macroscopic mats in shallow freshwater environments in the High Arctic and Antarctic. In these habitats, the communities are exposed to seasonal freezing and desiccation as well as to freeze-thawing and drying-rewetting cycles. Here, we characterized the annual cycles of two Phormidium communities in very shallow seepages located in central Svalbard. We observed the structure of the communities and the morphology, ultrastructure, metabolic activity, and viability of filaments and single cells. The communities overwintered as frozen mats, which were formed by long filaments enclosed in thick multilayered polysaccharide sheaths. No morphologically and/or ultrastructurally distinct spore-like cells were produced for surviving the winter, and the winter survival of the communities was not provided by a few resistant cells, which did not undergo visible morphological and ultrastructural transformations. Instead, a high proportion of cells in samples (85%) remained viable after prolonged freezing. The sheaths were the only morphological adaption, which seemed to protect the trichomes from damage due to freezing and freeze-associated dehydration. The cells in the overwintering communities were not dormant, as all viable cells rapidly resumed respiration after thawing, and their nucleoids were not condensed. During the whole vegetative season, defined by the presence of water in a liquid state, the communities were constantly metabolically active and contained <1% of dead and injured cells. The morphology and ultrastructure of the cells remained unaltered during observations throughout the year, except for light-induced changes in thylakoids. The dissemination events are likely to occur in spring as most of the trichomes were split into short fragments (hormogonia), a substantial proportion of which were released into the environment by gliding out of their sheaths, as well as by cracking and dissolving their sheaths. The short fragments subsequently grew longer and gradually produced new polysaccharide sheaths.

Centre for Polar Ecology Faculty of Science University of South Bohemia České Budějovice Czech Republic Botany Department Faculty of Science University of South Bohemia České Budějovice Czech Republic

Centre for Polar Ecology Faculty of Science University of South Bohemia České Budějovice Czech Republic Institute of Botany Academy of Sciences of the Czech Republic Třeboň Czech Republic

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