The Arctic and Antarctic regions are renowned for their harsh environmental conditions, including low temperatures, lack of liquid water and mineral nutrients, and intense solar radiation during the summer solstice period. However, the recent global warming is leading to extreme weather events, including periods of winter thaws, and the widespread melting of ice caps, snowbanks, and glaciers. These environmental changes have a serious impact on local polar communities. The diversity of polar cyanobacterial and microalgal species has recently been the subject of extensive study. Detailed ecophysiological and molecular studies reveal species distributions, either widely distributed (ubiquitous) and/or local (endemic) in a particular polar area and in particular habitat types. To survive, local endemic communities develop a variety of protective mechanisms that differ from those species living in more favourable conditions. In this review, prokaryotic cyanobacteria and eukaryotic microalgae are used as model organisms to discuss the hardening processes, i.e. how they are prepared and/or how they react to the most extreme periods during seasonal cycles, on the ecological, physiological, and molecular levels. While some information on the physiological and morphological adaptations of polar communities is reviewed here, there is a lack of information on the hardening of organisms at the molecular level. It is crucial to determine the mechanisms by which different organisms prepare for the most extreme and stressful conditions during seasonal cycles, involving freezing and desiccation, as well as other important ecological limiting factors.