Freezing tolerance of freshwater diatoms as a key to their success in polar regions

II ABSTRACT (ENGLISH VERSION) Diatoms are microalgae characterised by a golden-brown colour and ornamented silica shells. They thrive in various environments worldwide ranging from aquatic marine and freshwater to terrestrial. Together with cyanobacteria and green algae, they are important primary producers in both polar regions (Arctic and Antarctica). The polar environment is characterised by extreme natural conditions, which microalgae have to overcome, such as low and freezing temperatures, desiccation, long periods of dark and irregular nutrient and liquid water availability. Many microorganisms overwhelmed by unfavourable conditions use dormancy and create stress resistant stages. Yet, freshwater pennate diatoms are not known to form such morphologically distinct stages. Despite this fact, they prosper well in many polar habitats. This doctoral thesis presents a summary of the challenges of life in polar environments and reviews the current knowledge of survival strategies of microalgae with the focus on freezing stress. The conducted research provides a deeper insight into survival of freshwater pennate diatoms in the severe conditions of the polar environment. Their tolerance to freezing stress was evaluated experimentally under laboratory conditions and by field observations over a one- year period..