| Summary: | Earth’s oceans are predominantly cold, with nearly 90% of their volume having temperatures below 5 °C. Microorganisms commonly referred to as psychrophiles have adapted to the temperatures of these cold waters. The most extreme psychrophiles are found inside the sea ice of polar oceans, where bacterial growth can be observed down to -20 °C. Sea ice consists of a matrix of ice and high-saline water (brine) that provide a unique habitat for microbial communities. Microscopic algae and bacteria dominate these extreme environments, which are considered very stressful as they are characterised by large variations in salinity, low temperatures, and low radiation levels. However, the brine-filled channels also provide a platform from which microscopic algae remain in the euphotic zone and refugees from significant grazing, thereby enabling net autotrophic growth. As a result, sea ice hosts some of the highest chlorophyll a concentrations on the planet, and is one of the most important factors controlling primary production and bloom dynamics in polar areas. In this thesis, I focus on the ecophysiology of psychrophiles adapted to the sea ice environment. Physiological acclimation to environmental change needs to be studied in order to address how different stressors may influence organisms’ capacity to tolerate both naturally- and climatically-driven changes. Extremophiles growing close to their physiological limits may be especially susceptible to environmental stressors, such as rapid climate change. Therefore, a series of studies has been performed to investigate how environmental stressors, such as increased temperature and elevated CO2, affect microbial physiology and community structure in polar areas. The ecophysiology of sea ice microorganisms has been addressed in laboratory experiments (Papers I, II, and IV) and in field measurements (Paper III). In brief, relatively small changes in temperature had considerable effects on the physiology of sea ice diatoms, and indirectly affected the structure of sea ice ...
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