Environmental conditions in the Arctic are highly variable in space and they can fluctuate seasonally. As for all living organisms, Arctic microbes are affected by low temperatures. Consequently, Arctic microbes might experience low rates of biochemical reactions that can further result in lower metabolism. Long periods of freezing temperatures may slow or prevent microbial growth, reproduction and even lead to cell disruption by ice crystals. Some Arctic habitats experience low precipitation, which limits the amount of water present in the environment. Temperatures below freezing often results in reduced water availability. Many Arctic environments are characterised by low nutrient availability, e.g., nitrogen and phosphate.
In addition, Arctic environments undergo a strong seasonal fluctuation both in temperature and light regimes. They are characterized by extended periods of darkness during the polar night, preventing harvesting of light energy for photosynthesis for significant parts of the year. During polar day, the sunlight is available for equally long time causing extreme irradiation and cellular distress. However, (micro)organisms experience lower light intensities due to a lower angle of incoming solar radiation and a deeper atmosphere to penetrate, and higher levels of UVB due to an Arctic ozone hole, compared to other regions of the planet.
Microorganisms can thrive in Arctic regions and are present in abundance in all northern polar environments; from tundra (Lee et al., 2013) to taiga (Neufeld and Mohn, 2005), snow (Larose et al., 2013), glaciers (Anesio et al., 2009), permafrost (Mackelprang et al., 2011), air (Harding et al., 2011; Cuthbertson et al., 2017), marine (Ghiglione et al., 2012) and freshwater (Crump et al., 2012) ecosystems. In order to survive in Arctic regions, all living organisms have to be well adapted or highly resistant to cold, variable temperature and melt/thaw cycles, low nutrient and liquid water availability and seasonally variable UV radiation levels (Larose et al., 2013; Boetius et al., 2015).
