The primary climatic condition associated with the Arctic is low temperature, and therefore most reviews and fundamental studies of Arctic microbiology focus on low temperature and psychrophilic (cold-loving) microbes. However, other conditions are also specific for this environment, including extreme light/dark conditions in the Arctic summer and winter, respectively. The Arctic area also includes coverage, with 9-12 million km² of pack ice with average ice thicknesses of 3-4 m, with ridges up to 20 m thick. As described above the microbes in this ice are not only challenged by the subzero temperature, but also by high salt concentrations in the brine channels.

The temperature of the Arctic Ocean normally does not exceed 4-5°C, with cold (close to 0°C) low-salinity surface water and warmer high-salinity water (up to 5°C) below 50 m depth. The marine organisms in the Arctic are therefore consistently exposed to low temperatures. Numerous studies of microbial adaptions to low temperature have been published (e.g.  Brakstad et al. 2008; Helmke and Weyland, 2004; Knoblauch et al. 1999; Robador et al. 2009). As the temperature decreases with concomitant decrease in the thermal energy of a system (enthalpy), increased stability and rigidity of biomolecules occur. Proteins become less flexible, while the secondary structures of DNA and RNA become more stable, and the processes of DNA replication, transcription of RNA and translation of proteins are inhibited (Bakermans 2012). However, psychrophilic microorganisms have adapted to these low-temperature conditions in a number of ways:

• Enzymes in bacteria isolated from sea ice may be activated by the cold, with detectable catalytic activity well below the freezing point of seawater (Groudieva et al. 2004). The enzymes of psychrophilic microbes are characterized by increased flexibility of active sites resulting from structural modifications to address freezing temperatures.  These modifications may include less enzyme core hydrophobicity, increased surface polarity and fewer interactions between amino acids (hydrogen bonds, salt and disulphide bridges). These characteristics facilitate retention of enzymatic activities at low temperature (Bakersman 2012).
• Nucleic acids show adaption to low temperature at the sequence level in psychrophiles, as exemplified by increased uracil content in ribosomal RNA and dihydrouridine content in transfer RNA as a result of decreased temperature (Khachane et al. 2005; Dalluge et al. 1997).
• Microbial membrane structures may crystallize at low temperatures, impairing function. However, in psychrophiles, membrane fluidity is retained by increasing the proportion of unsaturated fatty acids and by desaturation of membrane lipids, resulting in unsaturated fatty acyl chains and less tightly packed lipid chains (Russel 2008).
• Microorganisms can be challenged in ice by reduced water availability due to the formation of ice crystals. However, psychrophiles can combat the effects of reduced water activity by the synthesis of compatible solutes such as sugars, amino acids, alcohols or cryoprotective proteins. Ice crystal formation can also be prevented by production of anti-nucleating materials (Bakersman 2012).
• Psychrophilic bacteria in brine channels are challenged by both low temperature and high salinity.  These may produce extracellular polymeric substances (exopolysaccharides), which have a cryoprotective role in the sea ice brine channels, as well as binding essential cationic trace metals to enhance halotolerance (Nichols et al. 2005).
• Microbes exposed to large drops in temperature may produce cold shock and cold acclimation proteins (Berger et al. 1996). Cold shock proteins are supposed to be involved in protein translation regulation, while cold acclimation proteins show high catalytic activity at low temperatures and rapid inactivation at moderate temperatures (Fukunaga et al. 1999).

In Arctic regions, with a long polar summer and a correspondingly long polar winter and associated widespread ice coverage, phototrophic organisms must tackle a number of challenges.  For example, phytoplankton communities responded rapidly and increased in numbers by an order of magnitude in the Beaufort Sea with the increased solar irradiation in May, despite the presence of sea ice. However, eukaryotic phototrophs somehow also persisted throughout the winter darkness (Terrado et al. 2008). Among phototrophic bacteria isolated from the Arctic Ocean were photoheterotrophic microbes, which are capable of both utilizing dissolved organic materials and harvesting light energy.  Photoheterotrophic taxa included coccoid cyanobacteria (Synechococcus and Prochlorococcus), aerobic anoxygenic phototrophic (AAP) bacteria, and proteorhodopsin (PR)-containing bacteria. In a study of photoheterotrophic microbes in the Arctic Ocean in the summer and winter it was observed that some of these bacteria (AAP and PR containing bacteria) decreased from summer to winter, in parallel with a threefold decrease in the total prokaryotic community, while the abundance of Synechococcus did not decrease in winter (Cottrell and Kirchman 2009). These data demonstrated the ability of phototrophs to switch from photosynthesis to heterotrophic activities during the winter.  Characterization of green light-absorbing PR from bacterial isolates from the Arctic Ocean showed a slower photocycle, relatively larger change in activation energy of the transition between the oxygen intermediate and the ground state compared to PR from bacterial phototrophs isolated from temperate seawater (Jung et al. 2008).

Photosynthesizing microbes have also been observed within sea ice. Marine algae seem to be more sensitive to brine salinity than bacteria and therefore are detected mainly in the lower parts of sea ice (lower salinity in winter associated with contact with seawater; Gradinger and Zhang 1997). Recent studies have also identified marine autotrophic bacteria in marine ice indicating the potential ability for ongoing use of dissolved organic materials (Díez et al. 2012).

Conditions in marine ice vary considerably from those in seawater. The temperature is lower in the ice relative to fluid water, and may drop as low as the air temperature during the polar winter. Transmission of light through the ice is severely reduced compared to seawater, and light may be entirely absent when the ice is covered by a snow cap. In first-year ice, brine inclusion networks are generated by salting-out processes, as described in previous sections. The brine channels of the ice provide liquid saline niches at subzero temperatures which enable microbial motility and metabolic activity (Junge et al. 2002; 2003; 2004; 2006). The brine channels also act as a matrix for transport of soluble organic matter and other nutrients.  In the brine channels the ice temperatures can be as low as -20°C and salinity conditions can be as high as 200 Practical Salinity Units (Salinity in normal seawater is 35 PSU). Though challenging to microbial life, laboratory experiments have shown that bacteria (Colwellia psychroerythraea) were able to incorporate amino acids into proteins at temperatures as low as -20°C in brines (Junge et al. 2006). This metabolic activity was associated with particles or with the surfaces of the brine channels (Junge et al. 2003; 2004). In addition, the ability to grow at subzero temperatures has been demonstrated in the bacterium Psychromonas ingrahamii, which was cultured at temperatures as low as ‑12°C (Breezee et al. 2004). The brine channels may become depleted in oxygen, for instance if microbial processes occur without oxygen renewal (seawater or photosynthesis), and anaerobic denitrification processes have been measured in marine ice (Rysgaard and Glud 2004). These studies therefore demonstrate that bacterial activity can take place in marine ice, both with respect to motility within the ice and active metabolism. Recently, Mykytzuk et al. (2013) presented a detailed characterization of the physiological adaptations that enable a halotolerant psychrophilic permafrost bacterium to grow and metabolize at temperatures as low as -15°C.  The implication is that halotolerant psychrophilic species may also exist in the ice.