It is frequently overlooked that the majority (>80%) of the Earth’s biosphere is cold and permanently exposed to temperatures below 5 °C (Rodrigues & Tiedje, 2008). Such low mean temperatures mainly arise from the fact that ~70% of the Earth’s surface is covered by oceans that have a constant temperature of 4°C below 1000 m depth, irrespective of the latitude. The polar regions account for another 15%, to which the glacier and alpine regions must be added, as well as the permafrost representing more than 20% of terrestrial soils. All these low temperature biotopes have been successfully colonized by cold-adapted microorganisms, termed psychrophiles (Margesin et al., 2008). These organisms do not merely endure such low and extremely inhospitable conditions but are irreversibly adapted to these environments as most psychrophiles are unable to grow at mild (or mesophilic) temperatures. Extreme psychrophiles have been traditionally sampled from Antarctic and Arctic sites, assuming that low temperatures persisting over a geological time-scale have promoted deep and efficient adaptations to freezing conditions. In addition to ice caps and sea ice, polar regions also possess unusual microbiotopes such as porous rocks in Antarctic dry valleys hosting microbial communities surviving at -60 °C (Cary et al., 2010), the liquid brine veins between sea ice crystals harboring metabolically-active microorganisms at -20 °C (Deming, 2002) or permafrost cryopegs, i.e. salty water pockets that have remained liquid at -10 °C for about 100 000 years (Gilichinsky et al., 2005). Psychrophiles and their biomolecules also possess an interesting biotechnological potential, which has already found several applications (Margesin & Feller, 2010). Cold exerts severe physicochemical constraints on living organisms including increased water viscosity, decreased molecular diffusion rates, reduced biochemical reaction rates, perturbation of weak interactions driving molecular recognition and interaction, strengthening of hydrogen bonds that, for instance, stabilize inhibitory nucleic acid structures, increased solubility of gases and stability of toxic metabolites as well as reduced fluidity of cellular membranes (D'Amico et al., 2006; Gerday & Glansdorff, 2007; Margesin et al., 2008; Rodrigues & Tiedje, 2008). Previous biochemical studies have revealed various adaptations at the molecular level such as the synthesis of cold-active enzymes by psychrophiles or the incorporation of membrane lipids promoting homeoviscosity in cold conditions. It was shown that the high level of specific activity at low temperatures of coldadapted enzymes is a key adaptation to compensate for the exponential decrease in