Permafrost is a common feature in polar regions. The uppermost permafrost layer, thawed in summer, is characterized by an extreme temperature regime from about +15°C to 35°C. Even so a diverse range of microorganisms have been discovered in this so-called active layer.The aim of the current study was to examine as to how the microbial populations within the different horizons of the active layer were adapted to the extreme variable temperature regime of the permafrost area. Thus, two soil samples were taken from the active layer on the island Samoylov in the southern Lena delta, Siberia: one surface-near sample (11-18 cm) and one permafrost-near sample (25-32 cm). Aliquots of each sample were incubated under 4 °C and 28 °C for about 4 weeks. Subsequently, the molecular cell membrane composition (phospholipids) was qualitatively and quantitatively evaluated using HPLC-ESI-MS/MS. To maintain the membrane fluidity at low temperatures, microbial cells can decrease their solid-liquid phase transition temperatures below the ambient temperature. They change their phospholipid fatty acid composition to more bulky-shaped cis-unsaturated fatty acids and/or to more shorter-chain fatty acid, because of the lower melting temperatures of unsaturated and shorter-chain fatty acids. The comparison of the phospholipid fatty acid (PLFA) distribution of the different horizons at 4 and 28 °C shows that the microbial population of both horizons does not incorporate significantly more unsaturated fatty acids under cooler conditions. In contrast to this the surface near as well as the permafrost near microbial communities reveal for both a distinct relative increase of short chain fatty acids of 7.3 and 10.3% in the 4 °C incubation experiment.In addition to this distinct chain length adaptation, the PLFA proportions of the microbial population of the active layer differ with the different depth horizons. The permafrost near microbial community shows, in general, a higher relative proportion of unsaturated and shorter chain fatty acids. This indicates a stronger adaptation to cooler environmental conditions, whereas the surface near population reveals a higher flexibility towards warmer temperature conditions.