The manuscript presents the results of experimental studies of the adaptive potential of microorganisms that are part of the bacterioplankton of the surface and bottom water layers of the Bureiskoe Reservoir in the zone of the landslide that descended in the winter of 2018. In the summer of 2022, the structure and activity of microbial complexes from area near the landslide were studied. More than 60 strains of microorganisms of different physiological groups were isolated. On the example of 4 strains isolated from different habitats (above and below the landslide body, surface and bottom layers of water) after 30 days of freezing at a temperature of –18°C, their viability and activity in the utilization of easily available nitrogen-containing organic substances were shown. In experimental cyclic freezing-thawing, two variants of thawing were used: slow thawing in a refrigerator from –18°C to +4°C; fast defrosting over a wide temperature range from –18°C to +23°C (at room temperature). Regardless of location, all strains grew vigorously in vitro with use of a readily available peptone carbon source before and after freezing. The maximum activity on peptone was shown by strain 40 BB (below the landslide body, bottom water) in the absence of substrate change. Utilization of peptone as a source of amino acids and peptides could be accompanied by activation of the protective function against cold stress. The selected strains of microorganisms differed in their ability to transform sodium humate molecules depending on the conditions of the freeze/thaw cycles. According to the spectral characteristics, significant changes in the aliphatic and aromatic components of the humate molecule occurred with the participation of strains 45 AB and 40 BB isolated from the bottom water. These strains were more active at low thawing temperatures, which actually corresponded to the in situ temperature of the bottom water layers, which is 4–6°C. Strain 13 BS from surface water sampled below the landslide body is characterized by active transformation of the aromatic component of humic substances in a wide range of thawing temperatures (from –18 to +23°C). The results indicate that in different regions during the thawing of permafrost and the influx of specific organic substances the specific mechanisms of formation of the quality of natural waters may manifest.
