Our studies showed that soil protozoa remain viable for tens and hundreds of thousands of years under conditions of subzero temperatures, oxygen deficiency, and lack of available water and food.
Protozoa are most often found in Holocene sediments (the first 2.5 m below the surface). The maximal depth of thawing in the regions explored can reach 0.8 m in extraordinarily warm years. Therefore, the age of protozoa at the upper permafrost boundary does not exceed a few hundred years. However, below the active layer, in the ice-cemented strata, the influence of environmental factors is largely restricted, and there is no aquifer or infiltration. Thermodiffusion and migration of protozoa through the films of non-frozen water are impossible too, since the size of protozoa is incommensurably larger than the thickness of these films, which is about 10-3 |im. The presence of thick icy veins directly indicates that the ice-containing sediments have never been unfrozen, i.e., the biota found could not penetrate into these layers. The biota also could not been introduced from the outside in the process of drilling: the technique of sterile core sampling has been proved many times in the microbiological studies of frozen strata. In view of the aforesaid, we can conclude that the viable protozoa species revealed in the permafrost strata were found in situ.
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