We guess that the conditions under which protozoa cysts were buried and passed into the frozen state, as well as the conservation regime, had a considerable impact on the formation of the fauna of "alive fossil" protozoa. The cysts of protozoa may
have been buried in the course of gradual deposit formation. In this case, cells should have experienced a dramatic long-lasting stress of freezing-thawing until they finally got frozen. This factor could have been crucial for the formation of a "survivor's community". Another variant of burial implies filtration with the flow of soil moisture from the active layer to the upper permafrost boundary, as described for bacteria (Spirina and Fedorov-Davydov 1998). In this case, the transition of cells to the frozen state should have taken much less time than it would have taken in the case of a gradual burial. The conditions in the permafrost strata are relatively stable, and the duration of cryoconservation and the protective mechanisms that protozoa possess play a key role in the selection of the most resistant organisms. Taxons highly tolerant to the extreme conditions of tundra ecotopes, i.e., r-strategist, would have been favored in both cases. Our observations confirm this conclusion: the fauna of soil protozoa isolated from the sediments of icy complex and the soils buried there is characterized by low species diversity, and consists of pioneer species adapted to the extreme environmental conditions.
In the permafrost sediments of the icy complex, we found fossil burrows that should be considered as special paleoecological objects. These are suslik (ground squirrel) burrows, which belong to a species of the subgenus Urocitellus (Gubin et al. 2003a, b; Zanina 2005). These rodents collected seeds and plant fruits from various biotopes, and stored them in the food chambers located at the upper boundary of permafrost sediments. Brought from the surface together with the plant material, protozoa cysts were kept in dry, well-aerated chambers that were protected from abrupt temperature drops, in which they froze in a little while. As a result, we see a large increase in the diversity of viable protozoa species in fossil burrows in comparison with the diversity found in the sediments of the icy complex.
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