The Abundance and Taxonomic Diversity of Filterable Forms of Bacteria during Succession in the Soils of Antarctica (Bunger Hills).

Previous studies have shown that a significant part of the bacterial communities of Antarctic soils is represented by cells passing through filters with pore sizes of 0.2 µm. These results raised new research questions about the composition and diversity of the filterable forms of bacteria (FFB) in Antarctic soils and their role in the adaptation of bacteria to the extreme living conditions. To answer such questions, we analyzed the succession of bacterial communities during incubation of Antarctic soil samples from the Bunger Hills at increased humidity and positive temperatures (5 °C and 20 °C). We determined the total number of viable cells by fluorescence microscopy in all samples and assessed the taxonomic diversity of bacteria by next-generation sequencing of the 16S rRNA gene region. Our results have shown that at those checkpoints where the total number of cells reached the maximum, the FFB fraction reached its minimum, and vice versa. We did not observe significant changes in taxonomic diversity in the soil bacterial communities during succession. During our study, we found that the soil bacterial communities as a whole and the FFB fraction consist of almost the same phylogenetic groups. We suppose rapid transition of the cells of the active part of the bacterial population to small dormant forms is one of the survival strategies in extreme conditions and contributes to the stable functioning of microbial communities in Antarctic soils.

Hypolithic communities shape soils and organic matter reservoirs in the ice-free landscapes of East Antarctica.

The soils of East Antarctica have no rhizosphere with the bulk of organo-mineral interactions confined to the thin microbial and cryptogamic crusts that occur in open or cryptic niches and are collectively known as biological soil crust (BSC). Here we demonstrate that cryptic hypolithic varieties of BSC in the Larsemann Hills of East Antarctica contribute to the buildup of soil organic matter and produce several types of continuous organogenous horizons within the topsoil with documented clusters of at least 100 m2. Such hypolithic horizons accumulate 0.06-4.69% of organic carbon (TOC) with isotopic signatures (δ13Corg) within the range of -30.2 - -24.0‰, and contain from 0 to 0.38% total nitrogen (TN). The properties of hypolithic organic matter alternate between cyanobacteria- and moss-dominated horizons, which are linked to the meso- and microtopography patterns and moisture gradients. The major part of TOC that is stored in hypolithic horizons has modern or centenary 14C age, while the minor part is stabilized on a millennial timescale through shallow burial and association with minerals. Our findings suggest that hypolithic communities create a "gateway" for organic carbon to enter depauperate soils of the Larsemann Hills and contribute to the carbon reservoir of the topsoil at a landscape level.

Alteration of rocks by endolithic organisms is one of the pathways for the beginning of soils on Earth.

Subaerial endolithic systems of the current extreme environments on Earth provide exclusive insight into emergence and development of soils in the Precambrian when due to various stresses on the surfaces of hard rocks the cryptic niches inside them were much more plausible habitats for organisms than epilithic ones. Using an actualistic approach we demonstrate that transformation of silicate rocks by endolithic organisms is one of the possible pathways for the beginning of soils on Earth. This process led to the formation of soil-like bodies on rocks in situ and contributed to the raise of complexity in subaerial geosystems. Endolithic systems of East Antarctica lack the noise from vascular plants and are among the best available natural models to explore organo-mineral interactions of a very old "phylogenetic age" (cyanobacteria-to-mineral, fungi-to-mineral, lichen-to-mineral). On the basis of our case study from East Antarctica we demonstrate that relatively simple endolithic systems of microbial and/or cryptogamic origin that exist and replicate on Earth over geological time scales employ the principles of organic matter stabilization strikingly similar to those known for modern full-scale soils of various climates.