Microbial Assemblages in Pressurized Antarctic Brine Pockets (Tarn Flat, Northern Victoria Land): A Hotspot of Biodiversity and Activity

Abstract

Two distinct pressurized hypersaline brine pockets (named TF4 and TF5), separated by a thin ice layer, were detected below an ice-sealed Antarctic lake. Prokaryotic (bacterial and archaeal) diversity, abundances (including virus-like particles) and metabolic profiles were investigated by an integrated approach, including traditional and new-generation methods. Although similar diversity indices were computed for both Bacteria and Archaea, distinct bacterial and archaeal assemblages were observed. Bacteroidetes and Gammaproteobacteria were more abundant in the shallowest brine pocket, TF4, and Deltaproteobacteria, mainly represented by versatile sulphate-reducing bacteria, dominated in the deepest, TF5. The detection of sulphate-reducing bacteria and methanogenic Archaea likely reflects the presence of a distinct synthrophic consortium in TF5. Surprisingly, members assigned to hyperthermophilic Crenarchaeota and Euryarchaeota were common to both brines, indicating that these cold habitats host the most thermally tolerant Archaea. The patterns of microbial communities were different, coherently with the observed microbiological diversity between TF4 and TF5 brines. Both the influence exerted by upward movement of saline brines from a sub-surface anoxic system and the possible occurrence of an ancient ice remnant from the Ross Ice Shelf were the likely main factors shaping the microbial communities.

Cultivable Bacterial Communities in Brines from Perennially Ice-Covered and Pristine Antarctic Lakes: Ecological and Biotechnological Implications

“…Similarly to previous culture-independent results [ 13 , 31 ], even if (as it was expected) at different relative percentages and distribution, the cultivable bacterial community was composed by Proteobacteria (namely Gamma- and Alphaproteobacteria), Bacteroidetes (only in BC brines), Actinobacteria (mainly in TF) and Firmicutes, with most phylotypes that are well known for inhabiting cold and salty environments [ 64 , 65 , 66 ]. Most isolates from TSA 100 and TSA 50 plates were affiliated to Gammaproteobacteria and Actinobacteria.…”

“…Despite major advances have been made in the last decades, our current knowledge of physiology, metabolism, microbial ecology and interactions in cold-adapted microorganisms remains quite limited and it is particularly scarce with regard to less investigated habitats, such as Antarctic lake brines [ 56 ]. Brines of perennially ice-covered Antarctic lakes represent ideal environmental models to gain important insights on the ecology of the microbial life in Antarctica, by an overall synergistic analysis of both taxonomic diversity and functional capacities and interactions within a pristine ecosystem [ 13 , 57 ]. Recent culture-independent studies have shown that Antarctic brines in Boulder Clay and Tarn Flat lakes harbour diverse prokaryotic taxa, also highlighting astrobiological implications due to the strong similarities (e.g., harsh values of temperature and salinity) between these cryogenic habitats on Earth and other planetary and celestial bodies [ 13 , 31 ].…”

“…Brines were collected from three perennially frozen Antarctic lakes, lying in two sites of the Northern Victoria Land, namely Tarn Flat (TF; coordinates: 75°4′ S 162°30′ E) and Boulder Clay (BC; coordinates: 74°44′ S–164°01′ E). Brine samples were collected near the ice blisters of the adjacent Lake 16 (L16; i.e., brines BC1 and BC2 from two different sampling points at a depth of 2.5 and 1.0 m, respectively) and Lake L-2 (i.e., brine BC3, depth 2.0 m) in the Boulder Clay area [ 30 , 31 ] and from a perennially frozen lake at the highest elevation of the Tarn Flat area, namely brines TF4 (between 3.78 and 3.98 m depth) and TF5 (between 4.10 and 4.94 m depth) along the same borehole [ 6 , 13 ]. All samples were collected in pre-sterilized polycarbonate bottles by using sterilized peristaltic pump and tubing.…”

“…However, mobilization processes, as well as the brine genesis, remain poorly understood [ 10 ]. Interestingly, the similarities of brine pockets with conditions retrieved on Mars, in terms of cold and dry climates, together with the development of highly saline microhabitat [ 11 , 12 ], lead researchers to question the common meaning of limits of life and brines are suggested as a model to study extra-terrestrial life [ 6 , 13 ].…”

“…Recently, brine lenses from Tarn Flat and Boulder Clay lakes were explored for prokaryotic and eukaryotic diversity and activities by the application of advanced culture-independent methods [ 13 , 29 , 30 , 31 ]. Exceptionally diverse (in terms of both ecophysiology and phylogenetic affiliation) prokaryotic communities, which included methanogens, strictly anaerobes, halophiles and (hyper)thermophiles, were characterized [ 13 , 31 ]. As a complementary research, bacterial isolates were also obtained from those brine samples.…”