Extraterrestrial thermophiles on icy moons of Jupiter and Saturn? The case of the subglacial Antarctic Lake Vostok

The objective was to search for microbial life in the subglacial Antarctic Lake Vostok by analyzing the natural accretion ice obtained by deep ice coring during the Russian campaign toward entering the lake. The ice samples to study originated from ice type I [1] and contained mineral inclusions. The study aimed to re-evaluate previous microbial finds obtained with Sanger sequencing using the high throughput Oxford Nanopore sequencing technology.

Lake Vostok is a giant (270 x 70 km, 15800 km2 area), deep (up to 1.3km) freshwater liquid body buried in a graben beneath a 4-km thick East Antarctic Ice Sheet with the temperature near ice melting point (around -2.5oC) under 400 bar pressure. It is exceptionally oligotrophic and poor in chemical ions, under the high dissolved oxygen tension (320 – 1300 mg/L) range, with no light, and sealed from the surface biota about 15 Ma ago [1].

The common Sanger sequencing technique previously discovered thermophiles in the subglacial Lake Vostok in analyzing bacterial 16S rRNA genes [2,3,4]. The ice samples included accretion ice segments at a depth of 3561m and 3607m containing sediment inclusions. As a result, in both samples, the facultative thermophilic chemolithoautotroph Hydrogenophilus thermoluteolus of beta-Proteobacteria, which originated from hot springs, was discovered. This finding suggested that a geothermal system exists beneath the cold-water body of Lake Vostok.

To clarify the presence of thermophiles in Lake Vostok, the accretion ice segments from 3607m (the thermophile mentioned above was detected here [2]), 3608m, 3607m (2 segments), and 3709m were retested by high throughput nanopore sequencing using the same genomic DNA and broader-in-cover degenerate primers for the v3-v4 region 16S rRNA genes. The nanopore controls (sham DNA isolation/negative PCR, nanopore reagents) were applied for the first time.

A dozen Ma reads were obtained for all five amplicons, but only one sample, 3608, showed thermophiles in records. For this sample of 1,643,669 reads analyzed, 88% of which were classified. Amongst them, 279 (0.02%) reads were assigned to moderate thermophile Meiothermus hypogaeus NBRC 106114 (Deinococcus-Thermus), isolated from a hot spring in Japan. No reads for this find were recorded in other ice samples and controls. This could mean that a new thermophile of Deinococcus-Thermus was discovered in the native accretion ice of Lake Vostok. The Hydrogenophilus thermoluteolus remained undetected due to the use of different primers.

Thus, the high throughput Oxford Nanopore sequencing technology provides a very efficient tool to record/prove the microbial content of the subglacial Antarctic water reservoirs in detail. Newly discovered meio-thermophiles might represent ingenious cell populations inhabiting faults offshore the subglacial Lake Vostok. They could provide prospects in searching for extraterrestrial thermophiles on Jupiter and Saturn's icy moons.

• Bulat S., Petit JR. (2023) Vostok, Subglacial Lake. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. Pp. 3206-12
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• Lavire, C. et al. (2006) Environmental Microbiology 8, 2106-14
• Bulat, S. (2016) Phil Trans Royal Soc A Math Phys Eng Sci 374 (2059), 20140292