Abstract
We report the first investigation of a deep
subpermafrost microbial ecosystem, a terrestrial analog
for the Martian subsurface. Our multidisciplinary team
analyzed fracture water collected at 890 and 1,130 m
depths beneath a 540-m-thick permafrost layer at the
Lupin Au mine (Nunavut, Canada). 14C, 3H, and noble
gas isotope analyses suggest that the Na–Ca–Cl, suboxic,
fracture water represents a mixture of geologically ancient
brine, ~25-kyr-old, meteoric water and a minor modern
talik-water component. Microbial planktonic concentrations
were ~103 cells mL−1. Analysis of the 16S rRNA
gene from extracted DNA and enrichment cultures
revealed 42 unique operational taxonomic units in 11
genera with Desulfosporosinus, Halothiobacillus, and
Pseudomonas representing the most prominent phylotypes
and failed to detect Archaea. The abundance of terminally
branched and midchain-branched saturated fatty acids (5
to 15 mol%) was consistent with the abundance of Grampositive
bacteria in the clone libraries. Geochemical data,
the ubiquinone (UQ) abundance (3 to 11 mol%), and the
presence of both aerobic and anaerobic bacteria indicated
that the environment was suboxic, not anoxic. Stable
sulfur isotope analyses of the fracture water detected the
presence of microbial sulfate reduction, and analyses of
the vein-filling pyrite indicated that it was in isotopic
equilibrium with the dissolved sulfide. Free energy
calculations revealed that sulfate reduction and sulfide
oxidation via denitrification and not methanogenesis were
the most thermodynamically viable consistent with the
principal metabolisms inferred from the 16S rRNA
community composition and with CH4 isotopic compositions.
The sulfate-reducing bacteria most likely colonized
the subsurface during the Pleistocene or earlier, whereas
aerobic bacteria may have entered the fracture water
networks either during deglaciation prior to permafrost
formation 9,000 years ago or from the nearby talik
through the hydrologic gradient created during mine
dewatering. Although the absence of methanogens from
this subsurface ecosystem is somewhat surprising, it may
be attributable to an energy bottleneck that restricts their
migration from surface permafrost deposits where they are
frequently reported. These results have implications for
the biological origin of CH4 on Mars.