Subsurface ice as a microbial habitat

Abstract

We determine the physicochemical habitat for microorganisms in subsurface terrestrial
\nice by quantitatively constraining the partitioning of bacteria and fluorescent beads
\n(1–10 m) between the solid ice crystals and the water-filled veins and boundaries around
\nindividual ice crystals. We demonstrate experimentally that the partitioning of spherical
\nparticles within subsurface ice depends strongly on size but is largely independent of
\nsource particle concentration. Although bacteria are shown consistently to partition to the
\nveins, larger particles, which would include eukaryotic cells, become trapped in the crystals
\nwith little potential for continued metabolism. We also calculate the expected concentrations
\nof soluble impurities in the veins for typical bulk concentrations found in natural
\nice. These calculations and scanning electron microscope observations demonstrate a concentrated
\nchemical environment (3.5 M total ions at 10 C) in the veins, where bacteria
\nwere found to reside, with a mixture of impurities that could sustain metabolism. Our
\ncalculations show that typical bacterial cells in glacial ice would fit within the narrow
\nveins, which are a few micrometers across. These calculations are confirmed by microscopic
\nimages of spherical, 1.9-m-diameter, fluorescent beads and stained bacteria in
\nsubsurface veins. Typical bacterial concentrations in clean ice (102–103 cells/mL) would
\nresult in concentrations of 106–108 cells/mL of vein fluid, but occupy only a small fraction
\nof the total available vein volume (0.2%). Hence, bacterial populations are not limited
\nby vein volume, with the bulk of the vein being unoccupied and available to supply energy
\nsources and nutrients.