We developed an experimental technique for observing in-situ within solid ice samples the extent to which microbes are partitioned to the liquid-water-filled veins during ice formation. The method involves freezing deionised water containing either fluorescent (ex. 469nm/em. 509nm) polystyrene beads (diameter 1µm-10µm) or cells of Clostridium vincentii (a psychrophilic, anaerobic bacterium isolated from a pond on the McMurdo Ice Shelf (Mountfort et al 1997)) stained with Acridine Orange in a glass ring mounted on a microscope coldstage, as shown in the sketch above right. We also determined the chemical habitat of the microbes by calculating the expected concentrations of soluble impurities in the veins for typical bulk concentrations found in natural terrestrial ice.
The calculations indicate a concentrated chemical environment (3.5M total ions at -10ºC) in the veins with a mixture of impurities that could sustain bacterial metabolism. The experiments show that the partitioning of spherical particles within subsurface ice depends strongly on size but is largely independent of ice formation rate or concentration; during ice formation, bacteria within ice are consistently partitioned to the veins whereas larger particles, which would include eukaryotic cells, become trapped in the crystals with little potential for continued metabolism.
Typical bacterial ice concentrations (102-103 cell/ml) would result in concentrations of 106-108cells/ml of vein fluid, but occupy only a small fraction of the total available vein volume (<0.2%). Hence bacterial populations are not limited by vein volume, with the bulk of the vein being unoccupied and available to supply energy sources and nutrients. Our calculations show that typical bacterial cells in glacial ice would fit within the narrow veins, which are a few µm's across. These calculations are confirmed by microscopic images of spherical, 1.9µm diameter, fluorescent beads and stained bacteria in subsurface veins.
Those involved in this work include Dr Heidy Mader, David Mallard (NERC CASE Student with British Antarctic Survey 2000-2005), Professor John Parkes & Dr Michala Pettitt (Leverhulme PDRA 2001-2003), Dr Jemma Wadham, Dr Eric Wolff (British Antarctic Survey, Cambridge)).
Funded by The Leverhulme Trust.