 Permanent ice
covers on Antarctic lakes in the McMurdo Dry Valleys, (76º 30’ - 78º 20’S,
160º - 164º E) are unique geophysical features of our planet which have only recently
been recognized as being capable of supporting resident microbial life. Previously, the
permanent ice covers have been considered primarily as physical barriers to wind mixing,
material transport, and solar radiation necessary for photosynthesis in the lake’s
water columns. The realization that these features support viable microbial habitats and
populations was only recognized once 1) anomalous nitrous oxide gas was measured in the
ice cover, 2) microalgal (primarily cyanobacterial) and bacterial cells were found in
tight association with resident sediment inclusions within the interior of several
permanent ice covers, and 3) historical temperature records were analyzed which indicated
that substantial amounts of liquid water (up to 70% by ice volume) were generated in the
regions of sediment inclusions in the ice during the summer months. Based on the
temperature records and bubble morphologies it was inferred that the absorption of
penetrating radiation by the sediments in the absence of conductive processes (which could
dissipate the resultant heat energy) led to localized ice melting and refreezing of
generated liquid water pockets (thus the arching bubble patterns). Together, these
findings show that the most geographically unique ice covers on this planet present an
extreme environment where microbial persistence and growth occurs along the fringe of the
physiocochemical gradients which are currently known to support life. |
