Cryobiology. 2006 Apr 26;: 16647051
Bacterial incorporation of leucine into protein down to -20 degrees C
with evidence for potential activity in sub-eutectic saline ice
formations.
Karen Junge , Hajo Eicken , Brian D Swanson , Jody W Deming
http://lib.bioinfo.pl/pmid:16647051

This is a great paper if the results are valid. I
would even call it a revolutionary paper, if
confirmed. The paper also gives some very nice
references to life surviving in subfreezing
temperatures.
In their Discussion section, the authors do examine
some possible explanations for the observations but
their primary conclusion is that there is some
metabolic activity going on even down to -196C (!)
They note that the low rates of incorporation of the
radiolabeled leucine at the low subfreezing
temperatures were in keeping with the results of Price
and Sowers that metabolic activity at subfreezing
temperatures is sufficient only for maintenance
activities (such as DNA repair) but not growth
(reproduction):

Temperature dependence of metabolic rates for
microbial growth, maintenance, and survival.
P. Buford Price and Todd Sowers
PNAS|March 30, 2004|vol. 101|no. 13|4631-4636
http://www.pnas.org/cgi/content/full/101/13/4631

As explanation of how metabolic activity could have
occurred even down to liquid nitrogen temperatures,
Junge et.al. note that at rapid freezing rather than
water turning to ice it can convert to a semi-liquid
glassy form. This is referred to vitrification. They
hypothesize that metabolism can still take place with
water in this glassy state.

However, another possibility is suggested by another
recent startling research finding: water can remain
liquid inside carbon nanotubes down to -265C, only 8
degrees above absolute zero!

Nanotube Water Doesn't Freeze -- Even At Hundreds Of
Degrees Below Zero.
Science Daily - ARGONNE, Ill. (May 13, 2005) -- "A new
form of water has been discovered by physicists in
Argonne's Intense Pulsed Neutron Source (IPNS)
Division. Called nanotube water, these molecules
contain two hydrogen atoms and one oxygen atom but do
not turn into ice -- even at temperatures near
absolute zero."
http://www.sciencedaily.com/releases/2005/06/050605183843.htm

Since this unusual capability of water is due to its
structure in its liquid compared to its solid forms,
this would probably also be true of other thin,
tubular macromolecules. The researchers note also that
some proteins have the function of conducting water
through the cell wall into the cell:

"Researchers ranging from biologists to geologists and
materials scientists are interested in water's
behavior in tightly confined spaces controlled by
hydrophobic -- water repulsing -- materials because
this situation is found in nature, for example when
tiny roots carry water to plants. Some membrane
proteins also face this challenge, including
aquaporin, which controls water flow through cell
walls."

Then conceivably some cellular proteins could have
the necessary structure to maintain the water in
liquid form down to cryogenic temperatures.
The nanotubes in this study were less than 2 nm
across however. Similar results though may hold at
larger diameters, though still at the nanoscale, at
higher cryogenic temperatures.
One type of protein to test this capability on for example
might be microtubules:

Microtubule.
http://en.wikipedia.org/wiki/Microtubule

These are 24 nanometers across. So there is some question
if the structural restrictions for the water phase change would
remain for water trapped in tubes at this larger diameter.
Even if it is shown that some biomolecules have the capability
to retain water in liquid form at cryogenic temperatures though,
there still would be the question of whether the radiotracer uptake
observed was due to metabolic activity at these temperatures.
In order to test this further I suggest a method devised by Lin Chao
as a test of astrobiology be used.

Defining Life
by Bruce Moomaw
Cameron Park - April 28, 2000
http://spacedaily.com/news/life-00w4.html

The Meaning of Life.
by Lin Chao
BioScience, March 2000, Vol. 50, No. 3, p. 245-250.

Chao notes that life has the ability to evolve when exposed to new
environments. He suggests successively transferring the putative life
forms to a new nutrient batch and observing whether or not they
become more efficient at utilizing the nutrients, in essence a test
to
see if they undergo natural selection and Darwinian evolution.
However, it appears in the Junge et.al. experiments that at the very
lowest temperatures, the microbes were only undergoing maintenance
activity; they weren't reproducing, which is required for the Lin
Chao
test.
The specific psychrophilic (cold-loving) bacteria used though can
reproduce at higher subfreezing temperatures. Then what might
work would be to perform the Lin Chao test at the lowest temperature
at which growth occurs. Then the strain selected (developed) by the
Lin Chao method would be better evolved to grow at this temperature,
and presumably it would show greater metabolic activity at the
cryogenic temperatures as well.

Bob Clark