Limits for microbial life at subzero temperatures

The limitations of terrestrial life are not well-defined or understood and have primarily been advanced through exploration and discovery of organisms living in extreme environments where life was not thought possible. Identifying the limits of life is hampered by our inability to define the essential nature of life, rather than just describing its properties (organization, energy use, growth, adaptation, response to stimuli, reproduction). Similarly, until the twentieth century, we could not define the essential nature of water (a molecule composed of two atoms of hydrogen and one atom of oxygen joined by covalent bonds), only its properties (colorless, solvent, liquid at specific temperatures and pressures). Additionally, when examining the limits of life, defining what is alive becomes more difficult. The metabolism of life is often separated into three classes: growth, maintenance, and survival (Morita 1997). Clearly an organism that is metabolizing actively enough to reproduce itself exhibits growth and is considered living. But what about organisms that are actively metabolizing but not reproducing? In this case, energy is consumed for repair and maintenance of cellular structures to preserve the integrity of the organism, but cannot sustain reproduction. Maintenance may also allow adaptation and evolution to changing conditions over very long time spans (other definitions of maintenance do not generally include evolution). Finally, organisms routinely survive conditions at which they cannot actively metabolize by forming completely inactive dormant states (such as spores) or with very weak intermittent metabolism for the repair of accumulated damage.