and can replicate at low, sub-zero temperatures if liquid water is present (Goordial
et al. 2013; Mykytczuk et al. 2013). Some have even hypothesized that due to the
slowing down of metabolic activity at cold temperatures, there is possibly no
low-temperature limit to life on Earth (Price and Sowers 2004). In many
cryoenvironments, microorganisms are the foundational life forms driving ecosystem function and, in many cases, are the only life present at all. These microbialbased ecosystems include microorganisms occupying multiple trophic levels and
niches (e.g. autotrophy, heterotrophy) and those in varying physiological states, for
example, metabolically active, replicating, dormant, or dead. Each of these physiological states plays an important role in ecosystem function.
This essay will discuss evidence of microbial “vital signs” in cryoenvironments—
evidence for both activity and cellular replication at sub-zero temperatures. In
particular, it will focus on one extreme site in the Antarctic McMurdo Dry Valleys,
where microorganisms are thought to be on the verge of the cold-arid limit of life. It
is an example of how microorganisms in different physiological states play a role in
cryoenvironmental ecosystem function, even when vital signs of active life or a
“pulse” is not detected. Finally, I will discuss what studying microbial life in
cryoenvironments can tell us about the limits of life on Earth and potentially beyond.
6.1.1 Microbial Replication and Activity at Sub-zero
Temperatures
Sub-zero temperatures affect microbial cells by causing lipids and proteins to
become rigid and inflexible, affecting membrane and protein conformation and
thus basic cellular function. Psychrophilic (cold-loving) and psychrotolerant (coldtolerant) microorganisms have evolved multiple mechanisms to adapt to these
conditions (Bakermans et al. 2009; Bakermans et al. 2011; Feller and Gerday
2003). For example, cold-adapted proteins have altered amino acid compositions,
favouring residues that result in higher protein flexibility via less tertiary bonding
and less densely packed hydrophobic cores (Raymond-Bouchard et al. 2018;
Goordial et al. 2016). Psychrophilic organisms also have increased copy numbers
of cold shock proteins and helicases for DNA and RNA processes such as replication
and transcription (Mykytczuk et al. 2013). Cell membranes may be enriched with
saturated and branched fatty acids to increase fluidity (Siliakus et al. 2017). In
addition, as a result of thermodynamics, kinetic energies are lower at cold temperatures—lowering reaction rates and transport of molecules. However, the primary
constraint to life at sub-zero temperatures may not be temperature itself, but the
availability of liquid water, necessary for all of life as we know it. At low temperatures water freezes, and thus water availability for microorganisms in these environments decreases sharply. During the freezing process, any solutes present are
squeezed and concentrated into thin brines within the frozen substrate, depressing
the freezing point of water and permitting small pockets of liquid, though salty,
116
J. M. Goordial
et al. 2013; Mykytczuk et al. 2013). Some have even hypothesized that due to the
slowing down of metabolic activity at cold temperatures, there is possibly no
low-temperature limit to life on Earth (Price and Sowers 2004). In many
cryoenvironments, microorganisms are the foundational life forms driving ecosystem function and, in many cases, are the only life present at all. These microbialbased ecosystems include microorganisms occupying multiple trophic levels and
niches (e.g. autotrophy, heterotrophy) and those in varying physiological states, for
example, metabolically active, replicating, dormant, or dead. Each of these physiological states plays an important role in ecosystem function.
This essay will discuss evidence of microbial “vital signs” in cryoenvironments—
evidence for both activity and cellular replication at sub-zero temperatures. In
particular, it will focus on one extreme site in the Antarctic McMurdo Dry Valleys,
where microorganisms are thought to be on the verge of the cold-arid limit of life. It
is an example of how microorganisms in different physiological states play a role in
cryoenvironmental ecosystem function, even when vital signs of active life or a
“pulse” is not detected. Finally, I will discuss what studying microbial life in
cryoenvironments can tell us about the limits of life on Earth and potentially beyond.
6.1.1 Microbial Replication and Activity at Sub-zero
Temperatures
Sub-zero temperatures affect microbial cells by causing lipids and proteins to
become rigid and inflexible, affecting membrane and protein conformation and
thus basic cellular function. Psychrophilic (cold-loving) and psychrotolerant (coldtolerant) microorganisms have evolved multiple mechanisms to adapt to these
conditions (Bakermans et al. 2009; Bakermans et al. 2011; Feller and Gerday
2003). For example, cold-adapted proteins have altered amino acid compositions,
favouring residues that result in higher protein flexibility via less tertiary bonding
and less densely packed hydrophobic cores (Raymond-Bouchard et al. 2018;
Goordial et al. 2016). Psychrophilic organisms also have increased copy numbers
of cold shock proteins and helicases for DNA and RNA processes such as replication
and transcription (Mykytczuk et al. 2013). Cell membranes may be enriched with
saturated and branched fatty acids to increase fluidity (Siliakus et al. 2017). In
addition, as a result of thermodynamics, kinetic energies are lower at cold temperatures—lowering reaction rates and transport of molecules. However, the primary
constraint to life at sub-zero temperatures may not be temperature itself, but the
availability of liquid water, necessary for all of life as we know it. At low temperatures water freezes, and thus water availability for microorganisms in these environments decreases sharply. During the freezing process, any solutes present are
squeezed and concentrated into thin brines within the frozen substrate, depressing
the freezing point of water and permitting small pockets of liquid, though salty,
116
J. M. Goordial