13.2 Low Temperature Stress [in progress]

One of the most intuitive stresses associated with freezing is low temperature stress. Even without considering the challenges associated with ice itself, there are already several challenges associated with temperature along that freeze-tolerant organisms must tolerate. This chapter section will ignore the problems associated with ice for the moment, and specifically focus on how organisms experience and mitigate the challenges associated with low temperatures during freezing and thawing.

When low temperature stress occurs

Low temperature stress occurs throughout the freeze-thaw process (Figure 13.2). As the temperatures begin to drop below a “normal” temperature for the organism, low temperature stress begins. As ice forms and thaws, the cell is still at a low temperature and must withstand the stresses associated with low temperature to survive. Therefore, low temperature is one of the most pervasive stresses associated with freezing, with the stress becoming more intense as temperatures get lower and lower.

Impact of low temperature stress on cells and their macromolecules

The severity of low temperature stress depends on how extreme the temperature change is and how long the cell is exposed to the environment. Mild temperature changes cause temporary damage to the cell, which can often be reversed or repaired. When exposed to extreme changes in temperature, changes to the cell can be permanent and potentially cause cell death.

When a cell is exposed to low temperatures, its protein function is inhibited. Mild changes in temperature temporality impair protein functioning. For example, enzyme catalysis slows down, impairing metabolic processes such as ATP synthesis. Transmembrane ion pumps cannot sustain proper activity, both due to the effect of low temperatures on protein function and the lack of ATP associated with metabolism at low temperatures. The cytoskeleton may also be impacted by low temperatures because the association of cytoskeletal proteins into filaments (e.g., G-actin in F-actin) can be disrupted by changes in temperature. Extreme low temperatures can cause proteins to denature (sometimes permanently) and aggregate.

[figure? Flow chart summarizing these impacts?]

Low temperature stress also affects cellular membranes. As temperature decreases, membranes become less fluid, impairing membrane function. If membranes are cooled to a sufficiently low temperature, they solidify (gel phase; see Chapter 12), which can cause permanent damage to the membranes. Lots of organisms will adjust their membrane composition to regulate membrane fluidity as temperatures change, but may not be able to do so if temperature changes are rapid or extreme.

Mechanisms that protect cells from low temperature stress

The major mechanism for protecting cells at low temperatures is stabilizing cells and their macromolecules. For example, cells can modify membrane composition to preserve membrane fluidity at low temperatures by incorporating phospholipids with unsaturated fatty acid chains into its membranes. Cells can also accumulate cryoprotectants like glycerol and other small molecules that protect protein and membrane structure when the cell is experiencing low temperature stress. Cells can accumulate chaperones, specialized proteins that can help stabilize or refold other proteins. Furthermore, cold-tolerant organisms can synthesize cold-adapted protein isoforms – versions of proteins that function well at low temperatures. If cold-induced damage occurs during the freeze-thaw cycle, cells likely have to repair that damage post-thaw via mechanisms discussed in Chapter 14.

[figures for cryoprotectants and chaperones?]