Life at low temperatures

The greater part of our planet is below 5°C: 'cold is the fiercest and most widespread enemy of life on earth' (Franks et al., 1990). More than 70% of the planet is covered with seawater: mostly deep ocean with a remarkably constant temperature of about 2°C. If we include the polar ice caps, more than 80% of earth's biosphere is permanently cold.

By definition, all temperatures below chilling injury the optimum are harmful, but there is usually a wide range of such temperatures that cause no physical damage and over which any effects are fully reversible. There are, however, two quite distinct types of damage at low temperatures that can be lethal, either to tissues or to whole organisms: chilling and freezing. Many organisms are damaged by exposure to temperatures that are low but above freezing point - so-called ectotherms and endotherms coexist: both strategies 'work'

'chilling injury'. The fruits of the banana blacken and rot after exposure to chilling temperatures and many tropical rainforest species are sensitive to chilling. The nature of the injury is obscure, although it seems to be associated with the breakdown of membrane permeability and the leakage of specific ions such as calcium (Minorsky, 1985).

Temperatures below 0°C can have lethal physical and chemical consequences even though ice may not be formed. Water may 'supercool' to temperatures at least as low as —40°C, remaining in an unstable liquid form in which its physical properties change in ways that are bound to be biologically significant: its viscosity increases, its diffusion rate decreases and its degree of ionization of water decreases. In fact, ice seldom forms in an organism until the temperature has fallen several degrees below 0°C. Body fluids remain in a supercooled state until ice forms suddenly around particles that act as nuclei. The concentration of solutes in the remaining liquid phase rises as a consequence. It is very rare for ice to form within cells and it is then inevitably lethal, but the freezing of extracellular water is one of the factors that prevents ice forming within the cells themselves (Wharton, 2002), since water is withdrawn from the cell, and solutes in the cytoplasm (and vacuoles) become more concentrated. The effects of freezing are therefore mainly osmoregulatory: the water balance of the cells is upset and cell membranes are destabilized. The effects are essentially similar to those of drought and salinity.

Organisms have at least two different metabolic strategies that allow survival through the low temperatures of winter. A 'freeze-avoiding' strategy uses low-molecular-weight polyhydric alcohols (polyols, such as glycerol) that depress both the freezing and the supercooling point and also 'thermal hysteresis' proteins that prevent ice nuclei from forming (Figure 2.8a, b). A contrasting 'freeze-tolerant' strategy, which also involves the formation of polyols, encourages the formation of extracellular ice, but protects the cell membranes from damage when water is withdrawn from the cells (Storey, 1990). The tolerances of organisms to low temperatures are not fixed but are preconditioned by the experience of temperatures in their recent past. This process is called acclimation when it occurs in the laboratory and acclimatization when it occurs naturally. Acclimatization may start as the weather becomes colder in the fall, stimulating the conversion of almost the entire glycogen reserve of animals into polyols (Figure 2.8c), but this can be an energetically costly affair: about 16% of the carbohydrate reserve may be consumed in the conversion of the glycogen reserves to polyols.

The exposure of an individual for several days to a relatively low temperature can shift its whole temperature response downwards along the temperature scale. Similarly, exposure to a high temperature can shift the temperature response upwards. Antarctic springtails (tiny arthropods), for instance, when taken from 'summer' temperatures in the field (around 5°C in the Antarctic) and subjected to a range of acclimation temperatures, responded to temperatures in the range +2°C to —2°C (indicative of winter) by showing a marked drop in the temperature at which they froze (Figure 2.9); but at lower acclimation temperatures still (—5°C, —7°C), they showed no such drop because the temperatures were themselves too low for the physiological processes required to make the acclimation response.

Acclimatization aside, individuals commonly vary in their temperature response depending on the stage of development they have reached. Probably the most extreme form of this is when an organism has a dormant stage in its life cycle. Dormant stages are typically dehydrated, metabolically slow and tolerant of extremes of temperature.

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