Resistance and capacity adaptation to low temperatures

In environments that are temporarily cold, most organisms show resistance adaptation to subzero temperatures, surviving in a dormant state until conditions favourable for activity and growth return. Plants do not generally grow at low temperatures and most terrestrial animals, and other organisms, enter a state of dormancy or cold stupor and become inactive. Metabolism may slow sufficiently at very low tem peratures for the organism to become cryptobiotic. Some freeze-avoid-ing insects, however, can remain active at very low temperatures. Mass aggregations of springtails are often observed moving across the surface of the snow in the European Alps ('snow fleas') even during the coldest part of the winter. The enzymes and physiological systems of these animals must be able to function at these low temperatures and they thus show capacity adaptation, as do psychrophilic microorganisms that can grow at low temperatures.

Organisms that live in environments that are permanently cold must maintain activity and growth at low temperatures. The seas of the Arctic and around the Antarctic are constantly at subzero temperatures. The fish, invertebrates and other organisms that live there must have capacity adaptation to the conditions.

cool applications

Studies of the responses of organisms to cold have yielded, or have the potential to produce, a wide range of practical applications. These include the control of pests by influencing or predicting their survival overwinter and improved methods for cryopreserving organisms and biological materials. Cold-adapted enzymes from psychrophilic microorganisms and polar fish may have applications in the food industry, in biotechnology, laundry detergents and in the treatment of wastewater. Cold-adapted microbes themselves are used for the cold fermentation of beer and wine and for the ripening of cheeses and other foods.

Ice nucleation-active microorganisms have been used experimentally in food processing where freezing at a high subzero temperature is required (such as the freeze concentration of soy sauce, coffee, non-heated jams and other foods). As well as improving the quality of food processed by freeze concentration or freeze drying, they can also improve the texture of frozen foods, presumably by influencing the size and shape of ice crystals. Ice-active bacteria (Pseudomonas syringae) are widely used commercially for the artificial production of snow by ski fields (there is even a company in subtropical southern Georgia,

USA, which will supply the locals with an otherwise unlikely 'white Christmas'). The bacteria are sold under the trade name 'SNOMAX©' to be added to the water used by snowmaking machines. This assists snowmaking by raising the critical temperature for snow formation by as much as 20 °C. A similar product (SNOMAX weather manager©) is sold for seeding clouds to encourage rainfall or snow. It may be possible to control freeze-avoiding insect pests by exposing them to ice-nucleating microorganisms that reduce their capacity to supercool. On the other hand, it may be possible to prevent frost injury to plants by controlling their associated populations of ice-nucleating microorganisms.

A wide variety of ice-active proteins are produced by cold-adapted organisms, which influence the formation and stability of ice. These include ice-nucleating proteins which produce ice formation at high subzero temperatures and antifreeze proteins that inhibit ice nuclea-tion. Antifreeze proteins produce a thermal hysteresis (a difference between the melting and freezing point in the presence of an ice crystal), but they also have the property of inhibiting recrystallisation. It is becoming clear, however, that some organisms (plants, molluscs and nematodes) produce proteins that inhibit recrystallisation but show little or no thermal hysteresis and cannot, therefore, be called antifreeze proteins. These may represent a third class of ice-active proteins, whose functions are to control the size, shape, location or stability of ice rather than the temperature at which it forms. Understanding how organisms control the properties, formation and stability of ice could find uses in a wide range of situations.

For the production of a creamy delicious ice cream, it is necessary for the freezing process to produce millions of tiny ice crystals, rather than fewer larger crystals which would give the ice cream a gritty texture. Ice-active proteins may be useful in controlling the size and nature of the crystals formed in ice cream and other frozen products. They may also be useful for inhibiting the ice recrystallisation that might cause the quality of the product to deteriorate. The nature of ice crystals formed in foods stored frozen also determines its quality when it is con sumed after thawing and/or cooking. The addition of fish antifreeze proteins to meat before freezing results in the formation of smaller ice crystals. A number of studies have looked at whether the addition of fish antifreeze protein could improve the cryopreservation of sperm, oocytes, liver tissue and a variety of other cells and tissues. These studies have met with mixed success but companies have been formed to develop and exploit the commercial potential of these proteins. Attempts to improve the frost resistance of plants by inducing them to express fish antifreeze proteins via genetic engineering have so far been unsuccessful. The recognition that some plants themselves produce ice-active proteins may help in the development of techniques to improve frost resistance in plants. Ice slurries, mixtures of ice and water, are used for cold storage and heat transportation. The formation of large ice crystals by recrystallisation could block the pipes through which the slurry flows. Antifreeze proteins are able to prevent this.

Our understanding of how some organisms are able to survive freezing conditions is increasing and, although the ability to freeze human bodies is still some way off, it is clear that the lessons we learn from these organisms are going to find some cool applications.

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