Many organisms experience environmental conditions that seem to us to be relatively 'normal', but some are able to survive or even thrive in conditions which we might regard as 'extreme'. This judgment is based on our own experience of our environment. The great majority of organisms live permanently in the sea. We would find more than a brief immersion beneath the waters of the sea an extreme stress, unless we had special equipment, but what we would find extreme is normal for marine organisms. There is clearly a problem with defining extreme conditions by our own experience. Can we develop less subjective criteria for determining what might be normal and what might be extreme for an organism?
Measuring the responses of organisms to changes in environmental conditions might provide the tools we need. Let us use the effect of temperature as an example. The responses of organisms to temperature are complex. The simplest response to temperature is that there is an optimum, at which activity, growth rates and metabolism are greatest, and a range of temperatures that an organism will survive (Figure 1.1). As the temperature increases or decreases from the optimum, the metabolism of the organism decreases. If the temperature becomes more extreme, the organism may display heat or cold stupor, in which movement becomes disorganised and normal processes are disrupted. Close to the limit of the tolerable range of temperatures, the organism will display heat or cold coma and cease activity altogether. Once the temperature limits are exceeded, it will die. Establishing the temperatures at which these changes in activity occur for an organism may allow us to determine what is 'normal' and 'extreme' with respect to temperature for that organism. Defining these transition temperatures may, however, not be easy because the organism may respond to changing temperature by initiating biological responses that extend the sur-vivable limits. In the case of low temperatures, the temperature at which metabolism ceases may not represent the point at which the organism dies.
There are important differences between the lethal effects of high and low temperatures. The damage caused by high temperature is destructive as proteins become denatured and other irreversible changes occur. The effect of low temperature may be rather different. As the temperature falls, metabolism slows (and if the temperature is low enough, it ceases) as the kinetic energy imparted to chemical reac-
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