The life box

Just as there are ranges for survival and activity with respect to temperature, the same is true for other environmental variables (such as salinity, conditions of acidity or alkalinity, oxygen concentration etc.). We could measure conditions (temperature, pH, salinity etc.) in the environment adjacent to an organism. If we did this lots of times over the lifespan of the organism, we could determine the range of conditions it has experienced. Some organisms move around, experiencing changes in environmental conditions, and conditions themselves change with time. Plotting these measurements in multidimensional space would thus define the overall range of conditions that the organism has experienced (its 'life box'; Figure 1.2). I have experienced — 26 °C in the Antarctic and +45 °C in the Australian desert and so my life box would extend between these two values for the parameter of temperature. If we did the measurements for all organisms of the same species, we could define the life box for that species, and thus the sort of habitats in which that species could live. If we did the measurements for all organisms of all species, we could define the life box for life in general. If conditions change such that an organism finds itself outside the life box for its species, it will die. If conditions are such that they are outside the life box for life in general, there will be no life. Ecologists call an organism's life box its 'ecological niche'. This is determined by both the physical characteristics of its environment (temperature, pH, water availability etc.) and its interactions with other organisms (predation, competition, disease, the food source it exploits etc.). I have used the term 'life box' to focus attention on the range of physical conditions that organisms can tolerate and because it might help us to identify what is extreme for organisms.

To decide what might be an extreme organism, it might be useful to think in terms of the life box for the majority of organisms or for the

figure 1.2 The life box of a hypothetical organism with respect to three environmental variables (x, y, z; e.g. temperature, pH, oxygen availability). Each point represents the conditions experienced and survived by the organism at a particular place and moment in time. The conditions experienced and survived at all places and times defines the life box for the organism or species of organism. If the conditions go beyond its life box, the organism dies.

figure 1.2 The life box of a hypothetical organism with respect to three environmental variables (x, y, z; e.g. temperature, pH, oxygen availability). Each point represents the conditions experienced and survived by the organism at a particular place and moment in time. The conditions experienced and survived at all places and times defines the life box for the organism or species of organism. If the conditions go beyond its life box, the organism dies.

majority of species of organism. An extreme organism would, therefore, be one that tolerates conditions beyond those tolerated by most organisms. The range of conditions for such an organism will be different from that for the majority of organisms, and it will have a life box which occupies a different theoretical space from that of most organisms (Figure 1.3). Such organisms have been called extremophiles (they love conditions which are far removed from the ordinary or average). The best-known examples are thermophilic bacteria that live associated with hot springs and deep-sea hydrothermal vents, where the temperatures they experience are much higher than those experienced

figure 1.3 The life box of an extremophile (O) compared with that of the majority of organisms. For the extremophile, the range of conditions that it will survive is different from that survived by the majority of organisms. Its life box thus occupies a different theoretical space.

by most organisms. There are extremophiles that colonise other types of extreme environments - such as very saline habitats (halophiles), acidic or alkaline conditions (acidophiles, alkaliphiles), low temperatures (psychrophiles) and high pressures (piezophiles).

There is, however, another group of organisms that can be considered to be extreme. In terms of the conditions in which they can maintain activity, their life box is the same as, or considerably overlaps, the life box of the majority of organisms. However, when conditions change to the point where the organism can no longer sustain metabolic activity, rather than dying, they cease metabolism and enter into an ametabolic dormant state. When conditions return to normal, they

figure 1.4 The life box of a cryptobiote, only here the symbols represent conditions under which the organism can metabolise rather than survive. Its life box is thus the same as, or considerably overlaps, the life box for the majority of organisms (see Figures 1.2 and 1.3). When conditions change to beyond those in which the cryptobiotic organism can metabolise (-•O--), metabolism ceases, but the cryptobiote can resume metabolism once conditions become favourable again.

resume activity. In other words, if we think of life in terms of metabolism, they have the capacity to step outside their life box and to become active again once conditions become favourable (Figure 1.4). This phenomenon has been called cryptobiosis (hidden life), anabiosis (renewed life) or latent life. Latent life is perhaps the most appropriate term, since, in the latent state, the capacity for life is present but is not apparent. Cryptobiosis is, however, the most commonly used term. Some cryptobiotic organisms can enter this state at any stage in their life cycle. Others have special survival and dispersal stages, which act as lifeboats that carry organisms to new habitats, or which enable them to survive periods unsuitable for growth. This ability enables them to survive in space and time until conditions are favourable. These lifeboats include spores, eggs, cysts, seeds and resistant larval stages.

Cryptobiotic organisms can survive a variety of environmental stresses. Some can survive the complete loss of their body water. This phenomenon has been called 'anhydrobiosis' (life without water). Other types of cryptobiosis include cryobiosis (extreme cold), thermo-biosis (heat), osmobiosis (osmotic stress, such as high salt concentrations) and anoxybiosis (lack of oxygen). A number of organisms enter a period of dormancy in which their activity levels are lowered in response to these types of environmental stresses. The lowering of metabolic rate may be to as little as 80 per cent of normal resting levels, but more typically to a point in the range of 5-40 per cent of resting levels. Cryptobiosis is distinguished from dormancy by resulting in a depression of metabolic rate to less than 1 per cent of resting levels or even ceasing altogether.

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