The word 'extremophile', coined by Bob MacElroy (1974) from the Latin 'extremus' and the Greek 'philos', literally means 'lover of extremes'. Unfortunately, even the definition is not as clear as one might think. Extreme is usually taken to refer to physical, chemical, or, rarely, biological conditions. But what is extreme?
There is an objective definition of extreme, owing to the fact that life on Earth is based on organic carbon with water as a solvent. Thus, conditions that disrupt the integrity or function of aqueous solutions of organic compounds are extreme.
Must extremophiles 'love' extreme conditions? Scientists tend to be inconsistent; when it comes to temperature, extremophiles thrive optimally at the extreme temperatures. However, for many extremes, from pH to radiation, the extremophile organisms grow well, or even grow better, under less-extreme conditions.
The ability to thrive under extreme conditions may be limited to particular life stages. For example, some species of frogs, turtles, and snakes can tolerate freezing of extracellular water, but only after the production of appropriate antifreeze molecules in the autumn (Storey and Storey, 1996).
Which taxa contain extremophiles? With attention focused on temperature and salt tolerance, biologists tend to equate Archaea with extremophiles. This is inaccurate, as many extremophiles are Eubacteria and Eukarya, and additionally an increasing number of non-extremophilic Archaea are being uncovered.
Reasons for studying extremophiles are listed in Table 6.1. Only the last five are primarily or exclusively the territory of astrobiologists: the future use of extremophiles in space - perhaps for life support, perhaps for terraforming; the biodiversity of life on Earth, mechanisms to survive extreme environments, and the limits for life in the universe.
It is interesting to note that extremophiles have been used to look for life in the Universe not as a recent construct of astrobiology, but rather as a nineteenth-century suggestion. Richard Proctor, a popular science writer for the public, was
Reason to study extremophiles Important to astrobiology food preservation no basic research; model organisms for basic research no biological warfare no biotech potential no future use in space yes biodiversity on Earth yes origin of life yes mechanisms of extremophile survival yes limits for life in the universe yes probably the first to connect the study of life in extreme environments and life on other planets (Proctor, 1870, Chapter 1):
If we range over the [E]arth, from the arctic regions to the torrid zone, we find that none of the peculiarities which mark the several regions of our globe suffice to banish life from its surface.
While this chapter will focus almost exclusively on microbes, even humans inhabit extreme environments, including outer space (for a fascinating review of human adaptations to extreme environments, see Ashcroft (2001)).
Was this article helpful?