Searching for Life in the Solar System ... And Beyond
A Research Discussion Meeting
London, UK - 31 October 1996

Micro-organisms in inhospitable environments

Don A. Cowan, Department of Biochemistry and Molecular Biology
University College London

The environmental conditions which sustain or limit the existence of microbial life on earth may be used predictively in a search for life on other planets. The "biological envelope" is currently defined by such criteria as the available nutrient and energy source, the presence of water, and specified limits of temperature, pH, salinity, and chemical toxicity.

This biological window is largely based on studies of "extremophiles", micro-organisms which have adapted to so-called extreme environments. Many of these studies have been carried out in only the past 10 years and it is likely that the window will widen as research continues.

The availability of liquid water may be the most important factor for biological growth (as opposed to survival). Microbial resting bodies may be resistant to temperatures below the freezing point or above the boiling pont of water (at normal pressures) but little or no growth occurs outside these limits. The environmental temperature is also critical; organisms capable of growth at 110 degrees C have been recently discovered but it is generally believed that carbon-based life is unlikely to exist at significantly higher temperatures (say, above 150 degrees C). The availability of certain carbon and nitrogen and other energy sources may also be a determinant for life although the known diversity of microbial metabolism indicates a high degree of adaptability to different nutrient and energy supplies.

Our current understanding of the limits of the "biological envelope" on earth will be illustrated with examples of known extremophilic micro-organisms.