Some considerations of the theoretical limits for living organisms

The study of the limits of life has been traditionally divided into two parts (a) extreme conditions for growth and metabolism, and (b) those conditions permitting survival in a dormant state. The variables most often considered have been temperature, radiation, availability of water and ageing. For growth and metabolism the most sensitive parts of living organisms appear to be those that involve labile metabolites, of which there is a great variety. For dormancy the stability of covalent bonds seems most important; here the C-C bond is likely to be less significant than C-O and C-N. But for both classes of damage the stability greatly depends on numerous factors (such as pH and oxygen concentration) that have seldom been considered in work so far. It is therefore desirable to explore the effects of combinations of adverse factors upon key metabolites and macromolecules. In addition, such studies may permit better estimates of the effects of very long time periods by extrapolation from experimental work.