The physical and chemical environments that are created and maintained by the proximity of a given planet to a star of a given class, as well as the magnetic, radiative and atmospheric dynamics that result from this association, represent the most fundamental controls on the ability of life to evolve and flourish. Once life has emerged the biosphere also plays a role in maintaining the planetary system through a number of complex feedback loops and geochemical cycling. However, our understanding of the prerequisites, subsequent requirements and limits of both ‘simple’ and advanced life has been shaped by the study of organisms on Earth. Therefore, our knowledge of the limitations on the ability of organisms to evolve is a product of the terrestrial environment in which, at present, all life has been found. In recent times our understanding of the physical and chemical boundaries for life, here on Earth and in the Solar System and beyond, have been radically revolutionised by the discovery of several species of ‘extremophile’; defined as organisms that can tolerate “…conditions that disrupt the integrity or function of aqueous solutions of organic compounds…” (pg. 114, Rothschild, 2007). This definition illustrates the importance of water as a solvent, and organic carbon as the basis of life on Earth. Extremophilic species can be found in the Archaea, Eubacteria and Eukarya taxa (Rothschild, 2007).


The study of extremophiles promises to provide a wealth of useful information and discoveries, both in the field of astrobiology and in other disciplines, and it is hoped that some of the fundamental questions in this area of science can be answered by continued research into species of organisms can exist on the edge of what is thought possible for life (Rothschild, 2007). For example, understanding of the chemical and physical mechanisms of extremophile survival could result in a number of useful applications on Earth and in space and the continual discovery of extremophilic species continues to extend the limits for life in the universe, as well as our understanding of the biodiversity of Earth (Rothschild, 2007). Meanwhile, other uses of extremophiles include food preservation, biological warfare, biotechnology, environmental biosensors, waste treatment, methane production, oil discovery, medical plastics and cancer detection, dietary supplements and other innumerable applications in industry, food production and preservation and healthcare and medicine (Rothschild, 2007; Satyanarayana, 2005)

Table 1 outlines some of the extreme environments in which species of hardy organisms have been found which include: in and around geysers, hotsprings and ‘black smokers’ on the ocean floor, within acidic mine drainage, salt lakes and nuclear reactors and waste (Rothschild, 2007; Southam et al., 2007).

Table 1: Classification of extremophiles and examples of ecosystems and organisms (Rothschild, 2007; Southam et al., 2007)

>  80 °C
60 – 80 °C
15 – 60 °C
< 15 °C

Ice or snow
Pyrolobus fumarii

pH > 9
Low pH
Soda lakes
Mine drainage
Cyanidium caldarium
Salt lakes
Artemia salina
Deep ocean

Nuclear reactors and waste
Deinococcus radiodurans
tolerates some O2
Cannot tolerate O2
Most of Earth

Deep ocean
Homo sapiens
Methanocaldococcus jannaschii
> 1 g
< 1 g


Tolerates vacuum

insects, seeds, tardigrades

Tolerates high levels of metals and concentrations of chemicals
Mine drainage
Cyanidium caldarium
Ferroplasma acidarmanus
Ralstonia spp.

Thermophiles and hyperthermophiles tolerate temperatures above which normal biomolecules such as chlorophyll (near 75°C) degrade; even nucleic acids break down well below 100 °C with some variation depending on base composition, polymer length and solvent concentration (Rothschild, 2007). Proteins that form structural compounds degrade with increasing temperature and enzymes begin to lose their catalytic ability both above and below an optimal temperature whilst soluble gases such as oxygen and carbon dioxide are less well absorbed into solvents (like water) at higher temperatures (Rothschild, 2007). On the other end of the temperature spectrum are psychrophiles that are able to withstand temperatures that are lower than would normally be conducive to biological functioning. Ice crystals can penetrate and destroy cell membranes and frozen water acts to spatially concentrate solutes to potentially toxic levels (Rothschild, 2007). The discovery of organisms able to withstand prolonged vacuum, extreme acceleration and deceleration (‘jerk’) and radiation exposure is contributing considerably to the concept of lithopanspermia and the possibility that life may have arrived on Earth from further afield via an asteroid or meteorite impact.

Life on Exoplanets

Extremophiles have extended the once rigid physical and chemical boundaries of life and I feel that this has important implications for both the habitable zone theory and for the existence of possible life on exoplanets in low mass star systems. If life were to exist on an exoplanet or its moon in an M-class star system the organisms are likely to be significantly dissimilar to life on Earth, potentially employing as-of-yet undiscovered metabolic pathways and utilising non-aqueous solvents and other hypothetical forms of biochemistry.  
However, scientists suggest that the universality of our biochemistry may confirm that the biological and physical limits of life of Earth may also apply elsewhere. For example, Pace (2001) states that:
“…it seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail. Thus, the 20 common amino acids are the simplest carbon structures imaginable that can deliver the functional groups used in life… Similarly, the five-carbon sugars used in nucleic acids are likely to be repeated themes…”
(Page 806)
Therefore, whilst extremophilic organisms may extend the parameters of our understanding, the general limits and basic requirements of life may be a universal trend and the habitable zone theory may be the instrument through which our search for life should be focussed. 

Pace, N.R. (2001). The universal nature of biochemistry. Proceedings of the National Academy of Science. 98 (3) pp. 805 – 808
Rothschild, L.J. (2007). Extremophiles: defining the envelope for the search for life in the universe. In: Pudritz, R., Higgs, P. and Stone, J. (2007). Planetary Systems and the Origins of Life. Cambridge: Cambridge University Press
Satyanarayana, T., Raghukumar, C. and Shivaji, S. (2005) Extremophilic microbes: Diversity and perspectives. Current Science 89(1) pp. 78 – 90
Southam, G., Rothschild, L.J. and Westall, F. (2007) The Geology and Habitability of Terrestrial Planets: Fundamental Requirements for Life. In: Fishbaugh, K.E., Logonné, P., Raulin, F., Des Marais, D.J. and Korablev, O. (2007) Geology and Habitability of Terrestrial Planets. London: Springer

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