Life Beyond Earth

Extrapolating Extra-Terrestrial Life

The adaptations of life on Earth to extreme environments and the potential analogues in the Sol system.

Author: Elsby, D.

Introduction

On Earth, microorganisms have consistently been found living in environments previously thought incapable of supporting life; life has adapted to survive in conditions initially believed too extreme for the chemical processes of life to occur. Before 1965 it was believed that life could not exist in temperatures greater than 73°C; Thomas Brock discovered the bacteria Thermus aquaticus which can survive at temperatures between 50-80°C (Brock, 1997). Before 1977 it was believed that sunlight was required as the initial energy source for any ecosystem; the discovery of the deeps sea hydrothermal vents, demonstrated life that can exist in the in environments completely isolated from the sun and, again, at temperatures lethal to most other life on Earth (Garagaud et al. eds 2011). In 2010, the bacterium GFAJ-1 was studied at Mono Lake, California, and was found to be capable of thriving in the presence of arsenic (Wolf-Simon et al. 2010), a substance toxic to most other life on Earth.

Given the diversity of environmental conditions present on Earth, and the abundance of life found to exist within them, it is not unrealistic to propose that life exists in similar conditions elsewhere within our solar system. The extreme environments of our solar system, the life within them and how it has adapted is still a new and exciting area of research that demands thorough scientific knowledge, but welcomes imagination.

What Does Life Need

Life, as we know it, has three fundamental requirements in order to exist – liquid water, a pool of nutrients and a source of energy.

Liquid water acts as a solvent and medium for crucial life processes. It was in water that the first cells formed and liquid water was the medium by which the first gametes were exchanged. In many species, this is still the case today. On earth, wherever we have found liquid water, we have found life.

In order for organisms to grow, they must have a pool of nutrients from which they build and renew molecular and cellular structures. On Earth, carbon, hydrogen, oxygen and nitrogen are crucial along with dozens of other elements in varying quantities e.g. phosphorus forms part of the backbone of DNA.

The medium and components of life are nothing without the energy to drive the processes of life. The majority of life on Earth relies upon the Sun for energy. Photosynthetic plants utilise the energy of sunlight to pull nutrients from the air and soil. As the primary producers of most ecosystems on the planet, nearly all other life relies on these plants as their own source of nutrients – the carbon cycle dictates that every carbon molecule in your body was, at one point, pulled from the air and fixed into a carbohydrate molecule by the photosynthesis of a plant.

As we now know, sunlight is not required for life. Miles from sunlight, on the ocean floor, the heat from the earth’s molten core powers the life found at the deep sea hydrothermal vents

Given the above, if we assume that life will only occur in the presence of water, energy and nutrients, then these environments, no matter how extreme, are where we should look to find life.

Extremophiles

Extremes of temperature, pressure, pH, and salinity are found all over the Earth and life can be found, usually in the form of microbial mats or biofilms, in every example (Chela-Flores, 2011; Seckbach and Oren, 2010). Extremophiles are simply organisms that have physiological adaptations which allow them to survive and thrive in environments out with the range of human survival (Horikoshi and Bull, 2011). Table 1 lists a small selection of extremophile categories and the environmental conditions on Earth to which they are adapted. The same general categorisations can be applied to organisms found elsewhere in the solar system living in similar environments.

Table 1: Extremophile Classification

Environmental Condition Classification
Temperature
0°C; Optimal Growth ≤15°C Psychrophile
Optimal Growth 80-113°C Hyperthermophile
Pressure
High pressure favours/required for growth
e.g. 1000 atm
Barophile
Water Activity
Tolerant of variations in solute concentrations in water Osmotolerant
Radiation
Able to resist ionising radiation Radioresistant

Temperature

Extreme heat and extreme cold each bring different problems to the process of life – extreme cold causes a reduction in the fluidity of cell membranes, while extreme heat causes the membrane to disassociate and enzymes to denature. Nevertheless, there is evidence of active life at temperatures from -20°C to over 121°C (Willey et al. 2009).

In psychrophiles, high concentrations of unsaturated fatty acids allow for continued cell membrane fluidity at lower temperatures due to the ‘kinks’ in the fatty acid chains.

In hyperthermophiles, proteins (enzymes) are folded in a way to maximise the hydrogen bonding within the structure. They tend to be less flexible, and thus less affected by heat, and can employ ‘chaperone proteins’ for added stability and protection of important molecules i.e. histones in DNA (Willey et al. 2009). Cell membranes can contain high quantities of saturated and, in archaea, highly-branched fatty acids, ether linkages and/or a monolayer structure, thus contributing to membrane strength.

Pressure

While there are still many facets of their biochemistry that are not yet understood, it has been demonstrated that barophiles survive extreme pressure by employing highly unsaturated membrane fatty acids (DeLong and Yayanos, 1985, 1986; Wirsen et al., 1987; Kamimura et al., 1993 cited in Kato and Bartlett, 1997) and altering cell membrane fluidity (Kato and Bartlett, 1997).

Water Activity

Maintaining the osmotic concentration of the cytoplasm is crucial in order to prevent cells from dehydrating or bursting. Methods observed include increasing the uptake of external solutes – or manufacture their own – that do not interfere with enzyme functions to increase the osmotic concentration of the cell. In hypotonic solutions some organisms use channels in their membranes as valves to prevent excessive solute build up in the cytoplasm (Willey et al. 2009). Whatever the chemistry, any organism present will likely use similar methods to maintain the water activity of the cell.

Radiation

Ionising radiation from the sun is a very real danger to life on any planet that lacks an ozone layer – or similar atmospheric shield – to protect against it. Ionising radiation breaks chemical bonds, polymerises molecules and directly damages DNA. In oxic environments it can create toxic hydroxyl radicals. Some organisms, like the bdelloid rotifers, protect against such damage by possessing multiple copies of their DNA. Arguably, the best strategy is simply to avoid direct exposure e.g. a subsurface existence.

Possible Locations for Life

It is likely that the adaptations mentioned above will be present in any similar environment found in our solar system which contains life. From existing robotic missions, satellite and spectrographic observations, there are several solar bodies that stand out as possibilities for harbouring such life.

Europa

Jupiter’s closest moon possesses a surface covered in a relatively thin (kilometres thick) water ice shell (Figure 2). It is believed that beneath this frozen crust, heated by tidal motion and the molten mantle of the moon itself, a salty, liquid ocean exists, some 150km deep and containing more water than all of Earth’s oceans combined (California Institute of Technology, 2012; Southam et al. 2007).

Figure 1: The geological structure of Europa.
(Image credit: jet propulsion lab, 2012)

Given the molten mantle, it is possible that conditions very similar to our own deep sea hydrothermal vents may exist on the floor of the Europa’s ocean. As such, we would expect to find barophilic thermophiles and/or hyperthermophiles near such vents. It is conceivable that the full spectra of temperatures may be present in the subsurface ocean – from the hydro thermal vents to the frozen surface of the moon.

It is also possible that life exists in the near freezing temperatures on the underside of Europa’s frozen shell – similar to the algae found on the arctic ice sheets.

In Earth’s deep-ocean sediment, methane hydrates have been discovered. These are a by-product of archaea metabolism, found only in cold, high pressure environments (Willey et al. 2009). If something similar exists within the oceans of Europa, they could be another source of energy – along with geothermal heat – to potential life.

Mars

Lacking the protection of Earth’s atmosphere, ionising radiation is a significant hurdle for life on Mars to surmount. A subsurface existence seems a plausible strategy. There is evidence of bacteria on Earth producing endospores that are resistant to the damaging effects of radiation (Willey et al. 2009), so it is not inconceivable that a similar strategy could be used on Mars. Hardy endospores could survive dispersion via the dust storms which frequently sweep the planet, ultimately spreading life far and wide to germinate once favourable conditions (e.g. buried under dust, near water) are reached.

There is clear evidence of water ice at the poles of Mars and potentially in great quantities beneath the surface as well, with indications suggestive of seasonal melting. Topographical analysis shows many features indicative of water erosion in Mars’s past (Figure 1), but today the planet is largely desert (Garagaud et al. 2011). At the poles, we could reasonably expect to find life similar to that found in the glaciers on earth (psychrophiles), however, the lack of water in its liquid state, both in the thin atmosphere and on the land, appears to be the primary limiting factor for life on Mars.

Seasonal fluctuations of methane have been recorded in the Martian atmosphere, suggesting the possibility of methanogenic life, though this could be the result of volcanic activity. It is thought that life did exist on Mars in the past (Garagaud et al. 2011), the question is whether it still exists today.

Conclusion

The lessons we learn by studying the diversity of environments on earth and the life that inhabits them can point us in the right direction in our search for other life in our solar system. Once we know where and, importantly, how life exists in an environment, we can both focus our efforts and extrapolate indicators. Despite studying our own planet’s biology for centuries, some of the most profound discoveries have only been made within the last several decades. Space exploration, exobiology in particular, is still in its relative infancy, but already we have identified promising targets that clearly warrant further, detailed investigation. It is clear that the potential for exciting discoveries is, like life itself, vast indeed.

References

Brock T. D., 1997. The Value of Basic Research: Discovery of Termus aquaticus and Other Extreme Thermophiles. Genetics Society of America. [e-journal] 146(4), p. 1207-1210. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 2nd March 2013].

Chela-Flores, J., 2011. The Science of Astrobiology. [e-book] Netherlands: Springer Science. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 26 Feb 2013].

Garagaud M., Amils R., Quintanilla J. C., Cleaves H. J., Irvine W.M., Pinti D.L., Viso M., eds. 2011. The Encyclopaedia of Astrobiology. [e-book] Berlin, Heidelberg : Springer Science. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 26 Feb 2013].

Horikoshi K., ed. 2011. Extremophiles Handbook. [e-book] Japan: Springer Science. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 2nd March 2013].

Jet Propulsion Laboratory, California Institute of Technology, 2012. Europa Study 2012 Report: Executive Summary. [pdf] Available at: <http://solarsystem.nasa.gov/europa/docs/ES%202012%20Report%20ES%20Ex%20Summary%20-%20Final%20-%2020120501.pdf&gt; [Accessed 2nd March 2013].

Kato C. and Bartlett D. H., 1997. The molecular biology of barophilic bacteria. Extremophiles. [e-journal] 1(3), p.111-116. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 3rd March 2013].

Prescott L. M., Harley J. P. and Klein D. A., 2005. Microbiology. New York, NY: McGraw-Hill.

Seckbach A. and Oren A., 2010. Microbial Mats: Modern and Ancient Microorganisms in Stratified Systems. [e-book] Netherlands: Springer Science. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 26 Feb 2013].

Southam G., Rothschild L. J. and Westall F., 2007. Fundamental Requirements for Life. In: Fishbaugh K.E., Lognonné P., Raulin F., Des Marais D. J. and Korablev O., eds. 2007. The Geology and Habitability of Terrestrial Planets. [e-book] Netherlands: Springer Science. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 26 Feb 2013]. p. 7-34.

Willey J. M., Sherwood L. M. and Woolverton C. J., 2009. Prescott’s Principles of Microbiology. New York, NY: McGraw-Hill.

Wolfe-Simon F., Blum, J. S., Kulp, T. R., Gordon, G. W., Hoeft, S. E., Pett-Ridge, J., Stolz, J. F., Webb, S. M., Weber, P. K., Davies, P. C. W., Anbar, A. D. and Oremland, R. S., 2010. A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. Science. [e-journal] 332. Available through: Edinburgh Napier University Library website: <http://nuin3.napier.ac.uk/&gt; [Accessed 28 Feb 2013].

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