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Defining the envelope for the search for life in the Universe

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Lynn Rothschild at National Aeronautics and Space Administration
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Abstract

The search for life in the universe relies on defining the limits for life and finding suitable conditions for its origin and evolution elsewhere. From the biological perspective, a conservative approach uses life on earth to set constraints on the environments in which life can live. Conditions for the origin of life, even on earth, cannot yet be defined with certainty. Thus, we will describe what is known about conditions for the origin of life and limits to life on earth as a template for life elsewhere, with a particular emphasis on such physical and chemical parameters as temperature, pH, salinity, desiccation and radiation. But, other life forms could exist, thus extending the theoretical possibility for life elsewhere. Yet, this potential is not limitless, and so constraints for life in the universe will be suggested.
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Highlights of Astronomy, Volume 15
XXVIIth IAU General Assembly, August 2009
Ian F. Corbett, ed.
c
International Astronomical Union 2010
doi:10.1017/S1743921310011026
Defining the envelope
for the search for life
in the Universe
Lynn J. Rothschild
Mail Stop 239-20, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
email: lynn.j.rothschild@nasa.gov
Abstract. The search for life in the universe relies on defining the limits for life and finding
suitable conditions for its origin and evolution elsewhere. From the biological perspective, a
conservative approach uses life on earth to set constraints on the environments in which life
can live. Conditions for the origin of life, even on earth, cannot yet be defined with certainty.
Thus, we will describe what is known about conditions for the origin of life and limits to life on
earth as a template for life elsewhere, with a particular emphasis on such physical and chemical
parameters as temperature, pH, salinity, desiccation and radiation. But, other life forms could
exist, thus extending the theoretical possibility for life elsewhere. Yet, this potential is not
limitless, and so constraints for life in the universe will be suggested.
1. Introduction
To find something one has to know where to look. From Earth to the edge of the
observable universe is about 46.5 billion light-years, so for the moment we must narrow
the search considerably to where it is possible for life to reside, while staying within our
technological bounds. Life is always likely to be based on organic carbon because carbon
is the fourth most common element in the universe, its chemical versatility, the discovery
of organic compounds elsewhere, and the fact that we are made of it (Rothschild 2009).
Thus, as a first order organic compounds must be stable and function in order for life
to exist, which provides a theoretic maximum envelope for life. Because Earth is only
one place that life is known thus far, the minimum envelope is derived by assessing the
environmental limits for life on Earth. But what a minimum envelope, as life swarms
over Earth in many niches that until recently seemed uninhabitable.
2. The extremes of life
The extreme environments and examples of organisms that inhabit them are listed in
Table 1. All of these environments at some point make it difficult for organic carbon to
stay intact and/or for a solvent – such as water for Earth-based life – to stay liquid (see
review in Rothschild 2009). Further, each environment can add other complexities. For
example, at low temperatures membranes loose their fluidity and enzymatic reactions
are slowed to the point that they cannot sustain life. Radiation and oxidative damage
are of particular interest as they provide physical and chemical limits to life, but also
act to mutate the genetic material. To this end, our lab has conducted experiments in
high ultraviolet environments from the Bolivian Altiplano to Mount Everest, and by
transporting biological samples to 33 km on high altitude balloons through Stanford’s
BioLaunch program.
697
698 L. J. Rothschild
Environment Type Definition Example
Temperature hyperthermophile growth >80
◦
C Pyrolobus fumarii-113
◦
C strain 121
thermophile growth 60-80
◦
C
mesophile growth 15-60
◦
C Homo sapiens
psychrophile growth <15
◦
C Psychrobacter, insects
pH alkaliophile pH >9 OF4 (10.5); 12.8?
acidophile low pH loving Cyanidium, Ferroplasma
Desiccation xerophile cryptobiotic tardigrades
anhydrobiotic
Salinity halophile 2-5 M NaCl Haloarcula, Dunaliella
Radiation high radiation Deinococcus radiodurans
Oxygen anaerobe cannot tolerate O
2
Clostridium
miroaerophil low levels of O
2
Methanococcus jannaschii
aerophile mid to high O
2
Homo sapiens
Pressure barophile/ pressure/ Shewanella viable at 1600 MPa
piezophile weight loving tardigrades
Vacuum tolerates vacuum tardigrades, insects, microbes, seeds
Gravity hypo/hypergravity <1g/>1g none known
Chemical gasses, metals tolerates CO
2
(Cyandium cadarium); Cu/As/
high levels Cd/Zn (Ferroplasma acidarmanus)
Electricity electric eel
Tab le 1 . Examples of extremophiles. Adapted from Rothschild & Mancinelli (2001).
3. Could it happen again?
To assess if life could arise again and inhabit these niches, once again the biodiversity of
life on Earth provides clues. When more than one organism has converged on a solution for
an environmental extreme, it gives us more confidence that this evolutionary adaptation
is not a one time event (Rothschild 2008). Multiple organisms have evolved to function
at low and high temperature, low and high pH and so on. This suggests that there may
be some universality – at least given the starting points of organic carbon and liquid
water as a solvent – to the extreme environments for life.
4. Where is the field heading?
As we find more locations in our solar system and beyond that meet single variable
constraints for life, attention must be paid to polyextremophiles, or organisms that can
cope with multiple extreme parameters. Will this show that all possible niche space is
occupied or that there are environmental combinations that, for some reason, cannot be
occupied?
References
Rothschild, L. J. 2008, Phil. Trans. Royal Soc. B 363, 2787
Rothschild, L. J. 2009, in: C. Bertka (ed.), Exploring the Origin, Extent and Future of Life,
(Cambridge: Cambridge University Press), p. 113
Rothschild, L. J. & Mancinelli, R. L. 2001, Nature 409, 1092
... A new domain of research came in front with the discovery of Taq polymerase, capable of withstanding high temperature without losing its activity, extracted from an extremophilic microbe, Thermus aquaticus, and now used in everyday PCR technique (Rothschild 2009). Thereafter several studies were performed in order to gain a deeper understanding of the extremophilic domain. ...
... These include areas with hot and cold temperature, acidity, alkalinity, high salinity, extreme radiation, high pressure, desiccation, and higher amounts of metals and nutrients. Such regions are distributed around the globe comprising volcanic regions, hot springs, deep-sea hydrothermal vents, polar regions, acidic and hypersaline lakes, soda lakes, nuclear contamination sites, desert and arid regions, etc. (Rothschild 2009). Figure 1 represents some of the known extreme environments found across the globe. ...
... Radiation is measured in "Gray" units, and a lethal dose of radiation is usually defined in terms of LD 50 , an amount of radiation able to kill 50% of the population of the test organism (Cox and Battista 2005). The ozone layer did not exist when the Earth was in an early stage of formation, but once it formed, it was able to prevent damaging radiation at wavelengths below 300 nm (Rothschild 2009); however, depletion of ozone has greatly affected the amount of radiation entering the atmosphere. Other sources of radiation are naturally occurring radioactive elements and man-made nuclear reactors (Rothschild 2009). ...
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Astrobiology seeks to expand our knowledge of life by investigating how microorganisms survive and thrive in extreme environments, allowing us to better assess the potential habitability of distant worlds. Recent developments in the study of extremophiles, solar system planetary exploration, and exoplanet discovery and analysis are providing fresh insights into astrobiology and the possible distribution of life on other planets. Extraterrestrial environments frequently feature extreme conditions spanning multiple factors simultaneously. In order to survive in such complex and extreme conditions organisms need to develop adaptations that enable them to withstand a wide range of challenges. Therefore, a critical next step toward understanding the true limits of habitability is the study of polyextremophiles, or microorganisms that can survive under multiple extreme conditions simultaneously. This chapter outlines various extreme environments on Earth and the types of extremophiles found in these conditions. This chapter also explores the interesting world of polyextremophiles and the strategies they have evolved that enable them to thrive in these environments. Polyextremophiles can serve as invaluable model organisms in astrobiology, offering insights into the possibilities of life beyond Earth and to gain valuable knowledge for future space exploration missions.
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View
... Exploration of extremophiles and their biodiversity in order to make them useful for developing processes and products to serve humanity begins with the categorization of these organisms (MacElroy 1974;Rothschild 2007). Studies have not even scratched the surface in identifying extremophiles from natural habitats: less than 1 % of the organisms have been identified and even fewer have been sequenced for their beneficial properties. ...
Extremophiles and Their Applications in Medical Processes
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Extremophiles are known to thrive under harsh environmental conditions. Many extremophilic bio-products are already used as life-saving drugs. Recent technological advancements of systems biology have opened the door to explore these organisms anew as sources of products that might prove useful in clinical, environmental and drug development.
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... Exploration of extremophiles and their biodiversity in order to make them useful for developing processes and products to serve humanity begins with the categorization of these organisms (MacElroy 1974;Rothschild 2007). Studies have not even scratched the surface in identifying extremophiles from natural habitats: less than 1 % of the organisms have been identified and even fewer have been sequenced for their beneficial properties. ...
Survival Mechanisms of Extremophiles
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  • Oct 2015
It is vital for extremophiles to cope with their environments making them viable to withstand under harsh environmental conditions. Extremophiles are known to adapt to the changes in their environment and surroundings that enable them to stabilize the changes in their homeostasis. The adaptability of extremophiles arrives from alteration of varying genes and proteins. Extremophiles produce extremolytes, which helps them to maintain their homeostasis such as ectoine-mediated mechanism, which is produced by halophiles and organisms alike. Evolutionary diversity, increased catalytic activity, amino acid accumulation, aggregation resistance strategies, resistance to cell death, activation of the nuclear factor, the use of heat shock proteins, and cellular compartmentalization, are all vital tools that extremophiles take on in order to conserve their genes.
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Each recent report of liquid water existing elsewhere in the Solar System has reverberated through the international press and excited the imagination of humankind. Why? Because in the past few decades we have come to realize that where there is liquid water on Earth, virtually no matter what the physical conditions, there is life. What we previously thought of as insurmountable physical and chemical barriers to life, we now see as yet another niche harbouring 'extremophiles'. This realization, coupled with new data on the survival of microbes in the space environment and modelling of the potential for transfer of life between celestial bodies, suggests that life could be more common than previously thought. Here we examine critically what it means to be an extremophile, and the implications of this for evolution, biotechnology and especially the search for life in the Universe.
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'Replaying the tape' is an intriguing 'would it happen again?' exercise. With respect to broad evolutionary innovations, such as photosynthesis, the answers are central to our search for life elsewhere. Photosynthesis permits a large planetary biomass on Earth. Specifically, oxygenic photosynthesis has allowed an oxygenated atmosphere and the evolution of large metabolically demanding creatures, including ourselves. There are at least six prerequisites for the evolution of biological carbon fixation: a carbon-based life form; the presence of inorganic carbon; the availability of reductants; the presence of light; a light-harvesting mechanism to convert the light energy into chemical energy; and carboxylating enzymes. All were present on the early Earth. To provide the evolutionary pressure, organic carbon must be a scarce resource in contrast to inorganic carbon. The probability of evolving a carboxylase is approached by creating an inventory of carbon-fixation enzymes and comparing them, leading to the conclusion that carbon fixation in general is basic to life and has arisen multiple times. Certainly, the evolutionary pressure to evolve new pathways for carbon fixation would have been present early in evolution. From knowledge about planetary systems and extraterrestrial chemistry, if organic carbon-based life occurs elsewhere, photosynthesis -- although perhaps not oxygenic photosynthesis -- would also have evolved.
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Exploring the Origin, Extent and Future of Life
  • Jan 2009
  • 113
  • L J Rothschild
Rothschild, L. J. 2009, in: C. Bertka (ed.), Exploring the Origin, Extent and Future of Life, (Cambridge: Cambridge University Press), p. 113
  • Jan 2008
  • 2787
  • L J Rothschild
Rothschild, L. J. 2008, Phil. Trans. Royal Soc. B 363, 2787