encyclopedia of life sciences || astromicrobiology

AstromicrobiologyAlfonso F Davila, Carl Sagan Center for the Study of Life in the Universe, Mountain View,

California, USA

Astrobiology is the scientific discipline that focuses on the

origin, evolution and distribution of life in the universe.

Most efforts aiming at understanding the origin of life on

Earth, and the possibility that life might exist elsewhere,

are directed to the study of microorganisms. This is so

because on Earth, microorganisms are the most abundant

and widespread forms of life, and are seemingly able to

colonise virtually every environment that can support life.

Furthermore, small and relatively simple cells are more

likely to originate first on a planet than large and complex

multicellular organisms, and are also more likely to be

transported from one planet to another in the process of

panspermia. Hence the term Astro(micro)biology. In this

article I introduce and summarise the main fields of

research in Astro(micro)biology.

Introduction

Astrobiology is the scientific discipline that studies theorigin, evolution and distribution of life in the Universe(DesMarais et al., 2008). Earth is the only planet known tohost an active, complex and widespread biosphere, andtherefore our understanding of the forces and conditionsthat control the origin and evolution of life in planetarybodies is largely based on what we know about the originand evolution of life in our own planet. If we look back atthe history of terran life, microorganisms appear as themost likely and widespread form of life in the Universe.This claim is supported by five facts: (i) life on Earth musthave originated as relatively simple unicellular micro-organisms; (ii) for 90%ofEarth’s history, the only forms oflife on Earth were individual cells, or colonies and aggre-gates of unicellular organisms; (iii) microorganisms havecolonised virtually every environment that can support lifeon Earth. This includes environments with extreme

temperatures, pH or salinity, where multicellular organ-isms are rare or absent; (iv)microorganisms aremore likelyto be transported from one planet to another in asteroidsandother cometary bodies, a process knownas panspermiaand (v) after three successful landings on other planetarybodies of the solar system (Moon, Mars and Titan), andafter having mapped the surface of several planets andmoons in detail, there has been no evidence ofmacroscopicforms of life (alas, nor microscopic!). Most experiments tounderstand the origin of life and the limits of life on Earth,which can then be extrapolated to other planets, are betterconducted withmicroorganisms, which generally speakingrespond faster to experimental conditions and providestatistically relevant results. Taking this into account, alltheories about the origin of life on Earth, and all efforts tosearch for life beyond our planet, are mostly focused onmicrobial life, whereas exceptions exist, as best exemplifiedby the SETI Institute (Search for Extraterrestrial Intelli-gence). Hence the term Astro(micro)biology. See also:Microorganisms; Origin of LifeAstromicrobiology is not just an academic term, but also

a discipline with a very specific set of hypotheses, meth-odologies and tools that strictly speaking apply mostly tomicroorganisms. The broader term of Astrobiology mightalso be used to include hypotheses and experiments relatedto the response and behaviour of higher organisms such asinsects, plants and humans, to the environments of space.In either case, studying the phenomenon of life in theUniverse requires an interdisciplinary approach thatcombines molecular biology, ecology, planetary science,astronomy, information science, space exploration tech-nologies and related disciplines. In the following text I willoutline and summarise some of the main focus areas in thefield.

Planetary Exploration

Searching for life on another planetary body has been, andcontinues to be, themain goal ofNASA’s and ESA’s Spaceprogrammes. Direct planetary exploration is fundamentalin the question of the origin and commonality of life in theUniverse, and currently, most efforts to search for lifebeyond Earth are focused on Mars. Other astrobiologicalsites in our Solar System include the moons Europa, TitanandEnceladus.Europa andEnceladus appear tohave largeamounts of liquid water underneath the ice-shells thatcover their surface. Titan, however, is the only planetary

Introductory article

Article Contents

. Introduction

. Planetary Exploration

. Origin and Evolution of Life

Online posting date: 15th September 2010

ELS subject area: Microbiology

How to cite:Davila, Alfonso F (September 2010) Astromicrobiology. In:Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester.

DOI: 10.1002/9780470015902.a0021899

This is a US Government work and is in the public domain in the United States of America.

ENCYCLOPEDIA OF LIFE SCIENCES. www.els.net. John Wiley & Sons, Ltd 1

http://dx.doi.org/10.1038/npg.els.0000460


body outside the Earth with liquids on the surface, albeit inthe case of Titan the liquids are composed of hydrocarbons(molecules of carbon and hydrogen), which are only stablein the liquid form at very low temperatures.

Mars is the prime target for the search for life beyond theEarth primarily because of the persuasive evidence for pastliquid water on the surface and indications of liquid wateractivity even in recent times. In addition Mars has anatmosphere that contains the essential elements, carbonand nitrogen, needed for life. Finally, the cold and dryconditions on Mars open the possibility that evidence forlife may be well preserved. The first, and so far the only,mission to search for life outside the Earth were the Vikingmissions (Figure 1). Both Viking landers were designed andequipped to search for extant microorganisms in theMartian soil. In the Labelled Release (LR) experiment theaddition of an aqueous solution of dilute organic com-poundswith radioactive 14C toMartian samples resulted ina rapid release of labelled gas. The process was virtuallyeliminated by prior heating of the samples at a sterilisingtemperature of approximately 1608C for 3 h, and wassubstantially reduced by heating to only 45–508C (LevinandStraat, 1981). The results from theLRexperimentwereconsistent with biological activity; however, the gas chro-matography-mass spectrometry (GC-MS) instrument onboard the Viking landers did not detect conclusively anyindigenous organics in any of the samples tested at levelsdown to what was thought to be the parts per billion range(Biemann, 1979), which argues against the presence of anysubstantial biomass in the soils. The current consensus isthat the Viking missions failed to detect Martianmicroorganisms.

In the 35 years since Viking, and particularly during thelast decade, robotic missions to Mars (both orbiters andlanders) have focused on characterising the physical,chemical and geologic environment, as a necessary pre-amble to search for life in an efficient and systematicmanner. Much of this effort has focused on understandingthe history of liquid water on the planet. Water is thenecessary ingredient for Earth’s type of life, and has playedan important role in the origin and development of life onEarth. Thus one of the primary focus of planetary explor-ation is concerned with planets having a liquid waterboundary layer, although the focus may expand to includeother planets or satellites as astro(micro)biology maturesas a discipline (Des Marais et al., 2008). NASA’s Mars

Exploration Rovers Spirit and Opportunity provided forthe first time evidence that liquidwater was once present onthe surface of the planet. This has been later confirmed bytwo instruments in orbit around the planet: the Observa-toire pour la Mineralogie, L’Eau, les Glaces et l’Activite(OMEGA) onboard Mars Express, and by the CompactReconnaissance Imaging Spectrometer forMars (CRISM)onboard Mars Reconnaissance Orbiter (MRO) (Pouletet al., 2005; Mustard et al., 2008). Both instruments haveprovided evidence of widespread sedimentary deposits onand near the surface, which are indicative of large amountsof liquid water on the surface ofMars in the past. Recently,NASA’s Phoenix Mission, which landed in the northernlatitudes, sampled and analysed water ice from the surfacesoil (Smith et al., 2009). This water ice is very similar to thatfound in polar environments on Earth, such as the Ant-arctic Dry Valleys (see later discussion). Ice in the polarregions on Earth is a repository of microorganisms andbiomarkers, which can be preserved for long periods oftime due to the constantly cold temperatures. Therefore,the ground ice analysed by the Phoenix lander would be anideal substrate in which to search for traces of a pastMartian biosphere, or even for frozen – albeit still viable –microbial cells. See also: Planetary Protection

Field analogue research

Some of the greater advancements in our understanding ofthe limits of life on Earth, and the potential for life else-where, come from research conducted in analogue envir-onments. Analogue environments are places on Earth thatshare physical, chemical, geological, mineralogical or anyother type of environmental similarity with another plan-etary body. More often than not, analogue environmentsare characterised by extreme environmental conditions,such as very high or very low temperatures, extreme dry-ness or salinity and extreme pH values. In this type ofenvironments the most common forms of life are micro-organisms, and their strategies to adapt to the specialconditions in their environments are oftenoruncommonoreven unique. Mars has the closest environmental con-ditions to Earth in the Solar System, and for that reasonmost of the field analogue research occurs in Marsanalogue environments. Other more local environmentssuch as asphalt lakes can be used as a first approximation tomore alien environments, such as the hydrocarbon lakes

Figure 1 Panorama image of Chryse Planitia on Mars taken by the Viking Lander 1. Image credit: NASA/JPL.

Astromicrobiology


ENCYCLOPEDIA OF LIFE SCIENCES. www.els.net. John Wiley & Sons, Ltd2


observed on Titan. However, the widely different envir-onmental conditions between Earth and Titan warrant acertain degree of caution when drawing comparisonsbetween them. Analogue environments can also be appliedto a certain period in the history of a planet. For example, itis currently accepted that Mars was a wetter planet in thepast, but much of the liquid water was likely acidic (lowpH) (Fairen et al., 2004). On Earth there are naturallyacidic aqueous environments, such as Rio Tinto (Spain)which represents a window to that particular period ofMars’ history (Amils et al., 2007).

With respect to present day Mars, the best analogueenvironments on Earth are the Atacama Desert in Chileand the Dry Valleys of Antarctica (Figure 2). The AtacamaDesert is the oldest and driest desert on Earth. Theextremely dry conditions that characterise some parts ofthe desert, with mean annual precipitations below 1mm,have persisted for the past 4–5 million years. This has

resulted in a very unique type of adaptation,which has onlybeen recognised in the past 5 years. Although soils in thedriest parts of the Atacama Desert have very low levelsof bacterial cells and even organic compounds, the interiorof salt rocks contain high concentrations of diverse types ofmicroorganisms (Wierzchos et al., 2006; Figure 3), whichtake advantage of the hygroscopic properties of the salt(Davila et al., 2008), that is, the capability of the salt toextract water vapour from the atmosphere and form aliquid solution in its interior. This newly discovered nicherepresents a potential new environment in which to searchfor life on Mars, where similar deposits of salts have beenidentified, and exemplifies the utility of analogue researchin guiding space exploration (Davila et al., 2010). See also:CyanobacteriaThe Antarctic Dry Valleys (ADV), on the other hand,

are the coldest and at the same time driest regions onEarth,which makes them an excellent analogue to the surface of

Figure 2 (a) Panorama of the Atacama Desert, Chile (Photograph courtesy of Jacek Wierzchos). (b) Aerial picture of University Valley, Antarctica

(Photograph courtesy of Dale T. Andersen).

Astromicrobiology




Mars. Early microbial studies of ADV soils, using mostlyculture-based approaches, revealed a low abundance ofmicroorganisms and a limited number of taxa. In contrast,recent molecular-based phylogenetic studies have reporteda wide diversity of phototrophic and heterotrophicmicrobial communities (Cary et al., 2010). Some of thesesoils are very similar to soils analysed by the PhoenixLander in the northern latitudes ofMars. Soils in the ADVand the northern latitudes on Mars have an upper layer ofso-called dry permafrost – dry soil that never sees tem-peratures above freezing – underlined by a layer if ice-cemented soil or massive ice. By studying the physical andchemical properties of the soils in theDryValleys, and eventhe type of microorganisms that might live or be preservedwithin the ice, researches can have a better idea of what toexpect and search for in soils on Mars.

Orbit experiments

Orbit experiments are conducted in the environment ofSpace and many of them are aimed at investigating issuesrelevant to astromicrobiology. Environmental conditionsin Space include vacuum, high solar and cosmic radiation,microgravity and extreme temperature fluctuations. Mostof these conditions can be reproduced in the laboratoryindividually, but it is difficult to reproduce them collect-ively and for long periods of time.Hence the justification toconduct experiments in orbit. Many of these experimentsare aimed at investigating the evolution of organic com-pounds and organisms outside the protective shield of theEarth’s atmosphere and magnetic field. These studiesinform us of the potential to transfer organic compounds(and microorganisms) between planets. There are a num-ber of platforms to conduct these types of experiments,such as nano- and small satellites, which are small, light-weight satellites that can autonomously carry relativelycomplex experiments in orbit such as growing specific typesof organisms (Figure 4). Other orbit experiments aim tostudy the physical, chemical and biological evolutionundergone by meteorites during atmospheric entry, and

from that understand the potential for the passive dis-semination of life between planets using meteorites asvehicles (lithopanspermia) (Fajardo-Cavazos et al., 2005).Finally, in the past years, a number of experiments havebeen conducted in the International Space Station (ISS) tostudy the stability of organic compounds and the surviv-ability of organisms in space. These studies provideimportant information regarding the positive and negativeresponses of living organisms to long-term periods inSpace, and how to mitigate the later. This is particularlyrelevant for future long-duration manned-missions inSpace.

Studies of exogenous materials

Aside from the rock samples returnedbyApollo astronautsfrom the Moon, meteorites represent the main source ofextraterrestrial material available to scientist. Perhaps thebest example of the role that meteorites can play in astro-microbiology is that of the Martian meteorite ALH84001.The ALH84001 meteorite (Figure 5) was launched intointerplanetary space after an impactor hit the surface ofMars around 16 million years ago. It landed on the Ant-arctic continent around 13 000 years ago, where it wasdiscovered by an American expedition in the Allan Hillsregion in 1984. In a scientific paper published in the journalScience in 1996, D. McKay and collaborators proposedseveral lines of evidence for biological activity present inthe meteorite, including: (1) carbonate globules within themeteorite with textures similar to bacterially induced car-bonate crystal bundle precipitates, (2) the presence ofcomplex organic compounds, specifically polycyclicaromatic hydrocarbons (PAHs), (3) the coexistence ofiron-oxides, iron-sulfides and carbonates, (4) ovoid andbean-shaped structures that resemble fossilised ancientmicrobes and (5) magnetite particles that could haveformed through controlled biomineralisation processes.The authors pointed out that none of their single obser-vations was itself conclusive for the existence of past life onMars. Each of the observation had reasonable alternative

Figure 3 (a) Halite (sodium chloride) knob from the Atacama Desert with endolithic colonisation (green). (b) Light microscopy image of the cell colonies

(arrows) inside the halite.

Astromicrobiology



nonbiological explanations, but the totality of theirobservations considered collectively, particularly in viewoftheir spatial associations, was claimed to constitute evi-dence for relic biological activity on Mars (McKay et al.,1996). The seminal paper byMcKay et al. (1996) inspired alarge number of studies aiming to prove or disprove theevidence for the hypothesis of past life on Mars. Althoughsome lines of evidence favouring the biogenic hypothesishave been put into question, the presence of chains ofmagnetic single-domain particles remains plausible, albeit

controversial. Irrespectively of their origin in ALH84001,these structures are still considered important biomarkersand should therefore be searched for in samples returnedfrom Mars in the future. See also: Magnetotaxis: Micro-bial; UltramicrobacteriaAnother source of extraterrestrial material is a sample

returnmission. Samples collected remotely and returned toEarth offer a series of advantages versus samples analysedin situ, such as the quality and quantity of analyses that canbe performed in laboratories onEarth compared to roboticmissions. Actively returned samples are also more inter-esting than meteorites (passive return) because we havemore context information of the locality where the samplewas taken from and background information of the pos-sible origin of the material. Further, the returned samplewill not undergo chemical and physical alterations unlikemeteorites. Up to this day, Stardust remains the onlysample returnmission carried successfully (Brownlee et al.,2003). The Stardust mission collected samples of a cometand returned them to Earth for laboratory analysis.Comets are ancient bodies of frozen ice and dust thatformed beyond our solar system and have undergone littlemodifications since then. Therefore it is expected that theycontainmany of the materials that the solar system formedfrom. In fact, one of the major discoveries of Stardust wasthe finding of glycine, which is an amino acid used by livingorganisms to make proteins. This has important impli-cations for Astrobiology because it shows that cometsprovide some of the necessary building blocks of life duringplanetary accretion. NASA is also planning a samplereturn mission to Mars for this decade or the next. Thereturn of Mars samples would allow more extensiveanalysis of the samples compared to robotic missions onthe planet. Also, the presence of the samples on Earth

Figure 4 Payload of the PharmaSat nano-satellite. Photograph courtesy of NASA/ARC/Christopher Beasley.

Figure 5 The ALH84001 Martian meteorite. Scale bar represents 1 cm.

Astromicrobiology






would allow scientific equipment to be used on storedsamples, even years and decades after the sample returnmission.

Studies under simulated conditions

The technological complexities and the cost involved inconducting scientific experiments in orbit or other planet-ary bodies, justify the use of simulation chambers in whicha particular extraterrestrial environment can be partiallyreproduced, such as the interstellar environment or thesurface of a planet like Mars. Experiments can be con-ducted in these simulation chambers to evaluate specificscientific hypotheses or to test the performance of instru-ments that will be part of future space missions. Forexample, accurate and detailed information about theatmospheric and surface conditions on Mars obtainedfrom lander and orbiter missions has allowed to createproxy-Martian conditions in the laboratory and subjectsamples to these conditions.Mars simulation chambers aretypically capable to simulatewith a high degree of accuracyMartian conditions including surface pressure down to 0.1millibars; surface temperatures between 21008C and to+208C; atmospheric gas composition; and ultraviolet(UV) and visible radiation environments (200–900 nm).Advanced chambers can also simulate diurnal changes insurface temperatures. These types of chambers can be usedto conduct microbiology, geology and geochemistry stud-ies, for example, to evaluate the risk of forward con-tamination of Mars with terrestrial microbiota duringrobotic missions; to investigate the suitability of the Mar-tian environment for supporting life, or to understandaqueous processes (i.e. evaporation/sublimation of liquidsolutions) under current and past Martian conditions.

Extrasolar planets

Exoplanets and exomoons are planetary bodies locatedoutside the solar system. Up to this day nearly 500 exo-planets have been detected, many in solar systems withmultiple planets. These planetary bodies can only bestudied with powerful telescopes, such as the Large Bin-ocular Telescope Interferometer that is currently underconstruction in Arizona. All of the exoplanets detected sofar are larger than Earth, but space missions such as theKeplerMission are already able todetectEarth-size planetsand measure directly gases consistent with life such asozone and methane, of terrestrial-like atmospheres onplanets around stars up to 50 light years away (Schulze-Makuch and Irwin, 2008). Other missions such as Terres-trial Planet Finder (TPF) will be able to sample theatmosphere of extrasolar terrestrial planets in great detail(Heap, 2010). Given their increasingly large numbers andthe diversity of possible environments, extrasolar planetsare a likely candidate to provide the first evidence of lifeoutside the Earth. However, the assessment of the habit-ability of extrasolar planets is limited by our observationalcapabilities. Currently, the easiest measurable parameters

for extrasolar planets are their semi-mayor axis, eccen-tricity, inclination, period and mass (Jones, 2008), but thisinformation alone is not indicative of the presence orabsence of life. The atmospheric composition of a planetcan be used to assess its habitability. The combined pres-ence of water, oxygen, methane and carbon dioxide aresuggestive of an environment appropriate for complex life.Another important limitation when assessing the habit-ability of extrasolar planets is discerning surface features.For example, the presence of dense clouds can obscuresurface characteristic and have an impact on the surfacehabitability (Kitzmann et al., 2010). As technology evolveswe will be able to conduct more detailed and sophisticatedanalysis of exoplanets and exomoons, and with them, theprobabilities of finding other habitable or inhabited worldswill increase.

Origin and Evolution of Life

One of the central questions in Astrobiology is: How doesLife form and evolve through time? Until we find extantorganisms in another planetary body, this questions canonly be addressed based on what we know about life onEarth. Unfortunately, one of the geologic processes thathave facilitated the origin and diversification of terrestriallife, plate tectonics, is also an efficient recycling mech-anisms of ancient geologic terrains, and therefore there is avery limited number of samples in the geologic record thatdate back around the times when life first appeared onEarth. Based on our understanding of life on Earth andsome common sense, we can safely conclude that life onEarth first appeared as relatively simple organisms, moresimilar to unicellular organisms or viruses than to complexmulticellular ones. Subsequent evolution and naturalselection resulted in an increasing diversity of sizes andforms, and in adaptations that lead to multicellularity andan escalation in structural complexity.We can assume thatlife originating in another planet will likely have a similarstart and will be affected by similar evolutionary processes.However, this will remain an assumption until we findanother example of life that does not share a commonorigin with terrestrial life.

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Lunine JI (2005) Astrobiology. A Multi-Disciplinary Approach.

San Francisco: Pearson Addison Wesley.

Sullivan WT III and Barros JA (eds) (2007) Planets and Life. The

Emerging Science of Astrobiology. Cambridge, UK: Cambridge

University Press.

Astromicrobiology



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