Download 123 Biodata of Joseph Seckbach, author of the chapter “The

Biodata of Joseph Seckbach, author of the chapter “The Relevance of Halophiles and
other Extremophiles to Martian and Extraterrestrial Environments” and the
“Foreword” of this book.
Professor Joseph Seckbach is the initiator and chief editor of Cellular Origins, Life in
Extreme Habitats and Astrobiology (COLE) book series, and author of several chapters
in this series. He earned his Ph.D. from the University of Chicago, Chicago, IL (1965)
and spent his postdoctoral years in the Division of Biology at Caltech (Pasadena, CA).
Then he headed at the University of California at Los Angeles (UCLA) a team for
searching for extraterrestrial life. Dr. Seckbach has been appointed to the faculty of the
Hebrew University (Jerusalem, Israel), performed algal research, and taught biological
courses. He spent his sabbatical periods in Tübingen (Germany), UCLA and Harvard
University, and served at Louisiana State University (LSU) (1997-1998) as the first
selected occupant of the John P. Laborde endowed Chair for the Louisiana Sea Grant
and Technology transfer, and as a visiting Professor in the Department of Life Sciences
at LSU (Baton Rouge, LA).
Among his publications are books, scientific articles concerning plant ferritin
(phytoferritin), cellular evolution, acidothermophilic algae, and life in extreme
environments. He also edited and translated several popular books. Dr. Seckbach is the
co-author (with R. Ikan) of the Chemistry Lexicon (1991, 1999) and other volumes,
such as the Proceeding of Endocytobiology VII Conference (Freiburg, Germany, 1998)
and the Proceedings of the Algae and Extreme Environments meeting (Trebon, Czech
Republic, 2000); see: http://www.schweizerbart.de/pubs/books/bo/novahedwig051012300-desc.ht). His recent interest is in the field of enigmatic microorganisms and
life in extreme environments.
E-mail: [email protected]
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N. Gunde-Cimerman et al. (eds.), Adaptation to Life at High Salt Concentrations in Archaea,
Bacteria, and Eukarya, 123-136.
© 2005 Springer. Printed in the Netherlands.
THE RELEVANCE OF HALOPHILES AND OTHER EXTREMOPHILES TO
MARTIAN AND EXTRATERRESTRIAL ENVIRONMENTS
JOSEPH SECKBACH
The Hebrew University of Jerusalem
Home: P.O.Box 1132, Efrat 90435 Israel
1. Introduction
Life in various forms appears almost all over Earth, and most organisms live in
environments considered “normal” from our anthropocentric point of view. There are
other organisms, however, that are exceptional, living in very harsh conditions—they
are termed extremophiles. The extremophiles live in extraordinary habitats; some of
them are not able to grow in more moderate conditions or even to tolerate them for short
periods. Such harsh environments are lethal for most other organisms. The majority of
known extremophiles are unicellular—members of the domains Archaea and the
Bacteria; there are also a few members of the Eukaryotes (protists and multi-cellular
forms) among these exceptional organisms. The study of extremophiles and their
habitats increases our understanding of the limits of life and the nature of the first
organisms on early Earth, and in addition gives us an insight into the possibilities of
extraterrestrial life (which is a part of the study of astrobiology).
Astrobiology, an interdisciplinary research field, is the new scientific branch of
studies that includes the origin of life, various parameters of terrestrial environmental
conditions and their inhabitants (extremophiles), astronomy, astrophysics and the
possibilities for extraterrestrial life. In essence, astrobiology is all encompassing. From
comparison of the physical and atmospheric conditions on some celestial bodies with
the inhospitable niches and environments on Earth, we find that some extremophiles
could serve as candidates or analogues for extraterrestrial life in the Solar System and
beyond. Thus, the study and understanding of extremophiles is valuable for
astrobiology. Among the current celestial places suitable as candidates for finding past
and perhaps present life are Mars, the Jovian satellites such as Europa, and – for
exploring initial stages of life as they may have occurred on young Earth – Saturn’s
moon Titan. Thus extremophiles, living at the edge of life, may serve as models for life
forms in or beneath the surface of Mars, Jupiter’s satellites (e.g. Europa), and other
planetary bodies in the Solar System and beyond.
2. The Extremophiles
Most extremophiles are (micro)organisms that thrive in habitats which for other
terrestrial life-forms are intolerably hostile or even lethal environments. They are
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classified (see below) according to the conditions in which they grow as thermophiles,
hyperthermophiles, psychrophiles, halophiles, acidophiles, alkaliphiles, barophiles, etc.
(Table 1). In addition to the various extremophilic factors one may add that microbial
spores can withstand a variety of different hostile conditions in their dormant stage for
long periods of time. There are some limits in long-term dormancy, namely, thermal
decomposition and ionizing radiation (McKay, 2000). Extremophiles thrive in hot
niches, ice, salt solutions, acid and alkaline media; some may grow in toxic waste,
organic solvents, heavy metals while extremophiles are found as well in several other
habitats that were previously considered inhospitable for life and toxic for organisms
(see Table 1).
TABLE 1. The extremophiles on Earth and possibly on extraterrestrial bodies
Environmental
parameter
Salinity
Type
Location and Conditions
Examples
Halophiles
Cyanobacteria, diatoms,
flagellates, Dunaliella;
Halobacteriaceae.
pH
Acidophiles
In salt lakes (neutral, and
alkaline pH). In salt solutions
up to saturation. Synthesis of
glycerol to balance the
external osmotic pressure
pH 0 - 4 some hot springs
Alkaliphiles
pH >9 in African soda lakes
Psychrophiles
-20oC - <15oC
Thermophiles
50oC - 80oC
Hyperthermophiles
Barophiles
80oC - >100oC
Weight lovers
Piezophiles
Pressure lovers
High level of
radiation
Temperature
Hydrostatic
pressure
Radiation,
Ultraviolet and
ionizing radiation
Thriving in
organic
solvents and under
various
atmospheres
Gravity
Desiccation
Picrophilus oshimae,
Cyanidium caldarium,
Dunaliella acidophila,
Natronobacterium, Spirulina
sp., fungi
Psychrobacter,
Polaromonas, cyanobacteria,
diatoms and icy green algae,
some insects. Cyanidium,
prokaryotes as Mastigocladus
Pyrolobus fumarii (113oC)
Prokaryotic strain MT41
tolerates >100 MPa.
Cyanobacteria, desert plants
Deinococcus radiodurans
CO2 gas
Thriving at pure gas
Organic toxic
solutions
solutions of damaging agents
Hyper-gravity
>1 g
Hypo-gravity
<1g
Xerophiles
Anhydrobiotic condition
Low water activity
Cyanidiaceae (grown under
pure CO2)
Such microorganisms are not
known on Earth (this factor
may be related to extraterrestrial environments)
Prokaryotes, extreme
halophiles or endoliths, fungi
and lichens (a symbiotic
system), nematodes
Based on Oren and Seckbach (2001), Seckbach and Oren (2004), and other sources.
EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS
127
Many of the extremophiles cope with more than one environmental parameter, and may
be termed as multi- or poly-extremophiles (Rothschild and Mancinelli, 2001). For
example, the unicellular rhodophytan Cyanidium caldarium (Seckbach, 1994, 1999;
Seckbach and Walsh, 1999) and its cohorts are thermo-acidophiles growing in an acid
media (pH 0.5 to 3.5) at elevated temperatures (maximal temperature 57oC); they
tolerate salinity range from 3 to 10% (Pinto et al., 1994), 1 N H2SO4 (Allen, 1959) and
thrive under pure CO2 (Seckbach, 1994). Likewise the barophiles/piezophiles, the
weight/pressure lovers are also either thermophiles, living near hydrothermal vents at
the bottom of the oceans, or psychrophiles (cold lovers) in the depth of the seas and
oceans.
Among the extremophiles are representatives of all domains of life [Prokarya
(Archaea, Bacteria) and Eukarya]. Most extremophiles are prokaryotes (Archaea and
Bacteria), but this group also includes eukaryotic cells (Roberts, 1999). Among the
Eukaryotes are protists (e.g., algae, fungi, protozoa) and multicellular forms living in
some of these harsh habitats. Some extremophiles are relegated to a particular habitat
and are not able to survive milder conditions, while other are more flexible and can
grow also in “normal” environments.
A number of these drastic environments may represent the primeval conditions that
existed for the pioneering microbes evolving during the early periods in young Earth.
For example, hyperthermophiles live in very warm or hot conditions and obtain their
energy by heterotrophic and/or chemotrophic (with inorganic chemicals) metabolic
pathways, while thermophiles of less extreme temperatures (i.e., below 74oC) may even
be photosynthetic. The ability of thermophiles and hyperthermophiles to cope with
such extreme environments may agree with the idea proposed by several scientists that
the first organisms evolved in warm zones. The thermophiles are also placed at the
roots of the archaeal branch in the tree of life based on molecular sequences. They may
have evolved in the primordial soup of little ponds, in hot springs, inside rocks or even
at the hydrothermal vents at the openings found around the black smokers at the bottom
of the oceans. The hydrothermal vents are located at mid-ocean ridges where plates
spread apart and spew out hot water containing dissolved minerals providing material
and energy for a rich diversity of extremophilic life. Other extremophiles may have
evolved at great depth of beneath the surface environments. Living in such places,
these microorganisms (if they existed) may have been protected from the impact of the
asteroid bombardment on the planet and shielded from high doses of UV irradiation
during the early stages of Earth’s evolution. Huge extraterrestrial asteroids and
boulders from space hit young Earth between 4.5 Ga and 3.8 Ga (bya = billion years
ago). During this period, some microbes may have found refuge by descending into
underground areas where the temperature could reach ~93oC and around hydrothermal
vents it could well over 100oC. There is some evidence that life appeared when this
impact ceased at 3.5 to 3.8 Ga (Glausiusz, 2004). The conditions below Earth’s surface
were suitable for thermophiles deriving their energy via chemoautotrophic oxidation of
inorganic chemicals, e.g., Acidithiobacillus thiooxidans oxidizes sulfur or sulfide to
sulfate, A. ferrooxidans oxidizes ferrous to the ferric iron, while others oxidize
hydrogen to water, nitrites to nitrates. Other microorganisms may have survived by
blasting into space and returning to Earth later when the conditions became milder. The
fact that there is almost no limit in the habitats on Earth brings the astrobiologists to
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propose that extremophiles may serve as models for candidates for extraterrestrial life in
planets and moons of the Solar System and beyond.
Some enzymes from extremophiles are tolerant of extreme parameters such as heat,
salinity, acidity, alkalinity, and high or low pressures. They have been introduced
recently into applied biotechnology systems and industry. We can find the
extremophiles and their cellular productions in food and dairy products, health food,
medical and pharmaceutical lines, antibiotics, detergents, in fermentation processes, and
so on.
While this chapter covers most extremophiles, it does not deal with microorganisms
thriving under anaerobic conditions. The anoxic atmospheres appeared as the original
natural environment, while the oxygenated atmosphere is a secondary evolution. The
oxidative atmosphere evolved after the appearance of the oxygenic photosynthesis
process by cyanoprokaryota (Avers, 1989; Shapiro, 1986).
Additional reviews on extremophiles can be found in various chapters in the series
Cellular Origins, Life in Extreme Habitats and Astrobiology; see:
www.springeronline.com, and in Horikoshi and Grant (1998), Roberts (1999),
Rothschild and Mancinelli (2001), and Seckbach (1999, 2000a, 2000b, 2003) and
Seckbach and Walsh (1999).
2.1. HALOPHILES – THE SALT LOVERS
In the Solar System our Earth is considered the sole water-endowed planet, some 71%
of its area are seas and oceans, mostly salt water. Similar water bodies have not yet
been proven to exist outside the Solar System. The dawn of the appearance of the first
organisms is unknown. They were most probably simple cells belonging to the
Prokaryotes and ruled on Earth for 2–2.5 Ga (until the evolution of the eukaryotic cell).
In this “Prokaryotic world” the Archaea and Bacteria were the sole organisms on Earth
and they occupied almost every niche on the surface and subsurface of Earth. The
prokaryotic microbes may have evolved 3.6 bya within saline solutions in the oceans; it
seems, therefore, that at present too, all organisms “remember their roots” and use salt
in their cells. Brines are the medium for halophilic microorganisms, and such
environments occur ubiquitously on Earth.
Halophiles represent a class of
extremophilic organisms that are also adapted to high salt solution. Among the
hypersaline halophiles are photosynthetic green algae (such as Dunaliella species),
diatoms, and others. The enigmatic Cyanidiaceae, a group of thermo-acidophilic red
algae, including Cyanidioschyzon merolae, Cyanidium caldarium and Galdieria
sulphuraria tolerate NaCl solutions from 3 to 10% of NaCl (Pinto et al., 1994;
Seckbach, 1994, and see above). Among the halophiles at different ranges of pH are
also cyanoprokaryota, green and purple Bacteria, sulfur-reducing Bacteria, and
methanogenic Archaea.
Most halophiles live in the marine environments (containing water of 3% salt),
others thrive in inland lakes within bodies of salty water, while the hyper-halophiles
grow in very high concentration salt solutions. The rod shaped halobacteria strains
grow within a concentration of ~ 30% salt as salt concentration is present in the Dead
Sea (Israel-Jordan) and in the Great Salt Lake (Utah, USA). Other may tolerate
solutions of saturated salt solutions. As the water evaporates from salty aqueous bodies,
high concentrations of salt solutions remain, then only halophiles will remain and obtain
EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS
129
these areas for themselves. Vreeland et al. (2000) revived bacteria that were enclosed in
salt crystals for 250 million years. Assuming this report is authentic and fully reliable,
we may have a new record of halophilic life in dry condition for vast periods.
The halophiles have to react against the high gradient gaps in osmotic pressure of
their external solution vs. their internal cytosol medium. They have to avoid
plasmolysis – the process where cell aqueous solution escapes due to the hypertonic
external solution that causes the cytoplasm shrinkage. To avoid the escape
(plasmolysis) of cell liquid, the halophiles balance these concentration differences of the
solutes inside and outside the cell membrane by synthesizing intracellular organic
compounds (e.g., glycerol, betaines, sucrose, K+ ions, etc.). These compounds are
synthesized and/or accumulated in response to osmotic stress at high salinity.
Halophiles may regulate the movement of solutions by other means as well (Oren,
2002). The cell membrane may also regulate the osmotic pressure on both sides of the
plasmalemma.
Some Antarctic lakes are salty at different concentration and some can reach saline
concentration close to the Dead Sea level. Such saline lakes do not freeze during the
Antarctic winter even when the temperature declines to -50oC. For further literature on
the halophiles, see Oren (1999, 2002) and the chapters in this volume.
2.2. LIVING AT THE END RANGES OF TEMPERATURE AND pH
2.2.1. Thermophiles and Hyperthermophiles
The organisms living in warm or hot habitats are the thermophiles and
hyperthermophiles, respectively. On the tree of life these microbes are placed close to
the root of the prokaryotic position. It has been reported that among organisms there
are some differences in heat tolerance (Table 1). For example, the upper temperature
limit for eukaryotes is 62oC, while the thermo-cyanoprokaryotes can occur at < 70oC,
thermophilic bacteria may thrive at 70oC to <100oC and hyperthermophilic Archaea can
be detected at a maximum of 113oC (observed in the archaeon Pyrolobus fumarii – see
also Seckbach and Walsh, 1999 and tables therein). This latter temperature point is
considered the highest current temperature for life and might support the source where
life has originated. Under special treatments and conditions the maximum temperature
of life and the edge of stability can reach higher values, such as 121-125oC (Kashefi and
Lovley, 2003; Seckbach and Oren, 2004). Such archaeal anaerobes gain their energy
from reducing Fe+3 into Fe+2 via formation of magnetite.
The thermophiles thrive in geothermic sources, such as in hot springs, hot geysers,
or at hydrothermal vents at the ocean floors. Black smokers on the sea floor expel hot
water at 350oC, gases, minerals, and other materials from the inside of the Earth. One
of the microbes associated with the walls of black smokers is Methanopyrus that
produces methane (CH4) gas.
2.2.2. The Psychrophiles Like Cold and Chilly Environments
Life cannot grow where liquid water does not exist. On the opposite scale from the
spectrum of the thermophiles are the psychrophiles (cold-loving organisms) that grow at
very cold ranges of temperature, even below freezing some of these may live in icy
layers. In snow and ice layers one can find eukaryotic diatoms, green algae (e.g.,
Chlamydomonas nivalis) that “paint” snow with their pigmentation (green, red, orange).
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JOSEPH SECKBACH
Psychrophilic microorganisms are in permafrost of Siberia, in the Arctic zones, and in
the icy depth close to the Vostok Lake (Antarctica). The Antarctic sea is populated with
Bacteria, algae, and diatoms. In Antarctica the endolithic microbes—among them
Bacteria, cyanobacteria, algae, and the symbiotic association-lichens live a few
millimeters below the surface of rocks at very cold temperatures. In addition, at the
ocean floor are worms dwelling in methane-enriched environments.
The psychrophiles can tolerate severe cold environments at the level of –20oC
(Rivkina et al., 2000; Junge et al., 2004). Furthermore, lichens survive Antarctica
winters at –65oC. Many psychrophiles are also barophiles (poly-extremophiles) and
such microorganisms possibly could survive on Mars when the air temperature reaches
-10oC during daytime throughout the warmest part of the year.
2.2.3. Acidophiles
Acidophilic members are among all three domains of life. Some may be phototrophic
others heterotrophic – all require acid media. Seckbach (2000a) presented a table of the
lower pH limits for different groups of organisms. Most of the thermo-acidophiles are
prokaryotes (Archaea and Bacteria) with the exception of some fungi and the algal
members. Their habitats are in warm, hot (such as occur at volcanic fields, solfatara
fields, hot springs), ambient and cold areas. Some acidic habitats are formed by
microorganisms, such as after oxidation of elemental sulfur or ferrous ions and other
compounds by bacteria (e.g., Acidithiobacillus thiooxidans, A. ferrooxidans), which
leads to accumulation of sulfuric acid and ferric ions that precipitate Fe(OH)3 with
release of protons in various environments, according to the following equation:
2 Fe2+ + 0.5 O2 + 5 H2O Æ 2 Fe(OH)3 + 4 H+
Acid environments occur around active geothermal vents and springs, abandoned
mines, or from sulfuric compounds’ contaminations and other polluted places with acid
wastes turning the area to acidic.
Several poly-extremophiles thrive in various ranges of pH, from milder solutions to
very acidic media. Among the prokaryotes are thermo-acidic organisms (Oren and
Seckbach, 2001; Seckbach and Walsh, 1999; Weiss Bizzoco, 1999; see tables therein).
The archaeal hyperacidophilic Picrophilus oshimae and Picrophilus torridus grow
down to pH –0.06, the lowest level of pH known to support life. Thermoplasma
acidophilum, another archaeon, grows at pH 1.8-2 (min. at 0.4). The eukaryotic
thermoacidophilic red alga Cyanidium caldarium thrives at pH 0-3.5 (Seckbach, 1994,
1999) and tolerates 1 N sulfuric acid (Seckbach, 1994, 2000b). Some green algae are
acidophiles, such as Dunaliella acidophila at pH 0.5-3.0 (Pick, 1999), Chlamydomonas
acidophila (pH 1-2) or Euglena mutabilis (pH 1-5). Among diatoms, Pinnularia
braunii grows at pH 0-4. Cyanobacteria are known to thrive at neutral to alkaline pH.
Yet there are some exceptions, such as the Oscillatoria and Spirulina discovered in
Bavarian lakes at acidic condition measured a pH of 0.06 to 2.9 (Steinberg et al., 1998)
but not shown to grow at these levels (Castenholz, personal communication). Hundreds
of lakes in Sweden and Canada showed cyanobacteria present even in the most acidic
lakes (Steinberg et al., 1998) which are very oligotrophic. There are three species of
fungi growing at pH near 0 (Schleper et al., 1995): Acontium cylatium, Cephalosporium
EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS
131
spp., and Trichosporon ceribriae, which grow at pH –0.06 while their optimum growth
is at pH ~ 0.5.
These acidic pH values are only external to the cells; the intracellular cyto-solution
(cytosol) keeps its pH level around neutral pH (Beardall and Entwisle, 1984; Oren and
Seckbach, 2001; Seckbach, 2000a). The cell membrane rejects and prevents the H+
from entering the cell by using a variety of mechanisms for also pumping the protons
out. Strong cyto-proton pump or low proton membrane permeability regulates the H+
ions and stops them from entering the plasmalemma. The internal neutral pH has to
protect some cyto-constituents like DNA, ATP, and chlorophyll from the acidification
damage. Sasaki et al. (1999) reported about intracellular extract solutions from four
species of Dictyotales to be highly acidic (at pH 0.5 to 0.9), similar values of pH were
observed in some species of Desmarestiales (both groups are brown algae, Phaeophyta).
Their intracellular pH measured to be 0.5-0.9, because their vacuoles contain sulfuric
acid. Apparent these low values are due to the vacuolar content, but the true
cytoplasmic pH is supposedly much higher.
Further data on acidophiles are reported in volumes 1, 2, and 6 of Cellular Origins,
Life in Extreme Habitats and Astrobiology (COLE) and elsewhere. See website:
www.spingeronline.com
2.2.4. Alkaliphiles
In some soils and soda lakes in east Africa and under dry climates (such as in Mojave
desert), the soil is very alkaline. There are microorganisms (Archaea, Bacteria,
cyanobacteria, and protists) that prefer to live on the high scale of the pH (alkaline
levels). Such high pH levels were found together with high salt solutions. In highly
alkaline-saline lakes, as in Mono Lake, CA, are diatoms, and green algae such as
Nanochloris, Chlamydomonas and Dunaliella. The cyanobacterium Spirulina platensis
is an obligate alkaliphile, and reaches a high population at pH values of 11 and above.
Plectonema nostocorum has been reported to grow up to pH 13 which is considered the
highest level at which life has been found. The diatoms are prominent components in
the biota of many alkaline lakes.
These alkaliphiles keep the highly alkaline pH outside the cell and pump ions in and
out the cells (similar to the acidophiles) to keep a lower and milder intracellular milieu
which might reach up pH 9 (Horikoshi, 1998; see table 6.2 there) while the external pH
level is much higher. For further data on alkaliphiles see Horikoshi (1998), Oren and
Seckbach (2001), and Seckbach and Oren (2004). For additional data on alkaliphiles
see Horikoshi and Grant (1998). The alkaline hypersaline lakes and their inhabitants
may serve as analogues for ancient microbial habitats on Mars (see below).
2.3. XEROPHILES—THE DRY LIVING AND DESICCATED ORGANISMS
Some microorganisms have the ability to grow in low water activity and tolerate water
stress. Low water availability can cause high concentration of salt and other solutes.
Deserts and dry valleys of Antarctica or the Siberian permafrost support a variety of
microorganisms in dry conditions. A survey (Davis, 1972) has shown that
cyanobacteria (e.g., Nostoc sp.) survive desiccation for a record of 107 years. The
diatom Nitzschia palea and the chlorophytes Pleurococcus sp. and Cystococcus sp. were
shown to be viable after 98 years of dry storage in a herbarium.
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Spores have a low if any rate of metabolism, and remain in long-term periods of
dormancy before being revived. Well known are the cases claiming of viable bacterial
spores resuscitated from the stomachs of insects imbedded in amber dated at 25–40
million years (Cano and Borucki, 1995). If the report (Vreeland et al., 2000) of the
restoration to life of bacteria that had been encapsulated for 250 million years in a salt
crystal is supported by unquestionable ground, then we have strong evidence on the
long period during which life can be persevered.
2.4. ORGANISMS THRIVING AT HIGH PRESSURE
Barophiles (weight loving) and piezophiles (pressure loving) microorganisms both in
the subsurface of Earth and in the depths of the ocean are obligated to live
heterotrophically, chemoautotrophically or chemolithotrophically, in darkness and at
high pressure. This elevated hydrostatic pressure can reach over a thousand
atmospheres (at the bottom of the ocean). For every 10 m depth in the water column the
hydrostatic pressure increases by about 1 atmosphere (0.1 MPa). The pressure at the
deepest point in the ocean (at the bottom of Mariana Trench near the Philippines) is
about 110 MPa. The barophilic bacteria are poly-extremophiles being psychrophilic in
addition. Among the Archaea are the baro-thermophiles observed in the hydrothermal
vents; Themococcus barophilus grows at temperatures up to 100oC. Our knowledge
about barophiles must be expanded for us to understand more about this fascinating
group. For additional discussion on deep-sea Bacteria, see Yayanos (2000).
2.5. MORE EXTREME PARAMETERS FOR MICROORGANISMS
There are other factors of extreme environments that support life under various
atmospheres. Some algae thrive under pure CO2 (Seckbach, 1994), and other
microorganisms grow under ammonia (Siegel, 1999) or on hydrogen gas (e.g., the
methanogenic Archaea). Microorganisms can grow in environments with chemicals,
heavy metals, organic solvents that are lethal for most other organisms. Another
parameter is gravity, which might be relevant more to life in space or in celestial places,
rather than to earthling organisms.
3. Astrobiology
In recent years the new multidisciplinary field of astrobiology has been established.
Currently, in this area there are numerous books, publication of a plethora of articles in
various journals and proceedings of conferences, meetings, workshops and a great deal
of research being conducted. The wide variety of astrobiology-related disciplines
include prebiotic chemistry, astronomy, astrophysics, origin of life, microfossil records,
evolution, diversity of microorganisms, extremophiles, geobiology, molecular biology,
extremophiles and extraterrestrial possibilities of life. It also includes life in the
Universe, discovery of new stars, planets and satellites and the search for extraterrestrial
intelligent life (SETI). Astrobiology has been referred also as bioastronomy,
exobiology, extraterrestrial life and the Russian term cosmobiology.
EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS
133
The extremophiles that are ubiquitous may be considered as candidates for
inhabiting celestial places. Furthermore, if there is life to based on silicon instead on
carbon as well in solvents other than water (non-aqueous solvents), then life may
perhaps exist on planets and satellites. The most popular extraterrestrial bodies within
the Solar System that may carry living forms (as we know it), or their precursors are
Mars and the Jovian moons such as Europa. Titan, the satellite of Saturn, may serve as
a prebiotic laboratory resembling the Earth’s prebiotic periods. Europa has a very cold
environment (minus 162oC) and from its image one can distinguish huge cracked shells
of ice floating and moving on water. These ice sheets may cover and float over an
ocean of warm water originating from Europa’s hydrothermal vents that may reach a
couple of hundred degrees Celsius. The surface appearance of Europa resembles
Alaska and Antarctica with their frozen seas. If Prokaryotes can survive such severe
conditions on Earth, why not there in the heavenly bodies? One can ask the question:
How unlikely is it that life does not exist elsewhere?
Similar to Jupiter, Saturn is a gas planet surrounded by hydrogen and helium (with
traces of methane and ammonia). Its largest moon Titan is covered by a reddish fog
over the icy continent and contains organic molecules including hydrocarbon oceans
(while no oxygen is present in its atmosphere). Similar prebiotic organic chemistry may
have led to life on Earth.
3.1. MARS
Our knowledge about Mars has increased but still not complete. Wherever there is free
liquid water and other certain elements, there should be life. Water makes all cells
“running” in their enzymatic reactions, natural processes and other functions in all cells.
There are still debates among the astrobiologists whether Mars contains briny water on
the upper layers of the surface (Möhlmann, 2004) or whether the Martian water is
within layers present deeper in the subsurface. Such water might be accumulated in
salty oceans warmed up by volcanic sources, while it could be also located as hydrated
minerals. On Mars, similar to Earth, liquid water should goes together with life since
water is vital for cell metabolism, enzymatic reactions, processes as photosynthesis, etc.
The recently discovered methane in the Martian atmosphere provides encouragement
for the scientists searching for life (Formisano et al., 2004). This detection of Martian
methane may not originated from chemical reaction but rather it is believed that it may
have evolved from biological sources
The images of Mars indicate that in the past it was warmer (during the caldera’s age)
and wetter, with plenty of running water in rivers, canyons, lakes and large oceans. The
bulk of the Martian water might be tied up in the subsurface, either as permafrost in the
craters in the polar latitudes, or in deep aquifers as reservoirs of water (McKay and
Marinova, 2001). Possibly some forms of life may have flourished there in the past.
Perhaps spores from the ancient “green Martian Planet” are still present as dormant
spores and might be revived when environmental conditions will be improved by
terraforming (Graham, 2004). If Martian surface conditions are at present noninhabitable for living forms, the subsurface (lithologies in subterranean areas) may offer
the only dwelling place for extant life forms. Therefore, finding of liquid water on Mars
is vital for the presence of contemporary life on this planet.
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JOSEPH SECKBACH
Today the red planet is dry, cold (-50oC to +12oC); the Martian salty solutions are
lowering the freezing point of its water so that microbes might grow in liquid water.
Mars also has a very low atmospheric pressure (~ 6 to 10 mbars) and the oxygen in the
atmosphere is very low (<0.15%). Its atmosphere contains 95% CO2, 2.7% N2, 1.6 %
Ar and other minute ingredients. The soil of Mars contains mineral ions such as: Na,
Mg, Ca, Cl and K. Organisms require sulfur, potassium, sodium, chloride, potassium,
iron, and trace elements, all thought to be present on Mars.
Since it would be difficult for organisms to grow at the surface regions in the
cryosphere and under UV radiation of Mars, if life does exist on Mars today, it could be
located in the subsurface where warmer ground waters may be present. The thin
Martian ozone layer does not significantly block UV rays, but the radiations of very
short wavelengths penetrate all the way to the surface. If some species of Bacteria or
Archaea may overcome Martian conditions (similar to those Bacteria existing in
permafrost – see Möhlmann 2004), then this planet might perhaps harbor psychrophilic
life. Also, microbial dormant spores may have life preserved there in 3.5 to 4 Ga old
sediments that were left over from when the planet was warmer and had running water.
Earthling Arctic and Antarctic life forms wait for the icy coverage to melt into liquid
water (conditions that might be similar on Mars).
Mars (and other planets’ moons) might carry life in hydrothermal lakes under the
surface or in ice-rich regions similar to the way microbial life is harbored in the icy
layer and in subsurface warm water which is assumed to exist in Vostok Lake
(Antarctica).
As the early water evaporated from Mars, a high concentration of dissolved salt was
left so that only halophiles remained and had the planet for themselves. By
understanding osmophily (halophily) and extremophiles on Earth we may detect life
that lives or has lived on other planets. Such osmophiles are best suited for life in
brines and evaporates. Halophiles may possibly occur in the Martian icy soil, as they
thrive in the hypersaline ponds of Antarctica. Therefore, a preparatory search for extant
and extinct life of Mars should be a focused on osmophiles or on their fossilized
remnants (Mancinelli, this volume). All the answers about Martian life can be found
only after samples from Mars will be brought to Earth.
Recently there have been some proposals to create a greenhouse effect for warming
Mars towards restoring habitable conditions on this planet. Such terraforming ideas for
Mars have been published (Friedmann and Ocampo-Friedmann, 1995; McKay and
Marinova, 2001). Only in time will we know if such proposals are realistic or just
wishful dreams.
Is the future export or transfer of microbes from Earth to Mars practical? Some
scientists believe that the red planet is currently too cold and dry to allow the survival of
any terrestrial organisms. Terrestrial life forms could perhaps survive there only after
warming up the planet to a minimal level. Such warming of Mars can take place by
using artificial greenhouse gases (Marinova et al., 2004). Mars appears to have
sufficient water, if warmed, to form large bodies, and to once again be capable of
harboring life. There might be a possibility that genetic engineering may “create” and
convert organisms and equip them with special features to resist extraterrestrial
environments such as those existing on Mars. If introduced on Mars they may assist in
the terraforming there.
EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS
135
Acknowledgements
I thank Prof. Richard Castenholz, Prof. David J. Chapman, Dr. Rocco Mancinelli, and
Dr. Christopher McKay for their efficient reviews on this chapter and for providing
practical suggestions. I appreciate the kind invitation of Prof. Nina Gunde-Cimerman to
take part in the Halophile-2004 conference in Ljubljana (Slovenia) and I am grateful for
the generous support bestowed upon my attendance there.
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