Astrobiology

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Astrobiology Friday, February 13, 2009 Life in
Ice
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Life in Ice

• ice is the natural state or predominant form of
  water in our solar system
• the surfaces of most planets and moons are
  currently at temperatures well below the freezing
  point of pure water, including two of the more
  promising sites in the search for traces of
  extraterrestrial life, Mars and Europa
• the amount of water-ice on Europa exceeds the
  volume of liquid water on Earth
• comets are icy bodies
• Earth may have undergone a series of complete or
  near-complete glaciations in its history i.e.,
  Snowball Earth

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Life in Ice

• the presence of the liquid phase of water,
  however, is essential to the prospering of life
  as we know it
• in our Solar System, only Earth allows for a
  planetary surface with abundant liquid water

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Life in Ice
Mars may have some liquid water in permafrost
beneath its surface, as well as having polar ice
caps composed of water ice, and evidence of
glaciers at lower latitudes during certain periods
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Life in Ice

• Europa and perhaps other moons of Jupiter have
  water oceans below their icy crusts

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Why Investigate Life in Ice?

• studies of frozen environments on Earth further
  our understanding of fundamental constraints on
  the evolution of life at low temperatures
• also provide valuable information on the
  microbial diversity, the mechanisms for long term
  survival and activity of microbial cells at
  subzero temperatures, and the origin, evolution,
  limits, and detectability of life on Earth (e.g.,
  snowball Earth) and possibly on other planets
  such as Mars and Europa

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Cryosphere

• the cryosphere is the portion of the Earth where
  water is in solid form as snow or ice
• the terrestrial cryosphere consists of two parts
  glaciosphere (snow and ice) and frozen ground
  (permafrost)

(Gilichinsky 2008)
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Cryosphere

• because of the extremely harsh climatic
  conditions, these frozen environments had been
  considered for a long time to be devoid of life
  or serving merely as repositories for
  wind-transported microorganisms trapped in the
  ice
• increasing number of recent studies on the
  microbial ecology and diversity of natural ice
  samples have shown that permanently frozen
  environments harbor abundant, live and diverse
  microorganisms that may be detected and recovered
  by cultivation
• the cryosphere is important not only as an
  integral part of the global climate system, but
  as one of the major habitable ecosystems of
  Earths biosphere and as the best analogue for
  the search of extraterrestrial life

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Life in Ice

• ice does not preclude the simultaneous presence
  of a liquid phase
• ice is rarely if ever formed from pure water
• the impurities in water on Earth (e.g., salts)
  allow for the presence of liquid water as a
  significant fraction of the ice volume
• like virtually all natural waters on earth, this
  remaining liquid is inhabited by microorganisms
• the impurity effect allowing for liquid water
  holds true even at temperatures approaching the
  average surface temperature of Mars today (-55oC)

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Phase Diagram of Water
Phase diagram of pure water at high pressures,
showing the stability fields of liquid water (L)
and of ices I through VII. Ice I is the dominant
form of ice on Earth. Atmospheric pressure on
Earth corresponds to 10-3 kbar and the bottom of
a 1-km thick glacier to 0.1 kbar.
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Life in Ice

• water is bound in the solid phase on Earth in
  five major types of ice formations

(Deming Eicken 2008)
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Life in Ice

• the total amount of liquid water present at
  sub-freezing temperatures, as a result of salt
  and other impurities, within all ice formations
  exceeds the volume of freshwater flowing in all
  rivers

(Deming Eicken 2008)
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Liquid Water and Life on Planets

• temperature scale for the presence of liquid
  water on earth and for observed enzyme activity
  and growth of microorganisms (Bacteria and
  Archaea)
• more complex organisms (Eukarya) occupy a more
  restrictive thermal range
• average surface temperatures on Mars (-55oC) and
  Europa (-160oC) are also shown.

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Phase Diagram for the System NaCl-H2O

• the importance of water as a prerequisite for
  life on Earth derives in large part from the
  polar nature of the water molecule and its role
  as a solvent for ionic and other compounds
• the same molecular properties and electrostatic
  forces governing the interaction between water,
  ionic compounds, and many other types of organic
  compounds are also critical for freezing-point
  depression that allows the survival and activity
  of microorganisms at subzero temperatures

A saturated solution of NaCl depresses the
freezing point of water to -21.2oC.
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Single Snow Crystal
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Structure of Ice
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Nucleation and Growth of Ice Crystals
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Ice Formation
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Life in Ice

• at the lowest temperatures of ice on Earth, and
  thus for our best analogues to the frozen
  environments on Mars and Europa, salt or organic
  impurities are essential to the presence of
  liquid water within the ice
• thermal gradients across an ice formation can
  even allow fluids to flow within the ice on a
  scale relevant to microorganisms

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Life in Ice

• from a microbial perspective, even a fraction of
  a microlitre of water contained within a block of
  ice represents a luxurious water world
• critical to that water being supportive of
  ongoing metabolism and growth, however, is the
  presence of connections between numerous liquid
  niches an open system allows for the essential
  exchange of nutrients and waste products by
  diffusion or advection (carrying of materials or
  molecules vis small-scale currents)

Trapped mineral fragments associated with
microbial communities appear inside ice
(Credit Kjell Ove Storvik/AMASE)
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Tree of Life

• highlights branches containing cold-adapted
  species, whether psychrophilic (thick black
  lines) or psychrotolerant (gray lines)

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Microbiology of Ice

• bacteria are known to be present in significant
  numbers in all types of natural ice formations on
  Earth
• the diversity of bacteria in frozen environments,
  unlike other extreme environments such as
  hydrothermal vents, is poorly known
• the question is still unresolved whether
  microbial cells simply survive trapped frozen in
  glacial ice for hundreds of thousands of years or
  are they metabolically active and responsible for
  certain natural processes,

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Characteristics of Snow and Glacier Ice as
Microbial Habitats

• glacier ice is a unique ecosystem preserving
  microbial life and past climate changes
  chronologically for hundreds of thousands of
  years
• most of the glacier ice on Earth is represented
  by the ice sheets of Greenland and Antarctica
  corresponding to about 10 of Earths terrestrial
  surface and containing 77 of the fresh water on
  the planet
• glacier ice depths range from few hundred meters
  to 34 km with a gradual increase of temperature
  with depth, e.g., at the South Pole it ranges
  from about -50C on the surface to -6C to -10C
  in the deepest layers
• there are two types of microbial habitats in
  glacier ice - the liquid veins and the thin
  liquid film on the surfaces of mineral grains

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Characteristics of Snow and Glacier Ice as
Microbial Habitats

• another specific microbial habitat existing on
  the surface of glacier ice are cryoconite holes
• they contain abundant populations of active
  living organisms
• since each one of these mini-environments is
  usually spatially separated, cryoconite holes are
  drawing attention as model systems for microbial
  activity and adaptation to cold

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• SEM images of microbial populations, present in a
  3,043-m-deep Greenland ice core sample A,B small
  sized cells of different morphologies and a
  diatom fragment with smaller cells attached to
  its surface in the melted ice C,D thin
  filamentous and small coccoid cells in a
  50-day-old liquid medium culture along with thick
  filaments and cells with unusual structure E
  glacier ice isolate (Cryobacterium) with
  pleomorphic cell shapes F glacier ice isolate
  (Sphingomonas) embedded in extracellular
  material. (Photo credit V. Miteva)

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The Subglacial Lake Vostok System, Antarctica

• geophysical surveys in the Antarctica have
  revealed the existence of 145 subglacial lakes
• of all the subglacial Antarctic lakes identified
  to date, Subglacial Lake Vostok is by far the
  largest with a surface area gt14,000 km2, volume
  of 5,400 1,600 km3, and maximum depth of 800 m

Location of Lake Vostok, Antarctica
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The Subglacial Lake Vostok System, Antarctica
Location of Lake Vostok, Antarctica
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Possible Chemically-Driven Biogeochemical
Reactions in Lake Vostok
The melting of the basal ice provides crushed
sulfide and iron minerals and organic material
from the bedrock, and glacial ice provides a
constant supply of oxidants (O2 and NO3-),
nutrients, and organic material. Microbes,
minerals, and organic carbon are removed from the
lake via the accretion ice (southern portion of
the lake). Shown are oxic and
anoxic chemolithotrophic reactions (i.e., metal
sulfide oxidation). Fault vents may be present in
the, which could introduce significant amounts of
thermal energy, geochemical energy, and organic
carbon to the lake.
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Microbial Life in Ice, Lake Vostok

• microscopic analyses of samples from the accreted
  Lake Vostok ice revealed several different
  bacteria including (A) a coccoid-shaped
  bacterium (far right), (B) a rod-shaped
  bacterium, (C) an SEM image of a coccoid
  bacterium shown at a magnification of 1.5 x 105,
  and (D) an SEM image of the same rod-shaped
  bacterium as in (B). Karl et al. (2008)

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Bacterial Abundance in Ice
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Bacterial Diversity

• the diversity of bacteria in frozen environments
  such as in glacier ice and snow, unlike other
  extreme environments such as hydrothermal vents,
  is poorly known
• most of them are psychrophilic and
  psychrotolerant, many are oligotrophic and
  pigmented
• the majority are related to species known for
  their versatile metabolic properties and high
  resistance to stress associated with their
  environment, such as long-term freezing,
  freeze-thaw cycles, desiccation, solar radiation,
  and occupation of microniches

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Lowest Known Temperatures for Bacterial Activity
in Ice
(Deming Eicken 2008)
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Bacteria in Permafrost

• most colonized part of the cryosphere is
  represented by modern frost-affected soils and
  permafrost with cells adsorbed on organic or
  mineral particles

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Bacteria in Permafrost

• the term permafrost designates the permanently
  frozen groundsoil or rock that remains at or
  below 0C for at least two consecutive years

• it reflects a thermodynamic balance between
  ground surface temperature, which is controlled
  by air temperature, and the geothermal gradient

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Bacteria in Permafrost

• significant numbers of viable ancient
  microorganisms are known to be present within the
  permafrost
• they have been isolated in both polar regions
  from the cores up to 400 m deep and ground
  temperatures of -27C
• the age of the cells corresponds to the longevity
  of the permanently frozen state of the soils,
  with the oldest cells dating back to 3 million
  years in the Arctic, and 5 million years in the
  Antarctic
• they are the only life forms known to have
  retained viability over geological time

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Anaerobic Bacteria in Permafrost

• permafrost contains both aerobic and anaerobic
  bacteria
• in addition, the reducing conditions within the
  permafrost are more favorable for the
  preservation of anaerobic bacteria

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Anaerobic Bacteria in Permafrost

• micrographs of methanogenic permafrost isolates.
  Methanosarcina mazei strain JL01 a phase
  contrast image, bar 10 mm b ultrathin section,
  bar 0.5 mm. Methanobacterium sp. Strain M2 c
  phase contrast image, bar 10 mm d ultrathin
  section, bar 0.5 mm. Methanobacterium sp. strain
  MK4 e phase contrast image, bar 10 mm f
  ultrathin section, bar 0.5 mm. Pph, polyphosphate
  inclusions Clc, cyst-like cells (Photo of N.
  Suzina)

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Environmental Conditions in Ice

• high salinity (up to 250 g l-1)
• low temperature (as low as 20o C)
• poor light
• lack of nutrition

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Coping Mechanisms

• form into cysts robust, thick-walled dormant
  cells (e.g., dinoflagellates)
• hibernation
• produce organic compounds that act as antifreeze
• in high salt environments, may take up additional
  inorganic ions or produce organic compounds to
  prevent water loss from osmosis
• under low light conditions, produce more of their
  primary pigment (chlorophyll)

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Environmental Conditions on Land

• no visible life on surfaces of soil and rock
• in certain rock types (e.g., sandstone, marble),
  a narrow subsurface zone provides a favorable
  microclimate for microorganism colonization
• survive by changing their mode of growth -
  growing into the pore space and fractures of
  rocks
• mobilize inorganic ions in minerals by leaching
  (biomineralization)

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Carbonate
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• cyanobacteria present on the cleavage surfaces of
  calcite crystals isolated from metamorphosed
  carbonate rocks of Marblehead, Antarctica

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Prokaryotic Limit to Growth and Survival