Requirements for Life
George Lebo
23 October 2012
AST 2037
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Life: What is it?
• Things with the ability to reproduce AND the ability to evolve
and adapt
• Why both of these?
• Flames can spread or “reproduce”, but they aren’t alive
• Crystals (i.e. salt) can also spread or grow, but they aren’t alive
either
• Only living things evolve – meaning develop adaptations to
their environment that improve their ability to continue
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2
Implications
• Need: an energy source
• Something to power the “doing” of things
• Including reproduction
• Need: means of reproduction
• Access to material components of life
• Way of passing on the information about the structure of
life (“genetic code”)
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Energy Source
• Want something easy to make, easy to store, capable of
making things happen in a “typical” environment
• Options:
• Nuclear energy? (Requires 10,000,000K and high
pressure)
• Solar energy? (Hard to store light)
• Thermal energy? (tends to “leak” out; hard to store)
• Kinetic energy? (hard to store)
• Chemical energy? Works!!
4
Genetic Code
• Need lots of ability for variation in the code (especially if
adaptation/evolution are important)
• Need ways of “writing” and “reading” code
• Likely solution: chemical coding (like DNA)
• Need large/complex chemical molecules
• What element is really good at making complex chemical
molecules?
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Medium of Life
• Solids? Chemical reactions are very slow in most solids
• Gas? Chemicals are often (not always) easily dispersed in
air/gas
• Liquid? Chemical reactions can proceed quickly, while
density of reacting materials stays high
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Solvent
• Chemical that can break
apart solids into liquid
phase
• Chemical that can separate
and mix apart many
complex structures into the
liquid phase
• What is the best solvent
known in the world?
• (Not molecular acid)
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Summary
• Need energy source and reproductive code
• Likely energy source: chemical energy
• Reproductive code: likely chemical, and requires complex
molecules/chains
• A little weaker: May have a preference for liquid phase?
• Probably need a powerful solvent
• At the risk of seeming Earth-centric: carbon does a great job
of storing chemical energy and forming complex molecules
suitable for reproduction; water is a GREAT solvent
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Extreme Life on Earth
George Lebo
23 October 2012
AST 2037
9
Life on Earth
• So far, we have focused on “normal” life on Earth
• The sort of standard critters, plants, and bacteria we are used
to
• We will use this as a standard “baseline” for evaluating
conditions for life to develop elsewhere
• But …
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The Goldilocks Syndrome
• Earth is “just right” for this
sort of life
• Conversely, standard life is
“just right” for Earth
• Does that mean that life can
ONLY be that way?
• Or is it just that, because we
live on Earth, we mostly see
“Earth-standard” life?
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“Extreme” Life on Earth
• There are forms of life on Earth which seem “extreme”
compared to standard life
• These forms of life show how far life deviates from
“normal” and still survives and reproduces
• This gives us some idea of the limitations of life in the
Universe (at least Earth-like life)
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•
Extreme Life: Aquifex
Aeolicus
In the 1960’s, biologists
were interested in studying
“how extreme” life could
be
• They knew that microbes
lived in water downstream
from hot springs in
Yellowstone National Park
• The springs themselves
reached temperatures of
~85C (185 F) – near the
boiling point of water
• The question: How far
upstream (close to the
hottest water) could
microbes survive?
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High Temps: So What?
• What’s the Big Deal about life
at high temperatures?
• Experience says that putting
living creatures in boiling hot
water kills them
• Mmmmm … lobster!
• How?
• Denaturing of the proteins
• High heat causes proteins
to lose some of their
structural/chemical
properties
• Breaks down the structure
of the living cells
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Aquifex Aeolicus Surprise
• Biologists discovered
bacteria in the hottest
parts of the hot springs
themselves
• These creatures survive –
even thrive and
reproduce!! – at ~85C
(185 F), near the boiling
point of water
• Picture shows microbial
mats (as in stromatolites)
in Yellowstone hot spring
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Aquifex Aeolicus Properties
• These are very small bacteria
• Prokaryotes
• Genome structure is only 1/3 as long (complex) as E. coli (a
model “simple” bacteria)
• Single DNA molecule in a circular chromosome
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Aquifex Aeolicus Metabolism
• A. aeolicus survives from H, O,
CO2, and mineral salts
• Requires oxygen for
respiration (so, not that
primitive)
• But … no need for sunlight,
nor sunlight-using food !!
• Purely chemical food source
(in the presence of thermal
energy from the water)
The colors of Prismatic Spring in
Yellowstone come primarily from
the hyperthermophile microbes in it
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Archaea
• Genetic diversity studies show
that A. aeolicus is one of the most
“divergent” bacteria known
• I.e. it has little in common with
many of the other bacteria
• This and others led to the reclassification of 3 “Domains” of
life on the basis of genetic linkage:
• Archea
• Bacteria
• Eukaryota
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Archaea
• Very small critters (~1 micron in length)
• No nucleus (like bacteria)
• Different tRNA from bacteria and Eukaryotes (which have
same tRNA as each other)
• Cell structure LOOKS like other cells, but made from different
chemicals
• All bacteria/eukaryotes use D-glycerol isomers; Archaea only
use L-glycerol
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Archaea & Extremophiles
• Archaea are typically “primitive” organisms
• Most single-celled “extremophiles” are members of archaea
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Chemosynthesis
• Energy generation NOT dependent on sunlight
• Often (but NOT always) depend on other critters
• A. aeolicus survives by pure chemosynthesis (no
photosynthesis; no eating other life forms)
• Types of chemosynthetic life:
• Methanogens (Methane)
• Halophiles (Salt)
• Sulfur reducers
• Thermoacidophile (i.e. Aquifex aeolicus)
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Methanogens
• Things that use chemosynthesis to survive, and produce
methane (CH4) as a by-product
• Well-known examples:
• Swamp gas bubbles (methanogen byproduct)
• Flatulence (bovine, human) – mmmm … Tijuana Flats!
• Methanogens typically only thrive (and only survive for long)
in environments where other “chemically aggressive”
elements (like O) are rare
• Methanogens have been found thriving as slime mats on deep
rocks below Earth’s surface (endoliths)
• Also found in extreme cold/dry desert environments
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Halophiles
• Microbes that survive by
chemosynthesis in VERY
salty water (i.e. 5x to 10x
that of ocean water)
• Locations:
• Great Salt Lake (Utah)
• Dead Sea (Israel/Jordan)
• Owens Lake (California)
• Evaporation estuaries in
San Francisco Bay
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Black Smokers – Sulfur Reducers
• Black smoker vents
• Found in deepest parts of the
ocean
• Volcanic, mineral-enriched
water outflows
• Rich in iron, sulfur
compounds
• Very little/no oxygen
• Discovered in the 1970s
• Temps as high as 750 F (!!)
• Does not boil, though, due to
extreme pressure at this depth
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Black Smoker Structure
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Black Smoker Ecology
• Deep sea exploration vehicles investigate black smokers in
the 1980’s
• Much to everyone’s surprise, they find LIFE !!
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Black Smoker Ecology
• Not just life – fully-developed ecosystems!
• Crabs, shrimp, clams, Pompeii worms
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Pompeii Worms
• Tube worms anchored near black smoker vents
• Bottom end has very high temps; top end more like 70F
• Hot water flows through tubes; length as much as 10 feet!
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Pompeii Worms
• “Hairy” back is heat-resistant microbe mat (symbiotic with
worm mucus)
• Red “feathers” include hemoglobin; separates hydrogen
sulfide from vent flow
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What feeds the ecosystem?
• Sulfur-reducing extremophile archaea!
• Metabolism centers on hydrogen sulfide (not oxygen, nor
CO2!)
• Pompeii worms (and some clams) seem to have symbiotic
relationship with microbes
• Worm “feathers” gather H2S and bring it into tube, where
billions of microbes live
• Microbes “digest” minerals with sulfur metabolism, releasing
CO2 byproduct
• Worm uses CO2 to digest minerals as well
• Other life forms live on microbes, worms, etc.
• Worms may live as long as 200+ years (!)
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Summary
• Life is weird
• Extremophiles are found everywhere from petroleum
reservoirs to the Dead Sea to hot springs to deep sea vents
• Most single-celled extremophiles are Archaea
• Genetically distinct from eukaryota and bacteria
• tRNA differences and chemical differences too
• Metabolism may be oxygen-independent (even oxygenphobic!)
• Black smoker ecosystems show tremendous diversity, with
basis in (and symbiotic relationships with) sulfur-reducing
Archaea
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