lassen astrobiology student intern program. as we discussed in the previous lectures, the tree of...
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ASSIGNMENT 3:
ENVIRONMENTAL LIMITS OF LIFE
Lassen Astrobiology Student Intern Program
As we discussed in the previous lectures, the tree of life is a way of grouping
organisms based on how similar their DNA sequences are, and thus how
they’re related on a molecular level. This phylogenetic classification system is
different than the historic method of grouping organisms based on their
observable characteristics or phenotype.
When organisms are grouped according to similarities in their DNA sequences, we find that all life forms on Earth can be placed into three broad groups or
Domains.
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Each Domain contains organisms that can tolerate environmental extremes. By studying these organisms, we can
begin to understand the environmental limits of life.
Understanding the environmental limits of life helps to direct our search for life
within and beyond our solar system.
Jupiter’s moon Europa
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Some organisms can tolerate high temperatures, like the microbes at
Lassen Volcanic National Park. These thermophiles are distributed across all
Domains of life.
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These microbes have special adaptations that allow them to withstand the high
temperatures of the hot springs, fumaroles, and mud pots.
For example, the cyanobacteria in the alkaline spring near the Drakesbad Guest
Ranch can modulate their cell membranes to make them more stable
at higher temperatures.
The cell membrane is composed of a lipid bilayer
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The fatty acids that make up the lipid bilayer can be modulated between
unsaturated and saturated.
The cell membrane is composed of a lipid bilayer. The lipid bilayer is
composed of fatty acids.
Unsaturated fatty acids are less stable at higher temperatures because the double
bonds introduce a “kink” into the long hydrophobic tails.
This “kink” prevents the fatty acid molecules from packing close together. When the microbe experiences higher
temperatures, the increased heat makes the “loose” membrane very mobile and
unstable, interrupting critical cellular transport mechanisms. Think of what
happens when you pour olive oil (composed of unsaturated fatty acids
that are liquid at room temperature) into a hot pan...it smokes.
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In contrast, straight chain saturated fatty acids can pack tightly together and do not become excessively mobile upon
heating. For example, you have to heat bacon grease (composed of saturated
fatty acids which are solid at room temperature) to melt it. Therefore,
bacon grease is more stable at higher temperatures, similar to the saturated
fatty acids in the cell membranes of thermophiles.
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The Domain Archaea has a different method of stabilizing their cell
membranes....they can synthesize lipids that span the whole membrane. This effectively turns the lipid bilayer into a lipid monolayer, which can withstand
high heat.
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Some organisms can tolerate acidic conditions, such as the microbes at
Devil’s Kitchen.
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One example of an acidophile in the Domain Archaea is Sulfolobus. This
microbe grows in springs of pH 2-3 and temperatures of 75-80°C, making it a
thermoacidophile.
Sulfolobus can thrive in acidic environments because it has developed proteins that are adapted to low pH.
These proteins have a number of amino acids with positive charges. These
positive charges prevent destabilization of the proteins in the presence of positively charged hydrogen ions
(acidity).
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Other acidophiles actively pump hydrogen ions out of the interior of the
cell to maintain a neutral pH
IN
Cell exterioracidic pH
Cell interiorneutral pH
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The Tree of Life is the outcome of the environmental selection pressures that organisms have experienced on Earth
over geologic time.
Mars has experienced a different environmental trajectory than Earth has. Given the nature of the early aqueous environments on the red planet, what do you think the Martian Tree of Life
would look like?
ASSIGNMENT 4:
END OF ASSIGNMENT 4
Lassen Astrobiology Student Intern Program