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. Next

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 Next

Some organisms can tolerate high temperatures, like the microbes at Lassen Volcanic National Park. These thermophiles are distributed across all Domains of life. Next

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 Next

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. Next

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. Next

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. Next

Some organisms can tolerate acidic conditions, such as the microbes at Devil’s Kitchen. Next

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). Next

Other acidophiles actively pump hydrogen ions out of the interior of the cell to maintain a neutral pH IN Cell exterior acidic pH Cell interior neutral pH Next

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?

Lassen Astrobiology Student Intern Program